The Pituitary Gland

Look in the mirror and imagine a line going from the bridge of your nose straight back, deep into the center of your head. If you were able to see inside yourself, you would find a tiny organ there about the size of a pinto bean. It is surrounded by bone and appears to be hanging from the base of the brain. This is your pituitary gland. Although one of the smallest endocrine glands, this organ has been known throughout history as the master gland. It retains this title for several reasons: First, it is a primary link between the nervous system and the endocrine system. Second, it produces and releases a wide variety of hormones that control the functioning of other endocrine glands. And finally, it directs some fundamental life processes. The pituitary determines how tall you will grow and how early you will embark on the journey through puberty.

The pituitary gland is comprised of two distinct segments. The front part is called the anterior lobe; the section closer to the back of the skull is the posterior lobe. Although both lobes together are only about the size of a small acorn, each functions as an independent gland with its own distinct activities. The posterior lobe makes up approximately 25% of the entire gland. It stores and secretes two hormones manufactured in a part of the brain called the hypothalamus. The anterior lobe comprises the remaining 75% of the pituitary.

Both sections of the pituitary are directly connected to, and controlled by, the hypothalamus, a section of the brain controlling the basic survival processes of hunger, thirst, sexual reproduction, and self-defense. A stem containing both neurons and small blood vessels connects the pituitary gland with the hypothalamus. Thus, the hypothalamus communicates with the pituitary in two ways: by nerve impulses and by chemical messengers. The anterior pituitary receives instructions from the hypothalamus in the form of releasing hormones. These chemical messengers produced by the hypothalamus then travel through the blood. A special network of vessels directs this blood through the anterior pituitary before it returns to the heart. In contrast, the posterior pituitary receives its messages through neural tissue and is controlled by nerve impulses.

All information that enters the brain must pass through the hypothalamus. The pituitary gland plays a key role in the ability of the hypothalamus to act on this information and initiate the proper physical response.

The major function of the anterior pituitary is tropic. This does not, however, have anything to do with climate, palm trees, or exotic fruits. In this case, the suffix -tropic is used to indicate hormones that affect other glands and organs. The anterior pituitary produces six crucial tropic hormones:

• Growth hormone (GH) or somatotropic hormone.
• Thyroid-stimulating hormone (TSH) or thyrotropic hormone.
• Adrenocorticotropic hormone (ACTH).
• Follicle-stimulating hormone (FSH), a gonadotropic hormone.
• Luteinizing hormone (LH), a gonadotropic hormone.
• Prolactin or lactotropic hormone (LTH).

These hormones determine the size of both the glands and organs they affect as well as the production and secretion of these glands' hormonal products. And, if the anterior lobe secretes an abnormal amount of one hormone or another, the results are often plainly visible.

Adrenocorticotropic hormone (ACTH) is the anterior pituitary hormone that directs the adrenal glands, whose function is essential to life. In a manner very similar to TSH, a delicate feedback mechanism controls levels of ACTH. The hypothalamus produces an ACTH-releasing factor that stimulates ACTH secretion from the pituitary into the bloodstream. Specially programmed receptors on the adrenal glands then pick up the ACTH. These receptors also send signals to the cell's nucleus to begin producing and secreting adrenal hormones. The hormones are released into the circulatory system, and their presence is communicated to the brain.

ACTH also helps to regulate the metabolism. These hormones include glucocorticoids, which help control the body's metabolism; mineralocorticoids, involved in regulating the amount of water in the body; and progesterone, a hormone crucial to reproduction because it prepares the uterus for the reception and preparation of a fertilized ovum.

Like many other hormones, ACTH is secreted in a circadian pattern, a period of time approximating 24 hours. Each person has his or her own internal circadian rhythm, which determines how much sleep that person needs, when he or she is most alert, and when he or she likes to eat. Disruptions of this rhythm can affect biological functions as well as moods and intellectual ability. Jet lag after a long flight across several time zones is probably the result of a disruption of the circadian pattern. For most people, ACTH is at its highest blood levels in the early morning. The adrenals are crucial to how well people handle stress, how rapidly they act and react, and whether or not they start the day with a boost of energy.

Gonadotropic hormones

The gonadotropic hormones are appropriately named for their target organs-the gonads, or sex organs. There are two main gonadotropic hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Both do double duty. They control the secretion of sex hormones and the production of mature sperm and eggs.

Although the growing fetus has high levels of FSH and LH, infants and small children produce very little. Then, however, comes puberty. Sexual maturation begins for girls at an average age of 12 1/2 and 1 or 2 years later for boys.

