Thymic rudiments from chick and mouse embryos have been cultured in diffusion chambers on the chick chorioallantois. In this situation their supply of blood-borne stem cells is cut off. Although rudiments from older embryos become fully lymphoid under these circumstances, primordia from early embryos fail to develop any lymphoid cells. Early chick rudiments will however develop completely if they are grafted directly to the chorioallantois where they receive a vascular supply.

It is concluded that stem cells first enter the chick thymic anlage at between 6 and 7 days' incubation and the mouse thymic rudiment between 10 and 11 days' gestation. During the following few days of development there is a rapid inflow of stem cells to the rudiments. Since it is likely that stem cells, once they have entered the thymic primordium, are capable of only limited proliferation, it must be concluded that an inflow of stem cells continues once full lymphopoiesis has begun, although perhaps at a reduced rate.

Finally, the importance of the interaction between stem cell and organ rudiment to normal thymic development is discussed in relation to the pathogenesis of thymic anomalies.

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