Epithelial Tissue

Epithelial tissue is essentially a sheet of tightly packed cells with very little material between them. It appears in the body in three settings: as a covering (the outer surface of an organ), as a lining (the inner surface of a cavity or tube), and as a gland (a specialised secretory structure). Collectively, epithelial tissues are responsible for protection, absorption, filtration, excretion, secretion and sensory reception — different sub-types specialise in different combinations of these jobs.

Epithelia are first split by how many cell layers they have. Simple epithelium has just a single layer of cells and is typically found wherever the body needs to move substances across a barrier efficiently — absorption, secretion or filtration. Its five recognised forms are: squamous (a single thin sheet of flattened, irregularly shaped cells lining the kidney glomeruli, the lung's air sacs, and the heart and vessel walls, well suited to diffusion and filtration where protection is not the priority); cuboidal (a layer of cube-shaped cells in kidney tubules, small-gland ducts and the ovary's surface, built for secretion and absorption); columnar (tall cells with basal nuclei lining the digestive tract from stomach to rectum, often bearing absorptive microvilli or mucus-secreting goblet cells); ciliated (columnar cells whose free surface carries beating cilia that sweep mucus along, found in the bronchioles, fallopian tubes, uterus, trachea and upper respiratory tract); and pseudostratified (columnar cells of unequal height whose nuclei sit at different levels, so a genuinely single-layered sheet looks deceptively multi-layered — it lines the trachea in its ciliated form and structures such as the epididymis in its non-ciliated form).

Some of the cuboidal or columnar cells specialise entirely for secretion, forming glandular epithelium. Glands are first divided by cell number into unicellular glands (a single isolated secretory cell, like a goblet cell in the gut lining) and multicellular glands (an organised cluster of secretory cells, like a salivary gland). By where they release their product, glands are exocrine (releasing mucus, saliva, earwax, oil, milk, enzymes and similar products through a duct) or endocrine (ductless glands that release hormones straight into the surrounding tissue fluid). Multicellular exocrine glands are further classified by their overall structure (simple or compound), by the shape of their secretory unit (tubular, alveolar/acinar, or tubulo-alveolar), and by how they release their product (merocrine, holocrine or apocrine). …