Blastocyst hatching in humans

The human oocyte is surrounded by the zona pellucida—an elastic, transparent extracellular matrix consisting of specific glycoproteins. The zona pellucida is preserved after fertilization and surrounds the developing human embryo for a few days. The embryo needs to get out of the zona pellucida before implantation to establish cell contacts between the trophectoderm and endometrial epithelium. The release of the embryo from the zona pellucida is carried out at the stage of the blastocyst and called zona hatching. During zona hatching the blastocyst breaks the zona pellucida and performs active movements to escape through a gap formed in the zona. While microscopic description of zone hatching is well known, biochemical and cytological basis of zone hatching remains poorly understood. The break of the zona pellucida occurs under the influence of two forces: mechanical pressure of the growing blastocyst on the zone and chemical dissolution of the zone material with secreted lytic enzymes. There is only one paper (Sathananthan et al., 2003), which describes the specialized cells in the trophectoderm that locally dissolve the zona pellucida, promoting the emergence of the hole for blastocyst release. Taking into account the singleness of the paper and the absence of further development of this subject by the authors in the following decade, the existence of specialized cells for zone hatching should be assumed with great care. Lytic enzymes, secreted by cells of the trophectoderm for dissolving the zona pellucida, are different. Depending on the species of the mammal, different classes of proteases participate in the zone hatching process: serine proteases, cysteine proteases, metal-loproteinases. Proteases, secreted by human trophectoderm, are not described. The mechanisms of the active movement during blastocyst hatching are investigated to a lesser degree. Only the involvement of the cytoskeleton of trophectoderm cells in the mechanism of blastocyst compression was shown, and the participation of desmosomes in the coordinated change in the form of trophectoderm cells during compression is suggested. This review summarizes literature data on the possible mechanisms of zone hatching in the development of human embryos, obtained in experiments in vitro, as well as in animal models.