Surface Epithelium

The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. The epithelium is classified according to the cells (squamous, cuboidal, columnar), the number of layers, and other unique characteristics either due to function (transitional epithelium allowing distention) or appearance (pseudostratified epithelium giving a false impression of multiple layers). The surface epithelium has multiple functions, which include protection, secretion, filtration, and sensory reception.

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Epithelial Tissue

Surface epithelium

A complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body:

  • Derived from all 3 germ layers:
  • One of 4 major basic tissue types (the other 3 include nervous, muscular, and connective tissues)

These cells exhibit polarity:

  • Apical/luminal pole: derived from the apex, faces the surface/lumen
  • Basal pole: derived from the base, attached to the 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 located below the epithelium

The surface epithelium does not possess blood vessels; therefore, nutrients and oxygen are received from adjacent 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.

Functions

  • Protection (provides covering surface or lining)
  • Secretion (release 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, sweat, mucus, and enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes) as found in glands
  • Absorption Absorption Absorption involves the uptake of nutrient molecules and their transfer from the lumen of the GI tract across the enterocytes and into the interstitial space, where they can be taken up in the venous or lymphatic circulation. Digestion and Absorption (substance intake as seen in the intestinal lining)
  • Excretion and filtration of substances
  • Sensory reception (detection of sensation) as found in olfactory epithelium

Related structures

  • Intercellular junctions:
    • Adhesion of cells is facilitated by cadherins (calcium-dependent adhesion molecules).
    • At the apical end, structures encircle the area (like a band around the cell):
      • Tight or occluding junctions (zonulae occludens): have tight ridges, preventing passive flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure of substances between cells
      • Adherent junctions (zonulae adherens): help stabilize both junctions and hold cells together.
    • Desmosomes (macula adherens) and gap junctions (communicating junctions):
      •  Structures bound to intracellular intermediate filaments, aiding the zonulae adherens
      • Additionally, gap junctions permit flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure of molecules.
    • Hemidesmosomes: anchor epithelial cells to the basal lamina (of the basement membrane)
  • Interdigitations: cell membrane Cell Membrane A cell membrane (also known as the plasma membrane or plasmalemma) is a biological membrane that separates the cell contents from the outside environment. A cell membrane is composed of a phospholipid bilayer and proteins that function to protect cellular DNA and mediate the exchange of ions and molecules. The Cell: Cell Membrane foldings that increase the surface area (e.g., for ion or water transport)
  • Basement membrane: 
    • Thin semipermeable sheet on which the basal surface of epithelia rests
    • Attachment of cells achieved with laminin and integrin
    • Parts:
      • Basal lamina: thin, sheetlike layer of fine fibrils found nearest the epithelial cells
      • Reticular lamina: thicker and more fibrous 
    • Function:
      • Attachment to underlying tissue
      • Separation of tissues
      • Filtration
      • Scaffold and guidance (repair)
      • Cell signaling
Characteristics of the epithelium

Epithelia have an apex, lateral borders, and a surface sitting on a basement membrane:
As seen in the image, these cells exhibit polarity (have apical domains and basal/basolateral domains). At the apical end, structures encircle the area (like a band around the cell) facilitating cell-to-cell adhesion (tight junctions and adherent junctions). Near the basement membrane, hemidesmosomes anchor the epithelia to the basal lamina. On the right is shown the histology of the intestinal epithelial lining with corresponding structures. On the apical domain, microvilli are seen.

Image by Lecturio.

Classification

By number of layers

  • Simple epithelium:
    • Consists of a single layer
    • Typically found where absorption, secretion, and filtration occur
  • Stratified epithelium:
    • Composed of ≥ 2 layers
    • Found in high-abrasion areas ( 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. Structure and Function of the Skin and lining of the mouth)
    • Name of cells in stratified epithelia based on shape of cells in apical layer
  • Pseudostratified epithelium:
    • All cells are attached to the basement membrane, but not all cells extend to the free surface.
    • The nuclei, being at different levels, give the epithelia a stratified appearance.

By shape of the cells

  • Squamous cells: thin or flattened cells
  • Cuboidal cells: cell width and thickness generally similar (cube-shaped)
  • Columnar cells: cells are taller than wide (column-shaped).
  • Transitional: shape of cells “goes through a transition”:
    • When the organ (e.g., bladder) is relaxed, cells appear cuboidal.
    • When the organ is distended, cells flatten.

