What Is The Function Of Epithelial Tissue – Epithelial tissue is a membranous tissue made up of cells that form various surfaces and linings throughout the body. Although most epithelial tissues are a type of tissue on the surface of body structures (eg, the skin or the lining of the intestines), some are also part of glandular tissue that produce and secrete products, such as mucus and proteins. Most epithelial tissues constantly regenerate new epithelial cells from epithelial stem cells. In this process, when the superficial layer of cells die, the new cells are formed in the deeper basal layers. Therefore, younger cells are found deeper than the older cells, which are more superficial.
Epithelial cells typically have three regions or domains in their structure: apical, basal, and lateral. The apical domain is located on the side of the lumen or external environment. This region often contains structures related to the function of the cell, such as finger-like projections called microvilli found on intestinal epithelial cells in the small intestine, which increase surface area for absorption and fluid transport; cilia, which are mobile to transport substances in the respiratory tract; and stereocilia to help with hearing and balance in the inner ear. The basal region is the lower edge of the cell and is adjacent to the basal lamina of the extracellular matrix, which separates the epithelial cell from the surrounding connective tissue. The lateral region is on the sides and typically allows connections with neighboring cells.
- 1 What Is The Function Of Epithelial Tissue
- 2 Is Epithelium A Connective Tissue
What Is The Function Of Epithelial Tissue
Epithelial cell junctions allow connections between neighboring cells, structural support and more. There are four main types of epithelial cell junctions located on the lateral sides: tight junctions, adherens, desmosomes, and gap junctions. There is one type of connection that exists between epithelial cells and underlying connective tissue, called hemidesmosomes.
Question Video: Recalling The Major Tissue Types In The Human Body
The surface epithelium is classified according to its number of layers, cellular structure and function. There are three epithelial cell shapes: cuboidal, columnar and squamous. Cuboid epithelial cells are shaped like a cube and have a spherical nucleus in the center. Colonic epithelial cells are tall, rectangular in shape, and have an oval-shaped nucleus in the basal region. Squamous epithelial cells are slightly rounded, often elliptical and flat in shape, with a small nucleus in the center.
Epithelial cells can also be organized into one or more layers. There are two types of epithelial tissues with only one layer of cells: simple and pseudostratified. Simple epithelial tissues have one layer of cells in a single line; an example of this tissue type is simple cuboidal epithelium. This contrasts with pseudostratified epithelial tissues, which contain cells with irregular shapes and sizes; an example of this tissue type is pseudostratified colonic epithelium. Both subtypes often include specialized cell types located between the epithelial cells, such as the mucus-secreting goblet cells found in a mucous membrane.
Epithelial tissues can also have two or more layers of cells, known as stratified epithelium. Stratified epithelial tissues have two additional classifications: keratinized and transitional. Keratinized describes an epithelium that is filled with a fibrous protein called keratin and where the most apical layers are dead, with minimal nucleus and cytoplasm. This is seen in keratinized stratified squamous epithelium. On the other hand, transitional epithelium consists of many layers that can become flat when stretched.
In contrast, glandular epithelium can be classified based on its location relative to the surface epithelium, its secretory mechanism, and what it secretes. Intraepithelial cells are those located between surface epithelium, while extraepithelial cells are located beneath the surface epithelium. There are two types of intraepithelial glands: goblet cells and secretory epithelium. Goblet cells are single epithelial cells that often appear as a tube with a large vacuole filled with mucus. In contrast, secretory epithelium consists of columnar cells that have cytoplasm filled with mucus, but without a vacuole. Extraepithelial cells have terminals that connect to the secretory ducts and are involved in secretion.
Solution: Epithelial Tissue
Glandular epithelium can be further classified depending on its mechanism of secretion. The three basic mechanisms for secretion include merocrine, holocrine, and apocrine secretion. Merocrine, also called eccrine, secretion is where exocytosis occurs to secrete products. They can be further divided into serous or mucous glands based on what they secrete. Serous cells produce proteins such as digestive enzymes, while mucous glands secrete mucus. Holocrine secretion occurs when the cells have completed growth and disintegrate, which releases the products stored within. Finally, apocrine secretion is when a product, such as a lipid droplet, is surrounded by a membrane and then released as a small package.
Surface epithelial tissue is found throughout the body covering all surfaces both inside and outside of the body. Each cell type is prominent in specific areas.
Single layer epithelial tissue is often found along the linings of organs. Simple cuboidal epithelium is typically found in the lining of renal tubules and glandular ducts. Simple colonic epithelium is often found throughout the digestive tract and part of the female reproductive tract, such as the fallopian tubes. Pseudostratified colonic epithelium is often located in the respiratory tract.
