The Endocrine Glands - The pituitary-master gland
The pituitary is often called the master gland because the hormones it secretes play an active part in controlling the activities of all the other endocrine glands. This impressive power is wielded from two little bumps at the base of the brain, about midway between the ears at eye level. The two parts, or lobes, are connected by a tiny bridge of tissue, the three structures together being about the size of a small acorn. The lobe lying toward the front of the head is the anterior lobe; the one at the back, the posterior lobe . Each lobe is really an independent gland in itself, with its own quite distinct activities.
The Posterior Lobe and the Hypothalamus
The posterior lobe, so far as is known, does not make any of its own hormones, but serves as a storehouse for two hormones manufactured by the hypothalamus . The hypothalamus, apart from having a role in controlling the body's autonomic nervous system, also functions as an endocrine gland, secreting its own hormones, and as a connecting link between the brain's cerebral cortex and the pituitary gland.
The posterior lobe of the pituitary releases the two hormones it receives from the hypothalamus, called vasopressin and oxytocin , into the bloodstream. Vasopressin plays a role in the fluid balance of the body; oxytocin is thought to pace the onset and progress of labor during childbirth.
The Anterior Lobe
The anterior lobe secretes no fewer than six known hormones, five of which act as stimulators of hormone production by other endocrine glands. The sixth, identified as somatotrophin in medical textbooks, is more popularly known as the growth-stimulating hormone or simply as the growth hormone. It controls the rate of growth and multiplication of all the cells and tissues in our bodies—muscle, bone, and all our specialized organs.
In rare instances, during childhood, the pituitary releases too much or too little somatotrophin. If too much is secreted, the result is an overstimulation of growth processes, causing a disorder known as gigantism . Victims of this disorder have been known to grow nine feet tall and weigh 500 pounds.
If too little somatotrophin is secreted, dwarfism results. This pituitary-type dwarf, of which Tom Thumb was one, is different from a dwarf suffering from a disorder of the thyroid (another endocrine gland, discussed below). The pituitary dwarf is usually well proportioned despite a miniature size, while the thyroid dwarf typically has short, deformed limbs.
Neither pituitary gigantism nor dwarfism affects basic intelligence. If oversecretion of somatotrophin occurs after full size has been reached—as, for example, because of a tumor affecting the pituitary, the condition known as acromegaly occurs. The bones enlarge abnormally, especially those of the hands, feet, and face.
Anterior Pituitary Hormones
Of the five anterior pituitary hormones that regulate other endocrine glands, one affects the adrenal glands, one the thyroid, and the remaining three the sex glands or gonads (the testicles in men and the ovaries in women). Each is identified by a set of initials derived from its full name, as follows:
- • ACTH , the adrenocorricotrophic/zormone, affects the production of hormones by the outer “bark” of the adrenal glands, called the adrenal cortex .
- • TSH , the thyroid-stimulating hormone , also known as thyrotrophin , causes the thyroid gland to step up production of its hormone, thyroxin .
- • FSH , the f ollicle-stimulating hormone, spurs production in women of estrogen, a sex hormone produced by the ovaries; and in men, of sperm by the testicles. Follicle here refers to the Graafian follicles in the ovary, which contain developing female egg cells whose growth is also stimulated by FSH. Graafian follicles have approximate counterparts in the male—tiny pouches (seminal vesicles) on either side of the prostate gland that store mature sperm cells.
- • LH , the l uteinizing h ormone, transforms a Graafian follicle, after the follicle has released a ripened egg cell, into a kind of tissue called corpus luteum . The corpus luteum, in turn, produces progesterone , a hormone that prepares the mucous membrane lining the uterus (the endometrium) to receive a fertilized egg.
- • LTH , the l actogenic h ormone, or luteotrophin , stimulates the mother's mammary glands to produce milk; LTH also joins with LH in promoting the production of progesterone by the sex glands.
The last three hormones mentioned – follicle-stimulating, luteinizing, and lactogenic—are sometimes called the gonadotrophic hormones because they all stimulate activity of the gonads.