The pituitary gland that hangs from the hypothalamus


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The pituitary gland that hangs from the hypothalamus

The pituitary gland, also called the pituitary gland, is a small organ that connects to the hypothalamus and hangs down from the brain. It’s in the brain, but it’s not actually the brain. What are the anterior and posterior lobes of the pituitary gland? Also, posterior pituitary hormones produced by neurons in the hypothalamus and anterior pituitary hormones supervised by the hypothalamus are explained in an easy-to-understand manner.

What is the pituitary gland? small organ that connects to the hypothalamus

Inside our head is an organ called the pituitary gland. It is located in the skull just between the eyes and at the back of the nose. It is 7 to 8 mm in size and weighs about 0.7 g. It is also called the “pituitary gland” because it hangs down like a protruding part of the brain, but it is not actually the brain (nerve tissue).

There is much debate as to which of the two names, pituitary gland and pituitary gland, is more appropriate. I recommend calling it the “pituitary gland,” as it may be misleading to call it “a part of the brain.” By the way, “pituita” in the English name means “mucus” in Greek, and when it was discovered, it seems that it was named so because it was thought to be an organ that only secreted mucus, but now it is used to produce hormones.・It is clear that it is a site related to secretion.

The pituitary gland hangs down and is connected to the hypothalamus in the diencephalon by a thin stalk-like structure called the pituitary stalk or pituitary stalk or funnel. Some of you may be worried that it won’t fall like a persimmon, but don’t worry. As shown in the figure below, the base of our skull has a bony depression called the turcica, in which the pituitary gland is located and protected.

As its name suggests, the pituitary gland hangs down from the lower part of the cerebrum and is located in a depression in the skull called the sella turcica (original drawing by the guide).

Read Also: Reuse of thirst and urine: Mechanism of the hypothalamus that protects the amount of water in the body

“Anterior lobe” and “posterior lobe” of the pituitary gland…actually completely different origins

The name of the part of the brain is often compared to a plant, and the central part of the brain is called the brain “stem”, and part of the cerebral neocortex is called the frontal “leaf”. As for the pituitary gland, if the part that connects to the hypothalamus is the “stem”, then the part beyond that is the “leaf”. The pituitary gland is divided into two parts, the anterior lobe and the posterior lobe.

If you get caught up in the general morphology, you tend to think that the pituitary gland has an anterior lobe and a posterior lobe. In my university classes, I teach that the anterior pituitary gland is the organ that produces and secretes hormones, and the posterior pituitary gland is part of the hypothalamus. In other words, the posterior pituitary is part (or extension) of the brain, but the anterior pituitary is not the brain.

Even in the process of brain development in a baby’s head, the anterior and posterior lobes have completely different origins. The anterior lobe is a group of secretory gland cells that migrated during the fetal period from the roof of the oral cavity, which will become the mouth and esophagus in the future. The anterior lobe of the adult pituitary gland in a mature adult is also called the “adenopituitary gland” because many hormone-producing and secreting cells are distributed. On the other hand, the posterior lobe is a nerve tissue formed by protruding part of the brain (bottom of the third ventricle). The posterior lobe of the pituitary gland in a mature adult lacks glandular cells and is also called the “neurohypophysis” because it contains nerve endings extending from the hypothalamus.

It was created by attaching a group of gland cells (anterior pituitary = adenohypophysis) that migrated from outside the brain to the part that protruded from the nervous tissue of the brain (posterior pituitary gland = neurogenic pituitary gland). , is the pituitary gland.

a posterior pituitary hormone produced by neurons in the hypothalamus

In 1895, when G. Oliver and E. A. Schaefer of England conducted research to find substances contained in the pituitary gland of animals, they discovered that an extract of the pituitary gland had the effect of raising blood pressure. I was. In 1928, researchers at Park Davis, Inc. in the United States succeeded in extracting two active ingredients from the posterior pituitary gland extract, and confirmed that it exerts uterine contraction and blood pressure-raising effects. The uterine contraction component (oxytocic fraction) was called pitocin, and the blood pressure increase component (pressor fraction) was called pitressin. Later, pitocin was given another name, oxytocin, and pitresin, vasopressin, and they became widely known as representative posterior pituitary hormones under these names.

Oxytocin, as originally discovered, is a hormone that primarily causes contractions of the uterus and is essential for causing labor pains when a pregnant mother gives birth to a child. In addition, it is also known to have the effect of producing breast milk and promoting memory, and it is also believed to be a hormone related to trust and affection based on recent research. The relationship between developmental disorders and oxytocin is also attracting attention, so I would like to explain it in detail in a later article.

