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Adipose Tissue: Histology

Adipose tissue is a specialized type of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology that has both structural and highly complex metabolic functions, including energy storage, glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance homeostasis Homeostasis The processes whereby the internal environment of an organism tends to remain balanced and stable. Cell Injury and Death, and a multitude of endocrine capabilities. Multiple hormones Hormones Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. Hormones: Overview and Types, growth factors, and cytokines Cytokines Non-antibody proteins secreted by inflammatory leukocytes and some non-leukocytic cells, that act as intercellular mediators. They differ from classical hormones in that they are produced by a number of tissue or cell types rather than by specialized glands. They generally act locally in a paracrine or autocrine rather than endocrine manner. Adaptive Immune Response are expressed by adipocytes and their associated stromal cells and macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation. There are three types of adipose tissue, white adipose tissue, brown adipose tissue, and beige or “brite” adipose tissue, which is a transitional form. Brown adipose tissue is present mostly in the fetus and in young children, and its main purpose is thermogenesis. Small deposits of brown adipose tissue persist into adult life. White adipose tissue is the major adipose tissue in adults, and it plays a role in several disease states, most notably obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity, metabolic syndrome Metabolic syndrome Metabolic syndrome is a cluster of conditions that significantly increases the risk for several secondary diseases, notably cardiovascular disease, type 2 diabetes, and nonalcoholic fatty liver. In general, it is agreed that hypertension, insulin resistance/hyperglycemia, and hyperlipidemia, along with central obesity, are components of the metabolic syndrome. Metabolic Syndrome, and type 2 Type 2 Spinal Muscular Atrophy diabetes Diabetes Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus.

Last updated: May 5, 2022

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Overview

Definition

Adipose tissue is a type of loose connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology composed mainly of cells called adipocytes.

Composition

  • Adipocytes 
  • Stromal vascular fraction (SVF):
    • Preadipocytes
    • Fibroblasts Fibroblasts Connective tissue cells which secrete an extracellular matrix rich in collagen and other macromolecules. Sarcoidosis
    • Vascular endothelial cells
    • Immune cells (major—adipose tissue macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation)

Types of adipose tissue

Two classic types:

  • White adipose tissue:
  • Brown adipose tissue:
    • Brown color due to increased vascularity and more mitochondria Mitochondria Semiautonomous, self-reproducing organelles that occur in the cytoplasm of all cells of most, but not all, eukaryotes. Each mitochondrion is surrounded by a double limiting membrane. The inner membrane is highly invaginated, and its projections are called cristae. Mitochondria are the sites of the reactions of oxidative phosphorylation, which result in the formation of ATP. They contain distinctive ribosomes, transfer RNAs; amino Acyl tRNA synthetases; and elongation and termination factors. Mitochondria depend upon genes within the nucleus of the cells in which they reside for many essential messenger RNAs. Mitochondria are believed to have arisen from aerobic bacteria that established a symbiotic relationship with primitive protoeukaryotes. The Cell: Organelles
    • Present in large amounts during fetal life (~ 5% of body mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast of newborn Newborn An infant during the first 28 days after birth. Physical Examination of the Newborn)
    • Persists into adult life in much smaller amounts
    • Contains a large amount of small lipid droplets Droplets Varicella-Zoster Virus/Chickenpox (multilocular)
    • Generates body heat Heat Inflammation

3rd, recently described, transitional type:

  • Beige, “inducible,” or “brite” (= “brown in white”) adipose tissue
  • Transitional/ transformation Transformation Change brought about to an organism’s genetic composition by unidirectional transfer (transfection; transduction, genetic; conjugation, genetic, etc.) and incorporation of foreign DNA into prokaryotic or eukaryotic cells by recombination of part or all of that DNA into the cell’s genome. Bacteriology phase, when white adipose tissue converts to brown adipose tissue (browning of white adipose tissue) or vice versa by differential gene Gene A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. Basic Terms of Genetics expression, in response to thermogenic needs

Anatomic distribution

  • Subcutaneous fat Subcutaneous fat Fatty tissue under the skin throughout the body. Erythema Nodosum: beneath the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions
  • Visceral fat: around internal organs
  • Yellow bone marrow Bone marrow The soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells. Red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes. Bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. Bone Marrow: Composition and Hematopoiesis
  • Intermuscular
  • Breast tissue
  • Distribution of brown adipose tissue: 
    • Fetus: prominent in the posterior cervical, axillary, suprailiac, and perirenal regions; also in the interscapular and deltoid regions 
    • Adults: persists around the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy, adrenals, aorta Aorta The main trunk of the systemic arteries. Mediastinum and Great Vessels: Anatomy, mediastinum Mediastinum The mediastinum is the thoracic area between the 2 pleural cavities. The mediastinum contains vital structures of the circulatory, respiratory, digestive, and nervous systems including the heart and esophagus, and major thoracic vessels. Mediastinum and Great Vessels: Anatomy, and neck Neck The part of a human or animal body connecting the head to the rest of the body. Peritonsillar Abscess
Adipose tissue

Subcutaneous adipose tissue

Image: “Adipose Tissue” by Bruce Blaus. License: Public Domain, edited by Lecturio.

Development

Embryonic development

White adipose tissue: 

  • From 14 to 24 weeks of gestation: 
    • Adipocytes differentiate from perivascular undifferentiated mesenchymal cells or stem cells and progenitor cells and express peroxisome proliferator–activated receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors gamma/retinoid X receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors (PPARɣ/RXR) transcription factors Transcription Factors Endogenous substances, usually proteins, which are effective in the initiation, stimulation, or termination of the genetic transcription process. Stages of Transcription.
    • Adipocytes that arise in subcutaneous sites are molecularly and functionally different from those that arise in visceral sites.
    • PPARɣ is considered the master regulator of adipogenesis.
  • Precursor lipoblasts accumulate small lipid vacuoles in their cytoplasm, which fuse to become larger typical unilocular adipocytes of white adipose tissue. 

Brown adipose tissue:

  • Arises from skeletal myogenic progenitor cells, with similar close relationship Relationship A connection, association, or involvement between 2 or more parties. Clinician–Patient Relationship with angiogenesis Angiogenesis Bartonella, and express the transcription factors Transcription Factors Endogenous substances, usually proteins, which are effective in the initiation, stimulation, or termination of the genetic transcription process. Stages of Transcription PR domain–containing 16 (PRDM16) and PPARɣ coactivator 1 (PGC-1)
  • Lipoblasts accumulate small lipid vacuoles, which remain separate to become typical multiloculated adipocytes of brown adipose tissue.

