Text for questions number74 to 76.

    During the process of embryonic development, cells become progressively restricted in their developmental potential and finally acquire the biochemical and morphological specialization necessary for their respective functions in an adult. Since enzymatic and structural proteins are required for the appearance and maintenance of this specialization, the differentiated state results from the synthesis and activity of cell-specific proteins during development.

    Since all cells of an organism contain the same genotype as the fertilized egg, cellular differentiation is the result of variable gene activity rather than selective gene loss. Thus, cellular specialization and cell-specific protein synthesis result from the expression of appropriately selected groups of genes in each cell type. As development proceeds, the progressive differentiation of cells is correlated with changes in the population of protein species within the embryo, which in turn reflect the accurate programming of the time and sequence of the biosynthesis of different proteins by the genome. In the absence of opportunities for genetic analysis, determining the mechanisms involved in the regulation of protein synthesis is key to understanding genome control during development

    The majority of studies on gene activity in embryogenesis have been done on the sea urchin system, where large numbers of embryos undergoing relatively synchronous development can be easily obtained. Also, sea urchins' permeability to radioactive isotopes and to inhibitors of RNA and protein synthesis provides a distinct advantage for study over an amphibian material. Especially well documented are the maternal programming of early development and the genomic control of later differentiation in the urchin. Maternal products, stored in the egg cytoplasm from oogenesis, can support development from fertilization through the hatching blastula stage; however, development from the mesenchyme blastula stage is dependent upon gene products synthesized under the direction of the embryonic genome.

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