Glandular (secretory) epithelia. Glands
Secretion is the main function of the glandular epithelia.
Secretion is the complex process that consists of four phases (secretory cycle):
1. Transport of substances from the blood through basement membrane to cell cytoplasm;
2. Synthesis of the secretory products on membranous organelles, formation of secretory granules in the Golgi apparatus;
3. Secretion of the products from the cell;
4. Cell restoration.
The mechanism of secretion (3-d phase) may be merocrine, apocrine or holocrine.
Merocrine (eccrine).This is the most common type of secretion mechanism. Membrane-bound secretory granules form in the cells and accumulate in the apex. The granules fuse with the plasma membrane and are secreted into the lumen of the gland at apical cell surface by exocytosis. E.g.: pancreatic acinar cells; the protein component of milk from mammary glands. Merocrine secretion can involve two types of product: serous, or mucoussecretions.
Apocrine– secretion of product and a small portion of apical cytoplasm, pinched off enclosed by a membrane derived from the plasma membrane. E.g: fat droplet secretion by mammary glands.
Holocrine. In these glands the cell fills with its secretory product while the nucleus becomes smaller. The cell dies and the entire cell is released and disintegrates, releasing its product into acinus. E.g: sebaceous glands of skin and nose, which lubricates the epidermis with an oily lipid-rich product.
Glands
Secretory epithelium forms glands. There are two types of glands in the body: exocrineandendocrine. Exocrine[Greek: exo, outside; krinein, to separate] glands secrete through ducts or directly onto the surface of the body or lumen of body cavities. Some ducts alter the nature of the secretion by active transport mechanisms. Exocrine glands produce diverse types of secretions: enzymes, lubricants, waste products, aqueous fluids for evaporation, oils, etc.
Endocrineglands [GK: endon; within, krinein, to separate] do not secrete into ducts, but rather secrete directly into the bloodstream. They are ductless. Endocrine glands secrete substances, which are collective known as hormones. Hormones have the property of acting at specific sites in the body called target organs. Over 50 hormones have been identified up to date.
Glands consist of two parts: parenchyma and stroma. Parenchyma [Greek: "anything poured in beside"] - the epithelial, secreting cells of the gland, which perform the gland's essential function. Stroma [Greek: "anything laid out for lying or sitting upon"] - supporting cellular framework of connective tissue which contains blood vessels and nerves and gives structure to the gland.