Thymus | ||
The thymus is a gland which is present in the infant and adolescent, and which has usually shrunk and been replaced by fibrous tissue by early adulthood. It lies in a space called the anterior mediastinum between the pericardium, the sac containing the heart, and the sternum, or breastbone. The thymus is pyramid shaped and of two parts, right and left lobes which are closely applied to each other. In the newborn, it weighs between 10 and 30 grams, and in the child it may extend from the level of the fourth rib up to the thyroid gland above. In the adolescent it reaches its maximum weight of 50-60 grams before beginning to shrink. Each lobe is separated by fibrous strands into a number of smaller lobules, each of which consists of an outer cortex and an inner medulla, which originally (in the developing foetus) come from the stem cells in the bone marrow. These cells mature into the white blood cells called T-lymphocytes, and are an essential part of our immune system. The cells are called T because they have been programmed by the Thymus. In addition to producing the original population of T-cells, which migrate to colonise the rest of the lymphoid tissue around the body, it contains other cells that may be involved in the production of antibodies in the condition myasthenia gravis. | ||
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