Final answer:
Thymocytes undergo positive and negative selection during thymic selection in the thymus to become single positive thymocytes expressing either CD4 or CD8. Positive selection occurs in the cortex as thymocytes interact with MHC molecules on thymic epithelial cells, while negative selection occurs in the medulla as thymocytes bind with self-antigens and undergo apoptosis.
Step-by-step explanation:
Thymocytes undergo a process called thymic selection in the thymus to become single positive thymocytes. In the cortex of the thymus, double negative thymocytes undergo positive selection, where they bind to MHC molecules on thymic epithelial cells. Thymocytes that interact appropriately with MHC molecules receive positive stimulation and move further in the maturation process. Thymocytes that don't interact appropriately undergo apoptosis.
After positive selection, the surviving thymocytes move to the thymic medulla where they encounter antigen-presenting cells (APCs) presenting self-antigens. Thymocytes that bind with self-antigens undergo apoptosis, a process known as negative selection. The thymocytes that successfully pass both positive and negative selection become single positive thymocytes expressing either CD4 or CD8, but not both.
Positive selection and negative selection together make up thymic selection, a three-step process that ensures only functional T cells leave the thymus into the peripheral bloodstream.