The organs usually grouped together as the endocrine system represent many structures with a great deal of morphological and functional diversity. The fact that they are grouped together reflects primarily the fact that they all secrete directly into the extracellular space for uptake by the blood stream, rather than delivering their products into ducts for transport to other sites, as in the exocrine glands. Endocrine glands may be large, distinct organs, such as those to be studied here, or scattered groups of cells, such as the pancreatic islets, or individual cells, such as those found in the mucosa of the digestive system. A structural feature common to all endocrine tissues is a very rich vascular supply. Furthermore, in spite of having origins from diverse kinds of tissues, almost all endocrine glands are structurally epithelial in appearance, i.e., they are composed of sheets, tubes, columns or clusters of closely contiguous cells. A less universal feature of endocrine cells is a content of granules suggesting the secretory function of the cells.

Learning Objectives:

• Understand the functional characteristics of endocrine cells and organs.
• Be able to distinguish the major endocrine glands in LM.
• Distinguish the regions, zones, and most important specific cells in the pituitary gland, adrenal glands, thyroid gland, parathyroid gland, and the pancreatic islets.
   

Pituitary gland