A new source of embryonic lymphocytes in the mouse.

The source of stem cells within the early mammalian embryo has not been identified. The yolk sac, once thought to provide the early stem cells in birds, is now seen more as a primitive effector organ capable of allorecognition, natural cytotoxic reactions and elaboration of cytokines. In lower vertebrates the source appears to be within a region delineated by the anterior limbs, foregut and mesonephros. That region defines the boundaries of the developing omentum, a fold in the peritoneum. As the omentum is known to develop lymphoid cells postnatally we have now examined the omental rudiment for the presence of lymphoid cell precursors. Our experiments provide evidence that the presumptive omentum of the 13-day mouse embryo is capable of histiotypic differentiation into a reticular organ containing a significant number of Thy 1+ lymphocytes, which suggests that the omentum may represent a new primary lymphoid organ in the mouse.