Endocrine glands

The processes occurring in the body are controlled not only by the nervous system, but also by the endocrine glands (endocrine organs). The latter include glands of various origins that have specialized in the process of evolution, are topographically isolated, have no excretory ducts and secrete the secretion they produce directly into the tissue fluid and blood. The products of the endocrine glands (organs) are hormones. These are highly biologically active substances that, even in very small quantities, can influence various functions of the body. Hormones (Greek hormao - I excite) have a selective function, i.e. they can have a very specific effect on the activity of target organs. Hormones provide a regulatory effect on the process of growth and development of cells, tissues, organs and the whole organism. Excessive or insufficient production of hormones causes severe dysfunctions in the human body and even leads to diseases.

Anatomically isolated endocrine glands can have a significant influence on each other. Since this influence is provided by hormones that are delivered to target organs with blood, it is customary to speak of humoral regulation of the activity of these organs. However, it is known that all processes occurring in the body are under constant control of the central nervous system (CNS). Such dual regulation of organ activity is called neurohumoral.

The currently generally accepted classification of endocrine organs is based on their origin from different types of epithelium.

1. Glands of ectodermal origin - from the epithelial lining of the pharyngeal intestine (branchial pockets). This is the so-called branchiogenic group of endocrine glands: the thyroid and parathyroid glands.
2. Glands of endodermal origin - from the epithelium of the trunk part of the embryonic intestinal tube: the endocrine part of the pancreas (pancreatic islets).
3. Glands of mesodermal origin: interrenal system, adrenal cortex and interstitial cells of the sex glands.
4. Glands of ectodermal origin are derivatives of the anterior part of the neural tube (neurogenic group): the pituitary gland and the pineal body (pineal gland).
5. Glands of ectodermal origin are derivatives of the sympathetic nervous system: the adrenal medulla and paraganglia.

There is another classification of endocrine organs, which is based on the principle of their functional interdependence.

1. Adenohypophysis group:
   1. thyroid gland;
   2. adrenal cortex (fascicular and reticular zones);
   3. the endocrine part of the sex glands - the testicles and ovaries. The central position in this group is occupied by the adenohypophysis, the cells of which produce hormones that regulate the activity of these glands (adrenocorticotropic, somatotropic, thyrotropic and gonadotropic hormones).
2. A group of peripheral endocrine glands whose activity does not depend on the hormones of the anterior pituitary gland:
   1. parathyroid glands;
   2. adrenal cortex (glomerular zone);
   3. pancreatic islets.

These glands are conventionally called self-regulating. Thus, the hormone of the pancreatic islets, insulin, reduces the level of glucose in the blood; increased glucose levels in the blood stimulate the secretion of insulin.

3. . Group of endocrine organs of "nervous origin" (neuroendocrine glands):
   1. large and small neurosecretory cells with processes that form the nuclei of the hypothalamus;
   2. neuroendocrine cells that do not have processes (chromaffin cells of the adrenal medulla and paraganglia);
   3. parafollicular, or K-cells of the thyroid gland;
   4. argyrophilic and enterochromaffin cells in the walls of the stomach and intestines.

Neurosecretory cells combine nervous and endocrine functions. They perceive nervous impulses and in response produce a neurosecretion, which enters the blood or is transported to target cells via nerve cell processes. Thus, hypothalamic cells produce a neurosecretion, which is delivered to the pituitary gland via nerve cell processes, causing an increase or inhibition of cell activity.

4. A group of endocrine glands of neuroglial origin (from the embryonic neural tube):
   1. pineal body;
   2. neurohemal organs (neurohypophysis and median eminence).

The secretion produced by the cells of the pineal body inhibits the release of gonadotropic hormones by the cells of the adenohypophysis and, thus, inhibits the activity of the sex glands. The cells of the posterior lobe of the pituitary gland do not produce hormones, but ensure the accumulation and release into the blood of vasopressin and oxytocin, which are produced by the cells of the hypothalamus.

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