Postembryonic development

  • Growth in number and size of adipocytes continues through puberty Puberty Puberty is a complex series of physical, psychosocial, and cognitive transitions usually experienced by adolescents (11-19 years of age). Puberty is marked by a growth in stature and the development of secondary sexual characteristics, achievement of fertility, and changes in most body systems. Puberty.
  • New adipocytes may continue to form throughout the lifespan, especially if overnutrition (hyperalimentation) occurs.
  • Stem cells and preadipocytes can divide; mature adipose cells Adipose Cells Fat Necrosis of the Breast cannot.
  • Healthy versus unhealthy fat deposits:
    • Subcutaneous white adipose tissue, especially that in the gluteal–femoral region, is considered metabolically healthy.
    • Visceral white adipose tissue is associated with inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation and increased risk of metabolic disease.

Gender Gender Gender Dysphoria differences

  • Begin in early childhood
  • Young girls have more adipose tissue than young boys
  • After puberty Puberty Puberty is a complex series of physical, psychosocial, and cognitive transitions usually experienced by adolescents (11-19 years of age). Puberty is marked by a growth in stature and the development of secondary sexual characteristics, achievement of fertility, and changes in most body systems. Puberty, the distribution and physiology differ.
  • Related to steroid hormone secretion Secretion Coagulation Studies

Females:

  • Estrogens and progesterone Progesterone The major progestational steroid that is secreted primarily by the corpus luteum and the placenta. Progesterone acts on the uterus, the mammary glands and the brain. It is required in embryo implantation; pregnancy maintenance, and the development of mammary tissue for milk production. Progesterone, converted from pregnenolone, also serves as an intermediate in the biosynthesis of gonadal steroid hormones and adrenal corticosteroids. Gonadal Hormones promote localization of more fat in the lower body.
    • Results in the gynecoid habitus of women (fat around hips, thighs, and buttocks)
    • These fat deposits are subcutaneous.
  • After menopause Menopause Menopause is a physiologic process in women characterized by the permanent cessation of menstruation that occurs after the loss of ovarian activity. Menopause can only be diagnosed retrospectively, after 12 months without menstrual bleeding. Menopause, women may store more fat around the waist and more visceral fat.
  • Women continue to accumulate fat relative to body weight throughout the teen years.
  • In premenopausal women, gluteal adipocytes are different because they: 
    • Undergo less lipolysis Lipolysis The metabolic process of breaking down lipids to release free fatty acids, the major oxidative fuel for the body. Lipolysis may involve dietary lipids in the digestive tract, circulating lipids in the blood, and stored lipids in the adipose tissue or the liver. A number of enzymes are involved in such lipid hydrolysis, such as lipase and lipoprotein lipase from various tissues. Nonalcoholic Fatty Liver Disease despite weight loss Weight loss Decrease in existing body weight. Bariatric Surgery because cells have more catecholamine-sensitive α2 receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors, which inhibit lipolysis Lipolysis The metabolic process of breaking down lipids to release free fatty acids, the major oxidative fuel for the body. Lipolysis may involve dietary lipids in the digestive tract, circulating lipids in the blood, and stored lipids in the adipose tissue or the liver. A number of enzymes are involved in such lipid hydrolysis, such as lipase and lipoprotein lipase from various tissues. Nonalcoholic Fatty Liver Disease
    • Have higher lipoprotein lipase Lipoprotein lipase An enzyme of the hydrolase class that catalyzes the reaction of triacylglycerol and water to yield diacylglycerol and a fatty acid anion. The enzyme hydrolyzes triacylglycerols in chylomicrons, very-low-density lipoproteins, low-density lipoproteins, and diacylglycerols. It occurs on capillary endothelial surfaces, especially in mammary, muscle, and adipose tissue. Lipid Metabolism (LPL) levels, so they capture more circulating triglycerides Triglycerides Fatty Acids and Lipids
    • Are larger

Males:

  • Fat in the lower body is reduced by androgens Androgens Androgens are naturally occurring steroid hormones responsible for development and maintenance of the male sex characteristics, including penile, scrotal, and clitoral growth, development of sexual hair, deepening of the voice, and musculoskeletal growth. Androgens and Antiandrogens, producing an android distribution of fat.
  • Store more abdominal (visceral) fat
  • More prone to “central” (visceral obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity)
  • Reach a peak in body fat content during early adolescence

Morphology and Histology

Morphology

White adipose tissue: 

  • Bright cadmium-like yellow, glistening, and greasy surface:
    • Yellow color imparted by carotenoids Carotenoids The general name for a group of fat-soluble pigments found in green, yellow, and leafy vegetables, and yellow fruits. They are aliphatic hydrocarbons containing 4 terpene subunits. Leukoplakia found in many vegetables and fruits 
    • Reflects the active metabolism of fat in white adipose tissue
  • Homogeneous Homogeneous Imaging of the Spleen diffuse appearance on cut section
  • Thin septae dividing poorly defined lobules Lobules Breasts: Anatomy
  • Thicker septae in areas of mechanical pressure (buttocks)
  • Variation in color indicates a pathologic process:
    • White or whitish yellow: fat necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage
    • Pale yellow: lipoma Lipoma A lipoma is a benign neoplasm of fat cells (adipocytes) and the most common soft tissue tumor in adults. The etiology is unknown, but obesity is a predisposing factor; genetics also play a role, with multiple lipomas occurring in various inherited disorders. Lipoma
    • Reddish to orangish yellow: angiolipoma Angiolipoma A benign neoplasm composed of a mixture of adipose tissue and blood vessels. Cowden Syndrome
    • Whitish yellow to white firmer nodules: liposarcoma

Brown adipose tissue: 

  • Reddish brown owing to abundant vascularity and numerous mitochondria Mitochondria Semiautonomous, self-reproducing organelles that occur in the cytoplasm of all cells of most, but not all, eukaryotes. Each mitochondrion is surrounded by a double limiting membrane. The inner membrane is highly invaginated, and its projections are called cristae. Mitochondria are the sites of the reactions of oxidative phosphorylation, which result in the formation of ATP. They contain distinctive ribosomes, transfer RNAs; amino Acyl tRNA synthetases; and elongation and termination factors. Mitochondria depend upon genes within the nucleus of the cells in which they reside for many essential messenger RNAs. Mitochondria are believed to have arisen from aerobic bacteria that established a symbiotic relationship with primitive protoeukaryotes. The Cell: Organelles within cells
  • Glandular lobulated appearance

Beige: lighter brown than brown adipose tissue

Microscopic appearance

White adipose tissue:  

  • A mature white fat cell Fat cell Cells in the body that store fats, usually in the form of triglycerides. White adipocytes are the predominant type and found mostly in the abdominal cavity and subcutaneous tissue. Brown adipocytes are thermogenic cells that can be found in newborns of some species and hibernating mammals. Lipoma is spherical; size: up to 120 μm in diameter (compare to RBC = 7 μm, lymphocyte = 12–16 μm, hepatocyte = 20–30 μm)
  • Cytoplasm: displaced and compressed to the perimeter by a single lipid vacuole; composed of 99% triglycerides Triglycerides Fatty Acids and Lipids
  • Nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles:
    • Displaced peripherally by a lipid vacuole
    • Oval, thin, and small, with central minute clear vacuole
  • Adipocyte Adipocyte Cells in the body that store fats, usually in the form of triglycerides. White adipocytes are the predominant type and found mostly in the abdominal cavity and subcutaneous tissue. Brown adipocytes are thermogenic cells that can be found in newborns of some species and hibernating mammals. Lipoma basement membrane Basement membrane A darkly stained mat-like extracellular matrix (ecm) that separates cell layers, such as epithelium from endothelium or a layer of connective tissue. The ecm layer that supports an overlying epithelium or endothelium is called basal lamina. Basement membrane (bm) can be formed by the fusion of either two adjacent basal laminae or a basal lamina with an adjacent reticular lamina of connective tissue. Bm, composed mainly of type IV collagen; glycoprotein laminin; and proteoglycan, provides barriers as well as channels between interacting cell layers. Thin Basement Membrane Nephropathy (TBMN): highlighted by reticulin and periodic acid–Schiff (PAS) stains 
  • Ill-defined lobules Lobules Breasts: Anatomy are separated by thin bands of collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology that are thicker in the buttocks.
  • Stroma: capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries: Histology (have intimate relationship Relationship A connection, association, or involvement between 2 or more parties. Clinician–Patient Relationship with each adipocyte Adipocyte Cells in the body that store fats, usually in the form of triglycerides. White adipocytes are the predominant type and found mostly in the abdominal cavity and subcutaneous tissue. Brown adipocytes are thermogenic cells that can be found in newborns of some species and hibernating mammals. Lipoma, similar to other endocrine organs), fibroblasts Fibroblasts Connective tissue cells which secrete an extracellular matrix rich in collagen and other macromolecules. Sarcoidosis, macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation, preadipocytes, stem cells
White adipose tissue

White adipose tissue: cells with flattened nuclei that are located peripherally

Image: “White adipose tissue” by OpenStax College, Anatomy and Physiology. OpenStax CNX. License: CC BY 4.0

Brown adipose tissue:

  • Organized into prominent lobules Lobules Breasts: Anatomy of cells
  • Lobules Lobules Breasts: Anatomy are divided by septa containing blood vessels and nerves.
  • Brown adipose tissue adipocytes: 
    • Multivacuolated and some univacuolated cells are present.
    • Size: 25–40 μm (size varies, depending on nutritional status)
    • Many large, round mitochondria Mitochondria Semiautonomous, self-reproducing organelles that occur in the cytoplasm of all cells of most, but not all, eukaryotes. Each mitochondrion is surrounded by a double limiting membrane. The inner membrane is highly invaginated, and its projections are called cristae. Mitochondria are the sites of the reactions of oxidative phosphorylation, which result in the formation of ATP. They contain distinctive ribosomes, transfer RNAs; amino Acyl tRNA synthetases; and elongation and termination factors. Mitochondria depend upon genes within the nucleus of the cells in which they reside for many essential messenger RNAs. Mitochondria are believed to have arisen from aerobic bacteria that established a symbiotic relationship with primitive protoeukaryotes. The Cell: Organelles (giving granular appearance to cytoplasm)
    • Nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles: spherical, centrally located in the multilocular cells
Brown adipose tissue

Brown adipose tissue:
Brown adipose tissue contains multivacuolated adipocytes and has a characteristic brown color due to marked vascularization (4–5x more than white adipose tissue) and numerous mitochondria Mitochondria Semiautonomous, self-reproducing organelles that occur in the cytoplasm of all cells of most, but not all, eukaryotes. Each mitochondrion is surrounded by a double limiting membrane. The inner membrane is highly invaginated, and its projections are called cristae. Mitochondria are the sites of the reactions of oxidative phosphorylation, which result in the formation of ATP. They contain distinctive ribosomes, transfer RNAs; amino Acyl tRNA synthetases; and elongation and termination factors. Mitochondria depend upon genes within the nucleus of the cells in which they reside for many essential messenger RNAs. Mitochondria are believed to have arisen from aerobic bacteria that established a symbiotic relationship with primitive protoeukaryotes. The Cell: Organelles. The adipocytes are smaller than the ones in white adipose tissue and the nucleus Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles is centrally located; white adipocytes are also scattered within normal brown adipose tissue.

Image: “Tecido adiposo multilocular brown adipose tissue” by Lucasmcorso. License: Public Domain

Beige fat: 

  • Intermediate microscopic appearance between white adipose tissue and brown adipose tissue
  • Varies depending on stage of induction

