Thyroid. The development of the thyroid gland. The structure of the thyroid gland. Thyroid hormones

The thyroid gland produces thyroid hormones thyroxine and triiodothyronine, as well as thyrocalcitonin (calcitonin). 

Thyroxine and triiodothyronine are powerful stimulants of oxidative processes in cells, and triiodothyronine is 5-10 times more active than thyroxine. These hormones enhance metabolism, protein synthesis, gas exchange, metabolism of carbohydrates and fats. Thyroid hormones have a significant effect on the development, growth and differentiation of cells and tissues. They accelerate the development of bone tissue. Thyroid hormones have a particularly large effect on the histogenesis of nerve tissue. With thyroid insufficiency, the differentiation of cells and brain tissues is inhibited, the human mental development is impaired.   

Thyroid hormones have a stimulating effect on the regenerative processes in the tissues. Thyroxine and triiodothyronine contain iodine as an essential ingredient, so the intake of iodine with drinking water and food in the body is necessary for normal thyroid function. The third iodine-containing thyroid hormone, thyrocalcitonin, is involved in the regulation of calcium and phosphorus metabolism. 

The development of the thyroid gland.

An embryo of the thyroid gland appears at the 4th week of embryogenesis in the form of a protrusion of the ventral wall of the pharyngeal intestine between the 1st and 2nd pairs of gill pockets. This protrusion turns into an epithelial cord with a thickening at the end. The thyroid gland is initially laid as an exocrine gland. Later, the duct connecting the gland to the pharynx is reduced, and only foramen cecum remains from this duct. The epithelial cord bifurcates at the end. During the 3rd month along the course of epithelial cords, constrictions form. In distinct segments of these cords, gaps appear. As the constrictions deepen, the epithelial cords disintegrate into individual follicles and cell islets. 

At the end of the 3rd month, the follicle epithelium differentiates. His cells – follicular endocrinocytes (thyrocytes) – begin to produce hormones that accumulate in the cavity of the follicle. The formation of new follicles and their development occur heterochronously. By the time of birth, the presence of follicles with a colloid (“colloidal type of structure”) and without a colloid with desquamation of the epithelium (“desquamative type of structure”) is noted. Between the follicles there are interfollicular islets of cells. In the process of development of the gland, along with the differentiation of the epithelium, the mesenchyme grows, which is converted into connective tissue. A stroma of the gland is formed, containing a dense network of capillaries. Nerve fibers penetrate the stroma.  

Derivatives of the 5th pair of gill pockets — cells of the so-called ultimobranchial bodies — also grow into the germ of the gland . These are C cells producing calcitonin. These are cells of a neuroectodermal nature, and they are introduced into the thyroid gland through ultimobranchial rudiments. 

Thus, the following cellular differs are involved in the formation of the structural and functional units of the thyroid gland : these are the leading cells of the follicle epithelium – follicular endocrinocytes that produce thyroxin and triiodothyronine; calcitoninocytes, or C-cells producing calcitonin and other peptides (somatostatin, thyroliberin, serotonin, etc.).  

The structure of the thyroid gland.

The gland , consisting of two lobes, is externally covered with a connective tissue capsule, from which partitions depart, dividing the parenchyma into lobules. The structural and functional unit of the thyroid gland is the follicle. The average follicle diameter is about 50 microns. Their shape is mainly round. Follicles are closed vesicles. Their wall is formed by a single-layer epithelium consisting of follicular endocrinocytes (thyrocytes). Among these cells in the form of small clusters are C-cells. They can be near the follicles, and between the follicles.

The follicle cavity is filled with the secretion product of thyrocytes – a colloid containing proteins – thyroglobulins. Outside, the follicles entwine around the network of blood and lymph capillaries. Between adjacent follicles there are interfollicular islets consisting of poorly differentiated endocrinocytes. 

Follicular endocrinocytes have a cubic shape and a rounded core. On the apical surface there are microvilli. Organelles that provide protein synthesis are well developed in the cytoplasm. Many free ribosomes forming polysomes. Neighboring endocrinocytes in the follicle wall are connected using tight contacts, desmosomes and interdigitations. 

characteristic feature of the histophysiology of the thyroid gland is the differently directed movement of secretory products: first into the follicle cavity, and then into the blood in the opposite direction. This is due to the active activity of follicular endocrinocytes. The secretory cycle of these cells consists of the following phases: uptake of the starting substances, synthesis of the secretion, its release into the follicle cavity as a colloid, iodination of the colloid, endocytosis of the iodinated colloid and its modification and excretion of the hormone through the basal part of the cell into the surrounding tissues and blood vessels and lymphatic capillaries. The production of thyroid hormones begins with the synthesis of thyroglobulin in the basal part of endocrinocytes.  

Containing thyroglobulin synthesis products coming from the endoplasmic reticulum to the Golgi complex and further to a secretory granule derived by exocytosis into the cavity of the follicle. First, one iodine atom is attached to the non-iodinated thyroglobulin, and then the second, as a result of which mono- and diiodothyronines are formed. Subsequent complexation gives tetraiodothyronine, or thyroxine. When one atom of iodine is cleaved from thyroxine, triiodothyronine is formed.  

Under normal conditions, the processes of colloid formation and its resorption are balanced. This balance, however, is disturbed by hypo- and hyperfunction of the thyroid gland. With hypofunction, hormone excretion is delayed. In this case, the follicles are usually large, there are many colloids in the follicle cavity, it is thick, does not have resorption vacuoles, the epithelium is represented by flat endocrinocytes. With hyperfunction, on the contrary, the excretion of the colloid hormone in the blood is enhanced. The colloid liquefies, its quantity is small, the epithelium of the follicle wall becomes high prismatic.  

Parafollicular endocrinocytes (C-cells, or calcitoninocytes) produce the protein hormone calcitonin. It lowers the level of calcium in the blood and is an antagonist of parathyrin. Calcitonin acts on osteoclasts of bone tissue, reducing their resorptive activity. C cells are lighter and larger than follicular endocrinocytes, and their share is 0.1%. Usually they lie alone or in small groups. In the cytoplasm of parafollicular endocrinocytes there are many argyrophilic or osmiophilic secretory granules containing calcitonin and other peptides.  

The stroma of the gland consists of loose fibrous connective tissue, in which mast cells, macrophages, and lymphocytes are often found. Thyroid fibroblasts have a number of unique properties, which is determined by the characteristics of the receptor-transduction system of cells. In particular, they can participate in the development of inflammation along with immunocompetent cells. Fenestrated type capillaries are well developed. Nerve fibers contain peptides characteristic of C cells.  

Reactivity . The physiological regeneration of the thyroid gland occurs by updating the follicle epithelial cells due to their mitosis. The source of the development of new follicles can be interfollicular cell islets. Intoxications, injuries, autoimmune processes, hereditary factors, etc. can lead to thyrotoxicosis or hypothyroidism. It should be borne in mind that for the normal functioning of the thyroid gland, sufficient intake of iodine is necessary. The thyroid gland has a high ability to regenerate after injuries. An example of an autoimmune thyroid disease is Hashimoto’s disease. It arises due to the fact that thyroglobulin penetrates the stroma of the thyroid gland and, being an antigen, causes an immune response of the body.

At the same time, the stroma of the gland greatly grows , it is infiltrated by lymphocytes and plasmocytes, and the thyroid gland follicles contain little colloid and gradually atrophy. 

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