zona prlucida degenarates after-
A. before cleavage
B. after cleavage
- During the first week before implantation, the conceptus (product of fertilization) undergoes cleavage to form the blastocyst.
- Cells of the inner cell mass become aggregated and form the embryoblast that will form the tissues and organs of the embryo.
- Cells of the outer cell mass become flattened and form the trophoblast that will contribute to the formation of the extraembryonic supportive membranes.
- Around this time, the zona pellucida degenerates resulting in “hatching” of the blastocyst from its protective, but a restrictive coat.
- Once the blastocyst is rid of the zona pellucida it is free to begin implantation into the wall of the uterus.
- This implies that the zona pellucida degenerates only after cleavage.
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