Q.What is an epithelium? Enumerate the characteristic features of different epithelia.

Epithelial tissue is a continuous sheet of tightly packed cells, held together with very little intercellular material, that covers body surfaces, lines internal cavities/tubes, or forms glands. As a group it handles protection, absorption, filtration, excretion, secretion and sensory reception, with individual sub-types specialising in particular combinations of these roles.

Epithelia are classified first by layer count. Simple epithelium (a single layer) is suited to absorption, secretion and filtration, and comes in five forms: squamous (thin, flattened cells; kidney glomeruli, lung air sacs, vessel linings — diffusion/filtration), cuboidal (cube-shaped cells; kidney tubules, gland ducts, ovary surface — secretion/absorption), columnar (tall cells with basal nuclei; digestive tract lining — absorption/mucus secretion), ciliated (columnar cells with motile cilia; bronchioles, fallopian tubes, uterus, trachea — mucus propulsion), and pseudostratified (columnar cells of unequal height whose staggered nuclei make a truly single layer look multi-layered; trachea/upper respiratory tract). Cells specialised purely for secretion form glandular epithelium, divided into unicellular glands (e.g. goblet cells) and multicellular glands, and further into exocrine glands (release product via a duct — classified by structure as simple/compound, by secretory-unit shape as tubular/alveolar/tubulo-alveolar, and by secretion mode as merocrine/holocrine/apocrine) versus ductless endocrine glands (release hormones into surrounding fluid). …