Final answer:
T cell development involves a process of positive and negative selection in the thymus. Double-negative thymocytes, which lack CD4 and CD8 molecules, undergo positive selection in the cortex by binding to self MHC molecules. Thymocytes that survive positive selection become double-positive cells expressing both CD4 and CD8 markers.
In the junction between the cortex and medulla, negative selection occurs to eliminate T cells that bind to self-antigens. The T cells that leave the thymus become single-positive cells expressing either CD4 or CD8, but not both.
Step-by-step explanation:
T cell development and differentiation in the thymus involves several steps. Initially, thymocytes are referred to as 'double negatives' because they do not bear the CD4 or CD8 molecules. In the cortex of the thymus, double-negative thymocytes undergo positive selection, where they bind to self MHC molecules.
Thymocytes that survive positive selection become 'double positives' as they express both CD4 and CD8 markers. Later, in the junction between the cortex and medulla, negative selection occurs, eliminating T cells that bind to self-antigens. Finally, the T cells that leave the thymus become single positives, expressing either CD4 or CD8, but not both.