The puzzling origin of lymphocytes

Abstract

In this issue of Blood, Yoshimoto et al show that yolk sac (YS) and para-aortic splanchnopleura (P-Sp) cells, early in development, can generate both adult and fetal type T lymphocytes. The most remarkable conclusion is that T lymphocytes are derived directly from the hemogenic endothelium, before the appearance of hematopoietic stem cells.