The endocrine system produces hormones that help maintain homeostasis and regulate reproduction and development. A hormone is a chemical messenger produced in one part of the body that affects target cells in another part of the body. Hormones have the following general characteristics:

1. Hormones are transported throughout the body in the blood.

2. Minute amounts of hormones can have significant influence on target cells.

3. Hormones may be steroids, peptides, or modified amino acids. Through the various sensory neurons, the brain—and especially a portion of the forebrain, the hypothalamus—monitors the external environment and internal conditions of the body.

As the master integrator of information, the brain may determine that some kind of action is necessary to maintain homeostasis or that conditions are appropriate to activate developmental changes. These actions are initiated by special neurosecretory cells that link the hypothalamus and the pituitary gland, a gland attached to the base of the hypothalamus. Neurosecretory cells are structured like neurons, but rather than secreting neurotransmitters into synapses that affect neighboring neurons, they secrete hormones into the blood. There are two halves, or lobes, of the pituitary. Their special associations with the hypothalamus are described below:

1. Posterior pituitary. Two hormones, ADH (antidiuretic hormone) and oxytocin, are produced by neurosecretory cells in the hypothalamus and are stored in the posterior pituitary and released as needed.

2. Anterior pituitary. Releasing hormones are produced by neurosecretory cells in the hypothalamus and secreted into the blood. This blood flows directly to the anterior pituitary where the releasing hormones stimulate the release of tropic hormones produced in the anterior pituitary. Tropic hormones are hormones whose target cells are other endocrine glands. Thus, they regulate hormone production by other glands. Because the pituitary gland controls the production of hormones by many other glands, it is often referred to as the “master gland.” Clearly, however, it is itself controlled by the hypothalamus.

In addition, hormones from the posterior pituitary do not influence other glands, but target specific body tissues.

The regulation of blood glucose concentration in the blood illustrates how the endocrine system maintains homeostasis by the action of antagonistic hormones. Among the cells of the pancreas that produce digestive enzymes, there are bundles of cells called the islets of Langerhans, which contain two kinds of cells, alpha (á) cells and beta (â) cells. These cells secrete hormones, as follows:

1. Beta cells secrete insulin. When the concentration of blood glucose rises (after eating, for example), beta cells secrete insulin into the blood. Insulin stimulates the liver and most other body cells to absorb glucose. Liver and muscle cells convert the glucose to glycogen (for storage), and adipose cells (which form a connective tissue) convert the glucose to fat. In this way, glucose concentration decreases in the blood.

2. Alpha cells secrete glucagon. When the concentration of blood glucose drops (during exercise, for example), alpha cells secrete glucagon into the blood. Glucagon stimulates the liver to release glucose. The glucose in the liver originates from the breakdown of glycogen and the conversion of amino acids and fatty acids into glucose. Another example of antagonistic hormones occurs in the maintenance of Ca+ in the blood. Parathyroid hormone (PTH) from the parathyroid glands increases Ca2+ in the blood by stimulating Ca2+ reabsorption in the kidney and Ca2+ release from the bones. Calcitonin from the thyroid gland has the opposite effect on the bones and kidneys. There are two methods by which hormones are known to trigger activities in target cells, as follows:

1. The hormone (usually a steroid) diffuses through the plasma membrane, through the cytoplasm, and into the nucleus. The hormone binds to a receptor protein in the nucleus. The receptor protein, in turn, activates a portion of the DNA that turns on specific genes.

2. The hormone (usually a peptide) binds to a receptor protein on the plasma membrane of the cell (receptor-mediated endocytosis). The receptor protein, in turn, stimulates the production of one of the following second messengers.

• Cyclic AMP (cAMP) is produced from ATP. Cyclic AMP, in turn, triggers an enzyme that generates specific cellular changes.

• Inositol triphosphate (IP3) is produced from membrane phospholipids. IP3, in turn, triggers the release of Ca2+ from the endoplasmic reticulum, which, in turn, activates enzymes that generate cellular changes. Animal Structure and Function