Biology

Pituitary or pituitary gland: structure, functions and hormones

The pituitary gland is also called the pituitary . It is the directing gland of the endocrine system and together with the hypothalamus (which is a structure of the brain and on which you can delve into here ) they are those that direct the body’s response to changes in the external environment in order to achieve balance ( homeostasis) with the external environment at all times.

What is the pituitary?

The pituitary gland is a gland with a singular characteristic, since although we consider it as a whole, it has two parts with different embryonic origins that we will see later.

Its size is very small, barely between 1 and 1.6 cm and weighs 6 grams with a shape similar to that of testicles and is attached to the hypothalamus through the infundibulum.

Location of the pituitary

It is located below the hypothalamus at the base of the brain and on the bony structure called the sella turcica that protects it. In a more profane way, it would be behind where the nose begins, in an area close to the brain, in this case the hypothalamus.

Structure of the pituitary gland

pituitary gland
pituitary gland

As we said before, the pituitary or pituitary gland has two different parts : on the one hand the neurohypophysis or posterior pituitary and on the other hand the anterior pituitary or anterior pituitary.

Both parts have two different embryonic origins. On the one hand the neurohypophysis arises from a thickening of the floor of the brain and on the other hand the anterior pituitary arises from an ectodermal evagination of the stomodeum , called Rathke’s bag.

Although we are not going to go deeper, this allows us to understand why the neurohypophysis is the neural part of the pituitary (because it comes from the brain, the brain in short) and the adenohypophysis is the most glandular part (because it comes from a different origin than the brain). ).

The neurohypophysis

The neurohypophysis is made up of three parts or portions:

  • Pars nervosa : it is the nerve portion or posterior lobe that is the largest
  • Infundibulum
  • Middle eminence: it is the point of union between the hypothalamus and the pituitary.

pituitary gland
pituitary gland

The adenohypophysis

The adenohypophysis is made up of two parts that make up between 75 and 80% of the total pituitary gland:

  • Pars distalis : distal portion and is the largest
  • Pars tuberalis : tuberal portion that is in the upper part and surrounds the infundibulum

The upper part of the pars tuberalis and the infundibulum make up what is called the pituitary stalk, which is the junction between the hypothalamus and the pituitary.

In some species, the pars intermedia or intermediate lobe can be found, which in the case of humans after birth has no functional importance and atrophies, ceasing to be a lobe.

In the anterior pituitary there are 5 types of cells, each of them specialized in a certain type of hormones:

  • somatotrophs : these are the cells that are responsible for secreting growth hormone, as its name suggests (soma-body)
  • thyrotropes : they are responsible for secreting thyroid hormones such as TSH as its name indicates thyroid – thyroid
  • lactotropes : they are responsible for secreting prolactin as indicated by its name lacto – milk
  • Gonadotropes : they are responsible for secreting hormones related to sexual cycles such as luteinizing hormone (LH) and follicle-stimulating hormone (FSH)
  • corticotropes : they are responsible for secreting hormones from the adrenal cortex such as ACTH.

Functions of the pituitary

The function of this gland is to regulate and secrete the hormones that are necessary to regulate the balance in the body. The two parts have slightly different functions.

The neurohypophysis is responsible for distributing the hormones synthesized by the hypothalamus to the rest of the target organs, while the adenohypophysis synthesizes and secretes hormones depending on the regulation that the hypothalamus performs through hypothalamic factors.

The neurohypophysis distributes hormones and the anterior pituitary synthesizes and distributes hormones.

Hormones secreted by the pituitary

The pituitary secretes hormones properly synthesized in the pituitary itself, specifically in the cells of the adenohypophysis, and is responsible for distributing two hormones synthesized by the hypothalamus through the neurohypophysis: oxytocin and vasopressin that you can find in this post.

Hormones of the anterior pituitary

The hormones secreted by the anterior pituitary are:

  • Growth hormone (GH) or somatotropin
  • Thyrotropin or thyroid stimulating hormone (TSH)
  • Follicle Stimulating Hormone (FSH)
  • Luteinizing hormone (LH)
  • Prolactin (PRL)
  • Adrenocorticotropic hormone (ACTH)
  • Melanocyte-Stimulating Hormone (MSH)

Growth hormone (GH) or somatotropin

It is secreted by somatotropic cells and is most abundantly secreted by the adenohypophysis.

Stimulates cell growth and replication by accelerating protein synthesis.

It acts on all cells but especially on skeletal and muscular development. For this reason, when there is some trauma, this hormone increases its secretion.

This hormone acts on the liver, where liver cells respond by synthesizing and secreting somatomedins (also called IGF or insulin-like growth factors) . Somatomedins are peptide hormones that act by stimulating protein synthesis and cell replication in striated muscle fibers, cartilage, and other target cells. Cartilage and muscle tissue can also produce IGFs due to somatotropin or GH.

