Epithelium 3 (1) PDF

B)Glandular epithelium Glandular epithelium : It is a special type of epithelium characterized by its secretory function. Glands originate from epithelial cells that leave the surface and penetrate into the underling connective tissue Copyright © McGraw-Hill Companies Classification of glandular epi...

B)Glandular epithelium Glandular epithelium : It is a special type of epithelium characterized by its secretory function. Glands originate from epithelial cells that leave the surface and penetrate into the underling connective tissue Copyright © McGraw-Hill Companies Classification of glandular epithelium: The glandular tissues are classified according to the presence or absence of ducts into: 1. Exocrine glands: They retain their connection with the surface by having ducts which carry their secretion to the surface of the epithelium e.g. salivary glands. 2. Endocrine glands: They are ductless glands and lose their connection with the surface. They secrete hormones which pass directly into the blood capillaries e.g. pituitary, thyroid and adrenal glands. 3. Mixed exocrine and endocrine glands: They consist of the two previous types in the same time e.g. pancreas (exocrine pancreas and Islets of Langerhans). Exocrine glands are morphologically classified 1. According to the number of cells: Unicellular glands formed of one cell e.g. goblet cell. Multicellular glands consist of numerous cells e.g. salivary glands. b- the mode of secretion: 1. Merocrine glands: The secretion leaves the cell by exocytosis & the cell membrane is left intact. No part of the cell is lost e.g. salivary glands. 2. Apocrine glands: During secretion, there is loss of the apical part of the cell e.g. apocrine sweat glands. 3. Holocrine glands: The cells after being full of secretion, they burst &become totally included in the secretion e.g. sebaceous glands. 2. According to the shape of the secretory part and the branching of ducts : I- Simple glands: a-Simple straight tubular glands e.g. intestinal glands. b-Simple coiled tubular glands e.g. sweat glands. C- Simple acinar. II- Simple branched glands: Simple branched tubular glands e.g. gastric glands. Simple branched acinar glands e.g. sebaceous glands Simple branched tubulo-acinar glands e.g. minor salivary gland III- Compound glands: Compound tubular glands e.g. lacrimal glands Compound acinar glands e.g. mammary glands and pancreas. Compound tubuloacinar glands e.g. major salivary glands. 3. According to the nature of secretion: Serous glands secrete serous fluid containing water, albumin and enzymes e.g. parotid gland. Mucous glands secrete mucin e.g. oesophageal glands Mixed seromucous salivary glands secrete serous and mucous secretion e.g. submandibular gland. C) Neuroepithelium (sensory epithelium): It is a hig hly specialized type organized during development for perception of external stimuli. It consists of: 1- Neuroepithelial. 2- Sustentacular (or supporting cells). 3- Basal cells (act as stem cells). Site and function: 1. Taste buds (taste). 2- Olfactory mucosa (smell) 2. Organ of corti in the inner ear (hearing). D) Myoepithelium : Special type of epithelial cells that are modified to contract. Branched cells with multiple processes, which contain contractile filaments (actin, myosin, and intermediate filaments ). Located around the basal parts of some epithelial secretory cells. They surround the glandular acini and help in squeezing them to discharge their secretion into the ducts, e.g. in sweat glands, salivary glands and mammary glands. Epithelial Cells Renewal Epithelial tissues are relatively labile structures whose cells are continuously renewed by mitotic activity and stem cells populations. The rate of renewal is variable; being fast in some organs as the small intestine &epidermis of the skin or slow as in the large glands. Copyright © McGraw-Hill Companies