The communication between cells is crucial in human biology. Cell-to-cell communication exists at various levels of complexity and distance. Endocrine signaling involves the regulated secretion of a hormone into the extracellular fluid. The hormone then diffuses into the vasculature, circulating throughout the body. Eventually, the hormone diffuses out of the blood vessels and binds to a specific receptor in the cells of the target organ. Since hormones spread throughout the body, one hormone can regulate the activity of multiple target organs. Conversely, cells often express receptors for multiple hormones.

The endocrine system consists of glands that regulate multiple organs in the body to meet the growth and reproductive needs of the organism and respond to internal fluctuations and stress. The major glands of the endocrine system include the pituitary gland (in association with hypothalamic nuclei), the pancreas, the parathyroid glands, and the gonads (testes or ovaries). These glands synthesize and secrete bioactive hormones, except for the gonads which also perform gametogenic functions. The placenta, a temporary organ, also plays a significant endocrine role.

In addition to dedicated endocrine glands, there are endocrine cells within non-endocrine organs. For example, the heart produces atrial natriuretic peptide, the liver produces insulin-like growth factor type 1 (IGF-1), the kidney produces erythropoietin, and various cell types in the gastrointestinal tract produce gastrointestinal hormones. The hypothalamus also contains nuclei that secrete neurohormones into capillaries associated with the pituitary gland.

Another subset of the endocrine system consists of cell types that express intracellular, ectoenzymes, or secreted enzymes. These enzymes modify inactive precursors or less active hormones into highly active hormones. For instance, angiotensinogen is converted into angiotensin II through two proteolytic cleavages. Vitamin D is activated through two hydroxylation reactions in the liver and kidneys, producing the highly bioactive hormone 1,25-dihydroxyvitamin D.

The endocrine system functions through various feedback loops. One type is a simple endocrine negative feedback loop, while the other involves the hypothalamus, anterior pituitary, and peripheral endocrine gland. Under each type of feedback loop, there are several major endocrine glands.

A releasing hormone is defined as a hormone that stimulates the release of another hormone, while a tropic hormone is a hormone that targets and stimulates the secretion of another endocrine gland.

Protein/peptide hormones, catecholamine hormones, steroid hormones, and iodothyronines (thyroid hormones) have different chemical natures and characteristics. These characteristics include the site of regulation (synthesis or secretion), the circulating form of the hormone, the subcellular localization of hormone receptors, and the metabolic clearance.

The concept of peripheral conversion is integrated with the function and action of a secreted hormone. Additionally, the intracellular steps associated with a hormone response in a target cell are also integrated.
Map out and differentiate a simple endocrine negative feedback loop and one involving the hypothalamus, anterior pituitary and peripheral endocrine gland, and list the major endocrine glands under each type of feedback loop.
3.
Define a releasing hormone and a tropic hormone.
4.
Explain the chemical nature and the characteristics of protein/peptide hormones, catecholamine hormones, steroid hormones, and iodothyronines (thyroid hormones). Include such characteristics as site of regulation (synthesis or secretion), circulating form of hormone, subcellular localization of hormone receptor, and metabolic clearance.
5.
Integrate the concept of peripheral conversion with the function/action of a secreted hormone.
6.
Integrate the intracellular steps associated with a hormone response in a target cell.
he ability of cells to communicate with each other is an underpinning of human biology. As discussed in , cell-to-cell communication exists at various levels of complexity and distance. Endocrine signaling involves (1) the regulated secretion of an extracellular signaling molecule, called a hormone, into the extracellular fluid; (2) diffusion of the hormone into the vasculature and its circulation throughout the body; and (3) diffusion of the hormone out of the vascular compartment into the extracellular space and binding to a specific receptor within cells of a target organ. Because of the spread of hormones throughout the body, one hormone often regulates the activity of several target organs. Conversely, cells frequently express receptors for multiple hormones.
The endocrine system is a collection of glands whose function is to regulate multiple organs within the body to (1) meet the growth and reproductive needs of the organism and (2) respond to fluctuations within the internal environment, including various types of stress. The endocrine system comprises the following major glands (
Fig. 38.1
Endocrine tissues of the pancreas Parathyroid glands
Pituitary gland (in association with hypothalamic nuclei)
Gonads (testes or ovaries) These endocrine glands synthesize and secrete bioactive hormones and, with the exception of gonads, which perform both endocrine and gametogenic functions, are dedicated to hormone production (
Table 38.1 ). A transitory organ, the placenta, also performs a major endocrine function.
In addition to dedicated endocrine glands, there are endocrine cells within organs whose primary function is not endocrine (see
Table 38.1 ). These include cells within the heart that produce atrial natriuretic peptide, liver cells that produce insulin-like growth factor type 1 (IGF-1), cells within the kidney that produce erythropoietin, and numerous cell types within the gastrointestinal tract that produce gastrointestinal hormones. There also exist collections of cell bodies (called nuclei) within the hypothalamus that secrete peptides, called neurohormones, into capillaries associated with the pituitary gland.
A third subset of the endocrine system is represented by numerous cell types that express intracellular enzymes, ectoenzymes, or secreted enzymes that modify inactive precursors or less active hormones into highly active hormones (see
Table 38.1). An example is the generation of angiotensin II from the inactive polypeptide angiotensinogen by two subsequent proteolytic cleavages (see ). Another example is activation of vitamin D by two subsequent hydroxylation reactions in the liver and kidneys to produce the highly bioactive hormone 1,25-dihydroxyvitamin D (vitamin D).
Configuration of Feedback Loops Within the Endocrine System