Vasopressin was named so because it was first discovered as a hormone that constricts blood vessels (vaso) and raises blood pressure (when blood pressure rises, the walls of blood vessels are strongly pushed = press), but it has been studied in detail. We have found that vasopressin has a more dominant action. In 1919, a researcher gave patients with diabetes insipidus an extract from the posterior pituitary gland, which reduced urine output. It was presumed that the posterior lobe of the pituitary contains a substance that reduces the amount of urine, that is, has an antidiuretic effect.

Oxytocin and vasopressin are collectively called posterior pituitary hormones, but they are not produced by the posterior pituitary gland. Again, there are no hormone-producing endocrine cells in the posterior lobe. As shown in the figure below, posterior pituitary hormones are produced by nerve cells in the paraventricular nucleus of the hypothalamus, and their axons extend to the posterior pituitary gland. Oxytocin and vasopressin produced by neurons in the paraventricular nucleus are transported through nerve axons, secreted from nerve endings in the posterior pituitary gland, enter blood vessels, and are transported throughout the body.

Hormones (vasopressin or oxytocin) released from the axon terminals of nerve cells extending from the hypothalamus to the posterior pituitary gland enter the bloodstream from the pituitary arteries and are carried throughout the body (original drawing by the guide) .Some of you may have wondered, “Are hormones secreted by nerve cells?” A bit of biology makes it easy to remember that nerve cells secrete neurotransmitters and endocrine glands secrete hormones, but that’s not true. Whether a neurotransmitter or a hormone is not defined by a substance, nor is it determined by which cell secretes it. When the secreted substance is used for one-to-one transmission at the synapse, it is said to have acted as a neurotransmitter. I worked as a In other words, it is determined whether it is a neurotransmitter or a hormone depending on how it behaves after being secreted. Oxytocin is a substance that nerve cells make and secrete, but it enters the blood and acts on the whole body, so it is a hormone.

hypothalamus that oversees many anterior pituitary hormones

The anterior pituitary gland, which is an aggregate of glandular cells, produces a wide variety of hormones, including growth hormone, prolactin, thyroid-stimulating hormone, adrenocorticotropic hormone, and gonadotropic hormone. Its main function is as follows.

・Growth hormone: Controls metabolism such as glycogenolysis and lipolysis in the liver. Promotes bone elongation and muscle growth.
・Prolactin (lactation hormone): Stimulates the mammary glands to promote the production and secretion of milk.
・Thyroid-stimulating hormone: Stimulates the thyroid gland and promotes the production of thyroid hormones.
・Adrenocorticotropic hormone: Stimulates the adrenal cortex and promotes the production of adrenocortical hormones such as cortisol.
Gonadotropic hormones (follicle-stimulating hormone and luteinizing hormone): Stimulate the testes, ovaries and reproductive organs to promote the production of sperm, eggs and sex hormones.

It is determined which gland cells produce which hormones. However, you cannot decide for yourself when and how much hormones are made and secreted. It is the brain that decides. Specifically, nerve cells such as the arcuate nucleus of the hypothalamus located above the pituitary gland instruct specific endocrine gland cells in the anterior pituitary lobe so that they can respond to changes in the environment while cooperating with the sensory system. is sent and controlled.

As shown in the figure below, hormones secreted from nerve cells in the hypothalamus enter the bloodstream of the epiphysial artery, are transported to the anterior pituitary gland via the pituitary portal vein, and act on the gland cells of the anterior pituitary gland.

Hormones secreted from nerve cells in the hypothalamus (hypothalamic hormones) enter the bloodstream of the epiphysial artery and are transported to the anterior pituitary gland via the pituitary portal vein. Various hormones are secreted from gland cells in the anterior pituitary gland that have been acted on by hypothalamic hormones, and are transported throughout the body by the blood flow out of the anterior pituitary gland (original diagram created by the guide).
Moreover, signals from neurons in the hypothalamus are equipped with accelerators and brakes. For example, growth hormone releasing hormone (GHRH or GRH) is used as a command from the hypothalamus to “make and secrete a lot of growth hormone.” Growth hormone inhibitory hormone (also known as somatostatin) is used as a command to stop the production and secretion of growth hormone from the hypothalamus. Ultimately, the timing and amount of production and secretion of growth hormone by gland cells in the anterior pituitary gland are influenced by the balance of these factors.

Unbeknownst to us, the hypothalamus continues to supervise the working force of hormones that circulate throughout the body, constantly, meticulously, issuing many commands to carry out its most important task of “sustaining life.”