GLANDS
Glands are anatomical structures or whole organs whose main functional components are secretory cells.
Their sizes vary from microscopic, for example the small salivary glands that are inserted inside the tongue, lips and cheeks, up to larger glands, such as the bigger salivary glands or the hypophysis, or even larger as the pancreas and the liver.
Main classification of glands: exocrine and endocrine glands
Exocrine glands have excretory ducts that conduct the secretion to the exterior of the body (for example: sweat and sebaceous glands) or to internal cavities of the body (for example: salivary glands, pancreas).
Components of the exocrine glands:
- A secretory portion made of secretory cells.
- One, or depending on the size and complexity of the secretory portion, of several excretory ducts that in large glands join into one one two ducts that convey the secreted material to their destination. In some glands the cells of the excretory duct can influence the composition of the secretion.
Smaller glands, as the copious tubular glands of the intestinal mucosa may be formed by both, secretory and excretory cells.
Endocrine glands do not have excretory ducts and transfer the secretion toward their surrounding extracellular space. From this space it moves into blood vessels that exist in large amounts in endocrine glands and the secretory product is distributed throughout the body.
The structure of both, exocrine and endocrine glands, includes 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 their excretory ducts when present, the three-dimensional arrangement of their secretory cells, the types of their secretory cells and of the secretory material, the mode of release of the secretion.