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Medical Students

Image: “Medical Students.” by Ximehernandez1409. License: CC BY-SA 3.0


Definition: What is Histology?

Histology is a component of microscopic anatomy

Histology (Greek: histos = tissue and logos = teaching) is the study of tissues. It belongs to the field of microscopic anatomy. It deals with the arrangement and structure of cells and their organization as a cellular unit in various tissue types.

Histology curriculum and a microscopy course are offered in the 1st and 2nd semesters of preclinical studies, which are especially relevant for the subsequent clinical phase, as they provide the requisite diagnostic skills based on tissue studies.

Histological technique

Image: Slice preparation for light microscopy: 1 glass slides (76 x 26 mm), 2 cover glass, in between: 3 colored organ section. By Rollroboter, License: CC BY-SA 3.0

General Histology

The 1st step entails identifying the specimen via direct observation, i.e., without a microscope, to determine the number of edges and any lumen visible.

The specimen is viewed under the lowest magnification, in order to obtain a general overview and develop a hypothesis eventually. The magnification can be increased steadily to capture the details, and thus confirm or disprove the hypothesis.

For orientation purposes, it is advisable to always return to the lowest magnification and repeatedly zoom in on the parts of interest. However, for a successful histology lesson, in addition to the identification of the tissue specimen, thorough background knowledge is essential.

Tissues

1. Epithelial tissue

Epithelium Classification

Image: Classification of Epithelium. By BruceBlaus, License: CC BY 3.0

Epithelium represents a group of cells that line various organs. The 3 principal shapes include:

  • Squamous
  • Cuboidal
  • Columnar

The presence of epithelium provides an important clue for tissue diagnosis. The type of epithelium further facilitates the identification of the tissue section.

In addition to the systematic structure, the distribution and function of all epithelial types are essential for the understanding of pathology.

2. Bone tissue

OsteonBone is also tissue and can be sliced into different sections. For physicians, it is important to understand the structure and function of a bone as well as the different bones in the human body.

3. Muscle tissue

Muscles define the body; therefore, it is vital to understand muscle function. Movement is the result of alternating contraction and relaxation of muscles, which account for approx. 30–40% of the total body weight. Muscles are primarily engaged in the conversion of chemical energy into mechanical energy to perform work.

Histologically, the muscles of the body can be classified into 3 different types:

Smooth muscles as visceral muscles (muscles of internal hollow organs) Smooth Musculature
Cardiac muscle or myocardium Cardiac muscle or myocardium
Striated or skeletal muscle as a component of active musculoskeletal system Striated or skeletal muscle as a component of the active musculoskeletal system

Image: Partial View of ‘the musculature of our body can be differentiated into 3 types’. By Phil Schatz, License: CC BY 4.0

A concise overview of the different types of muscle, their structure, their movement from their origin and insertion points, and the auxiliaries used are essential for any physician.

Cells

The cell is the functional unit of a living organism. Organisms such as bacteria and algae are unicellular while complex organisms such as human beings are multicellular. In addition, complex organisms contain an extracellular matrix and exhibit complex intracellular structures.

Animal Cell and Components

Image: Prototypical human cell. By philschatz, License: CC BY 4.0

The cell is the basic building block of all life, measuring only a few microns in size. Nevertheless, it accomplishes incredible feats as a biological unit. The numerous organelles, the cytoskeleton, and cell-cell junctions are popular topics in biology, biochemistry, and histology examinations.

Special Histology

Which organs contain which tissue types and in what composition and how do these cell groups function as a whole?

Blood

Hemopoiesis

Image: Hematopoietic system of bone marrow. By Phil Schatz, License: CC BY 4.0

Blood (Latin: sanguis, Greek: haima) is a suspension of various cells in a protein-containing saline solution. It accounts for 6–8% of body weight and has a pH value of 7.4. Blood performs numerous vital functions and is the key player in the entire organism.