Physiology

Lipid storage

  • White adipose tissue is the largest and most efficient store of energy (as triglycerides Triglycerides Fatty Acids and Lipids).
  • LPL is synthesized by adipocytes and transferred to the adjacent endothelial cells.
  • LPL on the luminal surface of endothelial cells hydrolyzes triglycerides Triglycerides Fatty Acids and Lipids from chylomicrons (from the intestine) and very-low-density lipoproteins Lipoproteins Lipid-protein complexes involved in the transportation and metabolism of lipids in the body. They are spherical particles consisting of a hydrophobic core of triglycerides and cholesterol esters surrounded by a layer of hydrophilic free cholesterol; phospholipids; and apolipoproteins. Lipoproteins are classified by their varying buoyant density and sizes. Lipid Metabolism (from the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy).
  • Most of the released free fatty acids Fatty acids Organic, monobasic acids derived from hydrocarbons by the equivalent of oxidation of a methyl group to an alcohol, aldehyde, and then acid. Fatty acids are saturated and unsaturated. Fatty Acids and Lipids (FFAs) are taken up by the fat cells and re-esterified with glycerol phosphate Phosphate Inorganic salts of phosphoric acid. Electrolytes within the adipocyte Adipocyte Cells in the body that store fats, usually in the form of triglycerides. White adipocytes are the predominant type and found mostly in the abdominal cavity and subcutaneous tissue. Brown adipocytes are thermogenic cells that can be found in newborns of some species and hibernating mammals. Lipoma to form triacylglycerol.
  • Triacylglycerols Triacylglycerols Fatty Acids and Lipids are stored within the adipocyte Adipocyte Cells in the body that store fats, usually in the form of triglycerides. White adipocytes are the predominant type and found mostly in the abdominal cavity and subcutaneous tissue. Brown adipocytes are thermogenic cells that can be found in newborns of some species and hibernating mammals. Lipoma’s lipid droplet.
  • Stored triacylglycerols Triacylglycerols Fatty Acids and Lipids are mobilized by lipase Lipase An enzyme of the hydrolase class that catalyzes the reaction of triacylglycerol and water to yield diacylglycerol and a fatty acid anion. It is produced by glands on the tongue and by the pancreas and initiates the digestion of dietary fats. Malabsorption and Maldigestion, which hydrolyzes stored triglycerides Triglycerides Fatty Acids and Lipids.
  • Mobilization of triacylglycerols Triacylglycerols Fatty Acids and Lipids is controlled by both neural and hormonal factors:
    • Norepinephrine Norepinephrine Precursor of epinephrine that is secreted by the adrenal medulla and is a widespread central and autonomic neurotransmitter. Norepinephrine is the principal transmitter of most postganglionic sympathetic fibers, and of the diffuse projection system in the brain that arises from the locus ceruleus. Receptors and Neurotransmitters of the CNS:
      • Activates lipase Lipase An enzyme of the hydrolase class that catalyzes the reaction of triacylglycerol and water to yield diacylglycerol and a fatty acid anion. It is produced by glands on the tongue and by the pancreas and initiates the digestion of dietary fats. Malabsorption and Maldigestion
      • Denervated fat will continue to deposit even in a state of starvation.
    • Insulin Insulin Insulin is a peptide hormone that is produced by the beta cells of the pancreas. Insulin plays a role in metabolic functions such as glucose uptake, glycolysis, glycogenesis, lipogenesis, and protein synthesis. Exogenous insulin may be needed for individuals with diabetes mellitus, in whom there is a deficiency in endogenous insulin or increased insulin resistance. Insulin: stimulates lipid synthesis Synthesis Polymerase Chain Reaction (PCR) and inhibits lipase Lipase An enzyme of the hydrolase class that catalyzes the reaction of triacylglycerol and water to yield diacylglycerol and a fatty acid anion. It is produced by glands on the tongue and by the pancreas and initiates the digestion of dietary fats. Malabsorption and Maldigestion
    • Glucagon Glucagon A 29-amino acid pancreatic peptide derived from proglucagon which is also the precursor of intestinal glucagon-like peptides. Glucagon is secreted by pancreatic alpha cells and plays an important role in regulation of blood glucose concentration, ketone metabolism, and several other biochemical and physiological processes. Gastrointestinal Secretions and growth hormone: activate lipase Lipase An enzyme of the hydrolase class that catalyzes the reaction of triacylglycerol and water to yield diacylglycerol and a fatty acid anion. It is produced by glands on the tongue and by the pancreas and initiates the digestion of dietary fats. Malabsorption and Maldigestion 
    • Thyroid hormones Thyroid hormones The 2 primary thyroid hormones are triiodothyronine (T3) and thyroxine (T4). These hormones are synthesized and secreted by the thyroid, and they are responsible for stimulating metabolism in most cells of the body. Their secretion is regulated primarily by thyroid-stimulating hormone (TSH), which is produced by the pituitary gland. Thyroid Hormones
      • First increase lipogenesis Lipogenesis De novo fat synthesis in the body. This includes the synthetic processes of fatty acids and subsequent triglycerides in the liver and the adipose tissue. Lipogenesis is regulated by numerous factors, including nutritional, hormonal, and genetic elements. Nonalcoholic Fatty Liver Disease
      • Then cause lipolysis Lipolysis The metabolic process of breaking down lipids to release free fatty acids, the major oxidative fuel for the body. Lipolysis may involve dietary lipids in the digestive tract, circulating lipids in the blood, and stored lipids in the adipose tissue or the liver. A number of enzymes are involved in such lipid hydrolysis, such as lipase and lipoprotein lipase from various tissues. Nonalcoholic Fatty Liver Disease
    • Cortisol Cortisol Glucocorticoids’s effects depend on the physiologic context.
    • Androgens Androgens Androgens are naturally occurring steroid hormones responsible for development and maintenance of the male sex characteristics, including penile, scrotal, and clitoral growth, development of sexual hair, deepening of the voice, and musculoskeletal growth. Androgens and Antiandrogens are anti-adipogenic and estrogens are pro-adipogenic.
  • The released FFAs may be re-esterified or released to the circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment and bound to albumin Albumin Serum albumin from humans. It is an essential carrier of both endogenous substances, such as fatty acids and bilirubin, and of xenobiotics in the blood. Liver Function Tests for transfer to other cells.

Endocrine function

Adipose tissue produces a number of hormones Hormones Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. Hormones: Overview and Types and cytokines Cytokines Non-antibody proteins secreted by inflammatory leukocytes and some non-leukocytic cells, that act as intercellular mediators. They differ from classical hormones in that they are produced by a number of tissue or cell types rather than by specialized glands. They generally act locally in a paracrine or autocrine rather than endocrine manner. Adaptive Immune Response.

