Thymus

The thymus (thi'mus) gland is a soft, bilobed structure enclosed in a connective tissue capsule (fig. 16.12). It is located within the mediastinum, anterior to the aortic arch and posterior to the upper part of the body of the sternum, and extends from the root of the neck to the pericardium. The thymus varies in size from person to person, and it is usually larger during infancy and early childhood. After puberty, the thymus shrinks, and in an adult, it may be quite small. In elderly persons, adipose and connective tissues replace the normal lymphatic tissue.

Connective tissues extend inward from the thymus's surface, subdividing it into lobules (fig. 16.13). The lobules contain many lymphocytes that developed from progenitor cells in the bone marrow. Most of these cells (thymocytes) are inactivated; however, some mature into T lymphocytes, which leave the thymus and provide

Diaphragm

Stomach

Figure

Diaphragm

Stomach ure 16.12

Figure

The thymus gland is bilobed and located between the lungs and superior to the heart. The spleen is located inferior to the diaphragm, posterior and lateral to the stomach.

Connective tissue

Lobule

Figure

A cross section of the thymus (20x). Note how the gland is subdivided into lobules.

Connective tissue

Lobule ure I6.I3

Figure

A cross section of the thymus (20x). Note how the gland is subdivided into lobules.

immunity. Epithelial cells within the thymus secrete protein hormones called thymosins, which stimulate maturation of T lymphocytes after they leave the thymus and migrate to other lymphatic tissues.

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.

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