EMBRYONIC HEMOGLOBIN SYNTHESIS
121
tion presented here does illustrate the main features of the process as
they are currently understood. The wealth of information on the structure, function, and inheritance of hemoglobin is beginning to be applied
to the search for the levels and types of controls operative in the differentiation of the first erythrocytes. The methods of explantation and culture of tissues, in vitro, have provided a picture, for both the amphibians
and birds, of some early phases of the process. During a period of rapid
cell multiplication and extensive morphogenetic movements, groups of
cells become determined, or committed, to eventually undergo erythropoiesis and Hb synthesis. The molecular bases of this commitment, and
the impingement of cell division, tissue associations, and tissue movements on the commitment remain largely unknown. The formation of
the primitive red blood cells and first detectable synthesis of hemoglobin
occurs early in the embryogenesis of the amphibian and the chick. The
regulation of this new biosynthesis, at least in the chick, does not seem
to immediately depend on nucleic acid synthesis, although the integrity
of nucleic acid synthesis may well be necessary at much earlier stages
preceding overt differentiation. In the instances reviewed here, and perhaps in nearly all vertebrates, the hemoglobin first made by the embryo
may be of more than one type. The embryonic hemoglobins are later
succeeded by the appearance of the adult hemoglobins. Again, the description of this change in hemoglobins during embryogenesis is becoming
clearer, but the controlling factors are not understood. Perhaps the
information is sufficient, however, to suggest that the appearance of the
first hemoglobin molecules and the later appearance of adult hemoglobin
may be regulated by several precisely timed and interrelated controlling
elements. As the complexity of the succession of stages of the differentiation process become more clear, and the molecular basis of the
individual regulation mechanisms become better understood, we shall
better be able to reconstruct the way the embryo attains a functioning
erythropoietic tissue.
Note Added in Proof
Since the completion of this article, a number of papers relevant to
the topics discussed in this review have appeared. The following papers
may be of special interest to the reader.
1. Regulation of Hb Biosynthesis
Attardi, G., Parnas, A., Hwang, M .-I. H., and Attardi, B. (1966). J. Mol. Biol.
20, 145.
Conconi, F. M., Bank, A., and Marks, P. A. (1966). J. Mol. Biol. 19, 525.
Lin, S.-Y., Mosteller, R. D., and Hardesty, B. (1966). J. Mol. Biol. 21, 51.
Schapira, G., Padieu, P., Maleknia, N., Kruh, J., and Dreyfus, J. C. (1966). J. Mol.
Biol 20, 427.
121
tion presented here does illustrate the main features of the process as
they are currently understood. The wealth of information on the structure, function, and inheritance of hemoglobin is beginning to be applied
to the search for the levels and types of controls operative in the differentiation of the first erythrocytes. The methods of explantation and culture of tissues, in vitro, have provided a picture, for both the amphibians
and birds, of some early phases of the process. During a period of rapid
cell multiplication and extensive morphogenetic movements, groups of
cells become determined, or committed, to eventually undergo erythropoiesis and Hb synthesis. The molecular bases of this commitment, and
the impingement of cell division, tissue associations, and tissue movements on the commitment remain largely unknown. The formation of
the primitive red blood cells and first detectable synthesis of hemoglobin
occurs early in the embryogenesis of the amphibian and the chick. The
regulation of this new biosynthesis, at least in the chick, does not seem
to immediately depend on nucleic acid synthesis, although the integrity
of nucleic acid synthesis may well be necessary at much earlier stages
preceding overt differentiation. In the instances reviewed here, and perhaps in nearly all vertebrates, the hemoglobin first made by the embryo
may be of more than one type. The embryonic hemoglobins are later
succeeded by the appearance of the adult hemoglobins. Again, the description of this change in hemoglobins during embryogenesis is becoming
clearer, but the controlling factors are not understood. Perhaps the
information is sufficient, however, to suggest that the appearance of the
first hemoglobin molecules and the later appearance of adult hemoglobin
may be regulated by several precisely timed and interrelated controlling
elements. As the complexity of the succession of stages of the differentiation process become more clear, and the molecular basis of the
individual regulation mechanisms become better understood, we shall
better be able to reconstruct the way the embryo attains a functioning
erythropoietic tissue.
Note Added in Proof
Since the completion of this article, a number of papers relevant to
the topics discussed in this review have appeared. The following papers
may be of special interest to the reader.
1. Regulation of Hb Biosynthesis
Attardi, G., Parnas, A., Hwang, M .-I. H., and Attardi, B. (1966). J. Mol. Biol.
20, 145.
Conconi, F. M., Bank, A., and Marks, P. A. (1966). J. Mol. Biol. 19, 525.
Lin, S.-Y., Mosteller, R. D., and Hardesty, B. (1966). J. Mol. Biol. 21, 51.
Schapira, G., Padieu, P., Maleknia, N., Kruh, J., and Dreyfus, J. C. (1966). J. Mol.
Biol 20, 427.