  • Leptin: 
    • Protein product Product A molecule created by the enzymatic reaction. Basics of Enzymes of the ob gene Gene A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. Basic Terms of Genetics
    • Acts on the hypothalamus Hypothalamus The hypothalamus is a collection of various nuclei within the diencephalon in the center of the brain. The hypothalamus plays a vital role in endocrine regulation as the primary regulator of the pituitary gland, and it is the major point of integration between the central nervous and endocrine systems. Hypothalamus to increase energy expenditure Energy expenditure Energy expenditure is the sum of internal heat produced and external work. Energy Homeostasis and decrease appetite 
  • Adiponectin:
    • Decreases gluconeogenesis Gluconeogenesis Gluconeogenesis is the process of making glucose from noncarbohydrate precursors. This metabolic pathway is more than just a reversal of glycolysis. Gluconeogenesis provides the body with glucose not obtained from food, such as during a fasting period. The production of glucose is critical for organs and cells that cannot use fat for fuel. Gluconeogenesis and stimulates glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance uptake
    • Modulates lipid catabolism
    • Obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity is associated with reduced levels of adiponectin.
  • Resistin:
    • Increases low-density lipoprotein Low-density lipoprotein A class of lipoproteins of small size (18-25 nm) and light (1. 019-1. 063 g/ml) particles with a core composed mainly of cholesterol esters and smaller amounts of triglycerides. The surface monolayer consists mostly of phospholipids, a single copy of apolipoprotein B-100, and free cholesterol molecules. The main ldl function is to transport cholesterol and cholesterol esters to extrahepatic tissues. Cholesterol Metabolism (LDL) levels
    • Associated with insulin Insulin Insulin is a peptide hormone that is produced by the beta cells of the pancreas. Insulin plays a role in metabolic functions such as glucose uptake, glycolysis, glycogenesis, lipogenesis, and protein synthesis. Exogenous insulin may be needed for individuals with diabetes mellitus, in whom there is a deficiency in endogenous insulin or increased insulin resistance. Insulin resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing
    • Proinflammatory effects
  • Aromatase: 
    • Converts androstenedione Androstenedione A delta-4 C19 steroid that is produced not only in the testis, but also in the ovary and the adrenal cortex. Depending on the tissue type, androstenedione can serve as a precursor to testosterone as well as estrone and estradiol. Androgens and Antiandrogens to estrone Estrone An aromatized C18 steroid with a 3-hydroxyl group and a 17-ketone, a major mammalian estrogen. It is converted from androstenedione directly, or from testosterone via estradiol. In humans, it is produced primarily by the cyclic ovaries, placenta, and the adipose tissue of men and postmenopausal women. Noncontraceptive Estrogen and Progestins, the major source of estrogen Estrogen Compounds that interact with estrogen receptors in target tissues to bring about the effects similar to those of estradiol. Estrogens stimulate the female reproductive organs, and the development of secondary female sex characteristics. Estrogenic chemicals include natural, synthetic, steroidal, or non-steroidal compounds. Ovaries: Anatomy in men and postmenopausal women
    • Expressed by the stromal cell fraction of adipose tissue and not the adipocytes
  • Interleukin-6 Interleukin-6 A cytokine that stimulates the growth and differentiation of B-lymphocytes and is also a growth factor for hybridomas and plasmacytomas. It is produced by many different cells including T-lymphocytes; monocytes; and fibroblasts. Interleukins (IL-6): (30% of IL-6 is from adipocytes, stromal cells, and resident macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation):
    • Stimulates hepatic triglyceride secretion Secretion Coagulation Studies and acute-phase reactants Acute-Phase Reactants Inflammation
    • Increases platelet number and activity
    • Increases expression of endothelial adhesion Adhesion The process whereby platelets adhere to something other than platelets, e.g., collagen; basement membrane; microfibrils; or other ‘foreign’ surfaces. Coagulation Studies molecules
    • Release Release Release of a virus from the host cell following virus assembly and maturation. Egress can occur by host cell lysis, exocytosis, or budding through the plasma membrane. Virology is induced by tumor necrosis factor Tumor necrosis factor Tumor necrosis factor (TNF) is a major cytokine, released primarily by macrophages in response to stimuli. The presence of microbial products and dead cells and injury are among the stimulating factors. This protein belongs to the TNF superfamily, a group of ligands and receptors performing functions in inflammatory response, morphogenesis, and cell proliferation. Tumor Necrosis Factor (TNF) α (TNF-α) (implicated in anorexia Anorexia The lack or loss of appetite accompanied by an aversion to food and the inability to eat. It is the defining characteristic of the disorder anorexia nervosa. Anorexia Nervosa of cancer).
  • Other cytokines Cytokines Non-antibody proteins secreted by inflammatory leukocytes and some non-leukocytic cells, that act as intercellular mediators. They differ from classical hormones in that they are produced by a number of tissue or cell types rather than by specialized glands. They generally act locally in a paracrine or autocrine rather than endocrine manner. Adaptive Immune Response
    • TNF-α:
      • Expressed in preadipocytes
      • Blocks differentiation to mature adipocytes
    • Fibroblast growth factor Fibroblast growth factor A family of small polypeptide growth factors that share several common features including a strong affinity for heparin, and a central barrel-shaped core region of 140 amino acids that is highly homologous between family members. Although originally studied as proteins that stimulate the growth of fibroblasts this distinction is no longer a requirement for membership in the fibroblast growth factor family. X-linked Hypophosphatemic Rickets 1: 
    • C3 and adipsin: proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis of the alternative complement pathway 
    • Plasminogen Plasminogen Precursor of plasmin (fibrinolysin). It is a single-chain beta-globulin of molecular weight 80-90, 000 found mostly in association with fibrinogen in plasma; plasminogen activators change it to fibrinolysin. It is used in wound debriding and has been investigated as a thrombolytic agent. Hemostasis activator inhibitor 1 ( PAI-1 PAI-1 A member of the serpin family of proteins. It inhibits both the tissue-type and urokinase-type plasminogen activators. Metabolic Syndrome): 
      • Potent inhibitor of the fibrinolytic system (thromboemboli increased)
      • Insulin Insulin Insulin is a peptide hormone that is produced by the beta cells of the pancreas. Insulin plays a role in metabolic functions such as glucose uptake, glycolysis, glycogenesis, lipogenesis, and protein synthesis. Exogenous insulin may be needed for individuals with diabetes mellitus, in whom there is a deficiency in endogenous insulin or increased insulin resistance. Insulin induces expression of PAI-1 PAI-1 A member of the serpin family of proteins. It inhibits both the tissue-type and urokinase-type plasminogen activators. Metabolic Syndrome by adipocytes.
      • Obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity is associated with elevated levels.

Heat Heat Inflammation production

  • Major function of brown adipose tissue: nonshivering thermogenesis; important for newborns
  • Brown adipose tissue is very vascular and regulated by sympathetic stimulation.
  • Many β1 and β2 adrenoreceptors regulate lipolysis Lipolysis The metabolic process of breaking down lipids to release free fatty acids, the major oxidative fuel for the body. Lipolysis may involve dietary lipids in the digestive tract, circulating lipids in the blood, and stored lipids in the adipose tissue or the liver. A number of enzymes are involved in such lipid hydrolysis, such as lipase and lipoprotein lipase from various tissues. Nonalcoholic Fatty Liver Disease and thermogenesis.
  • Brown adipose tissue has more and larger mitochondria Mitochondria Semiautonomous, self-reproducing organelles that occur in the cytoplasm of all cells of most, but not all, eukaryotes. Each mitochondrion is surrounded by a double limiting membrane. The inner membrane is highly invaginated, and its projections are called cristae. Mitochondria are the sites of the reactions of oxidative phosphorylation, which result in the formation of ATP. They contain distinctive ribosomes, transfer RNAs; amino Acyl tRNA synthetases; and elongation and termination factors. Mitochondria depend upon genes within the nucleus of the cells in which they reside for many essential messenger RNAs. Mitochondria are believed to have arisen from aerobic bacteria that established a symbiotic relationship with primitive protoeukaryotes. The Cell: Organelles than white adipose tissue, and these possess a unique decoupling protein called “thermogenin” (the protein product Product A molecule created by the enzymatic reaction. Basics of Enzymes of the UCP-1 gene Gene A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. Basic Terms of Genetics).
  • Thermogenin uncouples the oxidation of fatty acids Fatty acids Organic, monobasic acids derived from hydrocarbons by the equivalent of oxidation of a methyl group to an alcohol, aldehyde, and then acid. Fatty acids are saturated and unsaturated. Fatty Acids and Lipids from the generation of ATP.
  • The resultant energy is dissipated as heat Heat Inflammation.

