In humans, five days after the fertilization, the blastocyst performs zona hatching; the zona pellucida degenerates and decomposes, to be replaced by the underlying layer of trophoblastic cells.
The zona pellucida is a thick, transparent layer that surrounds the unfertilized egg and early embryo in mammals. It is composed of glycoproteins and proteoglycans and plays a critical role in fertilization and embryonic development.
The zona pellucida can be digested by enzymes called proteases, which break down the glycoproteins and proteoglycans that make up the layer. These enzymes are produced by the cells of the developing embryo and also by sperm during fertilization.
The zona pellucida may be digested or removed in assisted reproductive technologies, such as IVF, to facilitate fertilization. In natural conception, sperm must penetrate the zona pellucida to reach and fertilize the egg. In some cases, the zona pellucida may be too thick or resistant, making it difficult for sperm to penetrate. By removing or digesting the zona pellucida, this barrier is eliminated and increases the chances of successful fertilization.
In natural conception, the zona pellucida is typically digested by enzymes produced by the embryo, and the remnants are shed during implantation. In rare cases, the zona pellucida may not be digested or removed properly, leading to a condition called zonapellucida retention syndrome. This can cause infertility or recurrent pregnancy loss.
The removal or digestion of the zona pellucida may carry some risks for the developing embryo, such as damage to the embryo or a decrease in its ability to implant in the uterus. However, these risks are minimal and are carefully monitored by fertility specialists during assisted reproductive procedures. Additionally, natural processes like the shedding of the zona pellucida during implantation indicate that the embryo can develop and implant successfully without the zona pellucida intact.