The endocrine glands, to which the pituitary gland belongs, produce biologically active substances – hormones that are released directly into the blood and are carried with its current to the organs. The pituitary gland (from the Greek hypophysis – process) regulates the activity of other endocrine glands, affects the processes of reproduction, participates in the implementation of protective and adaptive reactions of the body. The pituitary gland is a part of the neurohumoral regulation system, stimulating or inhibiting the production of tropic hormones corresponding to the hormones of the gonads, adrenal glands and thyroid glands.
The structure and location of the pituitary gland
The pituitary gland has a rounded shape and a mass of about 0.5 g. It is located inside the skull, in a small depression at its base, and is attached to the brain (hypothalamus) with a thin stalk – a funnel (Fig. 1). This place is called the Turkish saddle . It is separated from the cranial cavity by a dense membrane – the saddle diaphragm, through a narrow opening in which the funnel passes. The size of the pituitary gland is judged by the size of the sella turcica on radiographs of the skull.
In the embryo, the pituitary gland develops from different primordia. Therefore, the pituitary gland, as it were, consists of two fused parts with different functions and structure. The growth of the pituitary gland stops by the end of puberty. The production of hormones by it occurs already at the earliest stages of development – at the 9-10th week of the formation of the embryo. The formation of the activity of the pituitary gland is associated with the development of the brain. The most important stages are 6-7 years and 10 years, when the production of hormones by the pituitary gland significantly increases.
In accordance with the development of the pituitary gland, of the two primordia, they distinguish, as already indicated, the anterior lobe – the adenohypophysis , and the posterior lobe – the neurohypophysis . The adenohypophysis is larger, its mass is 70–80% of the mass of the entire gland. The smaller neurohypophysis is a part of the brain called the hypothalamus.
Hypothalamic-pituitary
The pituitary gland is included in the system of neurohumoral regulation, which works according to the feedback principle. So, the lack of hormones in the blood of the gonads, adrenal glands or the thyroid gland stimulates the production of the corresponding tropic hormones. And the excess of the hormones of these glands in the blood inhibits the production of tropic hormones.
The humoral (carried out through the blood) regulation of body functions is under the direct control of the nervous system and is carried out in conjunction with it. The hypothalamus is central to this process. Due to the close interaction of the pituitary gland with the hypothalamus, a single hypothalamic-pituitary system is created that controls the functions of the body. It is in the nuclei of the hypothalamus that substances ( neurohormones ) are produced , which then enter the pituitary gland and contribute to the production of hormones. In the anterior lobe of the pituitary gland, neurohormones enter the system of blood vessels, and in the posterior lobe, along the processes of the nerve cells themselves. The pituitary gland, together with the hypothalamus, is the central link of the endocrine system and performs the function of integrating and coordinating the activity of the endocrine glands.
Pituitary hormones
The cells of the anterior pituitary gland produce tropic hormones that selectively regulate the activity of other endocrine glands and the development of the body as a whole.
Growth hormone (growth hormone) stimulates protein synthesis in organs and tissues and the growth of the body as a whole. For its action, the body must have a sufficient amount of carbohydrates and insulin (pancreatic hormone). Under the influence of somatotropic hormone, the breakdown of fats and their use in energy metabolism are enhanced .
Gonadotropic hormones stimulate the activity of the sex glands. One of them – follicle stimulating hormone (FSH) – promotes the development of follicles in the ovaries and the formation of sperm in the ovaries. Another, luteinizing hormone (LH), is required for the release of mature eggs from the follicle (ovulation) and the secretion of female and male sex hormones. In women, FSH and LH regulate the menstrual cycle.
Prolactin promotes breast growth and milk secretion, and stimulates the secretion of female sex hormones in the ovaries. In addition, he is responsible for the manifestation of parental instinct. Experiments on animals have shown that the administration of prolactin to males increases their interest in the young and, under certain conditions, can make their mammary glands secrete milk.
Adrenocorticotropic hormone (ACTH) stimulates the growth of the adrenal cortex and the production of numerous hormones. The secretion of ACTH by the pituitary gland is increased under stress.
Thyroid stimulating hormone (TSH) is essential for the development and normal functioning of the thyroid gland: it promotes the accumulation of iodine, increases the number of secretory cells and increases their activity. The secretion of TSH by the pituitary gland increases with insufficient levels of thyroid hormones in the blood, as well as with cooling. In the latter case, this leads to an increase in the production of heat in the body.
The posterior lobe of the pituitary gland secretes hormones that regulate the tone of the smooth muscles of the vessels ( vasopressin ) and the uterus ( oxytocin ) (Fig. 2). Vasopressin causes a contraction of vascular smooth muscles (mainly small arteries) and leads to an increase in blood pressure, regulates the reabsorption of water in the kidneys, which reduces urine output and increases urine density (therefore, another name for this hormone is antidiuretic hormone ). Oxytocin stimulates uterine contraction, especially at the end of pregnancy, and also affects milk flow. The presence of this hormone in the blood is a prerequisite for the normal course of labor .
Diseases due to a violation of the production of hormones
A change in the normal size of the pituitary gland leads to a disruption in the production of hormones and noticeable shifts in the state of the body. Hormones enhance or weaken various bodily functions. Both their deficiency (hypofunction) and excess (hyperfunction) lead to a change in the state of the body.
With hyperfunction of the anterior lobe of the pituitary gland in childhood, increased body growth is observed: a person becomes very tall. In such giants, whose height exceeds 2 m, a pituitary tumor may be detected during examination. If the pituitary gland is not active enough during the growth period (hypofunction), then growth retardation occurs and a stunted person (dwarf) is formed. In such people, skeletal ossification occurs later, the genitals and secondary sexual characteristics are poorly developed, they do not tolerate infectious and other diseases well.
In an adult, hyperfunction of the anterior pituitary gland leads to acromegaly – an excessive increase in the hands, feet, nose, tongue, facial bones, auricles, some organs of the chest and abdominal cavities – the lower jaw becomes long and wide, the nose thickens, the cheekbones and eyebrows protrude strongly … In addition, with acromegaly, the functions of other endocrine glands, in particular the genital and pancreas, are disrupted. With hypofunction of the anterior lobe of the pituitary gland in adults, a metabolic disorder is noted, which leads either to obesity (pituitary obesity) or to sudden weight loss (pituitary cachexia).
Hypofunction of the posterior lobe of the pituitary gland is the cause of diabetes insipidus (diabetes insipidus). At the same time, a large amount of urine is released (up to 4 liters per day), due to the inability of the kidneys to concentrate it, and a strong thirst arises.