The Chick Embryo Blastoderm System and Hemoglobin Synthesis

The Chick Embryo Blastoderm System and Hemoglobin Synthesis 

All aerobic cells contain heme and closely related iron porphyrins these serve as the prosthetic groups of hemoproteins such as cytochromes and catalase. The amount of heme made by a stem cell or proerythroblast is minute compared with that which is required for hemoglobinization to form an erythroid cell. In the differentiation of the proerythroblast to erythroid cell the synthesis of heme as well as of globin must be turned on fully. 

Already at its earliest stages the colorless potential hemocytoblast cells of the blastoderm contain all the enzymes that convert ALA to porphyrins at nonlimiting activities. This was shown by incubation of the blastoderm with ALA. Porphyrinogen intermediates accumulated within hours, some of which became autoxidized to porphyrins such as coproporphyrin and protoporphyrin. These porphyrins could be detected in cells with a fluorescence microscope. 

The early blastoderm cells contain the apparatus for synthesizing globin but do not do so until sufficient heme is made. This fact was shown by adding to the blastoderm in vitro both ALA and inhibitors of protein synthesis such as cycloheximide and puromycin. Under these conditions ALA was converted to heme because enzymes to convert ALA to heme were already present and were relatively stable. However, no hemoglobin was made because globin synthesis was blocked by inhibitors of protein synthesis. 

The above experiments demonstrate that the early cells of the blastoderm are poised to make hemoglobin they have mRNA that codes for globin and, in addition, ribosomes and other apparatus for globin synthesis. Yet they cannot make globin until ALA can be made in adequate amounts. The ALA-synthetase activity in these cells must be low and limiting. 

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C. The Chick Embryo Blastoderm System and Hemoglobin Synthesis... 

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