Shedding of the zona pellucida by the rat blastocyst

Abstract It is known that rat blastocysts shed their zonae pellucidae during the afternoon of day 5 of pregnancy. The aim of the present study was to determine whether one can detect any morphological changes in the zonae before, during, and after shedding. From 74 rats killed between 11:00 and 19:30 hours on day 5 pregnancy, 711 blastocysts were recovered: 407 were zona‐encased, 293 were zonafree, and 11 were half‐way out of the zona. Preshedding changes were observed in the zonae of 34 of the zona encased blastocysts. The changes were confined to the regions of the embryonic and abembryonic poles. It was concluded that the blastocyst can escape from its zona by way of either the embryonic or the abembryonic pole. The entire process of zona shedding seems to be very rapid, since only 45 blastocysts (i.e., 34 exhibiting preshedding changes and 11 being half‐way out of the zona), out of the 711 recovered, were observed during the process of shedding. Theoretically, for every zona‐free blastocyst, an empty zona should have been recovered. However, the 293 zona‐free blastocysts recovered were matched by only seven empty zonae. The reason for the rapid disappearance of the zona after shedding was not determined.