Other functions of adipose tissue

  • Insulation from heat Heat Inflammation and cold by the subcutaneous layer 
  • Protective padding and structural support of visceral organs, mammary tissue, periorbital Periorbital Orbital and Preseptal Cellulitis tissue, bone marrow Bone marrow The soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells. Red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes. Bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. Bone Marrow: Composition and Hematopoiesis, and in areas of mechanical stress (palms and soles of feet)

Clinical Relevance

  • Obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity: a disease of excessive fat accumulation by both hypertrophy Hypertrophy General increase in bulk of a part or organ due to cell enlargement and accumulation of fluids and secretions, not due to tumor formation, nor to an increase in the number of cells (hyperplasia). Cellular Adaptation and hyperplasia Hyperplasia An increase in the number of cells in a tissue or organ without tumor formation. It differs from hypertrophy, which is an increase in bulk without an increase in the number of cells. Cellular Adaptation of white adipose tissue: “Overweight” is defined as a BMI BMI An indicator of body density as determined by the relationship of body weight to body height. Bmi=weight (kg)/height squared (m2). Bmi correlates with body fat (adipose tissue). Their relationship varies with age and gender. For adults, bmi falls into these categories: below 18. 5 (underweight); 18. 5-24. 9 (normal); 25. 0-29. 9 (overweight); 30. 0 and above (obese). Obesity (the weight in kilograms divided by the square of the height in meters) of 25–29.9 and “obese” as a BMI BMI An indicator of body density as determined by the relationship of body weight to body height. Bmi=weight (kg)/height squared (m2). Bmi correlates with body fat (adipose tissue). Their relationship varies with age and gender. For adults, bmi falls into these categories: below 18. 5 (underweight); 18. 5-24. 9 (normal); 25. 0-29. 9 (overweight); 30. 0 and above (obese). Obesity ≥ 30. In the United States, the prevalence Prevalence The total number of cases of a given disease in a specified population at a designated time. It is differentiated from incidence, which refers to the number of new cases in the population at a given time. Measures of Disease Frequency of obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity is ~ 40%, and that rate is increasing. Obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity is usually due to a sedentary lifestyle and an increased caloric intake, but there are also uncommon secondary causes. Obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity is associated with a number of disabilities, including:
    • Hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension, hypercholesterolemia Hypercholesterolemia A condition with abnormally high levels of cholesterol in the blood. It is defined as a cholesterol value exceeding the 95th percentile for the population. Lipid Disorders, diabetes mellitus Diabetes mellitus Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus type 2 Type 2 Spinal Muscular Atrophy
    • Heart disease and dyslipidemia
    • Cancer
    • Osteoarthritis Osteoarthritis Osteoarthritis (OA) is the most common form of arthritis, and is due to cartilage destruction and changes of the subchondral bone. The risk of developing this disorder increases with age, obesity, and repetitive joint use or trauma. Patients develop gradual joint pain, stiffness lasting < 30 minutes, and decreased range of motion. Osteoarthritis, gout Gout Gout is a heterogeneous metabolic disease associated with elevated serum uric acid levels (> 6.8 mg/dL) and abnormal deposits of monosodium urate in tissues. The condition is often familial and is initially characterized by painful, recurring, and usually monoarticular acute arthritis, or “gout flare,” followed later by chronic deforming arthritis. Gout
    • Nonalcoholic fatty liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy disease, gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract: Anatomy disease, GERD GERD Gastroesophageal reflux disease (GERD) occurs when the stomach acid frequently flows back into the esophagus. This backwash (acid reflux) can irritate the lining of the esophagus, causing symptoms such as retrosternal burning pain (heartburn). Gastroesophageal Reflux Disease (GERD)
    • Kidney disease
    • Sleep Sleep A readily reversible suspension of sensorimotor interaction with the environment, usually associated with recumbency and immobility. Physiology of Sleep apnea, dementia Dementia Major neurocognitive disorders (NCD), also known as dementia, are a group of diseases characterized by decline in a person’s memory and executive function. These disorders are progressive and persistent diseases that are the leading cause of disability among elderly people worldwide. Major Neurocognitive Disorders, depression
    • Increased susceptibility to infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease, including coronavirus disease 2019 Coronavirus disease 2019 Coronavirus disease 2019 (COVID-19) is an infectious disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that mainly affects the respiratory system but can also cause damage to other body systems (cardiovascular, gastrointestinal, renal, and central nervous systems). ( COVID-19 COVID-19 Coronavirus disease 2019 (COVID-19) is an infectious disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that mainly affects the respiratory system but can also cause damage to other body systems (cardiovascular, gastrointestinal, renal, and central nervous systems). ), influenza Influenza Influenza viruses are members of the Orthomyxoviridae family and the causative organisms of influenza, a highly contagious febrile respiratory disease. There are 3 primary influenza viruses (A, B, and C) and various subtypes, which are classified based on their virulent surface antigens, hemagglutinin (HA) and neuraminidase (NA). Influenza typically presents with a fever, myalgia, headache, and symptoms of an upper respiratory infection. Influenza Viruses/Influenza, and bacterial and fungal infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease
    • Skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions changes, including stretch marks (striae), acanthosis nigricans Acanthosis nigricans A circumscribed melanosis consisting of a brown-pigmented, velvety verrucosity or fine papillomatosis appearing in the axillae and other body folds. It occurs in association with endocrine disorders, underlying malignancy, administration of certain drugs, or as in inherited disorder. Diabetes Mellitus (likely due to sustained hyperinsulinemia Hyperinsulinemia Diabetes Mellitus), hirsutism Hirsutism A condition observed in women and children when there is excess coarse body hair of an adult male distribution pattern, such as facial and chest areas. It is the result of elevated androgens from the ovaries, the adrenal glands, or exogenous sources. The concept does not include hypertrichosis, which is an androgen-independent excessive hair growth. Polycystic Ovarian Syndrome in women
    • Metabolic syndrome Metabolic syndrome Metabolic syndrome is a cluster of conditions that significantly increases the risk for several secondary diseases, notably cardiovascular disease, type 2 diabetes, and nonalcoholic fatty liver. In general, it is agreed that hypertension, insulin resistance/hyperglycemia, and hyperlipidemia, along with central obesity, are components of the metabolic syndrome. Metabolic Syndrome diagnosed if 3 of the following 5 conditions are met MET Preoperative Care:
      • Hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension
      • Low levels of high-density lipoproteins Lipoproteins Lipid-protein complexes involved in the transportation and metabolism of lipids in the body. They are spherical particles consisting of a hydrophobic core of triglycerides and cholesterol esters surrounded by a layer of hydrophilic free cholesterol; phospholipids; and apolipoproteins. Lipoproteins are classified by their varying buoyant density and sizes. Lipid Metabolism
      • Abdominal obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity
      • Increased triglyceride levels
      • Increased glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance levels
      • Mnemonic PHATS 3/5: Pressure up, HDL low, Abdominal obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity, Triglycerides Triglycerides Fatty Acids and Lipids up, Sugars up
  • Cellulite: a cosmetic problem that refers to linear depressed streaks (mattress phenomenon) or dimpling in the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions: Cellulite appears mostly on the thighs and buttocks, is more common in females, and results from an uneven dermal–hypodermal interface, with fibrous Fibrous Fibrocystic Change tissue surrounding the protruding adipous papillae Papillae Lips and Tongue: Anatomy. Cellulitis Cellulitis Cellulitis is a common infection caused by bacteria that affects the dermis and subcutaneous tissue of the skin. It is frequently caused by Staphylococcus aureus and Streptococcus pyogenes. The skin infection presents as an erythematous and edematous area with warmth and tenderness. Cellulitis is less common in men because they have a smoother, strand-free dermal interface in the thigh Thigh The thigh is the region of the lower limb found between the hip and the knee joint. There is a single bone in the thigh called the femur, which is surrounded by large muscles grouped into 3 fascial compartments. Thigh: Anatomy and buttock areas.
  • Lipodystrophies: rare congenital Congenital Chorioretinitis or acquired syndromes associated with a loss of healthy adipose tissue: Generalized lipodystrophy can be associated with diabetes Diabetes Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus, hepatic steatosis Hepatic Steatosis Imaging of the Liver and Biliary Tract, pancreatitis Pancreatitis Inflammation of the pancreas. Pancreatitis is classified as acute unless there are computed tomographic or endoscopic retrograde cholangiopancreatographic findings of chronic pancreatitis. The two most common forms of acute pancreatitis are alcoholic pancreatitis and gallstone pancreatitis. Acute Pancreatitis, and metabolic syndrome Metabolic syndrome Metabolic syndrome is a cluster of conditions that significantly increases the risk for several secondary diseases, notably cardiovascular disease, type 2 diabetes, and nonalcoholic fatty liver. In general, it is agreed that hypertension, insulin resistance/hyperglycemia, and hyperlipidemia, along with central obesity, are components of the metabolic syndrome. Metabolic Syndrome.
  • Fat necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage: condition associated with mechanical trauma (e.g., fat necrosis of the breast Fat necrosis of the breast Fat necrosis of the breast is an inflammatory, benign condition resulting from injury to the breast tissue. Forms of injury include blunt traumatic injury as well as trauma from surgical procedures, biopsies, and radiation therapy. Fat Necrosis of the Breast), inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation (e.g., associated with acute pancreatitis Acute pancreatitis Acute pancreatitis is an inflammatory disease of the pancreas due to autodigestion. Common etiologies include gallstones and excessive alcohol use. Patients typically present with epigastric pain radiating to the back. Acute Pancreatitis), or ischemia Ischemia A hypoperfusion of the blood through an organ or tissue caused by a pathologic constriction or obstruction of its blood vessels, or an absence of blood circulation. Ischemic Cell Damage (e.g., omental infarction). 