Blood components (%) Blood subcomponents (%) Type (%) Production site Main functions
Plasma (43–63%)  Water (92%) Liquid Absorbed through the intestinal tract or synthesized during metabolism Transport medium
Plasma proteins (7%) Albumin (54–60%) Liver Maintenance of osmotic concentration, and transport of lipid molecules
Globulin (35–38%) Alpha globulin—Liver Transport, maintenance of osmotic concentration
Beta globulin—Liver Transport, maintenance of osmotic concentration
Gamma globulin (Immunoglobulin)—Plasma cells Immune response
Fibrinogen (4–7%) Liver Coagulation during hemostasis
Regulatory proteins (< 1%) Hormones and enzymes Various sites Regulation of various body functions
Other solutes (1%) Nutrients, gases, and waste products Intestinal absorption, gas exchange in the respiratory tract, or cellular synthesis Several different functions
Main components (37–54%) Erythrocytes (99%) Erythrocytes Red bone marrow Gas transport, primarily oxygen and carbon dioxide
Leucocytes (< 1%), thrombocytes (< 1%) Granulocytes: neutrophils, eosinophils, and basophils Red bone marrow Innate immunity
Agranular leukocytes: lymphocytes, monocytes Lymphocytes: bone marrow and lymphatic tissue Lymphocytes:
adaptive immunity
Monocytes: red bone marrow Monocytes: innate immunity
 Thrombocytes (< 1 %) Megakaryocytes: red bone marrow Coagulation

Blood vessels: The wall structure of arteries and veins

Blood vessels move in and out of a histological section. They should be recognized as such during the differentiation of arteries from veins. The main difference lies in the wall structure.

comparison arteries and veins

Image: Structure of blood vessels. By Phil Schatz, License: CC BY 4.0

Circulatory system

The human circulatory system is the convective transport of respiratory gases, nutrients, hormones, and heat via blood, which is pumped from the heart through the blood vessels and returned to the heart.

cardiovascular circulation

Image: Cardiovascular circulation. By Phil Schatz, License: CC BY 4.0

The blood circulation changes rapidly but is indispensable for the maintenance of body functions. High blood pressure (arterial hypertension) is one of the most common diseases in society. It is therefore important for physicians to know and understand the circulatory system.

Respiratory system

schematic picture of the lung pleura

Image: The lung pleura. By OpenStax College, License: CC BY 3.0

Along with the intestines and the skin, the lung is another organ in direct contact with the outside world. It facilitates gas exchange by diffusion. Pulmonary gas exchange every second requires a transport system for waste gases and fresh air.

A space in which the walls are thin enough for gas diffusion is as important as a transport system for the gases in a chemically bound form. Histologists must closely analyze the management of these tasks by the individual structures.

The integumentary system

Structure of the skin

Image: Layers of skin. By Phil Schatz, License: CC BY 4.0

The skin, subcutaneous fat, hair, and nails protect us from hazardous environmental exposures. Towards this end, they must be sealed so that our body fluids are not depleted. The skin is the largest organ and weighs 3–10 kg. In an individual who is 170 cm tall, the surface area of the skin equals 1.8 m². The skin is also the site of vitamin D synthesis.

Kidney and excretory organs

The kidney is a vital excretory organ and controls the water and electrolyte balance in our body. Their precise mechanism of action can only be understood at the microscopic level. Each renal tissue type is designed for a specific purpose, and the sophisticated functions of the kidney can be traced to its histology.

Endocrine organs and hormonal system

In the absence of a hormonal system, our bodies would not function as a whole. Endocrine organs ensure coordinated communication between specialized individual systems for an appropriate and adequate response to environmental factors.

The control of body functions via the endocrine system is the subject of anatomy, physiology, and biochemistry. Even in clinical practice, a comprehensive understanding of the hormones and their functions is essential. Pituitary, thyroid, parathyroid, adrenal, and pineal glands represent endocrine organs, in the strictest sense of the term. Based on their characteristic structure, they are not difficult to identify histologically; however, a few elements should be considered.

