Final answer:
Thymocytes go through a series of differentiation processes, expressing different surface proteins. Double negative thymocytes in the cortex of the thymus do not bear CD4 or CD8 molecules and undergo positive selection. Later, they become double positive and go through negative selection to ensure tolerance to self-antigens.
Step-by-step explanation:
Thymocytes are immature T cells that go through a process of differentiation in the thymus. During this process, they express different surface proteins, such as CD2, CD5, CD4, and CD8.
Double negative thymocytes do not express CD4 or CD8 molecules. They are in the cortex of the thymus and undergo positive selection, where they bind to MHC molecules on thymic epithelia. This process helps eliminate T cells that might attack the body's own cells.
Later, thymocytes become double positive and express both CD4 and CD8 markers. They move to the junction between the cortex and medulla, where negative selection occurs. In negative selection, T cells that bind to self-antigens are eliminated, ensuring tolerance to one's own body tissues.