Mechanics of human blastocyst hatching in vitro.

Critical examination of 30 blastocysts by transmission electron microscopy at various stages of blastulation and hatching, has revealed the presence of specialized, plump, trophoblastic cells at the points of hatching, which seem to aid in initial breaking of the zona pellucida (ZP) and then widen its opening to permit the progressive emergence of the embryo in amoeboid fashion, when it acquires a characteristic dumb-bell shape. These cells are named 'zona-breaker' cells and their characteristics are described. Normally, trophoblast cells in expanding blastocysts are flattened (squamous), forming a continuous robust epithelium with specialized cell junctions. Bundles of tonofilaments anchor onto desmosomes, forming a terminal web. Proper expansion of blastocysts by intake of fluid into the blastocoele causes an increase in internal hydrostatic pressure that stretches the trophoblast epithelium leading to an enlargement of its volume two- to three-fold, consequently thinning the zona prior to hatching. This is an important prerequisite to normal hatching. The blastocysts usually hatch out at the pole opposite the inner cell mass (ICM), though a few hatch out at the embryonal pole or elsewhere. In all cases zona-breakers seem to play a vital role in the hatching process.