Endocrine glands and their primary hormones

Endocrine gland Associated hormones Chemical class Effect
Pituitary gland (anterior) Growth hormone Protein Promotes the growth of body tissues
Prolactin Peptide Promotes milk production
Thyroid-stimulating hormone (TSH) Glycoprotein Stimulates thyroid hormone secretion
ACTH Peptide Stimulates the release of hormones from the adrenal cortex
Follicle-stimulating hormone (FSH) Glycoprotein Stimulates gametogenesis
Luteinizing hormone (LH) Glycoprotein Stimulates gonadal androgen production
Pituitary gland (posterior) ADH Peptide Stimulates water absorption in the kidneys
Oxytocin Peptide Stimulates uterine contractions during childbirth
Thyroid gland Thyroxine (T4); triiodothyronine (T3) Amine Stimulates basal metabolic rate
Calcitonin Peptide Lowers blood Ca2+ levels
Parathyroid gland Parathyroid hormone Peptide Raises blood Ca2+ levels
Adrenals (cortex) Aldosterone Steroid Raises blood Na2+ levels
Cortisol, corticosterone, cortisone Steroid Raises blood sugar levels
Adrenals (medulla) Adrenaline, noradrenaline Amine Stimulates fight or flight response
Pineal Melatonin Amine Controls the sleep cycle
Pancreas Insulin Protein Lowers blood sugar levels
Glucagon Protein Raises blood sugar levels
Testes Testosterone Steroid Stimulates the development of male secondary sexual characteristics and sperm production
Ovaries Estrogen and progesterone Steroid Stimulate the development of female secondary sex characteristics and prepares the body for childbirth

Table: Endocrine glands and their major hormones. By Phil Schatz, License: CC BY 4.0

The immune system

Anatomy of the Lymphatic System

Image: Anatomy of the lymphatic system. By philschatz, License: CC BY 4.0

The lymphoid organs, which include the thymus, spleen, lymph nodes and tonsils, are exciting points of interest, and important topics in medicine. The lymphatic system plays a role in numerous physiological events throughout the body, especially in the complex defense mechanisms. Understanding the lymphatic system is crucial for the diagnosis of several pathologies.

Nervous system

Central and Peripheral Nervous System

Image: Central and peripheral nervous system. By philschatz, License: CC BY 4.0

The nervous system ‘only’ weighs 2 kg, which amounts to approx. 3% of the total body weight. It is a small but complex network of neurons and an even greater number of neuroglia. For all physicians, it is important to know the structure and physiology of the nervous system.

Sensory system

The sensory organs allow us to interact with our environment and to perceive things outside of our bodies. Their functions are fascinating and complex and are substantial topics in the study of medicine.

Despite the sheer diversity of the sense organs, they conform to basic principles of structure and function.

Digestive system and gastrointestinal tract

Digestive system

Image: Digestive system with liver. By Gordon Flynn, License: CC BY-SA 2.5

The digestive system is an immense challenge given the vast but subtle differences between the individual components.

The breakdown of nutrients and their assimilation depend on digestion. Nutrients are then transported into individual cells of the body via the bloodstream.

Pancreas

The pancreas

Image: Pancreas. By Phil Schatz, License: CC BY 4.0

The pancreas is both an exocrine and an endocrine organ. It is essential for digestion and metabolism of carbohydrates, which is why a loss of function leads to severe diseases.

Female Reproductive System—follicular, mucous and breast tissue

The female sex organs undergo periodic and recurring structural changes. In female sex organs, a number of physiological events occur under hormonal influence. Topics such as folliculogenesis in the uterus, the structure of the mammary tissue, and lactation are important from a medical perspective.

Male reproductive system

The understanding of the male reproductive system requires a comprehensive biological insight. The male reproductive organs comprising testis and vas deferens are composed of different sections.

Histological Stains

Staining is an additional microscopic procedure that enhances the contrast of the microbiological image to highlight tissues in focus. The different histological stains and dye stains used are discussed here.

Hematoxylin and Eosin (H&E) stain

islet of Langerhans

Image: Islet of Langerhans, haemalum-eosin stain. By Polarlys, License: CC BY-SA 3.0

Hematoxylin & Eosin (H&E) staining is used to stain the nucleus blue and the cytoplasm pale red. Collagen fibers are stained strong red, as are muscle and bone. Elastic fibers are stained brighter red. Erythrocytes are orange-to-red under H&E stain. The hyaline cartilage is stained blue.

Azan stain

Modified stained with azan

Image: Modified stained with azan: collagenous connective tissue blue. epididymis. By Rollroboter, License: CC BY-SA 3.0

The Azan staining is based on the use of 3 dyes: azocarmine, orange G and aniline blue. The nucleus is bright red and the cytoplasm is pale pink, under Azan stain. Collagen fibers appear blue, as does hyaline cartilage. Elastic fibers, muscle tissue, and bone are all stained red. Erythrocytes are dyed orange to red, just as they are with H&E stain.

