Final answer:
An uncommitted progenitor cell entering the thymus expresses the surface protein CD34. CD4 and CD8 are surface proteins acquired later in T cell development and are not found on progenitor cells just entering the thymus. As T cells mature, they become either CD4+ or CD8+.
Step-by-step explanation:
An uncommitted progenitor cell that just entered the thymus will express CD34 on its surface. CD34 is a marker of hematopoietic stem cells and progenitor cells, indicating that the cell is still in an immature state and has not yet begun to differentiate into a specific T cell lineage. As the progenitor cell matures in the thymus, it will eventually lose CD34 and acquire other surface proteins like CD4 or CD8.
CD4+ and CD8+ are not expressed by uncommitted progenitor cells. Instead, these surface proteins are acquired later during T cell development in the thymus. Naïve T cells that exit the thymus will express either CD4 or CD8, but not both, which are coreceptors that assist T cell receptors (TCR) in binding to Major Histocompatibility Complex (MHC) molecules on Antigen Presenting Cells (APCs).
CD4+ T cells bind to class II MHC and become helper T cells, while CD8+ cells bind to class I MHC and differentiate into cytotoxic T cells (CTLs).