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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 .

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