Glandular epithelium

Glands develop as down-growths of surface epithelium into the underlying connective tissue. Exocrine glands keep their connection to the surface as a duct; endocrine glands lose it and release hormones into the interstitial fluid and blood (e.g. thyroid, adrenal, pituitary, pancreatic islets). Some organs are mixed — the pancreas has exocrine acini and endocrine islets.

Exocrine glands are unicellular (the goblet cell) or multicellular. Multicellular glands are classified by their duct — simple (unbranched) or compound (branched) — and by the shape of their secretory units — tubular, acinar/alveolar (flask-shaped) or tubuloacinar: e.g. simple tubular (intestinal crypts), simple coiled tubular (eccrine sweat glands), simple branched acinar (sebaceous glands), compound tubuloacinar (salivary glands, pancreas, mammary gland). By product, serous cells secrete a watery, enzyme-rich fluid (dark basophilic base, apical zymogen granules — parotid, pancreas), mucous cells secrete viscous mucin (pale, foamy cytoplasm, flattened basal nuclei — sublingual, oesophageal glands), and mixed glands have both (submandibular, with serous demilunes). Myoepithelial cells — contractile epithelial cells between the secretory cells and the basement membrane — squeeze secretions into ducts (salivary, sweat, mammary glands; contract to oxytocin in the breast).

By mode of secretion: merocrine (eccrine) glands release products by exocytosis with no loss of cytoplasm (salivary, pancreatic acinar and most sweat glands); apocrine glands release membrane-bound portions of apical cytoplasm (lipid droplets of the lactating mammary gland; apocrine sweat glands of axilla and anogenital region are traditionally included, although they largely use merocrine secretion); holocrine glands release whole cells, which accumulate product, die and disintegrate (sebaceous glands, tarsal glands).