3.3: Epithelial Tissue

By the end of the section, you will be able to:

• Explain the structure and function of epithelial tissue
• Distinguish between tight junctions, anchoring junctions, and gap junctions
• Distinguish between simple epithelia and stratified epithelia, as well as between squamous, cuboidal, and columnar epithelia
• Describe the structure and function of endocrine and exocrine glands and their respective secretions

Most epithelial tissues are essentially large sheets of cells covering all the surfaces of the body exposed to the outside world and lining the outside of organs. Epithelium also forms much of the glandular tissue of the body. Skin is not the only area of the body exposed to the outside. Other areas include the airways, the digestive tract, as well as the urinary and reproductive systems, all of which are lined by an epithelium. Hollow organs and body cavities that do not connect to the exterior of the body, which includes, blood vessels and the heart, are lined by endothelium (plural = endothelia), which is a type of epithelium.

Epithelial cells derive from all three major embryonic layers. The epithelia lining the skin, parts of the mouth and nose, and the anus develop from the ectoderm. Cells lining the airways and most of the digestive system originate in the endoderm. The epithelium that lines vessels in the lymphatic and cardiovascular system derives from the mesoderm and is called an endothelium.

Characteristics of Epithelial Tissues

All epithelia share some important structural and functional features:

• Cellularity: This tissue is highly cellular, with no extracellular material present between cells. Adjoining cells are held tightly together by specialized intercellular connections between their cell membranes.
• Polarity: The epithelial cells exhibit polarity with differences in structure and function between the exposed surface of the cell and the attached surface adjacent to the underlying body structures.
• Attachment: The basal surface of an epithelium is anchored to the underlying connective tissue by a complex structure of collagen and proteoglycans that is produced jointly by both tissues.
• Avascularity: Epithelial tissues are nearly completely avascular. No blood vessels cross the basement membrane to enter the tissue and nutrients must come by diffusion or absorption from underlying tissues or the surface.
• Regeneration: Many epithelial tissues are capable of rapidly replacing damaged and dead cells by quickly and continuously undergoing mitosis. Sloughing off of damaged or dead cells is a characteristic of surface epithelium and allows our airways and digestive tracts to rapidly replace damaged cells with new cells.
• Innervation: Many sensory receptors are attached to the basal cells or are present in the underlying connective tissue to detect changes in the environment at both the external and internal surfaces of the body.

Generalized Functions of Epithelial Tissue

• Physical Protection: Epithelial tissues provide the body’s first line of protection from physical, chemical, and biological wear and tear.
• Selective Permeability: The cells of an epithelium can act as gatekeepers of the body controlling permeability and allowing selective transfer of materials across a physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features that allow the selective transport of molecules and ions across their cell membranes.
• Secretion: Many epithelial cells are capable of producing substances and releasing (secreting) them onto their apical surfaces. The epithelium of the small intestine releases digestive enzymes, for example. Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles. A glandular epithelium may contain many secretory cells, such as the alpha and beta cells of the pancreas that produce hormones.
• Sensation: Some epithelial tissues also provide sensation. Nerve supply to epithelial tissues can alert the body to pressure, pain, and temperature from external or internal stimuli. This can help the body to maintain homeostasis or avoid harmful situations in the environment.

Epithelium Structure

Since epithelia are located at all free surfaces they separate the outside of the body from the inside of the body and have distinct structural specializations that allow them to fulfill this unique role. Both the individual cells and the sheet of cells exhibit a characteristic polarity: they have an apical surface that is exposed and is in contact with the outside as well as a basal surface that is attached to an underlying proteinaceous layer called the basement membrane. This membrane allows the epithelial cells to hook onto one side of it and the underlying connective tissue, called the lamina propria, to attach to the other (Figure \(\PageIndex{1}\)). This keeps the epithelium firmly anchored in place and helps prevent distortions and tearing.

Figure \(\PageIndex{1}\): ABC's of an Epithelium. From the exterior toward the interior the structures of an epithelium are: Apical surface, Basal surface and Basement membrane, Connective Tissue of the lamina propria. (Image credit: "ABC's of an Epithelium" by Jennifer Lange is licensed under CC-BY-NC-SA 4.0, modified from original by Cancer Research UK via Wikimedia Commons.)

Epithelial cells also have a polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces. Particular structures found in some epithelial cells are an adaptation to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical surface. The transport channels within the cell membranes also differ between the two sides. For example, if you want to absorb a glucose molecule you have ingested you need two transporters - one to bring the molecule into the cell on the apical side and another to kick it out of the cell so it can enter into the bloodstream. If both transporters were exactly the same you could only, for example, move sugar into the cell!

Cell to Cell Junctions

Cells of epithelia are closely connected and are not separated by intracellular material. Three basic types of connections allow varying degrees of interaction between the cells: tight junctions, anchoring junctions, and gap junctions (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Types of Cell Junctions. The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions. (Image credit: "Types of Cell Junctions" by OpenStax is licensed under CC BY 3.0.)

• At one end of the spectrum is the tight junction, which separates the cells into apical and basal compartments. Tight junctions essentially "stitch" adjacent cell together, forming an impermeable barrier.
• An anchoring junction includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of cells where they provide strong and flexible connections. There are three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens.
  • Desmosomes occur in patches on the membranes of cells. The patches are structural proteins on the inner surface of the cell’s membrane. The adhesion molecule, cadherin, is embedded in these patches and projects through the cell membrane to link with the cadherin molecules of adjacent cells. These connections are especially important in holding cells together.
  • Hemidesmosomes, which look like half a desmosome, link cells to the extracellular matrix, for example, the basal lamina. While similar in appearance to desmosomes, they include the adhesion proteins called integrins rather than cadherins.
  • Adherens junctions use either cadherins or integrins depending on whether they are linking to other cells or matrix. The junctions are characterized by the presence of the contractile protein actin located on the cytoplasmic surface of the cell membrane. The actin can connect isolated patches or form a belt-like structure inside the cell. These junctions influence the shape and folding of the epithelial tissue.

In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells.

Classification of Epithelial Tissues

Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed (Figure \(\PageIndex{3}\)). Cell shapes can be squamous (flattened and thin), cuboidal (boxy, as wide as it is tall), or columnar (rectangular, taller than it is wide). Similarly, the number of cell layers in the tissue can be one—where every cell rests on the basal lamina—which is a simple epithelium, or more than one, which is a stratified epithelium and only the basal layer of cells rests on the basal lamina with the remaining layers attaching to the more basal cells. Pseudostratified (pseudo- = “false”) describes tissue with a single layer of irregularly shaped cells that give the appearance of more than one layer. Transitional describes a form of specialized stratified epithelium in which the shape of the cells can vary.

Figure \(\PageIndex{3}\): Epithelial Tissues. Simple epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells. (Image credit: "Epithelial Tissue" by OpenStax is licensed under CC BY 3.0.)

Simple Epithelia

The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in simple squamous epithelium (Figure \(\PageIndex{3}\) and Figure \(\PageIndex{4}\)) have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell. The endothelium is the epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and it is made up of a single layer of squamous cells. Simple squamous epithelium, because of the thinness of the cell, is present where rapid passage of chemical compounds is observed. The alveoli of lungs where gases diffuse, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue. The mesothelium is a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal organs. Its primary function is to provide a smooth and protective surface. Mesothelial cells are squamous epithelial cells that secrete a fluid that lubricates the mesothelium.

Figure \(\PageIndex{4}\): Histology of Simple Epithelia @ 400x. All simple epithelia have a single layer of cells between the lumen and the basal side and all cells attach to the basement membrane. Individual cells are outlined in yellow to illustrate their characteristic shape: A. simple squamous epithelium in the kidney, B. simple cuboidal epithelium in the kidney, C. simple columnar epithelium in the salivary glands. (Image credit: “Histology of Simple Epithelia" by Jennifer Lange is licensed under CC-BY-NC-SA 4.0, images provided by the Regents of the University of Michigan Medical School under CC-BY-NC-SA 4.0 © 2022.)

In simple cuboidal epithelium (Figure \(\PageIndex{3}\) and Figure \(\PageIndex{4}\)), the nucleus of the box-like cells appears round and is generally located near the center of the cell. These epithelia are active in the secretion and absorption of molecules. Simple cuboidal epithelia are observed in the lining of the kidney tubules and in the ducts of glands.

In simple columnar epithelium (Figure \(\PageIndex{3}\) and Figure \(\PageIndex{4}\)), the nucleus of the tall column-like cells tends to be elongated and located in the basal end of the cells. Like the cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules. Simple columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract. Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the Fallopian tubes and parts of the respiratory system, where the beating of the cilia helps remove particulate matter.

Pseudostratified columnar epithelium (Figure \(\PageIndex{3}\) and Figure \(\PageIndex{5}\)) is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighboring cells appear at different levels rather than clustered toward the basal side. The arrangement gives the appearance of stratification; but in fact all the cells are in contact with the basement membrane, although some do not reach the apical surface. Pseudostratified columnar epithelium is found in the respiratory tract, where some of these cells have cilia.

Figure \(\PageIndex{5}\): Histology of Pseudostratified Columnar Epithelium @ 400x. Although it appears to have multiple layers because of the postion of the nuclei, all cells attach to the basement membrane. (Image credit: "Histology of Pseudostratified Columnar Epithelium" by Jennifer Lange, is licensed under CC-BY-NC-SA 4.0, image provided by the Regents of the University of Michigan Medical School under CC-BY-NC-SA 4.0 © 2022).

