Developmental Biology of Mammalian T-Cell Progenitors: From Early Lymphoid Progenitors to Thymus-Colonizing Cells

This review gives a general overview of the recent advances in the field of fetal hematopoiesis and T-cell development. Although it is now well recognized that early lymphoid progenitors first emerge in the fetal liver (FL) where active B lymphopoiesis take places, the identity of early T-cell precursors, as well as the mechanisms of thymus colonization, has long been controversial. Here we discuss the experimental evidence supporting the concept of prothymocyte in both human and mouse species, as well as its major implications regarding lineage relationships among the immune system and general hematopoietic organization.