Lesson Explainer: The Pituitary Gland Biology

In this explainer, we will learn how to describe the structure and the function of the pituitary gland in the human body.

The pituitary gland (or hypophysis) is often referred as the “master gland” of the human body. This pea-sized gland only weighs about half a gram in adults but affects almost all aspects of our lives: vital functions like our growth and metabolism, our reproductive systems, and even our social behaviors or empathy! Let’s understand what makes this little gland so special.

The pituitary gland belongs to the endocrine system, a series of glands in the body that secrete chemical messengers called hormones. Hormones are produced in response to changes that happen to the body, for example, an increase of glucose in the blood after you drink a sugary drink, a change in the environment like a drop of temperature, or even if you unexpectedly meet a grizzly bear and need to flee.

Hormones mostly travel throughout the entire body via the bloodstream to reach their target cells. Once hormones reach their target, they change the functions or the activity of the cells, so these adapt to the needs of the body or to the changes in the environment. Thus, hormones are important contributors of homeostasis, which is the fine balance our body maintains so that all the biological processes function in harmony.

The pituitary gland secretes a large variety of hormones that are released directly into a rich network of blood vessels within the gland, as you can see in Figure 1. The targets of the pituitary hormones are very diverse. They have an effect on many tissues and organs in the body (bones, muscles, skin, and kidneys) and also on other endocrine glands (thyroid gland, adrenal gland, and sexual glands) that in turn release hormones targeting other organs. As a result, pretty much every cell in the body is directly or indirectly regulated by the pituitary gland! This partly explains why the pituitary gland is often considered the “master gland.”

The pituitary gland is a unique gland divided into two main parts in humans, but most species have a third intermediate lobe. As you can see in Figure 1, the pituitary gland is located under the brain, attached to a region called the hypothalamus by a stalk named infundibulum. This structure connects the gland by blood vessels and nerve fibers, especially the posterior part.

One particularity of the posterior lobe (or neurohypophysis) is that it is very closely connected to the nervous system. In fact, the posterior lobe of the pituitary gland acts as a storage place for hormones produced by specialized neurons of the hypothalamus, as you can see in Figure 1.

The anterior lobe (or adenohypophysis) is the biggest lobe. It is separated from the posterior lobe by an intermediate middle region, which is only a thin layer in humans. This region, called pars intermedia, varies among living organisms: it is absent in birds and is poorly defined in some mammals, but it constitutes a third lobe in some species like fish and amphibians, where it controls the changes of skin colors for camouflage. In humans, this part secretes the melanocyte-stimulating hormone (MSH), which has functions related to hunger, metabolism, and skin coloration.

Now, let’s have a look in more detail at the functions of these two distinct parts of the pituitary gland.

The posterior pituitary secretes two types of hormones into the blood vessels: antidiuretic hormone (ADH, also known as vasopressin) and oxytocin. As we have seen before in Figure 1, these hormones are synthesized by different groups of neurons in the hypothalamus and are stored in the posterior pituitary. When these neurons of the hypothalamus receive nerve impulses from other brain regions, their hormones are secreted in the blood vessels.

ADH has two main effects that both result in controlling blood pressure. ADH is a major regulator of kidney functions: it increases water reabsorption by the kidney, which decreases the amount of urine produced (hence the term antidiuretic in the name of this hormone, which means to “reduce urination”). Besides, the other effect of ADH is stimulating the contraction of the muscles in the blood vessel wall, which causes vasoconstriction (hence the other name of this hormone, vasopressin).

Oxytocin is released during childbirth to induce uterus contraction and during breastfeeding to stimulate the release of milk from the mammary glands. Both ADH and oxytocin also act on the brain and influence our social relations like the mother–infant attachment or the bonding between romantic partners. When a dog and its owner gaze at each other, or when two people snuggle up, oxytocin is also released. Oxytocin is sometimes referred to as the “love hormone” for this reason.

Example 1: Recalling Which Structure in the Human Body Produces ADH and Oxytocin

ADH and oxytocin are released by the posterior pituitary gland. In what structure are they produced?

1. The adrenal gland
2. The frontal lobe
3. The heart
4. The hypothalamus
5. The pancreas

Answer

The posterior pituitary gland is directly connected to the hypothalamus by a stalklike structure called the infundibulum, which contains nerve axons from neurons located in the hypothalamus. These nerves terminate in the posterior lobe of the pituitary gland that serves as a storage place for the hormones they produce. The figure below illustrates in orange and green how these neurons are organized with their cell bodies in the hypothalamus and their terminals in the posterior lobe of the pituitary gland.

The hypothalamus is a control center in the brain that coordinates the nervous system and the endocrine system. It receives information from nerves about the entire body and the environment. Based on this, it produces an endocrine response to adapt the body to the new needs or changes.

