Pituitary Gland and Its Relation to the Hypothalamus

Pituitary Gland: Two Distinct Parts-The Anterior and Posterior Lobes. The pituitary gland (Figure 75-1), also called the hypophysis, is a small gland—about 1 centimeter in diameter and 0.5 to 1 gram in weight— that lies in the sella turcica, a bony cavity at the base of the brain, and is connected to the hypothalamus by the pituitary (or hypophysial) stalk. Physiologically, the pituitary gland is divisible into two distinct portions: the anterior pituitary, also known as the adenohypophysis, and the posterior pituitary, also known as the neurohypophysis. Between these is a small, relatively avascular zone called the pars intermedia, which is almost absent in the human being but is much larger and much more functional in some lower animals.

Embryologically, the two portions of the pituitary originate from different sources—the anterior pituitary from Rathke's pouch, which is an embryonic invagination of the pharyngeal epithelium, and the posterior pituitary from a neural tissue outgrowth from the hypothalamus. The origin of the anterior pituitary from the pharyngeal epithelium explains the epithelioid nature of its cells, and the origin of the posterior pituitary from neural tissue explains the presence of large numbers of glial-type cells in this gland.

Six important peptide hormones plus several less important ones are secreted by the anterior pituitary, and two important peptide hormones are secreted by the posterior pituitary. The hormones of the anterior pituitary play major roles in the control of metabolic functions throughout the body, as shown in Figure 75-2.

• Growth hormone promotes growth of the entire body by affecting protein formation, cell multiplication, and cell differentiation.

• Adrenocorticotropin (corticotropin) controls the secretion of some of the adrenocortical hormones, which affect the metabolism of glucose, proteins, and fats.

• Thyroid-stimulating hormone (thyrotropin) controls the rate of secretion of thyroxine and triiodothyronine by the thyroid gland, and these hormones control the rates of most intracellular chemical reactions in the body.

• Prolactin promotes mammary gland development and milk production.

• Two separate gonadotropic hormones, follicle-stimulating hormone and luteinizing hormone, control growth of the ovaries and testes, as well as their hormonal and reproductive activities.

The two hormones secreted by the posterior pituitary play other roles.

• Antidiuretic hormone (also called vasopressin) controls the rate of water excretion into the urine, thus helping to control the concentration of water in the body fluids.

• Oxytocin helps express milk from the glands of the breast to the nipples during suckling and possibly helps in the delivery of the baby at the end of gestation.

Anterior Pituitary Gland Contains Several Different Cell Types That Synthesize and Secrete Hormones. Usually, there is one cell type for each major hormone formed in the

Control

Hypothalamus

Hypophysial stalk Posterior pituitary

Hypothalamus

Hypophyseal Stalk

Hypophysial stalk Posterior pituitary

Anterior pituitary

Pars intermedia

Figure 75-1

Anterior pituitary

Pars intermedia

Figure 75-1

Pituitary gland.

Pituitary Gland Form

Cellular structure of the anterior pituitary gland. (Redrawn from Guyton AC: Physiology of the Human Body, 6th ed. Philadelphia: Saunders College Publishing, 1984.)

Thyroid . gland

Growth

Increases blood glucose level

Anterior pituitary gland

Corticotropin

Promotes secretion of insulin

Follicle

Adrenal cortex

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Essentials of Human Physiology

Essentials of Human Physiology

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