  • Fat embolism Fat embolism Blocking of a blood vessel by fat deposits in the circulation. It is often seen after fractures of large bones or after administration of corticosteroids. Nonthrombotic Embolism: an embolus composed of a cluster of adipocytes that enters the bloodstream and results in the blockage of blood flow Blood flow Blood flow refers to the movement of a certain volume of blood through the vasculature over a given unit of time (e.g., mL per minute). Vascular Resistance, Flow, and Mean Arterial Pressure: Fat emboli can present as pulmonary emboli after fractures of the long bones Long bones Length greater than width. Bones: Structure and Types owing to the release Release Release of a virus from the host cell following virus assembly and maturation. Egress can occur by host cell lysis, exocytosis, or budding through the plasma membrane. Virology of bone marrow Bone marrow The soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells. Red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes. Bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. Bone Marrow: Composition and Hematopoiesis fat.
  • Calciphylaxis: skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions and subcutaneous fat Subcutaneous fat Fatty tissue under the skin throughout the body. Erythema Nodosum necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage, with small blood vessel necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage, calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes precipitation, and thrombosis Thrombosis Formation and development of a thrombus or blood clot in the blood vessel. Epidemic Typhus: Calciphylaxis is a complication of dialysis Dialysis Renal replacement therapy refers to dialysis and/or kidney transplantation. Dialysis is a procedure by which toxins and excess water are removed from the circulation. Hemodialysis and peritoneal dialysis (PD) are the two types of dialysis, and their primary difference is the location of the filtration process (external to the body in hemodialysis versus inside the body for PD). Peritoneal Dialysis and Hemodialysis and renal failure Renal failure Conditions in which the kidneys perform below the normal level in the ability to remove wastes, concentrate urine, and maintain electrolyte balance; blood pressure; and calcium metabolism. Renal insufficiency can be classified by the degree of kidney damage (as measured by the level of proteinuria) and reduction in glomerular filtration rate. Crush Syndrome and is also seen in primary hyperparathyroidism Hyperparathyroidism Hyperparathyroidism is a condition associated with elevated blood levels of parathyroid hormone (PTH). Depending on the pathogenesis of this condition, hyperparathyroidism can be defined as primary, secondary or tertiary. Hyperparathyroidism This condition is often lethal.
  • Panniculitis Panniculitis General term for inflammation of adipose tissue, usually of the skin, characterized by reddened subcutaneous nodules. Erythema Nodosum: inflammatory infiltrates in the subcutaneous adipose tissue: There are many causes of panniculitis Panniculitis General term for inflammation of adipose tissue, usually of the skin, characterized by reddened subcutaneous nodules. Erythema Nodosum, including steroid withdrawal, diabetes mellitus Diabetes mellitus Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus, tuberculosis Tuberculosis Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis complex bacteria. The bacteria usually attack the lungs but can also damage other parts of the body. Approximately 30% of people around the world are infected with this pathogen, with the majority harboring a latent infection. Tuberculosis spreads through the air when a person with active pulmonary infection coughs or sneezes. Tuberculosis, pancreatic disease, α1-antitrypsin deficiency, and autoimmune diseases Autoimmune diseases Disorders that are characterized by the production of antibodies that react with host tissues or immune effector cells that are autoreactive to endogenous peptides. Selective IgA Deficiency. Inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation may involve eosinophils Eosinophils Granular leukocytes with a nucleus that usually has two lobes connected by a slender thread of chromatin, and cytoplasm containing coarse, round granules that are uniform in size and stainable by eosin. Innate Immunity: Phagocytes and Antigen Presentation, neutrophils Neutrophils Granular leukocytes having a nucleus with three to five lobes connected by slender threads of chromatin, and cytoplasm containing fine inconspicuous granules and stainable by neutral dyes. Innate Immunity: Phagocytes and Antigen Presentation, granulomas Granulomas A relatively small nodular inflammatory lesion containing grouped mononuclear phagocytes, caused by infectious and noninfectious agents. Sarcoidosis, and histiocytes Histiocytes Macrophages found in the tissues, as opposed to those found in the blood (monocytes) or serous cavities (serous membrane). Chronic Granulomatous Disease with lymphophagocytosis, among others. Panniculitis Panniculitis General term for inflammation of adipose tissue, usually of the skin, characterized by reddened subcutaneous nodules. Erythema Nodosum may sometimes resolve without treatment, but it is often managed with antibiotics and anti-inflammatory drugs.
  • Mesenteritis inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation (mesenteric panniculitis Panniculitis General term for inflammation of adipose tissue, usually of the skin, characterized by reddened subcutaneous nodules. Erythema Nodosum): usually benign Benign Fibroadenoma condition presenting as space-occupying lesion consisting of chronic inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation with fat necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage and proliferation of myofibroblasts Myofibroblasts Spindle-shaped cells with characteristic contractile proteins and structures that contribute to the wound healing process. They occur in granulation tissue and also in pathological processes such as fibrosis. Hypertrophic and Keloid Scars involving single or multiple areas of the mesentery Mesentery A layer of the peritoneum which attaches the abdominal viscera to the abdominal wall and conveys their blood vessels and nerves. Peritoneum: Anatomy: Mesenteritis inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation often presents with fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever, but it can also be asymptomatic. This condition mostly affects middle-aged males. Prognosis Prognosis A prediction of the probable outcome of a disease based on a individual’s condition and the usual course of the disease as seen in similar situations. Non-Hodgkin Lymphomas is excellent, with most lesions regressing within 2 years.
  • Tumors:
    • Hibernoma: a benign Benign Fibroadenoma neoplasm of brown adipose tissue: Hibernoma is the only pathologic lesion of brown adipose tissue. Tumors mostly occur in sites corresponding to the distribution of brown adipose tissue—the interscapular area, neck Neck The part of a human or animal body connecting the head to the rest of the body. Peritonsillar Abscess, mediastinum Mediastinum The mediastinum is the thoracic area between the 2 pleural cavities. The mediastinum contains vital structures of the circulatory, respiratory, digestive, and nervous systems including the heart and esophagus, and major thoracic vessels. Mediastinum and Great Vessels: Anatomy, and axilla Axilla The axilla is a pyramid-shaped space located between the upper thorax and the arm. The axilla has a base, an apex, and 4 walls (anterior, medial, lateral, posterior). The base of the pyramid is made up of the axillary skin. The apex is the axillary inlet, located between the 1st rib, superior border of the scapula, and clavicle. Axilla and Brachial Plexus: Anatomy. Hibernoma presents as a painless slow-growing subcutaneous mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast, and it may achieve a size of up to 23 cm. Treatment is surgical excision.
    • Lipoma Lipoma A lipoma is a benign neoplasm of fat cells (adipocytes) and the most common soft tissue tumor in adults. The etiology is unknown, but obesity is a predisposing factor; genetics also play a role, with multiple lipomas occurring in various inherited disorders. Lipoma: a benign Benign Fibroadenoma neoplasm of adipocytes (white adipose tissue): Lipoma Lipoma A lipoma is a benign neoplasm of fat cells (adipocytes) and the most common soft tissue tumor in adults. The etiology is unknown, but obesity is a predisposing factor; genetics also play a role, with multiple lipomas occurring in various inherited disorders. Lipoma is the most common soft tissue Soft Tissue Soft Tissue Abscess tumor Tumor Inflammation in adults. Many variants of lipoma Lipoma A lipoma is a benign neoplasm of fat cells (adipocytes) and the most common soft tissue tumor in adults. The etiology is unknown, but obesity is a predisposing factor; genetics also play a role, with multiple lipomas occurring in various inherited disorders. Lipoma exist, including angiolipoma Angiolipoma A benign neoplasm composed of a mixture of adipose tissue and blood vessels. Cowden Syndrome, atypical lipoma Lipoma A lipoma is a benign neoplasm of fat cells (adipocytes) and the most common soft tissue tumor in adults. The etiology is unknown, but obesity is a predisposing factor; genetics also play a role, with multiple lipomas occurring in various inherited disorders. Lipoma, myxolipoma, and spindle cell lipoma Lipoma A lipoma is a benign neoplasm of fat cells (adipocytes) and the most common soft tissue tumor in adults. The etiology is unknown, but obesity is a predisposing factor; genetics also play a role, with multiple lipomas occurring in various inherited disorders. Lipoma. Lipoma Lipoma A lipoma is a benign neoplasm of fat cells (adipocytes) and the most common soft tissue tumor in adults. The etiology is unknown, but obesity is a predisposing factor; genetics also play a role, with multiple lipomas occurring in various inherited disorders. Lipoma usually presents as a painless soft tissue Soft Tissue Soft Tissue Abscess nodule Nodule Chalazion. Diagnosis is usually established clinically and supported by tissue biopsy Biopsy Removal and pathologic examination of specimens from the living body. Ewing Sarcoma. The usual treatment is surgical excision, but smaller asymptomatic lesions may not require any treatment.
    • Liposarcoma: a rare malignant tumor Tumor Inflammation originating from adipocytes. This tumor Tumor Inflammation is a kind of soft tissue Soft Tissue Soft Tissue Abscess sarcoma. Symptoms depend on the site of origin. Clinically, liposarcoma may be difficult to distinguish from a lipoma Lipoma A lipoma is a benign neoplasm of fat cells (adipocytes) and the most common soft tissue tumor in adults. The etiology is unknown, but obesity is a predisposing factor; genetics also play a role, with multiple lipomas occurring in various inherited disorders. Lipoma. Diagnosis is established with biopsy Biopsy Removal and pathologic examination of specimens from the living body. Ewing Sarcoma, and the mainstay of treatment is complete surgical excision.

