Anatomical And Morphological Relationships

The anatomy and histology of the thymus gland are pictured in Figure 17-3. The gland lies above the heart, as shown in Figure 17-3A, and is very much larger in

Lymphatic Organs: Thymus

Position of thymu lymphocytes

Lymphatic Organs: Thymus

Position of thymu

Anatomy Gland Yearbook

lymphocytes

Thymus in 2-year-old child

Thymus in adult figure 17-3 Anatomy and histology of the thymus gland. (A) Locations and comparative sizes at different stages of growth. Reproduced from Kahle, W., Leonhardt, H., and Platzer, W. (1978). "Color Atlas and Textbook of Human Anatomy," Vol. 2, p. 93. Year Book Medical Publishers, Chicago, IL. (B, p. 686) Histological organization of the thymus. The thymus is encapsulated and divided into lobules by septa. Densely packed dividing lymphocytes form a network of epithelioid cells in the cortex to the medulla. There are fewer lymphocytes in the medulla that contain more bone marrow-derived interdigitating cells. There is a close association of the developing lymphocytes with epithelial and interdigitating cells. Functions of the corpuscle structures are unknown. Reproduced from Roitt, I. M., Brostoff, J., and Male, D. K. (1985). "Immunology," p. 14.3. Gower Medical Pub. Ltd.

Thymus in 2-year-old child

Thymus in adult figure 17-3 Anatomy and histology of the thymus gland. (A) Locations and comparative sizes at different stages of growth. Reproduced from Kahle, W., Leonhardt, H., and Platzer, W. (1978). "Color Atlas and Textbook of Human Anatomy," Vol. 2, p. 93. Year Book Medical Publishers, Chicago, IL. (B, p. 686) Histological organization of the thymus. The thymus is encapsulated and divided into lobules by septa. Densely packed dividing lymphocytes form a network of epithelioid cells in the cortex to the medulla. There are fewer lymphocytes in the medulla that contain more bone marrow-derived interdigitating cells. There is a close association of the developing lymphocytes with epithelial and interdigitating cells. Functions of the corpuscle structures are unknown. Reproduced from Roitt, I. M., Brostoff, J., and Male, D. K. (1985). "Immunology," p. 14.3. Gower Medical Pub. Ltd.

the newborn than in the adult. Shrinkage of the gland with age to adult status is the result of increasing function of the adrenal cortex with higher circulating levels of glucocorticoid hormones. Thymocytes are sensitive to the action of glucocorticoids mediated by the glucocorticoid receptor, and, as a result, the cells involute by a process of programmed cell death (specifically, apoptosis) under hormonal action. The histology of the thymus is shown in Figure 17-3B. Besides a capsule, there are two main layers of cells: the cortex and the medulla. Thymocytes are similar to lymphocytes of other lymphoid organs and to those in the circulation.

The thymus arises from an epithelial outgrowth of the third and fourth branchial pouches to form the thymus gland. The human fetal thymus is predominantly lymphoid, a pattern that is preserved throughout childhood. Later the lymphoid component decreases, particularly in the cortex, under the influence of endogenous adrenal glucocorticoids (see Chapter 10). The lymphoid cells either divide and die in the thymus gland or populate the peripheral lymphatic organs. Lobules constitute the histological cores of the thymus gland. The lobular structure is not evident until the third intrauterine month (Figure 17-4). Up to that time, the thymus appears as an epithelial organ. The periphery of the lobes is bounded by a capsule that originates the connective tissue septa. Each lobule is composed of an outer darkly staining cortex and an inner paler staining medulla, as visualized with hema toxylin and eosin (Figure 17-4). The thymus gland proportion to body weight is greatest at birth, but its absolute mass is greatest at puberty. Subsequently, there is involution and the thymic parenchyma is replaced by fat. This organ is the primary source of T lymphocytes.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment