Histone Gene Expression: Progeny of Isolated Early Blastomeres in Culture Make the Same Change as in the Embryo

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Abstract
Stage-specific changes in histone synthesis during sea urchin development reflect the expression of different sets of genes. The three kinds of blastomeres formed at the 16-cell stage are the earliest "determined" cells and fall into three distinct size classes. At this stage the cells synthesize only "early" histones. Such blastomeres can survive and divide in culture after being separated from the embryo, whether or not they are permitted to aggregate. With or without reaggregation, cultured progeny cells of each type of isolated blastomere perform the same changeover of histone synthesis as takes place in the intact embryo, that is, they begin spontaneously to synthesize a new set, the "late" histone variants. Normal contact relations among cells of the embryo are, therefore, not required for this programmed change in gene expression.

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