Mechanics of human blastocyst hatching in vitro

Abstract

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.

Time-Lapse Videomicrographic Observations of Blastocyst Hatching in Cattle

“…These results suggested that the quality of frozen-thawed cattle embryos is comparable to that of fresh embryos and that there could be a relationship between the hatching pattern of blastocysts and the viability of embryos after transfer. [13][14][15]. The process of blastocyst hatching has been extensively studied in mice [16][17][18][19][20][21][22][23] and rats [23][24][25][26], and it was reported that the process of blastocyst hatching could be classified into 6 types in the mouse and 5 types in the rat, according to the site of protrusion of trophectoderm cells from the zona pellucida, the mode of slitting in the zona pellucida and the state of blastocyst contraction at the time of escape from the zona pellucida [23].…”

“…Since then, the process of blastocyst hatching has been morphologically observed not only in the rabbit [2,3], but also in the golden hamster [4][5][6], guinea pig [7][8][9], rhesus monkey [10], cat [11], pig [12] and human [13][14][15]. The process of blastocyst hatching has been extensively studied in mice [16][17][18][19][20][21][22][23] and rats [23][24][25][26], and it was reported that the process of blastocyst hatching could be classified into 6 types in the mouse and 5 types in the rat, according to the site of protrusion of trophectoderm cells from the zona pellucida, the mode of slitting in the zona pellucida and the state of blastocyst contraction at the time of escape from the zona pellucida [23].…”

Effect of the site of assisted hatching on vitrified-warmed blastocyst transfer cycles: a prospective randomized study

“…Firstly, the hatching process in vivo differs from that of in vitro. In addition to the increased internal pressure, "protease" with zonalysis activity from the trophectoderm and (or) the uterus [19][20][21], repeated collapses and expansions of the blastocyst [22] and sphincter-like zona-breaker cells [23] are involved in the hatching process. Data from hamster and mouse showed that successful hatching in vitro was dependent on a sufficiently high number of embryonic cells, which enables blastocyst expansion and zona shedding.…”

“…Some studies demonstrated that natural hatching sites of human blastocysts were usually close to the ICM [22,27], but others found the human embryos hatched from the abembryonic pole in most cases and rarely at the ICM pole through video observation and critical examination of 30 human blastocysts by transmission electron microscopy [23,28]. Study of blastocyst transfer in fresh cycles also demonstrated that only 38.9 % of all the hatching blastocysts showed a zona breach close to the ICM [29].…”

Time‐lapse observations to analyze the effects of assisted hatching

“…Since then, the process of blastocyst hatching has been extensively documented [5][6][7][8]. Massip et al [9] have reported repeated contractions and re-expansions during mouse blastocyst development, and these may have some influence on hatching.…”