The intracellular pH of the sea urchin embryo increases 0.3 pH units between 1 and 4 min after fertilisation. The increase in pH is required for initiating development. The increase results from an exchange of extracellular Na+ for intracellular H+.
Silkmoth follicles, arranged in a precise developmental sequence within the ovariole, yield pure and uniform populations of follicular epithelial cells highly differentiated for synthesis of the proteinaceous eggshell (chorion) . These cells can be maintained and labeled efficiently in organ culture ; their in vitro (and cell free) protein synthetic activity reflects their activity in vivo . During differentiation the cells undergo dramatic changes in protein synthesis . For 2 days the cells are devoted almost exclusively to production of distinctive chorion proteins of low molecular weight and of unusual amino acid composition . Each protein has its own characteristic developmental kinetics of synthesis . Each is synthesized as a separate polypeptide, apparently on monocistronic messenger RNA (mRNA), and thus reflects the expression of a distinct gene . The rapid changes in this tissue do not result from corresponding changes in translational efficiency . Thus, the peptide chain elongation rate is comparable for chorion and for proteins synthesized at earlier developmental stages (1 .3-1 .9 amino acids/sec) ; moreover, the spacing of ribosomes on chorion mRNA (30-37 codons per ribosome) is similar to that encountered in other eukaryotic systems .
The results of sperm binding experiments reveal no change in the sperm binding properties of the egg surface coat at fertilization of Urechis caupo eggs. When fertilized eggs are reinseminated, sperm continue to attach to the egg surface coat. The acrosomal tubules of supernumerary sperm are observed in the perivitelline space closely apposed to the egg membrane. Thus, the polyspermy block in Urechis eggs involves neither alteration of sperm binding sites nor inhibition of the acrosome reaction. Our results suggest that the block is at the level of sperm-egg membrane fusion.
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