By function

Note that there are tissues that have both:

  • Covering epithelium: line the cavities or organs
  • Glandular epithelium Glandular epithelium Glandular epithelia, composed of epithelial tissue, are specialized structures that play a role in the production and release of enzymes, hormones, sweat, oil, and mucus in organisms. The secretion and release of these substances are prompted by either external or internal stimuli. Glandular Epithelium: secretory
Types of epithelium

The types of epithelia:
The differentiation and classification of the types are based on the shape of the cell and the layers. Simple epithelium indicates 1 layer of cells. Stratified epithelium indicates multiple layers. Pseudostratified epithelium gives a false impression of > 1 layer owing to different nuclei levels. Transitional epithelium is a type in which the shape of cells “go into transition” depending on the organ function (distention versus relaxation, as seen in the urinary bladder).

Image: “Types of epithelium” by U.S. National Cancer Institute’s Surveillance. License: Public Domain, edited by Lecturio.

Simple Epithelium

Simple squamous epithelium

  • Characteristics:
    • Single layer of flattened cells
    • Disc-shaped nuclei (most prominent structure)
    • Sparse cytoplasm
  • Typically found in lining of vessels, regulating passing substances to the tissue(s)
  • Located in:
    • Air sacs/alveoli of lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs
    • Lining of the heart
    • Blood vessels and lymphatic vessels
    • Renal loops of Henle
    • Cornea
  • Special naming:
    • Endothelium: simple squamous epithelium lining blood vessels and lymphatic vessels
    • Mesothelium: simple squamous epithelium found in large body cavities (secreting serous fluid)
Simple squamous epithelium

Structure noted in simple squamous epithelium (single layer of flattened cells)

Image: “Simple squamous epithelium” by Phil Schatz. License: CC BY 4.0
Epithelial tissues simple squamous epithelium

A whole mount of simple squamous epithelium

Image: “Epithelial Tissues Simple Squamous Epithelium” by Berkshire Community College Bioscience Image Library. License: CC0 1.0

Simple cuboidal epithelium

  • Single layer of cube-like cells
  • Cuboidal shape allows increased mitochondria and other organelles Organelles A cell is a complex unit that performs several complex functions. An organelle is a specialized subunit within a cell that fulfills a specific role or function. Organelles are enclosed within their own lipid bilayers or are unbound by membranes. The Cell: Organelles needed for functions such as active transport Active transport The movement of materials across cell membranes and epithelial layers against an electrochemical gradient, requiring the expenditure of metabolic energy. The Cell: Cell Membrane, secretion, and absorption. 
  • Found in:
    • Ducts and secretory portions of glands
    • Kidney tubules
Simple cuboidal epithelium

Simple cuboidal epithelium: 1 layer of cuboidal cells

Image: “Simple cuboidal epithelium” by Phil Schatz. License: CC BY 4.0
Simple cuboidal epithelium

Cross section of a kidney tubule showing a layer of cuboidal cells

Image: “Epithelial Tissues Simple Cuboidal Epithelium” by Berkshire Community College Bioscience Image Library. License: CC0 1.0

Simple columnar epithelium

  • Single-layer tall cells
  • Often with cilia or microvilli
  • Mostly involved in absorption and secretion
  • Found in:
    • Ciliated epithelium in the bronchi, fallopian tubes Fallopian tubes The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The fallopian tubes receive an ovum after ovulation and help move it and/or a fertilized embryo toward the uterus via ciliated cells lining the tubes and peristaltic movements of its smooth muscle. Posterior Abdominal Wall, and uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Posterior Abdominal Wall
    • Smooth (nonciliated) epithelium in the digestive tract 
    • Lining of the 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
Simple columnar epithelium

Simple columnar epithelium:
Shown is a layer of columnar cells with cilia

Image: “Simple columnar epithelium” by Phil Schatz. License: CC BY 4.0
Simple columnar epithelium

Simple columnar epithelium lining the intestinal tract

Image: “Epithelial Tissues Simple Columnar Epithelium” by Epithelial Tissues: Simple Columnar Epithelium. License: CC0 1.0

