Meckel's diverticulum. I. Extramedullary myelopoiesis in the yolk sac of hatched chickens (Gallus domesticus).

We have described an extramedullary myelopoietic tissue located in the wall of the yolk sac. This myelopoietic site is absent at hatch and it produces only granulocytic and monocytic cells during the regression of the yolk sac from 2 to 7 weeks of age. The granulocytic and monocytic cells are located in two distinct zones. The monocytic cells migrate close to the lumen of the yolk sac where they may fuse and form giant cells. There are no mature granulocytes present. Cell migration has not been observed. Therefore, the fate and function of the young granulocytic cells are unknown. In this extramedullary myelopoietic tissue, the absence of erythrocyte and thrombocyte formation may indirectly suggest that these cells develop together in sinuses that are lacking in the wall of the yolk sac.