Sensory: Many of the more complex sensory receptors of the nervous system are derived from specialized epithelia called neuroepithelia (e.g., the rods and cones of the retina, olfactory receptors of the nose, taste receptors on the tongue, etc.). Sensory receptors function by converting mechanical, chemical, or electromagnetic signals from the environment into nerve impulses which can be processed by the nervous system.
Contractility: Some very specialized epithelial cells (myoepithelia) contain the contractile proteins myosin and actin similar to muscle. Myoepithelia are associated with the ducts of sweat, salivary, lacrimal, and mammary glands and assist in the secretory process.
sensory, contractility
Absorption
Epithelial cells are found in those organs (e.g., small intestine) which are involved in absorption of substances important for life. These cells often have microscopic projections on the apical surface of their plasma membranes called microvilli which increase cell surface area in order to facilitate absorption.
Groups of epithelial cells lining the small intestine are organized into larger finger-like structures called villi which project into the lumen of the gut to further increase its surface area and aid the process of nutrient absorption.
Functions and distribution:
Barrier: Epithelial tissue commonly functions as a covering or lining for organs and other tissues (e.g., skin, mucous membranes, intestinal tract, pleural cavity, etc.). In this way, epithelial cells serve as selective barriers between the environment and the internal structures of the body. They protect underlying tissues from drying, and from mechanical and chemical injury. Tight junctions between individual cells play an important role in the barrier function of epithelium. Some barrier epithelial cells have motile cilia that propel fluid or particulate matter over tissue surfaces (e.g., cells lining the bronchi).
Endocrine glands
Glandular tissues that have no ducts opening onto a surface or into a cavity. Most endocrine glands secrete their products (hormones) across basement membranes into connective tissues where they are absorbed by nearby blood vessels and transported to target organs (e.g., pituitary, and adrenal glands).
Exocrine glands can also be classified by their secretion mechanism.
Merocrine glands - Glands that secrete substances by the process of exocytosis (fusion of cytoplasmic vesicles with the plasma membrane resulting in the release of their contents into the extracellular space without compromising the integrity of the cell membrane). Sweat glands (eccrine sweat glands) and salivary glands are merocrine in nature. The lining of the respiratory and digestive tracts contain "goblet cells" - glandular epithelia that synthesize and secrete a complex glycoprotein called mucus. Mucus provides a protective function as well as serving as a lubricant.
Apocrine glands - Glands that secrete by shedding the apical portion of their cytoplasm into a duct (e.g., mammary glands). Sweat glands in the axillae, perianal region, and external genitalia are also apocrine in nature.
Holocrine glands - Glands that secrete by shedding entire cells from the lining of a duct (e.g., sebaceous glands).
8 types distinguished by morphology of secretory portion
8 types distinguished by morphology of secretory portion (tubular [coiled versus straight] versus acinar [simple versus compound]) and branching versus straight excretory portions (which may also secrete bicarbonate)
simple tubular: large intestine: single, straight; lined by goblet (mucous) cells
simple coiled tubular: sweat glands; terminal secretory portion lined by simple cuboidal epithelium; followed by nonsecretory (excretory) duct lined by stratified cuboidal epithelium
simple branched tubular: stomach; secretory portions converge into unbranched duct (wider diameter); lined by mucus-secreting cells.
simple acinar: rounded secretory unit; pockets in epithelial surface; secretory cells (e.g., mucus-secreting glands of penile urethra)
simple branched acinar: several acini emptying into single excretory duct (stratified epithelium); sebaceous gland
compound branched tubular: = duct branched; secretory portions tubular and branched; duodenal (Brunner's) glands
compound acinar: secretory units are acinar and drain into branched ducts; pancreas; branched excretory ducts of increasing diameter lined by simple cuboidal epithelium
compound tubulo-acinar: 3 types secretory units: branched tubular; branched acinar (serous cells); branched tubular (mucous cells) with acinar end pieces ([serous] demilunes); submandibular salivary gland
myoepithelial cells: sometimes embedded in basement membrane; may aid secretion of acinus glands
striated ducts: striations due to mitochondria lined up along folds of basal membrane; transport Na and bicarbonate; cells high cuboidal to columnar
intercalated ducts come between acini or acini and striated ducts; cells low cuboidal
Exocrine glands
Glandular tissue that have ducts that open onto a body surface or into a body cavity. Examples include sebaceous, sweat, and mammary glands. Salivary glands and certain glandular structures of the pancreas that secrete digestive enzymes are examples of exocrine glands secreting into a body cavity. The secretions of exocrine glands can be mucous (viscous), or serous (thin, watery). The ducts of exocrine glands may be simple straight tubes, or can be arranged as more complex branching tubes. Some of the more complex branching duct systems may terminate in multiple sac-like structures called acini giving them a "bunches of grapes" appearance.
Glandular Epithelium
Glandular Epithelium Columnar epithelium basic histology with goblet cells is called glandular epithelium. Some parts of the glandular epithelium consist of such a large number of goblet cells that there are only a few normal epithelial cells left. Columnar and cuboidal epithelial cells often become specialised as gland cells which are capable of synthesising and secreting certain substances such as enzymes, hormones, milk, mucus, sweat, wax and saliva. Basic histology Unicellular glands consist of single, isolated glandular cells such as the goblet cells. Sometimes a portion of the epithelial tissue becomes invaginated and a multicellular gland is formed. Multicellular glands are composed of clusters of cells. Most glands are multicellular including the salivary glands.
Transitional epithelium
Transitional epithelium in basic histology blog - Also called uroepithelium. Stratified epithelial tissue with unique dome-shaped surface cells giving it a cobblestoned appearance. Found exclusively in the urinary system (lining of bladder and ureter). They serve as a barrier preventing the exposure of underlying tissues to urine. They are also able to stretch in response to bladder filling. Basic histology
Pseudostratified epithelium
Pseudostratified epithelium - basic histology Intermediary between simple and stratified epithelium. Consists of one layer of irregularly shaped and sized cells attached to a basement membrane. Because these cells vary in height, their nuclei are at different levels, giving the appearance (under the microscope) of more than one layer of cells. Found in lining of the trachea, bronchi, pharynx, nasal cavity, and urethra in basic histology blog
Stratified Epithelium
Stratified epithelim in basic histology Where body linings have to withstand wear and tear, the epithelia are composed of several layers of cells and are then called compound or stratified epithelium. The top cells are flat and scaly and it may or may not be keratinised (i.e. containing a tough, resistant protein called keratin). The mammalian skin is an example of dry, keratinised, stratified epithelium. The lining of the mouth cavity is an example of an unkeratinisied, stratified epithelium.
Ciliated Columnar Epithelium
Ciliated Columnar Epithelium These are simple columnar epithelial cells, but in addition, they posses fine hair-like outgrowths, cilia on their free surfaces. These cilia are capable of rapid, rhythmic, wavelike beatings in a certain direction. This movement of the cilia in a certain direction causes the mucus, which is secreted by the goblet cells, to move (flow or stream) in that direction. Ciliated epithelium is usually found in the air passages like the nose. It is also found in the uterus and Fallopian tubes of females. The movement of the cilia propel the ovum to the uterus.
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