In children this hormone is responsible for increasing the speed of muscle and skeleton growth in children, and in adults to prevent the loss of muscle tissue.

This hormone also increases lipolysis in adipose tissue avoiding the use of amino acids as an energy source.

It is released in periodic pulses every few hours especially during sleep. It is controlled by GHRH and GHIH, which are both hypothalamic factors.

The hormone secretion of growth is enhanced by: lowering fatty acids and increased blood amino acid, physical exercise, stress, non – REM sleep and other hormones such as glucagon, estrogen, cortisol and insulin.

It is inhibited by a low concentration of amino acids in the blood, REM sleep, affective deficiency, obesity, low levels of thyroid hormones and growth hormone (GH) due to negative feedback.

Thyrotropin or thyroid stimulating hormone (TSH)

It is secreted by thyrotropic cells and acts on the thyroid gland by inducing the release of thyroid hormones (triiodothyroninia or T3 and thyroxine or T4) and stimulating the growth of the thyroid gland . It is regulated through TRH, which is a hypothalamic factor controlled by the concentration of T3 and T4.

Follicle Stimulating Hormone (FSH)

It is one of the gonadotropin hormones (that is, they act on the gonads: ovaries and testes). secreted by gonadotropic cells and its function is to stimulate the development of ovules or oocytes in the ovaries of mature women and in men it maintains sperm production in the testes .

In addition, FSH also stimulates the secretion of estrogens that occurs in the follicles of the ovaries where the eggs mature. Estrogens are female sex hormones, the most important being estradiol.

The release of this hormone is induced by the hypothalamic factor GnRH (gonadotropic releasing hormone) and its inhibition is produced by the concentration of these hormones by negative feedback.

Luteinizing hormone (LH)

It is the second gonadotropin hormone secreted by gonadotropic cells, its function is to induce ovulation in women and stimulate the secretion of progestins (mainly progesterone) in the ovaries, which are responsible for repairing the body for possible pregnancy : they prepare the breasts for the secretion of milk and the uterus for the implantation of a fertilized egg.

The function of this hormone in men is to stimulate the synthesis of male sex hormones (androgens, the most important being testosterone) in the testes, specifically in the Leydig cells.

Prolactin

Prolactin is secreted by lactotropic cells. This hormone is responsible for stimulating the development of the mammary glands and the production of milk. Although it has a dominant effect, the mammary glands are regulated by other hormones as well: estrogens, progesterone, growth hormone, glucocorticosteroids, and hormones produced by the placenta.

During the fetal period, it is also responsible for stimulating growth. In other animal and bird species it regulates sexual and reproductive behavior.

In men the role of prolactin is not very clear, although its high secretion is known to cause erectile dysfunction.

Remember that prolactin acts on milk production, not on the expulsion of milk as oxytocin does (another hormone released by the neurohypophysis).

The prolactin is regulated by hormonal domapina or inhibiting prolactin (PIH) . Shortly before the start of menstruation (remember that the onset of menstruation is the first day of bleeding), the decrease in estrogen causes a drop in PIH secretion, so prolactin is secreted at the beginning of menstruation, when the The estrogen level rises, the PIH returns to its normal levels, and the prolactin level returns to low. As the duration of prolactin is short, there is no time for milk to be secreted during menstruation.

Adrenocorticotropic hormone (ACTH)

It is secreted by corticotropic cells and stimulates the release of steroid hormones by the adrenal gland. These hormones are called glucocorticosteroids and they regulate the metabolism of cortisol .

Hypothalamic factor, corticotropin-releasing hormone, or CRH, stimulates ACTH secretion. It is stimulated by stress, low blood glucose, trauma, and interleukin (a substance produced by macrophages). Its inhibition is carried out by negative feedback according to the concentration of glucocorticoids.

Melanocyte-Stimulating Hormone (MSH)

It is secreted by corticotropic cells and is the only hormone released by the middle part of the anterior pituitary. Its function is to stimulate the melanocytes of the skin, increasing the rate of production and distribution of melanin .

It is only secreted during fetal development, in young children, and in pregnant women. Otherwise, if it appears, it can be associated with pathological processes.

In lower vertebrates it acts by regulating the distribution of melanin in the skin, as for example in amphibians.

You can see how hypothalamic factors act and are regulated in this YouTube video:

Other hormones in animals

There are other hormones secreted by the anterior pituitary gland that appear in other species such as lipotropin (LPH) and beta-endorphin. Lipotropin is responsible for mobilizing lipids in other vertebrates, but in humans its role is not very important.

Hormones of the neurohypophysis

The neurohypophysis is responsible for distributing the two hormones synthesized by the hypothalamus to the rest of the body to its target organs: oxytocin and vasopressin or antidiuretic hormone (ADH).

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