Pituitary Hormones and Their Control by the Hypothalamus

Anterior and Posterior Lobes of The Pituitary Gland

The pituitary gland ( Figure 76-1 ), 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 skull—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 portions is a small, relatively avascular zone called the pars intermedia, which is much less developed in humans but is larger and much more functional in some 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 major peptide hormones plus several other hormones of lesser known importance 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 76-2 .

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

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

• Thyroid-stimulating hormone (thyrotropin) controls the secretion rate 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 water concentration in the body fluids.

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

The 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 anterior pituitary gland. With special stains attached to high-affinity antibodies that bind with the distinctive hormones, at least five cell types can be differentiated ( Figure 76-3 ). Table 76-1 provides a summary of these cell types, the hormones they produce, and their physiological actions. These five cell types are as follows:

• 1.

  Somatotropes —human growth hormone (hGH)

• 2.

  Corticotropes —adrenocorticotropic hormone (ACTH)

• 3.

  Thyrotropes —thyroid-stimulating hormone (TSH)

• 4.

  Gonadotropes —gonadotropic hormones, which include both luteinizing hormone (LH) and follicle-stimulating hormone (FSH)

• 5.

  Lactotropes —prolactin (PRL)

Table 76-1

Cells and Hormones of the Anterior Pituitary Gland and Their Physiological Functions

Cell	Hormone	Chemistry	Physiological Action
Somatotropes	Growth hormone (GH) (somatotropin)	Single chain of 191 amino acids	Stimulates body growth; stimulates secretion of insulin-like growth factor-1; stimulates lipolysis; inhibits actions of insulin on carbohydrate and lipid metabolism
Corticotropes	Adrenocorticotropic hormone (ACTH) (corticotropin)	Single chain of 39 amino acids	Stimulates production of glucocorticoids and androgens by the adrenal cortex; maintains size of zona fasciculata and zona reticularis of cortex
Thyrotropes	Thyroid-stimulating hormone (TSH) (thyrotropin)	Glycoprotein of two subunits, α (89 amino acids) and β (112 amino acids)	Stimulates production of thyroid hormones by thyroid follicular cells; maintains size of follicular cells
Gonadotropes	Follicle-stimulating hormone (FSH)	Glycoprotein of two subunits, α (89 amino acids) and β (112 amino acids)	Stimulates development of ovarian follicles; regulates spermatogenesis in the testis
	Luteinizing (LH) hormone	Glycoprotein of two subunits, α (89 amino acids) and β (115 amino acids)	Causes ovulation and formation of the corpus luteum in the ovary; stimulates production of estrogen and progesterone by the ovary; stimulates testosterone production by the testis
Lactotropes-mammotropes	Prolactin (PRL)	Single chain of 198 amino acids	Stimulates milk secretion and production

About 30% to 40% of the anterior pituitary cells are somatotropes that secrete growth hormone (GH), and about 20% are corticotropes that secrete ACTH. Each of the other cell types accounts for only 3% to 5% of the total; nevertheless, they secrete powerful hormones for controlling thyroid function, sexual functions, and milk secretion by the breasts.

Somatotropes stain strongly with acid dyes and are therefore called acidophils. Thus, pituitary tumors that secrete large quantities of hGH are called acidophilic tumors.

Posterior Pituitary Hormones Are Synthesized by Cell Bodies in the Hypothalamus

The bodies of the cells that secrete the posterior pituitary hormones are not located in the pituitary gland but are large neurons, called magnocellular neurons, located in the supraoptic and paraventricular nu clei of the hypothalamus. The hormones are then transported in the axoplasm of the neuron’s nerve fibers passing from the hypothalamus to the posterior pituitary gland. This mechanism is discussed later in the chapter.

Hypothalamic-Hypophysial Portal Blood Vessels of the Anterior Pituitary Gland

The anterior pituitary is a highly vascular gland with extensive capillary sinuses among the glandular cells. Almost all the blood that enters these sinuses passes first through another capillary bed in the lower hypothalamus. The blood then flows through small hypothalamic-hypophysial portal blood vessels into the anterior pituitary sinuses. Figure 76-4 shows the lowermost portion of the hypothalamus, called the median eminence, which connects inferiorly with the pituitary stalk. The median eminence is the functional link between the hypothalamus and the anterior pituitary gland. Small arteries penetrate into the median eminence and then additional small vessels return to its surface, coalescing to form the hypothalamic-hypophysial portal blood vessels. These vessels pass downward along the pituitary stalk to supply blood to the anterior pituitary sinuses.

Growth Hormone Promotes Growth of Many Body Tissues

GH, also called somatotropic hormone or somatotropin, is a small protein molecule that contains 191 amino acids in a single chain and has a molecular weight of 22,005. It causes growth of almost all tissues of the body that are capable of growing. It promotes increased sizes of the cells and increased mitosis, with development of greater numbers of cells and specific differentiation of certain types of cells such as bone growth cells and early muscle cells.

Figure 76-5 shows typical weight charts of two growing littermate rats; one received daily injections of GH, and the other did not receive GH. This figure shows marked enhancement of growth in the rat given GH, in the early days of life and even after the two rats reached adulthood. In the early stages of development, all organs of the treated rat increased proportionately in size; after adulthood was reached, most of the bones stopped lengthening, but many of the soft tissues continued to grow. Once the epiphyses of the long bones have united with the shafts, further lengthening of bone cannot occur, even though many other tissues of the body can continue to grow throughout life.

Growth Hormone has Several Metabolic Effects

Aside from its general effect in causing growth, GH has multiple specific metabolic effects ( Figure 76-6 ), including the following: (1) increased rate of protein synthesis in most cells of the body; (2) increased mobilization of fatty acids from adipose tissue, increased free fatty acids in the blood, and increased use of fatty acids for energy; and (3) decreased rate of glucose utilization throughout the body. Thus, in effect, GH enhances body protein, decreases fat stores, and conserves carbohydrates.