Human Anatomy Part 2 - K. A. Diubenko, A. K. Kolomiitsev, Yu. B. Chaikovskyi 2008

Special Part
Hematopoietic and Immune System Organs - General Overview
Primary lymphoid organs {organa lymphoidea primaria} - Thymus

The Thymus is a central organ of lymphopoiesis and immunogenesis, as various types of T-lymphocytes develop within it from Bone Marrow precursors, actively participating in immune responses and stimulating lymphocyte development in other Organs of The Immune System.

The thymus is located behind the Sternum in the anterior Mediastinum, along the Trachea, covered by the m. sternohyoideus and m. sternothyroideus. The anterior surface of the gland lies adjacent to the sternum, and laterally to the mediastinal Pleura and internal thoracic artery (Fig. 115). The posterior surface of the gland is adjacent to the Pericardium, aortic arch, and brachiocephalic Veins. In the case of a multilobar form, the Thymus gland may extend through the superior thoracic aperture and penetrate into the pretracheal space of the neck, reaching the lower border of the thyroid Cartilage.

It consists of the right and left lobes, lobus dexter et sinister, which are vertically elongated and can reach the upper border of The Thyroid Gland. Sometimes an intermediate lobe is wedged between the main lobes. The thoracic part of the thymus widens and reaches the III–IV intercostal space, The Heart, and the pericardium. The thymus is characterized by significant variation in its external shape; sometimes it consists of 3–4 lobes, and it may resemble a fork or a leaf of the thyme plant, from which its name is derived.

The gland reaches a significant size in childhood (at 2 months) and its maximum weight during Puberty. After the age of 18, the gland begins to decrease in size, and from the age of 25, it undergoes age-related involution, being replaced by adipose tissue. However, even in old age, remnants of the thymus gland can still be found within its Structure.

Externally, the thymus is covered by a Connective Tissue capsule, from which septa extend to divide the gland into lobules measuring 4-10 mm. No Lymphatic vessels penetrate the thymus capsule, which indicates that Lymph does not pass through the thymus (A. Ham, D. Cormack, 1983).

The thymic lobules are not completely separated from each other and in some places anastomose. Within the lobules of the gland, the cortex and medulla are distinguished (Fig. 116). The cortex is located at the periphery of each lobule and is densely packed with lymphocytes, while the medulla is located more centrally and does not contain A large number of lymphocytes.

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Fig. 115. Thymus gland, thymus (infant under one year of age)

Fig. 116. Diagram of the Internal Structure of a child's thymus gland. Hematoxylin and eosin staining

The thymic cortex is a site of intensive lymphocyte production, especially in the fetus and during the early postnatal period. Unlike the Lymph Nodes AND Spleen, where lymphoid tissue practically disappears in the absence of Antigens entering the body (for example, under sterile conditions), lymphocyte production in the thymus occurs continuously. The intensity of this process depends neither on The amount of antigens nor on the number of lymphocytes in the peripheral Blood. The thymus is thus an autonomous "lymphocyte factory" (A. Ham, D. Cormack, 1983).

The thymic medulla is composed of small, medium, and large T-lymphocytes, and T-lymphoblasts, which are surrounded by epithelioreticulocytes and macrophages, though they are less densely packed. A characteristic morphological feature of the medulla is the presence of Hassall's corpuscles (thymic corpuscles), which are formed by the concentric layering of stellate medullary epithelioreticulocytes. The stroma of the lobules is formed by reticular Epithelial Tissue. Its Cells are stellate (branched) and contact each other; a few fibroblasts are also present among the epithelial cells.

The thymus produces the hormone thymosin, which regulates hematopoiesis in peripheral Hematopoietic organs and creates the necessary conditions for the maturation of T-lymphocytes (thymocytes). In addition, the thymus secretes several other active substances into the blood: a Calcitonin-like factor, which decreases calcium concentration in the blood, an Insulin-like factor, which reduces blood sugar levels, and growth factor.

Blood supply. The Blood supply to the thymus is provided by Branches of the inferior thyroid and internal thoracic Arteries. Venous blood from the gland drains into the left brachiocephalic, internal thoracic, and inferior thyroid veins.