Introduction

The Endocrine System consists of glands, groups of cells scattered in certain organs, as well as individual cells scattered in the parenchyma of the body.

Prior to your study of the endocrine system, it might be helpful to review a few concepts regarding glandular tissue. Epithelium that mainly functions in secretion is often arranged into structures known as glands. Glands are simply invaginations of surface epithelium into the underlying connective tissue. Glands which maintain contact with the surface via a system of ducts are called exocrine or ducted glands. The glands which have lost their duct system and are isolated in connective tissue are called endocrine glands.

Endocrine glands secrete their products, which are called hormones, directly into the blood vascular system. Fenestrated capillaries which lie in the reticular fiber-rich stroma of these endocrine glands enable them to secrete their products (hormones) directly into the bloodstream. These hormones can then act on target organs which, in some cases, are far removed from the site of secretion.

The endocrine glands, then, are characterized by an absence of ducts, and a high degree of vascularity. In addition to the individual endocrine organs, there are also cells and cell groups which secrete hormones within the tissue of the gonads, kidneys, stomach, intestines, and placenta. These have been already been studied in the context of those organ systems.

In addition to true endocrine glands and cells, there are other types of glands or cells that secrete hormone-like substances. Paracrine glands or cells secrete products which influence the secretion of another product by a local gland or cell. Autocrine glands or cells secrete products which influence the secretion of another product by the same cell. This laboratory exercise, however, will focus on endocrine organs, particularly the pituitary gland, the thyroid gland and the adrenal glands.

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