Polarization of blastomeres in the cleaving rabbit embryo

Cellular polarization is believed to be a crucial event in the differentiative divergence of the two cell lineages leading to the blastocyst in rodent embryos. This study was undertaken to determine if rabbit embryos exhibited cellular polarization prior to blastocyst formation and to define the embryonic stage at which polarization was first apparent. Polarity was assayed by observation of the pattern of binding of FITC-Con A to dissociated blastomeres from three stages of rabbit embryos. Scanning electron microscopy on the dissociated cells confirmed the fluorescence results. Fifty-one percent of blastomeres in 38- to 66-cell rabbit embryos exhibited an intense pole of FITC-Con A binding and a single pole of microvilli. Only 2% of blastomeres at the 17- to 34-cell stage were similarly polarized and none were polarized at the 8- to 16-cell stage. In addition, during attempts to remove the mucin coat and zona pellucida from the rabbit embryos prior to their dissociation, it was found that the protease sensitivity of these coats also changed at the 38- to 66-cell stage. Prior to this time, although the mucin coat disappeared after 30 min in 0.5% pronase, the zona required approximately 1.5-2.5 hr in pronase for even partial removal. At the 38- to 66-cell stage, pronase dissolved the mucin coat within 10 min and the zona pellucida within 20 min. The zona was resistant to 0.1% proteinase K at all stages examined.