Histology Glandular Epithelia PDF

Histology Glandular Epithelia Epithelia with secretory function: Glandular Epithelia Glandular epithelium: - Epithelium with secretory function. - Generally cuboidal or columnar. Glandular epithelium is the main component of the secretory portion of glands, the portion which produces and sec...

Histology Glandular Epithelia Epithelia with secretory function: Glandular Epithelia Glandular epithelium: - Epithelium with secretory function. - Generally cuboidal or columnar. Glandular epithelium is the main component of the secretory portion of glands, the portion which produces and secretes the secretory product. A gland can either be a single secretory cell, such as the goblet cells found in the digestive and respiratory tracts, or a group of secretory cells that form a bigger gland. The secretory cells in the glandular epithelium may synthesize, store, and release proteins (e.g., in the pancreas), lipids (e.g., adrenal, sebaceous glands), or complexes of carbohydrates and proteins. Epithelia with secretory function: Glandular Epithelia Formation of glandular epithelium The epithelium that forms the glands is formed in embryonic development by the formation of epithelial cells in the connective tissue beneath the epithelium. At first the glands are all connected to the epithelium of the surface through a channel known as a the duct. Later on the glands will be differentiated between endocrine and exocrine according in the absence or presence of ducts and vice versa.  Glandular Epithelia The essential function of a gland is secretion. Glands are of two main types: a) Exocrine glands: - are glands that retain their connection with the surface epithelium from which they originated via duct. - This connection takes the form of tubular ducts through which the secretions pass to reach the surface. b) Endocrine gland: - are those whose connection with the surface from which they originated was obliterated during development. - These glands are therefore ductless, and their secretions are picked up and transported to their site of action by the blood stream.  Exocrine gland Exocrine glands may be classified according to the number of cells: - Unicellular structures, e.g. the goblet cells, found in several types of epithelia (the columnar epithelium of the mucosa of the small intestine of Bufo. - Multicellular structures composed of large or smaller numbers of cells By their shape, multicellular glands are divided into two classes: - Spherical or ovoid constituting on acinus or alveolus - Long and of even diameter forming a tubule.  Exocrine gland Classification of multicellular exocrine glands: Multicellular exocrine glands are classified on the basis of two morphological factors in their architecture: - The shape of the secretory portion of the glands. - The modification of the duct system of the gland.  Exocrine gland As to the shape of the secretory portion: - A gland which has a tubular secretory portion is said to be tubular. Simple tubular unbranched gland simple - A gland whose secretory portion is saccular in acinar gland form is termed alveolar.  Exocrine gland On the basis of the modifications of the duct system of the gland: simple branched - A simple gland is one which has a single, tubular gland unbranched or branched duct. - A compound gland is one whose duct system compound is constituted of many ducts that connect with branched tubular one another in an ascending order of gland complexity so as to form one main duct.  Exocrine gland On the basis of this classification, the following types of glands can be distinguished: - Simple Tubular Gland: the gland is lined wholly with secretory epithelium and discharged directly on the epithelial membrane e.g., the small mammalian intestine. - Simple Coiled Tubular Gland: the gland has a coiled secretory portion from which arises a relatively long duct e.g., sweat gland of human. - Simple Branched Tubular Gland: the secretory portion of gland is forked into a number of tubules. A duct may not be present e.g., the glands of the mammalian stomach and some of the uterine glands.  Exocrine gland - Simple Alveolar Gland: this gland is generally composed of a saccular secretory portion with a duct. It does not generally occur in mammals e.g. poison and mucous glands of the Bufo skin. - Simple Branched Alveolar Gland: the secretory portion of the gland is divided into several small saccules which discharge into a duct e.g., the sebaceous glands of the human skin.  Exocrine gland - Compound Tubular Gland: this gland consists of a variable number of simple, branched tubular glands, the ducts of which generally contribute to formation of an elaborate duct system e.g., kidney and testis. - Compound Tubulo-Alveolar Gland: this gland is essentially a compound tubular gland, the tubular secreting portions of which are terminally sacculated e.g., the salivary glands and the pancreas.  Exocrine gland Additional classifications are based on the type of secretion the gland produces: - Serous—watery (e.g., parotid gland) - Mucous—viscous (e.g., minor salivary glands of the palate) - Mixed—serous and mucous (e.g., sublingual gland)  Exocrine gland Exocrine glands vary in the way they discharge their secretions. They are classified into three principal categories: Merocrine glands: - this type is found in the pancreas and salivary glands. - In this case, secretions diffuse through the outer tip of the cells. Apocrine glands: - in this case the apical parts of the cells are pinched off thus liberating the glandular secretion. - The cells then restore their apical portions, e.g. mammary glands. Holocrine glands: - this type is presented in the sebaceous glands found in the skin of the mammals in which the glandular cells burst altogether, liberating their secretory products. - The ruptured cells are then replaced by other cells produced through the division of non-secretory cells found in the outer wall of the gland  Endocrine glands Endocrine glands are the group of glands that do not have a duct system (lost the connection to their original epithelium) and thus, release their secretions directly into the bloodstream. The secretions of endocrine glands are called hormones. Endocrine secretions have far-reaching effects because they are distributed throughout the body by the bloodstream. Thin-walled blood capillaries adjacent to endocrine cells absorb their secreted hormone products for transport in the blood to target cells throughout the body.  Endocrine glands Histologically, the endocrine glands are of three types: - Trabecular type: In this type, the cells are arranged in cords like in the liver and the adrenal glands. - Follicular type: In this type, the cells form spherical structures like in the thyroid glands. - Disseminated type: In this type, the cells are placed in groups or separately in other organs like the Leydig cells in the testes.  Endocrine glands Hormones can be: - Proteins/peptides (e.g., insulin, growth hormone) - Lipid derivatives - derived from cholesterol or fatty acids (e.g., steroids) - Amino acid derivatives - derived from tyrosine or tryptophan (e.g., epinephrine, melatonin, thyroid hormones)  Endocrine glands Hormone effects are described as: - Endocrine - released into blood and act at long distances - Paracrine - diffuse a short distance through tissue fluids to nearby cells - Autocrine - act on the same cell that produced the hormone  Amphicrine glands The term amphicrine (endo-exocrine) cells refers to cells which show both exocrine and endocrine features. An amphicrine gland is a gland that has both exocrine (secretion of substances into dedicated channels) and endocrine (secretion of substances into the blood) functions. When the same cells have these two functions, the gland is said to be “homotypic amphicrine”, as is the case for the hepatocytes of the liver. When different types of cells each have one of these two functions, the gland is called "heterotypic amphicrine": this is the case of the pancreas, whose cells of the islets of Langerhans are endocrine, while the acinar cells are exocrine.