Avian primary erythropoiesis; accessory pathways of erythrocyte maturation in chick yolk sack.

In the population of chick primary erythroblasts the quota of the G2-phase (tetraploid) cells is greatly increased probably due to the fact that a part of erythroblasts is reversibly blocked at this phase. An increased number of G2 cells is also detected in the population of reticulocytes at later stages of embryogenesis and there is no cell-division in about half of the G2 cells until the 8th day the erythrocytes being exclusively diploid. The primary erythrocytes are probably formed from two types of precursors: normally dividing and specialized cells, and cells blocked in the G2-phase some of which do not complete mitosis before they develop into erythrocytes. Haemoglobinization is far more intensive in primary erythroid cells as compared with erythrocytes of definitive normal erythropoiesis.