Pseudostratified, Stratified, and Transitional Epithelia

Pseudostratified columnar epithelium

  • Characteristics:
    • Cells vary in height, with all cells resting on the basement membrane but only some reaching all the way to the apical surface.
    • Nuclei placed at different heights
    • Some have cilia.
  • Functions in secretion and absorption
  • Found in:
    • Ciliated epithelium of the trachea Trachea The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx and inferiorly becomes the bronchial tree within the lungs. The trachea consists of a support frame of semicircular, or C-shaped, rings made out of hyaline cartilage and reinforced by collagenous connective tissue. Trachea
    • Part of the upper digestive tract
Pseudostratified epithelium

Pseudostratified epithelium:
Shown is a single layer of columnar cells, with varying levels of nuclei

Image: “Pseudostratified epithelium” by Phil Schatz. License: CC BY 4.0
Cross-section of pseudostratified columnar epithelium

Pseudostratified columnar epithelium found in the trachea Trachea The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx and inferiorly becomes the bronchial tree within the lungs. The trachea consists of a support frame of semicircular, or C-shaped, rings made out of hyaline cartilage and reinforced by collagenous connective tissue. Trachea

Image: “Cross-section of pseudostratified columnar epithelium” by OpenStax College. License: CC BY 3.0

Stratified squamous epithelium

  • Most widespread of the stratified epithelia
  • Composed of several layers, providing protective function
  • Free surface cells are squamous.
  • Deeper layer cells are cuboidal or columnar.
  • Found in areas subjected to wear and tear
  • Found in:
    • Skin, which is keratinized or filled with keratin:
      • These cells will lose the organelles Organelles A cell is a complex unit that performs several complex functions. An organelle is a specialized subunit within a cell that fulfills a specific role or function. Organelles are enclosed within their own lipid bilayers or are unbound by membranes. The Cell: Organelles and nuclei as they flatten and accumulate keratin.
      • Cells move toward the surface, becoming metabolically inactive and are sloughed off.
    • Esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus, mouth, vagina Vagina The vagina is the female genital canal, extending from the vulva externally to the cervix uteri internally. The structures have sexual, reproductive, and urinary functions and a rich blood supply, mainly arising from the internal iliac artery. Vagina, Vulva, and Pelvic Floor, which have nonkeratinized cells (retain the nuclei)
Stratified squamous epithelium

Illustration of stratified squamous epithelium (multiple layers)

Image: “Stratified squamous epithelium” by Phil Schatz. License: CC BY 4.0
Epithelial tissues stratified squamous epithelium

Histologic picture of stratified squamous epithelium

Image: “Epithelial Tissues Stratified Squamous Epithelium” by Berkshire Community College Bioscience Image Library. License: CC0 1.0

Stratified columnar epithelium and stratified cuboidal epithelium

  • Both are rare and have limited distribution.
  • Stratified columnar epithelium:
    • Secretory and protective function
    • Found in:
      • Male urethra Male Urethra A tube that transports urine from the urinary bladder to the outside of the body in both the sexes. It also has a reproductive function in the male by providing a passage for sperm. Urinary Tract
      • Some glandular ducts
  • Stratified cuboidal epithelium is found in ducts of large glands (e.g., sweat glands, mammary glands).
Parotid gland

Stratified cuboidal epithelium (seen on the left) is visible in a duct surrounded by 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 in the parotid gland.

Image: “WVSOM Parotid Gland1” by Wbensmith. License: CC BY 3.0

Transitional epithelium

  • Characteristics:
    • Surface layer of umbrella cells (dome-like cells)
    • Allows for stretching of the organs as they distend
    • Urothelium: transitional epithelium in the urinary tract Urinary tract The urinary tract is located in the abdomen and pelvis and consists of the kidneys, ureters, urinary bladder, and urethra. The structures permit the excretion of urine from the body. Urine flows from the kidneys through the ureters to the urinary bladder and out through the urethra. Urinary Tract
  • Found in:
    • Urinary bladder
    • Urethra
    • Ureters
Transitional epithelium

Transitional epithelium

Image: “Transitional epithelium” by Phil Schatz. License: CC BY 4.0
Transitional epithelium

Transitional epithelium found in the urinary bladder

Image: “urinary bladder, urothelium, haemalum-eosin stain” by Polarlys. License: CC BY 2.5

