Early origins of definitive erythroid cells in the chick embryo.

The early maturation stages of definitive erythroid cells are observed in the embryonic circulation of the chick yolk sac at 4.5--5 days of incubation. Light and electron microscope observation of the mesoderm of the yold sac membrane indicate that individual presumptive precursors of the definitive-line are present as early as 2 days of incubation and give rise to sequestered populations of immature erythroblasts within sinusoids during the period of 2.5-6 days incubation. Such isolated populations of definitive-line erythroblasts eventually connect with the established capillary circulation of yolk sac membrane but a large proportion of the erythroblasts temporarily remain associated with the endothelium prior to free circulation.