Human Anatomy and Physiology - N. I. Fedyukovich 2003

Cells and Tissues
Tissues
Epithelial Tissue

Epithelial Tissue (epithelium) covers the entire outer surface of the body in humans and animals, lines the mucous membranes of hollow Internal Organs (Stomach, intestines, Urinary Tract, Pleura, Pericardium, Peritoneum), and forms a major component of endocrine and exocrine glands. It is subdivided into covering (surface) and secretory (glandular) epithelium. Epithelial tissue is involved in METABOLISM between the Organism and the external environment, performs protective Functions (Skin epithelium), as well as secretion, absorption (intestinal epithelium), excretion (renal epithelium), and gas exchange (lung epithelium), and exhibits a high regenerative capacity.

Depending on the number of Cell layers and the shape of individual Cells, epithelium is classified into stratified—keratinized and non-keratinized, transitional—and simple—simple columnar, simple cuboidal, and simple squamous (mesothelium) (Fig. 3).

In squamous epithelium, the cells are thin and flattened with sparse Cytoplasm; a disc-shaped Nucleus is centrally located with irregular borders. Squamous epithelium lines the pulmonary alveoli, capillary and vascular walls, and Heart chambers, where its thinness facilitates the diffusion of various substances and reduces fluid flow friction.

Cuboidal epithelium lines the ducts of many glands and forms renal tubules, performing a secretory function.

Columnar epithelium consists of tall, narrow cells. It lines The Stomach, intestines, Gallbladder, and renal tubules, and is also found in The Thyroid Gland.

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Fig. 3. Various types of epithelium:

A — simple squamous; B — simple cuboidal; C — columnar; D — simple ciliated; E — pseudostratified; F — stratified keratinized

Cells of the ciliated epithelium are typically cylindrical in shape, with numerous cilia on their free surfaces; it lines the oviducts, cerebral ventricles, spinal canal, and respiratory tract, where it facilitates The transport of various substances.

Pseudostratified epithelium lines the urinary tract, Trachea, and respiratory passages, and is part of the mucous membrane of the Nasal cavity.

Stratified Epithelium consists of multiple cell layers. It covers the outer surface of the skin and lines the mucous membrane of the Esophagus, the inner lining of the Cheeks, and the Vagina.

Transitional epithelium is found in organs subject to significant stretching (Urinary Bladder, Ureter, renal pelvis). The thickness of transitional epithelium prevents urine from leaking into surrounding Tissues.

Glandular Epithelium constitutes the bulk of those glands in which epithelial cells participate in the synthesis and release of substances essential to the body.

There are two types of secretory cells: exocrine and endocrine. Exocrine cells release their secretion onto a free epithelial surface and through ducts into a cavity (such as the stomach, intestines, or respiratory tract). Glands whose secretions (Hormones) are released directly into the Blood or Lymph are called Endocrine glands (pituitary, thyroid, Thymus, Adrenal Glands).

Based on their Structure, exocrine glands can be tubular, alveolar, or tubulo-alveolar.