Signals for the production of FSH and LH by the pituitary are also initiated in the hypothalamus. No one is absolutely certain how the hypothalamus determines when to begin stimulating the release of gonadotropins. There is, however, a definite correlation between the onset of puberty and the maturation of bones. A look at any grade school class will confirm that girls generally grow faster than boys. This discrepancy probably relates to the tendency for girls to enter puberty before boys.

The testes, or male sex organs, are under the control of both FSH, which promotes the production of sperm, and LH, which stimulates the production of testosterone. Although this hormone is often called the male sex hormone, this is not entirely true. LH also stimulates women's adrenal glands and ovaries to produce a small quantity of testosterone. Similarly, estrogen, labeled the female sex hormone, is produced not only in women's ovaries but also, in smaller quantities, by the adrenal glands and testes. When it comes to sex hormones, it is a question of quantity, not exclusion.

From the onset of puberty, males continuously secrete testosterone to ensure the production of mature sperm. These hormone levels are directed by a negative feedback mechanism linking the brain, the pituitary, and the body.

In females, sex hormone secretions are also directed by the release of gonadotropic hormones, but levels are cyclical rather than constant. Each cycle produces and releases only one mature egg. The length of this cycle, known as the menstrual cycle, averages 28 days. FSH levels peak at the time of ovulation. Both FSH and LH have to be present in the blood for the ovaries to produce estrogen.

Prolactin

The sixth anterior pituitary hormone discovered so far is prolactin, or lactotropic hormone. Prolactin stimulates the production of milk in the mammary glands. Of course, this normally occurs only in females who have just given birth. At this time, a complex array of hormones signals the hypothalamus to begin releasing prolactin. Many researchers believe that a prolactin-inhibiting factor (PIF) which helps to prevent untimely lactation also exists. A decline in PIF following the birth of a child is thought to temper its inhibition.

Although nursing infants are interested in only one thing-getting fed, as they suckle and empty their mothers' milk-laden breasts-they are also actually stimulating the continued production of prolactin, both maintaining an immediate, steady supply of mother's milk and securing their next meal.

Although an infrequent occurrence, an oversupply of prolactin may result in an abnormally high production of breast milk and the prevention of menstruation.

The posterior pituitary

The posterior pituitary secretes two hormones: oxytocin (OT) and antidiuretic hormone (ADH). Neither of these, however, is manufactured there. Both are actually products of the hypothalamus.

Oxytocin
The hypothalamus produces oxytocin and ADH and directs them to the posterior pituitary, where they are separately stored in membrane bound granules. Release of these hormones is totally dependent upon nerve impulses from the hypothalamus to the posterior pituitary.
In women, oxytocin works in conjunction with prolactin in providing milk to nursing infants. Oxytocin stimulates contraction of the milk ducts within the breasts, which helps eject the milk. This is known as the letdown reflex. It also has an important function in stimulating contractions of the uterus during labor. Therapeutically, oxytocin is used to induce labor and also to stop uterine bleeding. It is not clear what role, if any, oxytocin plays in men.

Antidiuretic hormone
The main job of antidiuretic hormone (ADH), also known as vasopressin, is water conservation. By helping the kidneys to conserve water, ADH maintains proper fluid levels in the body. As water passes through the tubules in the kidneys, it can either be reabsorbed by the body or excreted as urine. Water is reabsorbed at the proper level when there is a sufficient amount of ADH present.
If ADH secretions are too low, a disorder called diabetes insipidus can occur. The most common symptom associated with diabetes insipidus is the excretion of enormous quantities of urine-frequently as much as 10 quarts a day. This is not only a great inconvenience but can also readily lead to dehydration. Fortunately, diabetes insipidus is generally treatable with antidiuretic hormone replacement.
Alcohol is one of many factors that can depress the secretion of ADH. Often people who have had too much to drink find themselves having to urinate frequently and wake up "dying of thirst". They have temporarily dehydrated themselves by upsetting their hormonal balance. Pain, sleep, exercise, nicotine, barbiturates, and morphine are just a few of the things that have the opposite effect of increasing the production and release of ADH.
The pituitary gland plays a principal role in determining how tall a child will grow, when a girl will start to menstruate, and whether or not a man and a woman can conceive a child; and an imbalance in its secretion may influence the body's ability to metabolize food, nurse an infant, or even to get out of bed in the morning. It has certainly earned its title of the master gland; its unimpressive size belies its tremendous significance to human life.