Elastica stain

Elastic fibers

Image: Elastic fibers in mammary glands colored by Weigert’s elastic stain. Elastic fibers are blue. By R.Denzer, License: CC BY-SA 3.0

Elastic fibers and membranes react selectively with resorcinol-fuchsin or orcein. Their histological sections appear as pale violet (resorcinol-fuchsin) or reddish-brown (orcein).

Van Gieson’s stain

Colored bronchi of ostrich

Image: Colored bronchi of ostrich, after Van Gieson. By Lycaon.cl, License: CC BY-SA 4.0

Van Gieson’s stain is a mixture of iron hematoxylin, picric acid, and acid fuchsin. It dyes the nucleus black-brown and the cytoplasm yellowish. Collagen fibers appear red. Elastic fibers, hyaline cartilage, muscle tissue, bone, and red blood cells stain yellow with this dye.

Crossmon stain

The Crossmon stain consists of iron hematoxylin, orange G and light green SF. It can be used to stain the nucleus black-brown, while the cytoplasm appears reddish. Collagen fibers take on a green hue. Elastic fibers and bones are light-red. Erythrocytes are stained orange with Crossmon.

Special stains

A number of histochemical stains provide a glimpse into dynamic cellular events. For instance, carbohydrate components such as glycoproteins or glycogen (PAS reaction), negatively charged components such as glycosaminoglycans or hyaluronic acid (Alcian blue stain), or lipids (fatty dyes such as Sudan III) represent the site of their natural location within cells and tissues.

Finally, immunohistochemical methods are used to detect specific proteins or peptides based on antigen-antibody reaction.

In-situ hybridization is the method of choice to identify specific DNA or RNA sequences. Radioactively or non-radioactively labeled samples are used to localize the histological specimens appropriately.

Study Table: Commonly used stains

Stain General application Nucleus Cytoplasm Red blood cells Collagen fibers Special stains
Hematoxylin Commonly used stain, often paired with eosin Orange, cyan, blue or green Blue/brown /black N/A N/A Nucleic acids: blue;
endoplasmic reticulum: blue
Eosin Commonly used stain, often paired with hematoxylin N/A Pink Orange/red Pink Elastic fibers: pink;
collagen fibers: pink;
reticular fibers: pink
Toluidine blue Commonly used stain Blue Blue Blue Blue Mast cell granules: purple
Masson’s trichrome stain Connective tissue Black Red/Pink Red Blue/Green Cartilage: blue/green;
muscle fibers: red
Mallory´s trichrome stain Connective tissue Red Pale red Orange Dark blue Keratin: orange;
cartilage: blue;
bone matrix: dark blue;
muscle fibers: red
Elastica stain Elastica fibers Blue/black N/A N/A N/A Elastic fibers: blue/black
Azan stain Separates cells from extracellular components Red/purple Pink Red Blue Muscle fibers: red;
cartilage: blue;
bone matrix: blue
Silver stain
Reticular fibers, nerve fibers, fungi N/A N/A N/A N/A Reticular fibers: brown/black;
nerve fibers: brown/black;
fungi: black
Wright stain
Blood cells Bluish/purple Bluish/purple Red/Pink N/A Neutrophil granules: purple/pink;
eosinophil granules: bright red/orange;
basophil granules: dark purple/violet;
thrombocyte granules: red/purple
Orcein stain Elastic fibers Dark blue N/A Light red Pink Elastic fibers: dark brown;
mast cell granules: purple;
smooth muscle: light-blue
PAS reaction Basal membrane, localization of carbohydrates Blue N/A N/A Pink Glycogen and other carbohydrates: magenta

Table: Common laboratory stains. By Wikipedia.

Histology is a subject central to medicine and covers areas of anatomy, biology, physiology, even through to biochemistry. A basic understanding of histology facilitates the learning of other essential subjects to appreciate pre-clinical findings covering not only individualized compartments but the tissue as an integrated whole.

Histology mnemonics:

  • Titin sounds like Titan and is the largest protein in the body.
  • T-cells come from the thymus.
  • B-cells come from the bone.
  • Plexus submucosus (Meissner) is responsible for the mucosa centrally.
  • Plexus myentericus (Auerbach) innervates muscles externally.
  • Osteoclasts chew up bone.
  • Osteoblasts build the bone.
  • C-cells synthesize calcitonin, which reduces Ca2+-levels in plasma.
  • Parathyroid hormone provides the body with Ca2+ (in blood).
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