For example, when you are exercising on a hot day without drinking enough water, your hypothalamus detects that more water must be retained in your body. In response, hypothalamic neurons secrete antidiuretic hormone (ADH, also known as vasopressin) from the posterior lobe of the pituitary gland into the bloodstream. ADH reaches the cells of the kidney to make them reabsorb water and reduce the volume of urine.

Oxytocin is another hormone released by the hypothalamus. For example, when a baby suckles at the nipple of their mother’s breast, sensory impulses pass the information from the nipple to the brain. The information arrives to hypothalamic neurons, which respond by releasing oxytocin in the bloodstream. Oxytocin then stimulates the mammary glands to release milk.

Therefore, the answer should be D, the hypothalamus.

In contrast with the posterior pituitary that acts as a storage place for hormones produced in the hypothalamus, the anterior pituitary is the production place of a number of anterior pituitary hormones. The hypothalamus regulates the anterior pituitary by secreting some hormones named “releasing hormones,” which target the cells of the anterior pituitary gland. Hormones that affect the activity of other endocrine glands are called tropic hormones. As we will see in the rest of this explainer, the anterior pituitary secretes many tropic hormones.

What are the hormones secreted by the anterior pituitary? Let’s review some of them and their functions with the help of Figure 2.

Growth hormone (GH) stimulates the growth of our bones and muscles and has wide effects and functions in the body. During childhood, an abnormal overproduction of GH can result in gigantism (the person becomes unusually tall), whereas an insufficient GH production can lead to a restricted growth, a condition also known as dwarfism. Nowadays, growth problems linked to deficits of growth hormones can be treated by injections of growth hormones.

In adulthood, the main role of GH is to regulate metabolism. In other words, GH regulates how our food is converted into energy, bones, fat, and muscles. In particular, GH stimulates the synthesis of protein in muscles, which is an anabolic function of GH. Because GH helps in growing muscles, synthetic GH is listed as a doping substance, a supplement that enhances athletic performance and is prohibited in sports competitions. In adults who are suffering from a rare disorder called acromegaly, excess of GH causes abnormal bone growth that enlarges the hands and feet and thickens facial features (the skin and soft tissue of the tongue, nose, and lips). It deepens the voice by enlarging the vocal cords and sinuses. It also causes heart and vision problems.

Thyroid-stimulating hormone (TSH, also called thyrotropin) stimulates the thyroid, an endocrine gland located at the front of the neck. TSH has a tropic effect on the thyroid, which is activated to produce a variety of hormones that regulate various functions in our bodies: energy balance, blood pressure, muscle tone, protein synthesis, and also growth and development during childhood. The thyroid is thus an important gland for our growth and general well-being. Imbalances in our thyroid functions can have a myriad of consequences on our health.

Adrenocorticotropic hormone (ACTH) is usually released by the anterior pituitary in response to various sources of physical or psychological stress. ACTH is also a tropic hormone because its target is another endocrine gland: the adrenal glands located on top of each kidney. ACTH specifically targets the adrenal cortex, which is the outer layer of the adrenal gland (cortex means “bark” in Latin). When this cortex is stimulated by ACTH, it produces a hormone called cortisol, which triggers a variety of responses that modify our metabolism and immune responses.

In males and females, gonadotropins (Gn) include two types of hormones (FSH and LH) secreted by the anterior pituitary to regulate the activity of the sexual glands, or gonads. In females, gonadotropins target the ovaries, while in males, gonadotropins target the testes. Gonads do not only produce gametes but also release sexual hormones. As indicated by their name, gonadotropins have a tropic effect on the gonads by stimulating them to secrete hormones.

In males and females, FSH and LH are essential for the development of sexual organs during puberty and for the function of the reproductive systems. In males, one of the various roles of the luteinizing hormone (LH) is to enhance the production of testosterone by stimulating the development of the interstitial tissue of the testes. One main function of the follicle-stimulating hormone (FSH) is to stimulate the maturation of sperm cells in male testes. In females, FSH plays an important role in the maturation of egg cells in the ovaries. During each menstrual cycle, FSH has a role in the maturation of one ovarian follicle that evolves in the Graafian follicle. LH stimulates the production of estrogen and progesterone, by activating the formation of the corpus luteum after ovulation. As a consequence, disturbances in the releases of LH and FSH can contribute to infertility in both males and females.

All the hormones we have reviewed so far have tropic effects. The following examples have nontropic effects, which means they stimulate tissues that are not endocrine glands. Prolactin targets cells of the mammary gland in breasts to stimulate the production of milk in females during lactation. -endorphins are typically released by the anterior pituitary in response to pain and act on the nervous system to attenuate the pain signals. They are nicknamed the “feel-good” hormones because they cause the euphoric effect that is felt after exercising, listening to music, or eating chocolate.

Table 1 sums up the hormones of the pituitary gland that we have reviewed in this explainer.

To finish, let’s recap some of the key points we have covered in this explainer.