The thymus gland lies in the upper part of the mediastinum behind the sternum and extends upwards into the root of the neck. It weighs about 10 to 15 g.(about half an ounce) at birth and begins to grow until the individual reaches puberty when it begins to atrophy. It’s maximum weight is around 30 - 40g (around 1 to 1.5 ounces) by the age of 40 it has returned to it’s weight at birth. The thymus consists of two lobes connected by areolar tissue. The lobes are enclosed in a fibrous capsule which dips into their substance dividing them into lobules that consist of an irregular branching framework of epithelial cells and lymphocytes.

Function

Lymphocytes originate from haemocytoblasts (stem cells) in red bone marrow. Those that enter the thymus mature and develop into activated T-lymphocytes i.e. able to respond to antigens encountered elsewhere in the body. They then divide into two groups :

	those that enter the blood, some of which remain in circulation and some lodge in other lymphoid tissue
	those that remain in the thymus gland and are the source of future generations of T-lymphocytes.

The maturation of the thymus and other lymphoid tissue is stimulated by thymosin, a hormone secreted by the epithelial cells that form the framework of the thymus gland. Involution of the gland begins in adolescence and, with increasing age the effectiveness of T- lymphocyte response to antigens declines.