What Cells Are In The Epithelial Tissue – This section is edited and adapted from Chapter 4.2 “Tissue Hierarchy: Epithelial Tissue” of the OpenStax open source book “Anatomy and Physiology 2e” (the original text is available for free at https://openstax.org/details/books/anatomy- Acquisition) and -Physiology-2e).

Epithelial tissue, also called epithelium (plural: epithelial cells), is the sheet of cells that covers the outer surface of the body, lines the inner cavities and channels, and forms certain glands.

What Cells Are In The Epithelial Tissue

What Cells Are In The Epithelial Tissue

The skin is not the only exposed area of ​​the body. Other areas include the airways, digestive tract, and urinary and reproductive systems, all of which are covered by epithelium. Hollow organs and body cavities that are not connected to the outside of the body, including blood vessels and serous membranes, are lined with endothelial cells (plural = endothelial cells), a type of epithelial cell.

Basement Membrane: What Is It, How It’s Formed, And More

All epithelial cells share some important structural and functional characteristics. This tissue is highly cellular, with little or no extracellular material between cells. Neighboring cells form special intercellular connections between their cell membranes, called cell junctions. Epithelial cells exhibit polarity, with differences in structure and function between their exposed or apically-facing (top) surface and their basal (bottom) surface that is closer to underlying body structures. The basal lamina is a mixture of glycoproteins and collagen that provides attachment sites for the epithelium, separating it from the underlying connective tissue. The basal lamina is attached to the reticular layer, which is secreted by the underlying connective tissue to form the basement membrane that helps hold it all together (Figure 4.3).

Figure 4.3: Drawing of the basal part of the epithelium. The basement membrane provides attachment sites for epithelial cells and underlying connective tissue. The basement membrane consists of the basal and reticular laminae and is secreted by the underlying connective tissue (Source: Modified from work by P. Stanka (CC BY-SA 4.0; Source: https://commons.wikimedia.org) /wiki/file: Histologie_Basalmembran_(2).jpg).

Epithelial tissue is almost completely avascular (without blood vessels). For example, without blood vessels crossing the basement membrane into the tissue, nutrients must diffuse or be absorbed from the underlying tissue or surface. Many epithelial tissues are capable of rapidly replacing damaged and dead cells. The shedding 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.

Epithelial tissue is the body’s first line of defense against physical, chemical, and biological wear and tear. Epithelial cells act as the body’s gatekeepers, controlling permeability and allowing materials to selectively cross physical barriers. All substances entering the body must cross the epithelium. Some epithelial cells often possess structural features that allow selective transport of molecules and ions across the cell membrane. Many epithelial cells are capable of secreting and releasing mucus and specific compounds onto their apical surfaces. For example, the epithelium of the small intestine releases digestive enzymes. Cells lining the respiratory tract secrete mucus that traps incoming microorganisms and particles. The glandular epithelium contains many secretory cells.

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Epithelial tissues are classified based on the shape of the cells and the number of cell layers formed (Figure 4.4). Cell shape can be squamous (flat and thin), cubic (boxy, as wide as high), or columnar (rectangular, taller than wide). Likewise, the number of cell layers in a tissue can be one where each cell is on top of the basal layer (which is a simple epithelium), or multiple cell layers (which is a stratified epithelium), and only Basal cells are located on the basal layer. blade. Pseudostratification (pseudo-= “false”) describes tissues with a single layer of irregularly shaped cells that give the appearance of more than one layer (Fig. 4.4). Transitional describes a specialized form of stratified epithelium in which the shape of the cells can change.

Figure 4.4: Epithelial tissue cells Simple epithelial tissue consists of a single layer of cells, stratified epithelial tissue consists of multiple layers of cells.

An example of a simple squamous epithelium is the endothelium that lines lymphatic vessels or the lymphatic system and blood vessels. The single layer of cuboidal epithelium is active in the secretion and absorption of molecules. These epithelial cells are found in the lining of glandular ducts and renal tubules. Simple columnar epithelium also secretes and absorbs molecules. These epithelial cells form in parts of the digestive system and parts of the female reproductive tract. Ciliated columnar epithelium is composed of simple columnar epithelial cells that have cilia (small hair-like structures). They are found in the lining of the fallopian tubes (part of the female reproductive system) and the respiratory system, where cilia help remove small particles. Pseudostratified columnar epithelium is an epithelium that appears to be stratified but is actually composed of a single layer of columnar cells of irregular shape and varying sizes. In pseudostratified epithelium, the nuclei of adjacent cells occur at different levels rather than being clustered basally. This arrangement gives the appearance of layers; however, all cells are actually in contact with the basal layer, although some do not reach the top. Pseudostratified columnar epithelium is found in the respiratory tract, some of whose cells are ciliated.

What Cells Are In The Epithelial Tissue

Both simple columnar epithelium and pseudostratified columnar epithelium are heterogeneous epithelia because they include other cell types interspersed among the epithelial cells. For example, goblet cells are mucus-secreting single-cell “glands” interspersed among the columnar epithelial cells of the mucosa (Figure 4.5).

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Figure 4.5: Goblet cells. In the lining of the small intestine, columnar epithelial cells are interspersed with mucus-secreting goblet cells (arrows). Photomicrograph (LM × 1600) Courtesy of the Board of Trustees of the University of Michigan Medical School © 2012.

Stratified epithelium is composed of multiple layers of stacked cells. This epithelium protects against physical and chemical wear. Stratified epithelium is named after the shape of the apical layer closest to free space. Stratified squamous epithelium is the most common type of stratified epithelium in the human body. The apical (upper) cells are squamous, while the basal layer contains columnar or cuboidal cells. The top layer may be covered with dead cells filled with keratin. Human skin is an example of this type of dry, keratinized, stratified squamous epithelium. The lining of the oral cavity is an example of nonkeratinized stratified squamous epithelium. Stratified cuboidal epithelium and stratified columnar epithelium are also found in some glands and ducts but are less common in humans. Another type of stratified epithelium is the transitional epithelium, so called because the top cells gradually change shape as the bladder fills with urine. It is found only in the urinary system, specifically the ureters and bladder. When the bladder is empty, this epithelium is coiled and has cuboidal apical cells with a convex, umbrella-shaped apical surface. As the bladder fills with urine, the epithelial cells lose convolution and the apical cells transform from cuboidal to squamous. When the bladder is empty, it looks thicker and more layered; when the bladder is full and distended, it looks more stretched and less layered.

A gland is a structure composed of one or more cells modified to synthesize and secrete chemicals. Most glands are composed of epithelial cell populations. Glands can be classified as endocrine glands, ductless glands, which release secretions directly into surrounding tissues and fluids (endo- = “inside”), or exocrine glands, which secrete their contents into the external environment (exo- = ” External”) ).

Endocrine glands secrete hormones. Hormones are released into the interstitial fluid surrounding gland cells, diffuse into the bloodstream, and are delivered to cells that have receptors that bind the hormones. The endocrine system is part of the major regulatory system that coordinates the regulation and integration of body responses. Some examples of endocrine glands include the anterior pituitary gland, thymus, adrenal cortex, and gonads.

Epithelial Tissue And Connective Tissue Proper

Exocrine glands release their contents to the external environment either directly or through ducts leading to the epithelial surface. Mucus, sweat, saliva, and breast milk are all examples of eccrine gland secretions. Secretions within the lumen of the gastrointestinal tract (technically outside the body) also fall into the exocrine category. Exocrine glands are classified as unicellular or multicellular. Unicellular exocrine glands are dispersed single cells, such as goblet cells, found in the mucosa of the small and large intestines (Fig. 4.5).

Exocrine glands, which release their contents through ducts, can be classified based on their structure and duct shape. Pipes can be single or branched. Figure 4.6 shows a summary of these glands and their classification.

Exocrine glands are also classified according to the nature of the substances they release. Serous glands produce a watery, plasma-like secretion that is rich in enzymes such as amylase, an enzyme that breaks down starch and releases it in saliva. Mucous glands secrete a watery to viscous product rich in mucin. Mucin is a so-called glycoprotein whose function

What Cells Are In The Epithelial Tissue

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