Both simple and pseudostratified columnar epithelia can include additional types of cells interspersed among the epithelial cells. For example, a goblet cell is a mucous-secreting unicellular “gland” interspersed between the columnar epithelial cells of mucous membranes (Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{6}\): Goblet Cells. A. Goblet cells are specialized for the production and secretion of the protein mucin that becomes mucous when it is mixed with water. B. In the lining of the small intestine, goblet cells are scattered amoungst the columnar cells. LM × 1000. (Image credits: A. "Goblet Cell" by OpenStax is licensed under CC BY 3.0; B. "Goblet Cell Histology" by Jennifer Lange is licensed under CC-BY-NC-SA 4.0, micrograph provided by the Regents of the University of Michigan Medical School under CC-BY-NC-SA 4.0 © 2022.)

Stratified Epithelia

A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical wear and tear. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space.

Stratified squamous epithelium (Figure \(\PageIndex{7C}\)) is the most common type of stratified epithelium in the human body. The apical cells are squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells filled with the protein keratin. The epidermis of mammalian skin is an example of this dry, keratinized, stratified squamous epithelium. The lining of the oral cavity is an example of an non-keratinized, stratified squamous epithelium.

Stratified cuboidal epithelium and stratified columnar epithelium (Figure \(\PageIndex{7.A.B}\)) can be found in certain glands and ducts. Both are uncommon in the human body.

Figure \(\PageIndex{7}\): Stratified Epithelia Histology. All stratified epithelia have multiple layers of cells between the apical surface and basal surface, with the superficial cells attaching to the deeper cells instead of to the basement membrane. A. stratified cuboidal epithelium of a sweat gland, B. stratified columnar epithelium of the male urethra, C. stratified squamous epithelium of the vagina. (Image credit: “Histology of Stratified Epithelia" by Jennifer Lange is licensed under CC-BY-NC-SA 4.0, images provided by the Regents of the University of Michigan Medical School under CC-BY-NC-SA 4.0 © 2022.)

Another kind of stratified epithelium is transitional epithelium (Figure \(\PageIndex{8}\)), so-called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine. It is found only in the urinary system, specifically the ureters and urinary bladder. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella shaped, apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition from cuboidal to squamous. It appears thicker and more multi-layered when the bladder is empty, and more stretched out and less stratified when the bladder is full and distended.

Figure \(\PageIndex{8}\): Transitional Epithelium. Transitional epithelium is found lining organs of the urinary system. These organs are able to stretch because the rounded epithelial cells will flatten out. (Image credit:“Histology of Transitional Epithelium" by Jennifer Lange is licensed under, image provided by the Regents of the University of Michigan Medical School under CC-BY-NC-SA 4.0 © 2022.)

 

Table \(\PageIndex{1}\): Summary of Epithelial Tissues. The cell shape and number of layers determines an epithelial tissue's location and function.

Tissue	Location	Function
Simple Squamous Epithelium	Air sacs of lungs, tubules of the kidneys, and lining of the heart, blood vessels, and lymphatic vessels.	Allows material to pass through by diffusion and filtration, and secretes lubricating substance
Simple Cuboidal Epithelium	In ducts and secretory portions of small glands and in the kidney tubules	Secretes and absorbs
Simple Columnar Epithelium	Ciliated tissues are in bronchi, uterine tubes, and uterus Smooth (nonciliated tissues) are in the digestive tract, gallbladder	Absorbs; it also secretes mucous and enzymes May have goblet cells
Pseudostratified Columnar Epithelium	Ciliated tissue lines the trachea and much of the upper respiratory tract Nonciliated tissue is found in the middle regions of the male urethra	Secretes mucus, ciliated tissue moves mucus Has goblet cells
Stratified Squamous Epithelium	Lines the esophagus, mouth, anus, vagina, and distal urethra; forms epidermis of the skin	Protects against abrasion
Stratified Cuboidal Epithelium	Sweat glands, salivary glands, and the mammary glands	Secretory tissue
Stratified Columnar Epithelium	The ducts of some glands	Secretes and protects
Transitional Epithelium	Lines the urinary bladder, proximal urethra, and the ureters	Allows the urinary organs to expand and stretch

Glandular Epithelium

A gland is a structure made up of one or more cells modified to synthesize and secrete chemical substances. Most glands consist of groups of epithelial cells. A gland can be classified as an endocrine gland, a ductless gland that releases secretions directly into surrounding tissues and fluids (endo- = “inside”), or an exocrine gland whose secretions leave through a duct that opens directly, or indirectly, to the external environment (exo- = “outside”).

