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Epithelial Cells

Illustration of the dividing epithelium cell surrounded by epithelium tissue. Spindle apparatus rotates inside the cell. The rotation is a result of astral microtubules pulling towards tri-cellular-junctions (TCJ), signaling centers localized at the regions where three cells meet. Graphic credit: Margo. raichman – Own work, CC BY-SA 4.0,https://commons.wikimedia.org/w/index.php?curid=52087985



Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs. An example is the epidermis, the outermost layer of the1) ___. There are three principal shapes of epithelial cell: squamous, columnar, and cuboidal. These can be arranged in a single layer of cells as simple epithelium, either squamous, columnar, cuboidal, pseudo-stratified columnar or in layers of two or more cells deep as stratified (layered), either squamous, columnar or cuboidal. All glands are made up of epithelial 2) ___. Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport, and sensing.


Epithelial layers contain no blood 3) ___, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane. Cell junctions are well-employed in epithelial tissues. Epithelial tissues have as their primary functions:


1. Protect the tissues that lie beneath from radiation, desiccation, toxins, invasion by pathogens, and physical trauma

2. Regulation and exchange of chemicals between the underlying tissues and a body cavity

3. Secretion of hormones into the circulatory system, as well as the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts

4. Provide sensation


Cells of epithelial tissue are tightly packed and form a continuous sheet. They have almost no intercellular spaces. All epithelia is usually separated from underlying tissues by an extracellular fibrous basement membrane. The lining of the mouth, lung alveoli and kidney tubules all are made of epithelial 4) ___. The lining of the blood and lymphatic vessels are of a specialized form of epithelium called endothelium. Epithelium lines both the outside (skin) and the inside cavities and lumina of bodies. The outermost layer of human skin is composed of dead stratified squamous, keratinized epithelial cells. Tissues that line the inside of the mouth, the esophagus, the vagina, and part of the rectum are composed of nonkeratinized stratified squamous epithelium. Other surfaces that separate body cavities from the outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells. Other 5) ___ cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands. The outer surface of the cornea is covered with fast-growing, easily regenerated epithelial cells. A specialized form of epithelium – endothelium forms the inner lining of blood vessels and the heart, and is known as vascular endothelium, and lining lymphatic vessels as lymphatic endothelium. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum.


In arthropods, the integument, or external “skin“, consists of a single layer of epithelial ectoderm from which arises the cuticle, an outer covering of chitin the rigidity of which varies as per its chemical composition.


Epithelial tissue rests on a basement membrane, which acts as a scaffolding on which epithelium can grow and regenerate after injuries. Epithelial tissue has a nerve supply, but no 6) ___ supply and must be nourished by substances diffusing from the blood vessels in the underlying tissue. The basement membrane acts as a selectively permeable membrane that determines which substances will be able to enter the epithelium. It is known that some corneal diseases and degenerations have a significant heritable background. Because the density of the corneal endothelial cells is strongly 7) ___, this knowledge should stimulate future genetic studies to identify genes and pathways that are involved in determining corneal endothelial cell density (ECD) which might in turn lead to future treatments to prevent epithelial cell (EC) loss.


There is evidence that a major pressure driving evolution of CNS blood-tissue barriers was the selective advantage given by fine control (homeostasis) of the brain ionic microenvironment. The blood-brain barrier (BBB) is a diffusion barrier, which impedes influx of most compounds from blood to brain. Three cellular elements of the brain microvasculature compose the BBB-endothelial cells, astrocyte end-feet, and pericytes (PCs). Tight junctions (TJs), present between the cerebral endothelial cells, form a diffusion barrier, which selectively excludes most blood-borne substances from entering the 8) ___. At the blood brain 9) ___, the endothelial cells do not act alone, but function within a well organized ‘neurovascular unit’ (NVU), a modular structure integrating the local neuronal population and its associated astrocytic glia with the cells forming the microvascular tube providing blood flow, the endothelium and pericytes, and in arterioles also smooth muscle. Microglia, the resident immune cells of the CNS, are associated with the NVU, in quiescent state in normal physiology, but becoming activated in pathology.


When researchers today refer to HERS, they are including the studies, done over a century ago, of Oskar Hertwig. HERS stands for: Hertwig’s Epithelial Root Sheath. Today, NIH clinical reports in tandem with developmental and evolutionary studies re-introduce HERS as the ultimate governor of the periodontal ligament, the regulator of its width and homeostasis and the shield against resorption and ankylosis. From an evolutionary 10) ___ perspective, HERS appears to have evolved first to provide elastic anchorage for and mediate eruption of amphibian teeth and then may have evolved to facilitate the formation of a non-mineralized periodontal ligament in crocodiles and mammals and maintain its functional integrity. During development, HERS fenestration allows mesenchymal cells from the dental follicle to penetrate the epithelial barrier and deposit cementum. A part of this function may be related to the induction of acellular cementogenesis, and future studies will provide definitive answers to address this important issue.


ANSWERS: 1) skin; 2) cells; 3) vessels; 4) tissue; 5) epithelial; 6) blood; 7) heritable; 8) brain; 9) barrier; 10) biology


Sources: NIH.gov; /www.frontiersin.org; Wikipedia



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