Endocrine glands and tissues display a diversity in their gross morphological and histological patterns. This is particularly apparent when comparing species from phyletically distant groups. In some instances the physiological significance of these differences has been recognized but in most this is not so and may be related to the initial pattern of embryonic growth. If, however, one intuitively suspects a close relationship between structure and function, then the lack of a known correlation may merely reflect our ignorance.
The endocrines may display several different types of morphological Variation (see, for instance, Pang and Schreibman, 1986; Matsumoto and Ishii, 1992). The positions of the endocrine glands in the body may not be the same. The Variation can be of a minor nature, such as that seen with the ultimobranchial bodies, which can be situated near the heart or the thyroid gland. In some fish, however, thyroid tissue may vary in position from the branchial region to the kidney. Endocrine cells may show varying degrees of association and be scattered as individual cells, in small segments, or “islets.” or be closely associated as a compact gland enclosed in a capsule. Such aggregation of an endocrine tissue is commonly seen as one ascends the evolutionary (or the phyletic) scale. In addition, different endocrine tissues may display diverse associations with each other, as for instance the conglomeration of chromaffin and interrenal (or adrenocortical) tissue in the adrenal gland. Their relationship to the neural and vascular tissues can be very important. Pituitary tissues usually cannot function properly if they are transplanted to other parts of the body (ectopic transplant) or if the small blood vessels between the gland and the brain are cut. The major blood vessels not only carry hormones away from endocrine tissues but also supply them with nutrients and Controlling Stimuli. The pattern of the vasculature within the gland can also be important for its correct functioning.
The types of cell that make up an endocrine gland are, not surprisingly, similar in homologous glands among the vertebrates. Such similarities, as reflected by their microscopic anatomy (size, shape, the presence of inclusions, granules, etc.) and their reactions with dyes (tinctorial relationships), serve to aid in their identification. Antibodies to specific hormones are used to identify the cells where they are formed.