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 Home to epithelia!

Glands Glands of the body are classified as either exocrine or endocrine types. Exocrine glands are glands thatretain ducts to body surfaces. During development, endocrine glands lose their contacts to embryologicalsurfaces(ducts) and become isolated as small blocks of tissues. Endocrine glands are therefore referredto 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 thegland and flow toward the surface to which the duct is in contact. Since endocrine glands lack ducts, theproduct is released across the cell membrane into interstitial spaces around the cells. Diffusion of theproduct into capillaries follows.

Exocrine Glands 

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 bodysurface. Both types of glands are buried in deeper tissues but their products appear on a superficialsurface. 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 andcompound types. Note there are tubular and alveolar types!

Here is a branched alveolar type, a sebaceous gland associated with a hair follicle!

Modes of Secr etion 

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.

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Cells that secrete products via themerocrine method form membrane-bound secretory vesicles internalto the cell. These are moved to the apical surface where the vesicles coalesce with the membrane on theapical surface to release the product.Most glands release their products in way.

In those glands that release product via theapocrine 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 thismanner.

The third type of secretory release,holocrine, involves death of the cell. The secretory cell is releasedand as it breaks apart, the contents of the cell become the secretory product. This mode of secretionresults 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.

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 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 asglandular structures found in tissues can be identified as clusters of tightly packed cells with very littleintercellular 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 mosttypical type found in the body and ducts of exocrine glands.

Here is a view of simple cuboidal cells of aduct in cross-section and longitudinal section!

Endocrine Glands 

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 cellsembedded within the larger exocrine portion of this organ.

In lacking ducts, endocrine cells release their secretory products into the interstitial spaces around thecells. The hormones diffuse into nearby capillaries and are then carried to all parts of the body. Onlywhen 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 someblood vessels and a nerve!

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