Question

Define lymphocyte development . Mention what happens during lymphocyte development.

Answer 1

In the embryo lymphocytes are derived from mesenchymal cells present in the wall of the yolk sac, in the liver and in the spleen. These stem cells later migrate to bone marrow. Lymphocytes formed from these stem cells (bone marrow) enter the blood. Depending on their subsequent behaviour they are classified into two types :

• Some of them travel in the bloodstream to reach the thymus. Here they divide repeatedly and undergo certain changes. They are now called T-lymphocytes. These T-lymphocytes, that have been processed in the thymus re-enter the circulation to reach lymphoid tissue in lymph nodes, spleen, tonsils, and intestines. In lymph nodes T-lymphocytes are found in the diffuse tissue around lymphatic nodules. In the spleen they are found in white pulp. From these masses of lymphoid tissue many lymphocytes pass into lymph vessels, and through them they go back into the circulation. In this way lymphocytes keep passing out of blood into lymphoid tissue (and bone marrow), and back from these into the blood. About 85% of lymphocytes seen in blood are T-lymphocytes.
• Lymphocytes of a second group arising from stem cells in bone marrow enter the blood-stream,but do not go to the thymus. They go directly to lymphoid tissues (other than the thymus). Such lymphocytes are called B-lymphocytes. In contrast to T-lymphocytes that lie in the diffuse lymphoid tissue of the lymph nodes and spleen, B-lymphocytes are senn in lymphatic nodules. The germinal centers are formed by actively dividing B-lymphocytes, while the dark rim of lymphatic nodules are formed by dense aggregation of B-lymphocytes. Like T=lymphocytes, B-lymphocytes also circulate between lymphoid tissues and the blood stream.

Schematic reoresentation of circulation of T-lymphocytes and B-lymphocytes are given below :