Home to epithelia!
Glands of the body are classified as either exocrine or endocrine types. Exocrine glands are glands that retain ducts to body surfaces. During development, endocrine glands lose their contacts to embryological surfaces(ducts) and become isolated as small blocks of tissues. Endocrine glands are therefore referred to as "ductless" glands.
Endocrine and exocrine glands secrete various products. These include hormones, enzymes, metabolites, and other molecules. In exocrine glands, products of these cells collect in the duct of the gland and flow toward the surface to which the duct is in contact. Since endocrine glands lack ducts, the product is released across the cell membrane into interstitial spaces around the cells. Diffusion of the product into capillaries follows.
Most glands of the body are exocrine types with ducts connecting to anatomical surfaces. Contrast your salivary glands that open into the oral cavity with sweat glands that deposit their product on the body surface. Both types of glands are buried in deeper tissues but their products appear on a superficial surface. Connecting the glands to the surfaces are ducts!
A great deal of variation can be found in the design of glands. They are classified into simple and compound types. Note there are tubular and alveolar types!
Here is a branched alveolar type, a sebaceous gland associated with a hair follicle!
Modes of Secretion
Secretory cells of exocrine glands release their products into ducts in three different ways. The mode of secretion can be classified as merocrine, apocrine, or holocrine.
Cells that secrete products via the merocrine method form membrane-bound secretory vesicles internal to the cell. These are moved to the apical surface where the vesicles coalesce with the membrane on the apical surface to release the product. Most glands release their products in way.
In those glands that release product via the apocrine method, the apical portions of cells are pinched off and lost during the secretory process. This results in a secretory product that contains a variety of molecular components including those of the membrane. Mammary glands release their products in this manner.
The third type of secretory release, holocrine, involves death of the cell. The secretory cell is released and as it breaks apart, the contents of the cell become the secretory product. This mode of secretion results in the most complex secretory product. Some sweat glands located in the axillae, pubic areas, and around the areoli of the breasts release their products in this manner. Sebaceous glands also are of this type.
Regardless of gland type, structural complexity, or mode of secretion, epithelia are the secretory cells of all glands. Epithelia also form the ducts that connect the glands to the surface. Remember this as glandular structures found in tissues can be identified as clusters of tightly packed cells with very little intercellular space(an epithelial characteristic). When ducts are present and cut in longitudinal or cross-section, epithelial cells are also seen making up these structures. Simple cuboidal epithelia are the most typical type found in the body and ducts of exocrine glands.
Here is a view of simple cuboidal cells of a duct in cross-section and longitudinal section!
Endocrine glands are the hormone producing structures of the body. Some, like the thyroid are large and obvious. Others, for instance the islet cells of the pancreas, are small islands of endocrine cells embedded within the larger exocrine portion of this organ.
In lacking ducts, endocrine cells release their secretory products into the interstitial spaces around the cells. The hormones diffuse into nearby capillaries and are then carried to all parts of the body. Only when the hormones encounter a "target organ" do they exert an effect.
Notice the islet cells in this view of the pancreas! A pancreatic duct shown earlier is visible with some blood vessels and a nerve!
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