Endocrine Glands

The secretions of endocrine glands are called hormones. Hormones are released into the interstitial fluid, diffused into the bloodstream, and delivered to targets, in other words, cells that have receptors to bind the hormones. The endocrine system is part of a major regulatory system coordinating the regulation and integration of body responses. A few examples of endocrine glands include the anterior pituitary, thymus, adrenal cortex, and gonads (ovaries and testes).

Exocrine Glands

Exocrine glands release their contents through a duct that leads to the epithelial surface. Mucous, sweat, saliva, and breast milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts. Secretions into the lumen of the gastrointestinal tract, technically outside of the body, are of the exocrine category.

Glandular Structure

Exocrine glands are classified as either unicellular or multicellular. The unicellular glands are scattered single cells, such as goblet cells, found in the mucous membranes of the small and large intestine.

The multicellular exocrine glands known as serous glands develop from simple epithelium to form a secretory surface that secretes directly into an inner cavity. These glands line the internal cavities of the abdomen and chest and release their secretions directly into the cavities. Other multicellular exocrine glands release their contents through a tubular duct. The duct is singular in a simple gland but in compound glands it is divided into one or more branches (Figure \(\PageIndex{9}\)).  The shape of the duct can be either tubular (uniform diameter) or alveolar (rounded pockets).  In tubular glands, the ducts can be straight or coiled.  Combinations of tubes and pockets are known as tubuloalveolar (tubuloacinar) compound glands. In a branched gland, a duct is connected to more than one secretory group of cells.

Figure \(\PageIndex{9}\): Types of Exocrine Glands. Exocrine glands are classified by their structure. (Image credit: "Types of Glands" by OpenStax is licensed under CC BY 3.0.)

Methods and Types of Secretion

Exocrine glands can be classified by their mode of secretion and the nature of the substances released, as well as by the structure of the glands and shape of ducts (Figure \(\PageIndex{10}\)). Merocrine secretion is the most common type of exocrine secretion. The secretions are enclosed in vesicles that move to the apical surface of the cell where the contents are released by exocytosis. For example, watery mucous containing the glycoprotein mucin, a lubricant that offers some pathogen protection is a merocrine secretion. The eccrine glands that produce and secrete sweat are another example.

Figure \(\PageIndex{10}\): Modes of Glandular Secretion. (a) In merocrine secretion, the cell remains intact. (b) In apocrine secretion, the apical portion of the cell contining the product is released. (c) In holocrine secretion, the cell is destroyed as it releases its product and the cell itself becomes part of the secretion. (Image credit: "Modes of Secretion by Glands" by OpenStax is licensed under CC BY 3.0.)

In apocrine secretion the secretory product accumulates near the apical portion of the cell. That portion of the cell and its secretory contents pinch off from the cell and are released. Some sweat glands of the axillary region are classified as apocrine glands. Both merocrine and apocrine glands continue to produce and secrete their contents with little damage caused to the cell because the nucleus, rER and Golgi regions remain intact after secretion.

In contrast, the process of holocrine secretion involves the rupture and destruction of the entire gland cell. The cell accumulates its secretory products and releases them only when it bursts. New gland cells differentiate from cells in the surrounding tissue to replace those lost by secretion. The sebaceous glands that produce the oils on the skin and hair are holocrine glands/cells (Figure \(\PageIndex{11}\)).

Figure \(\PageIndex{11}\): Sebaceous Glands. These glands secrete oils that lubricate and protect the skin. They are holocrine glands and they are destroyed after releasing their contents. New glandular cells form to replace the cells that are lost. LM × 400. (Image credit: "Sebaceous Glands" by OpenStax is licensed under CC BY 3.0/ Micrograph provided by the Regents of University of Michigan Medical School © 2012.)

 

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Concept Review

In epithelial tissue, cells are closely packed with little or no extracellular matrix except for the basal lamina that separates the epithelium from underlying tissue.

The main functions of epithelia are: protection from the environment, coverage, secretion and excretion, absorption, and filtration.

Cells are bound together by tight junctions that form an impermeable barrier. They can also be connected by gap junctions, which allow free exchange of soluble molecules between cells, and anchoring junctions, which attach cell to cell or cell to matrix.

The different types of epithelial tissues are characterized by their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia. Single cell layers form simple epithelia, whereas stacked cells form stratified epithelia.

Very few capillaries penetrate these tissues.

Glands are secretory tissues and organs that are derived from epithelial tissues.

• Exocrine glands release their products through ducts.
• Endocrine glands secrete hormones directly into the interstitial fluid and blood stream. 

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Review Questions

Query \(\PageIndex{1}\)

Query \(\PageIndex{2}\)

 

Critical Thinking Questions

Query \(\PageIndex{3}\)

 

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Glossary

Query \(\PageIndex{4}\)

Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology