Human Anatomy and Physiology (with Age-Related Characteristics of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

Structure of the Human Body
Tissues
Epithelial Tissue

Cells and their derivatives combine to form Tissues. A tissue is an evolutionarily established system of cells and intercellular substance sharing a common origin, Structure, and function. Based on morphological and physiological characteristics, four types of tissues are distinguished in The Human Body: epithelial, connective, muscular, and nervous.

The epithelium of epithelial tissue forms the outer layers of the Skin, lines the mucous membranes of hollow Internal Organs and the surfaces of serous membranes, and forms glands. Accordingly, a distinction is made between covering epithelium and Glandular Epithelium.

Covering epithelium occupies a barrier position in the body, separating the internal environment from the external one. It protects the body from external influences and facilitates METABOLISM between the body and the environment.

Glandular epithelium forms glands that vary in shape, Location, and function. Epithelial cells (glandulocytes) of the glands synthesize and secrete substances called secretions, which participate in various bodily Functions. Therefore, glandular epithelium is also referred to as secretory epithelium.

Covering epithelium forms a continuous sheet of tightly packed cells joined by various types of Cell junctions. Epithelial cells always rest on a basement membrane rich in carbohydrate-protein-lipid complexes, which determine its selective permeability. The basement membrane separates epithelial cells from the underlying Connective Tissue. Epithelia are richly supplied with nerve fibers and receptor endings that transmit signals about various external influences to the Central Nervous system. Nourishment of covering epithelial cells occurs via diffusion of tissue fluid from the underlying connective tissue.

Based on The Relationship of epithelial cells to the basement membrane and their position on the free surface of the epithelial sheet, simple and stratified epithelia are distinguished (Fig. 5). In Simple Epithelium, all cells rest on the basement membrane, whereas in Stratified Epithelium, only the deepest layer is in contact with the basement membrane. Simple epithelium in which The Cell nuclei are located at the same level is called simple (or uniserial). Epithelium with cell nuclei located at different levels is called pseudostratified. Stratified epithelium can be non-keratinized (stratified squamous non-keratinized) or keratinized (Cytology/practical/35.html">Stratified squamous keratinized), in which the superficial cells undergo keratinization, turning into horny scales. Transitional epithelium is so named because its structure changes depending on the distension of the walls of the organ it lines (for example, the epithelial lining of the Urinary Bladder mucosa).

According to their shape, epithelial cells are classified as squamous, cuboidal, and columnar. Epithelial cells have a basal part facing the basement membrane and an apical part directed toward The surface of the covering epithelial layer. The Nucleus is located in the basal part, while the apical part contains cell Organelles, inclusions, including secretory granules in glandular epithelium. The apical part may feature microvilli—cytoplasmic outgrowths in specialized epithelial cells (or cilia in the ciliated epithelium of the respiratory tract).

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Fig. 5. Diagram of epithelial tissue structure:

A — simple squamous epithelium (mesothelium); B — simple cuboidal epithelium; C — simple columnar epithelium; D — ciliated epithelium; E — transitional epithelium; F — non-keratinized stratified squamous epithelium

When damaged, covering epithelium is capable of rapid regeneration through mitotic Cell Division. In simple epithelium, all cells retain The ability to divide, whereas in stratified epithelium, only the basally located cells do. Epithelial cells, proliferating intensively at the edges of the injury, migrate over the wound surface, thereby restoring the integrity of the epithelial lining.