Histology Definition

Epithelial tissues are physically separated from underlying connective tissues by a basement membrane (also called the basal lamina). The portion of an epithelial cell attached to the basement membrane is called its basal surface. The opposite side - facing the external environment, or lumen of a body cavity, is its apical surface. Basement membranes are composed of a special type of collagen and a substance called laminin (see below). The basement membrane helps epithelial cells orient themselves in relation to other tissues. After epithelial injury (e.g., an abrasion), the basement membrane serves as a scaffolding upon which new cells attach themselves during healing.

Epithelium: Surface and Glandular

Epithelium is one of the primary tissues. Typically it consists of cells that are closely opposed to one another and occur in various morphological types, thus producing tissues with rather scanty intercellular substance. The epithelial tissues are divided into two major groups: namely, the covering and lining epithelial membranes and second the glandular epithelium.

Maximum cell-to-cell contact; minimum extracellular material; cell junctions; supported by basement membrane (basal lamella); not penetrated by blood vessels; derived from all embryonic germ layers, including mesoderm (endothelia and mesothelia)

Polarity: basal vs apical (luminal); lateral; specialized plasmalemma, appendages and junctional complexes

glycocalyx (apical) vs basement membrane (bm: basal lamina secreted by epithelium + lamina reticularis secreted by connective tissue)

Epithelia: thelia = nipple; epithelia = upon the nipple (convex [surface layer: epidermis, intestinal lining]); endothelia = within the nipple (concave [lining blood and lymph vessels and heart; cornea]); mesothelia = epithelia derived from splanchnic and somatic mesoderm lining body cavity (peripheral serosa) and covering organs (visceral serosa) within peritoneal, plural- and cardiac cavities.