ACTION OF PROGESTERONE AS A CLEAVAGE INHIBITOR OF RABBIT OVA IN VITRO

Summary. The manner whereby progesterone acts as an inhibitor of cleavage of mammalian ova was studied using rabbit ova in vitro. Progesterone blocked cleavage when it was present in the culture medium at concentrations of 10 µg/ml more, and exerted this same effect on all stages up to the morula. The growth of blastocysts was not affected. The action was reversible: ova resumed cleavage within 2 to 3 hr after removal to progesterone-free media. The reversal was time-specific and was used to phase the subsequent cleavage of ova inhibited for varying periods of time. The inhibition was overcome by increasing the concentrations of amino acids or of the serum component in the medium. Supplementing the progesterone media with oestradiol did not reverse the inhibition. Autoradiographs of sections of ova that were kept in C-labelled progesterone for 8 hr, showed silver grains concentrated on the surface of the ovum and the zona pellucida. It is concluded that progesterone blocks cleavage by limiting the supply of protein or amino acids and thus inhibiting protein synthesis within the ovum. Presumably this is done by the progesterone aggregating on the surface of the cell or its protective coating. The inhibitory action appears to be timed in relation to some event that precedes mitosis by an hour or two. It is therefore suggested that the protein synthesis involved is related to formation of the mitotic apparatus or to chromosome condensation.