GLANDS
Glands are structures formed by secretory epithelial cells.
Their sizes vary from microscopic, for example the small salivary glands that are inserted in the tongue, lips and cheeks, up to larger glands, such as the salivary glands or the hypophysis, or even larger as the pancreas and the liver.
Despite the definition of gland described here, some authors call the goblet cells, dispersed among other epithelial cells as unicellular glands.
Main classification of glands: exocrine glands and endocrine glands
Exocrine glands have excretory ducts that conduct their secretion product to the outside of the body (for example:sweat or sebaceous glands) or to internal cavities of the body (for example:salivary glands, pancreas).
Exocrine glands always have two components:
- A secretory portion – formed by secretory cells.
- One or more excretory ducts that convey the secreted material. In some glands the cells of the excretory duct can influence the composition of the secretion.
Endocrine glands do not have excretory ducts. They transfer their secretory products toward their surrounding extracellular space and from this space the products are transferred into blood vessels that exist in large amounts in endocrine glands. In this way the secretory product is distributed throughout the body.
The structure of both, exocrine and endocrine glands, include other components such as: blood vessels, connective tissue, and nervous tissue.
About the classification of glands. Glands are classified according to several criteria that will be presented as you study this chapter: the presence or absence of excretory ducts, the branching of the excretory ducts when present, the three-dimensional arrangement of their secretory cells, the types of their secr etory cells and of the secretory material, the mode of release of secretion.