References

  1. Brooks, J. S. J. (2020). Adipose Tissue. In Mills, S. E. (Ed.), Histology for Pathologists (5th ed., Chap. 6). 
  2. Harvey, I., Boudreau, A., Stephens, J. M. Adipose tissue in health and disease. Open Biology, 10(12), 200291. https://doi.org/10.1098/rsob.200291
  3. Pawlina, W., Ross, M.H. (2020). Adipose Tissue. In Histology: A Text and Atlas with Correlated Cell and Molecular Biology (8th ed., Chap. 9)
  4. Akalestou, E., Genser, L., Rutter, G. A. (2020). Glucocorticoid Metabolism in Obesity and Following Weight Loss. Frontiers in Endocrinology, 11. https://doi.org/10.3389/fendo.2020.00059
  5. Stimson, R. H., Anderson, A. J., Ramage, L. E., et al. (2017). Acute physiological effects of glucocorticoids on fuel metabolism in humans are permissive but not direct. Diabetes, Obesity and Metabolism, 19(6), 883–891. https://doi.org/https://doi.org/10.1111/dom.12899
  6. Toward a consensus view of mammalian adipocyte stem and progenitor cell heterogeneity. (2020). Trends in Cell Biology, 30(12), 937–950. https://doi.org/10.1016/j.tcb.2020.09.\
  7. Perreault, L., Laferrère, B. (2020). Overweight and obesity in adults: Health consequences. UpToDate. Retrieved January 23, 2021, from https://www.uptodate.com/contents/overweight-and-obesity-in-adults-health-consequences
  8. Divoux, A., Moutel, S., Poitou, C., Lacasa, D., Veyrie, N., Aissat, A., Arock, M., Guerre-Millo, M., Clément, K. (2012). Mast cells in human adipose tissue: Link with morbid obesity, inflammatory status, and diabetes. Journal of Clinical Endocrinology and Metabolism, 97(9), E1677–E1685. https://doi.org/10.1210/jc.2012-1532

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