On the yolk sac of the cat. II. Erythropoietic phases, ultrastructure of aging primitive erythroblasts, and blood vessels.

The phase of primitive erythropoiesis in the feline yolk sac lasts from the 14th to the 20th day after malting. The globular nucleated primitive erythroblasts are formed extravascularly to some extent, but they can be clearly distinguished from the endoderm. They do not undergo a denucleation and are still present in the circulating blood on the 45th day. Aging primitive erythroblasts are characterized by a loss of polysomes, by the appearance of long intracytoplasmic electron-lucent channels, and by a nuclear pyknosis which can turn into a karyolysis. Definitive erythropoiesis begins around the 17th day but, even by the 19th day, it is not particularly prominent. It ends around the 45th day. It is almost exclusively intravascular. The distinction of immature primitive erythroblasts from erythroblasts of the definitive series is difficult, because it is based upon only slight differences in the heterochromatinization, in the nuclear-cytoplasmic ratio, and in the organelle content of the cells. In the definitive series, the nuclear divisions follows the law of the rhythmical halving of the nuclear volume. The cells exhibit more clearly identifiable maturation stages here, and the 'checkerboard nucleus' is more distinct. The vascular endothelium is largely attenuated and moderately fenestrated; it lacks a distinct basement membrane. Organelle-rich adventitial cells are found in close apposition.