The organs grouped together as the endocrine system represent many structures with a great deal of morphological and functional diversity. They are grouped together because their secretions are carried to target tissues and organs primarily by the blood stream, rather than by ducts as occurs in exocrine glands. Endocrine glands may be large, distinct organs (e.g. pituitary and adrenal glands), scattered groups of cells (e.g. pancreatic islets), or individual cells (e.g. mucosa of 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 epithelioid (see images below), meaning they are composed of sheets, tubes, columns, or clusters of closely contiguous cells. In contrast to exocrine glands where secretory granules are readily visible, the visible presence of secretory granules is less common in endocrine glands.

The learning objectives for this unit are:

• Identify the adenohypophysis (anterior pituitary) and neurohypophysis (posterior pituitary gland) and their unique cellular organization.
• Identify the cortex and medulla of the adrenal gland and their associated cell types.
• Identify thyroid follicles, colloid, and parafollicular cells in the thyroid gland.
• Identify parathyroid tissue and its chief and oxyphil cells.
• Recognize the structure of the pineal gland and explain its variability.
   

Pituitary gland