Clinical Relevance

  • Ichthyosis: dermatologic disease in which abnormal keratinization occurs. Ichthyosis is caused by an autosomal dominant Autosomal dominant Autosomal inheritance, both dominant and recessive, refers to the transmission of genes from the 22 autosomal chromosomes. Autosomal dominant diseases are expressed when only 1 copy of the dominant allele is inherited. Autosomal Recessive and Autosomal Dominant Inheritance mutation Mutation Genetic mutations are errors in DNA that can cause protein misfolding and dysfunction. There are various types of mutations, including chromosomal, point, frameshift, and expansion mutations. Types of Mutations in the filaggrin gene, which results in 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. Structure and Function of the Skin barrier dysfunction. Clinical presentation features rough, dry, and scaly 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. Structure and Function of the Skin, with symptoms worsening during cold, dry months. The diagnosis is usually clinical but often aided with a 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. Structure and Function of the Skin biopsy showing hyperkeratosis and a diminished stratum granulosum. 
  • Transitional cell carcinoma: malignancy affecting the urinary bladder. Transitional cell carcinoma is the most common cancer involving the urinary system. The predominant histologic type is urothelial (transitional cell) carcinoma in the United States and Europe. Risk factors include genetics Genetics Genetics is the study of genes and their functions and behaviors. Basic Terms of Genetics, smoking, opium use, and occupational carcinogen exposure. Presentation is usually with painless hematuria, and the diagnostic approach includes cystoscopy, urine cytology, and biopsies. Transurethral resection of bladder tumor, radical cystectomy, and chemotherapy are among the treatment options, which depend on the stage. 
  • Barrett’s esophagus: replacement of stratified squamous epithelium with gastric columnar epithelium in the esophagus caused by chronic 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. This is associated with an increased risk of esophageal adenocarcinoma. Esophagogastroduodenoscopy would show proximal displacement of the squamocolumnar junction (Z-line) from the gastroesophageal junction (GEJ). Biopsies reveal columnar epithelium and goblet cells in the distal esophagus. Treatment is with proton pump inhibitors and lifestyle modifications. 
  • Cervical cancer Cervical cancer Cervical cancer, or invasive cervical carcinoma (ICC), is the 3rd most common cancer in women in the world, with > 50% of the cases being fatal. In the United States, ICC is the 13th most common cancer and the cause of < 3% of all cancer deaths due to the slow progression of precursor lesions and, more importantly, effective cancer screening. Cervical Cancer: very common cancer affecting women. Most cases of cervical cancer are due to HPV HPV Human papillomavirus (HPV) is a nonenveloped, circular, double-stranded DNA virus belonging to the Papillomaviridae family. Humans are the only reservoir, and transmission occurs through close skin-to-skin or sexual contact. Human papillomaviruses infect basal epithelial cells and can affect cell-regulatory proteins to result in cell proliferation. Papillomaviridae: HPV. The region around the external os is lined by nonkeratinized stratified squamous epithelium continuous with that of the vagina Vagina The vagina is the female genital canal, extending from the vulva externally to the cervix uteri internally. The structures have sexual, reproductive, and urinary functions and a rich blood supply, mainly arising from the internal iliac artery. Vagina, Vulva, and Pelvic Floor. This area meets the mucus-secreting columnar epithelium of the endocervix in the transformation zone. Exposure of the transformation zone to HPV HPV Human papillomavirus (HPV) is a nonenveloped, circular, double-stranded DNA virus belonging to the Papillomaviridae family. Humans are the only reservoir, and transmission occurs through close skin-to-skin or sexual contact. Human papillomaviruses infect basal epithelial cells and can affect cell-regulatory proteins to result in cell proliferation. Papillomaviridae: HPV can lead to squamous differentiation into squamous intraepithelial lesion (leading to squamous cell carcinoma Squamous cell carcinoma Cutaneous squamous cell carcinoma (cSCC) is caused by malignant proliferation of atypical keratinocytes. This condition is the 2nd most common skin malignancy and usually affects sun-exposed areas of fair-skinned patients. The cancer presents as a firm, erythematous, keratotic plaque or papule. Squamous Cell Carcinoma). The endocervical columnar epithelium can develop into glandular intraepithelial lesions (which can progress to adenocarcinoma).

References

  1. Kierszenbaum, A., Tres, L. (2019). Histology and Cell Biology: An Introduction to Pathology, 5th ed. Elsevier.
  2. Mescher A.L.(Ed.). (2021). Epithelial tissue. Chapter 4 of Junqueira’s Basic Histology Text and Atlas, 16th ed. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=3047&sectionid=255120320
  3. Mescher A.L.(Ed.), (2021). The female reproductive system. Chapter 22 of Junqueira’s Basic Histology Text and Atlas, 16th ed. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=3047&sectionid=255122848

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