Line-restricted hemoglobin synthesis in chick embryonic erythrocytes.

The presence of embryonic hemoglobin in early definitive erythrocytes was checked by indirect immunofluorescence assay, using specific antibodies raised against embryonic Hb P. As positive control we used anti-Hb A which reacted with the alpha A chain shared by the minor embryonic Hb E and the adult Hb A. The assay was performed using blood smears from embryos between 6 and 15 days of incubation and yolk sac sections from embryos between 4 and 6 days. Hb P was never detected in the definitive line in circulating erythrocytes or in maturing erythroblasts still sequestered in the blood islands of the yolk sac. The expression of the 'specific' embryonic genes is thus restricted to the primitive line (as the 'specific' adult beta gene is restricted to the definitive line), and the hemoglobin switch is the result of the progressive substitution of the primitive line by the definitive one.