Thymus

Situation

The thymus gland lies in the upper part of the mediastinum

behind the sternum and

extends upwards into the root of the neck.

10 to 15 g. at birth and begins to grow until the individual reaches puberty

Then it begins to atrophy.

Structure

It's maximum weight is around 30 - 40g (around 1 to 1.5 ounces) by the age of 40 Consists of two lobes connected by areolar tissue.

Fibrous capsule - septae divides the gland into lobules

Lobules consist 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

Thymosine is 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.