Stratified squamous epithelium is the most common type of stratified epithelium, and is keratinized along the skin and non-keratinized in areas such as the lining of the oral cavity. The stratified cuboidal and columnar epithelia are also located within some glands and ducts, but are very rare.
Is Epithelium A Connective Tissue
Glandular epithelial tissue is found scattered throughout surface epithelium and in various glands. Goblet cells are commonly found along the intestines, nasal mucosa, and respiratory tract. Secretory epithelial cells are often found along the stomach and cervical canal. Extraepithelial glands are often found within organs, such as the pancreas, and glands, such as the salivary glands of the mouth and the sweat glands of the skin.
There are three primary functions of epithelial tissue: acting as a protective barrier, excreting substances, and absorbing substances. The main function of specific tissues varies according to the specific location and type of cell. Protective barriers are typically formed by stratified epithelium and goblet cells. The stratified epithelium protects the body from physical and chemical damage by creating a barrier of keratinized cells. Goblet cells are involved in maintaining a protective layer of surface tissue, such as the inner lining of the stomach, through secretions such as mucus and other substances that provide a barrier. Secretion and absorption are often performed by simple epithelia for molecules that require active transport across a cell membrane, while extraepithelial glands are involved in direct drainage into ducts.
Because of the high turnover rates of epithelial cells, the most common disease affecting epithelial tissues is cancer. A benign tumor of epithelial origin is called an adenoma or papilloma, typically appearing along the skin or digestive tract. However, when tumors of epithelial origin have spread beyond the basement membrane, they are then identified as malignant and referred to as carcinomas. The most common carcinomas worldwide are lung adenocarcinomas, colon adenocarcinomas and thyroid papillary carcinomas.
Because epithelial cells are the most common type of cell to undergo metaplasia, a process in which cells change their cell type, there is a risk that the cells will become cancerous. Metaplasia often occurs in response to environmental changes and stress. For example, when gastroesophageal reflux disease (GERD) is uncontrolled, the continuous acid exposure causes the esophageal epithelial cells to turn from squamous cells to columnar cells that produce mucus. If this continues, there is a higher risk of developing cancers.
Chapter 4: Epithelial Tissue
Many other disorders can affect epithelial tissues, from those that develop in the digestive tract, such as celiac disease, to those on the surface of the skin, such as human papillomavirus.
Epithelial tissues make up many surfaces and linings throughout the body, and may include surface and glandular tissues. Surface epithelial tissue is further classified by the number of layers, its structure and function. Additionally, glandular epithelium is classified by its location to the surface epithelium, its secretory mechanism, and secretory substance. Surface epithelium is found throughout linings and tracts of the body, while glandular epithelium can be found between surface cells and within glands. The function of epithelium varies by cell type and location, but for the most part, epithelial tissue provides a site for protection, secretion, and absorption. Epithelial cells are often associated with cancers, such as adenocarcinoma of the colon.
Biga, L. M., Dawson, S., Harwell, A., Hopkins, R., Kaufmann, J., LeMaster, M., Matern, P., Morrison-Graham, K., Quick, D., & Runyeon, J. (2019). 4.2 Epithelial tissue. Anatomy & Physiology (first ed.). OpenStax/Oregon State University.
Blanpain, C., Horsley, V., & Fuchs, E. (2007). Epithelial stem cells: turning over new leaves. In Cell, 128(3), 445–458. DOI: 10.1016/j.cell.2007.01.014
Transitional Epithelium Tissue
Knoop, K. A., & Newberry, R. D. (2018). Goblet cells: multifaceted players in immunity at mucosal surfaces. Mucosal Immunology, 11(6), 1551-1557. DOI: 10.1038/s41385-018-0039-y
Kurn, H., & Daly, D.T. (2021). Histology, Epithelial Cell. In StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing LLC. Retrieved November 11, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK559063/
McLean, W. H., & Irvine, A. D. (2007). Disorders of keratinization: from rare to common genetic diseases of skin and other epithelial tissues. The Ulster Medical Journal, 76(2), 72-82.
University of Leeds. (2021). Epithelium: Functions of the Two Important Types of Epithelium. In The Histological Guide. Retrieved November 11, 2021, from http://www.histology.leeds.ac.uk/tissue_types/epithelia/epithelia_function.phpEpithelial tissue or Epithelium (plural = epithelium) is
Solved: Types Of Epithelial Tissues Distinctive Characteristics Type Function Location Squamous Simple Stratified Cuboidal Columnar Simple Stratified Pseudostratified Transitional
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