Epithelial Tissue- Definition

Membranous tissue composed of cells called epithelial tissue forms the body's numerous surfaces and linings. Some epithelial tissues are also a part of glandular tissue, which produces and secretes substances like mucus and proteins. However, most epithelial tissues are tissue on the surface of biological structures (for example, the skin or the lining of the intestines). Most epithelial tissues regularly produce fresh epithelial cells from epithelial stem cells. When the cells in the top layer of the process die, new cells are created in the deeper basal layers. Because older cells are more surface, younger cells are found deeper.

Apical, basal, and lateral are the three regions or domains that epithelial cells normally have in their structure. The apical domain is found on the lumen's or exterior environment's side. Structures related to the function of the cell are frequently found in this area, such as stereocilia, which aid in hearing and balance in the inner ear, and microvilli, which are finger-like projections found on intestinal epithelial cells in the small intestines and increase surface area for absorption and fluid transport. The extracellular matrix's basal lamina and the basal area of the cell's bottom edge, which divides the epithelial cell from the surrounding connective tissue, are located next to each other. The lateral area, which is on the sides, usually permits interactions with nearby cells.

Epithelial cell junctions enable various functions, including connections between nearby cells and structural support. Tight junctions, adherents, desmosomes, and gap junctions are lateral surfaces' primary epithelial cell connections. Hemidesmosomes are one kind of link that exists between epithelial cells and the underlying connective tissue.

The Epithelial Cell

Membrane-bound Organelles and proteins are often distributed unevenly across the apical and basal surfaces of epithelial cells. Some epithelial cells have features as an adaptation to particular functions. For instance, cilia are microtubule-supported extensions of the apical cell membrane. The fluids and particles can flow around the surface thanks to the coordinated beating of these extensions. The brain's ventricles are lined with these ciliated epithelia, which aid in cerebrospinal fluid circulation. They also line the respiratory system, which aids in the removal of dust and infections from the respiratory tract.

Glycoproteins and collagen are secreted by epithelial cells near underlying connective tissues, forming the basal lamina. The basement membrane, which is formed by the interaction of the basal lamina with the reticular lamina secreted by the underlying connective tissue, aids in securing the layers together.

Classification of Epithelial Tissues

There are two main types of epithelium: Surface and Glandular.

The number of layers, cellular makeup, and function of the surface epithelium are all categorized. There are three types of epithelial cells: squamous, columnar, and cuboidal. A spherical nucleus is located in the center of cuboidal epithelial cells with a cube-like structure. Tall, rectangular, and with an oval-shaped nucleus in the basal region, columnar epithelial cells exhibit these characteristics. Squamous epithelial cells have a tiny nucleus in the middle and a slightly rounded form that is frequently elliptical and flat.

Additionally, epithelial cells can be arranged into one or more layers. Simple and pseudostratified epithelial tissues both have a single layer of cells. Simple cuboidal epithelium is an example of this tissue type, with one layer of cells arranged in a single line. Contrastingly, pseudostratified epithelial tissues, such as pseudostratified columnar epithelium, comprise cells of variable sizes and shapes. Both subtypes frequently contain specific cell types situated in the crevices of epithelial cells, such as the mucus-secreting goblet cells in mucous membranes.

Stratified epithelium refers to tissues comprising two or more cell layers. Keratinized and transitional epithelial tissues are two additional categories for stratified epithelial tissues. The term "keratinized" refers to the epithelium that is keratinized or filled with keratin and in which the outermost, most apical layers are dead, with little nucleus or cytoplasm. The keratinized stratified squamous epithelium exhibits this. Contrarily, the transitional epithelium comprises several layers that, when stretched, may become flat.

On the other hand, the glandular epithelium can be categorized according to where it is about the surface epithelium, how it secretes, and what it secretes. Extraepithelial cells are found beneath the surface epithelium, whereas intraepithelial cells are found in the spaces between the surface epithelium. Goblet cells and secretory epithelium are the two varieties of intraepithelial glands. Goblet cells are solitary epithelial cells that frequently resemble tubes with a large vacuole filled with mucus. Contrarily, the secretory epithelium comprises columnar cells lacking vacuoles that contain a mucus-filled cytoplasm. Extraepithelial cells are secreted and have terminals connecting to the excretory ducts.

The glandular epithelium can be further divided into several categories depending on how it is secreted. Merocrine, holocrine, and apocrine secretion are the three fundamental processes for secretion. Exocytosis occurs to produce materials during merocrine, also known as eccrine secretion. Based on the substance they emit, they can also be classified as serous or mucous glands. Proteins, like digesting enzymes, are produced by serous cells, whereas mucous glands secrete mucus. When cells reach the end of their development cycle and begin to disintegrate, the products they have been storing are released. Last, apocrine secretion occurs when a substance, such as a lipid droplet, gets encircled by a membrane and released as a little package.

Location of Epithelial Tissue

The body's surfaces, both inside and outside, are all covered by surface epithelial tissue. Different cell types are more noticeable in particular areas. Along the linings of organs, single-layered epithelial tissue is frequently present. Normal locations for simple cuboidal epithelium include the lining of glandular ducts and kidney tubules. The female reproductive system, including the fallopian tubes and the digestive tract, frequently contains simple columnar epithelium. In the respiratory tract, the pseudostratified columnar epithelium is frequently found.

The most prevalent type of stratified epithelium is the stratified squamous epithelium, which is keratinized along the skin and nonkeratinized in places like the lining of the mouth cavity. Very few glands and ducts contain the stratified cuboidal and columnar epithelia.

Various types of glands and surface epithelium contain glandular epithelial tissue. Goblet cells are frequently observed along the intestines, nasal mucosa, and respiratory tract. Epithelial cells that secrete substances are frequently observed along the cervical canal and stomach. Organs like the pancreatic and glands like the salivary and sweat glands in the skin frequently contain extraepithelial glands.

Functions of Epithelial Tissue

Epithelial tissue has three main purposes: acting as a protective barrier, secreting chemicals, and absorbing substances. The primary function of particular tissues varies depending on the location and cell type. Goblet cells and stratified epithelium frequently form protective barriers. The stratified epithelium fortifies the body with keratinized cells to act as a barrier against physical and chemical harm. Through secretions like mucus and other substances that act as a barrier, goblet cells are important in maintaining a protective layer of surface tissue, such as the stomach's inner lining. Simple epithelia frequently secrete and absorb chemicals that must actively cross a cell membrane, while extraepithelial glands are responsible for direct draining into ducts.

Common Disorders that Affect the Epithelial Tissue

Cancer is the most frequent condition affecting epithelial tissues because epithelial cells have high turnover rates. Adenomas or papillomas are benign tumors of epithelial origin that often develop along the skin or digestive tract. The term "carcinoma" describes malignant tumors of epithelial origin that have progressed past the basement membrane. Lung, colon, and thyroid papillary carcinomas are the three most prevalent carcinomas worldwide.

There is a chance that epithelial cells could develop into cancer since they are the sort of cell that undergoes metaplasia, a process in which cells change their cell type, the most frequently. Stress and environmental changes frequently cause metaplasia. For instance, untreated gastroesophageal reflux disease (GERD) results in the esophageal epithelial cells changing from squamous cells to columnar cells that secrete mucus due to ongoing acid exposure. If this keeps happening, there is a larger chance of getting cancer. Epithelial tissues can also be impacted by various diseases, from those that arise on the skin's surface, like the human papillomavirus, to those that affect the digestive system, like celiac disease.

Important Facts about Epithelial Tissue

Surface and glandular tissues may be included in epithelial tissues, which comprise many of the body's surfaces and linings. The number of layers, the nature of its structure, and its purpose are used to categorize surface epithelial tissue further. Additionally, the glandular epithelium is divided into groups based on how close it is to the surface epithelium, how it secretes, and what it secretes. The body's linings and tracts are covered in the surface epithelium, whereas the glandular epithelium can be found between surface cells and the inside glands. Depending on the cell type and location, epithelia serve various purposes, but in general, epithelial tissue serves as a site for absorption, secretion, and protection. Cancers like colon adenocarcinoma are frequently linked to epithelial cells.