Surface morphology of the zona pellucida surrounding human blastocysts obtained after in vitro fertilization

Human zona pellucida (ZP) is maintained up to the blastocyst stage prior to hatching. In in vitro fertilized (IVF) embryos, it eventually acts as a morphodynamic interface between the cultured embryo and its microenvironment. Ultrastructural data on the ZP of IVF blastocysts are scarce in humans. We employed correlated phase contrast microscopy (PCM) and scanning electron microscopy (SEM) to study retrospectively the ultrastructural morphology of the ZP outer surface of 20 IVF human blastocysts from 16 Japanese patients (28-44 years of age, average 36.7+/-4.2) with a history of infertility. Blastocysts were derived from conventional in vitro fertilization (cIVF) (n = 10) and from intracytoplasmic sperm injection (ICSI) (n = 10). Both cIVF and ICSI groups included "clear blastocysts" (n = 5) and "dark blastocysts" (n = 5). By PCM, the clear blastocysts exhibited a regular, round-shaped contour and consisted of clear and voluminous cells. By SEM, they displayed a spongy ZP with numerous fenestrations formed by networked filaments. By PCM, dark blastocysts appeared irregularly shaped and often collapsed, and comprised dark cells and debris. By SEM, their ZP were smooth with remnants of compact fenestrations. In conclusion, viable blastocysts presented a normal ZP outer surface ultrastructure, whereas unhealthy blastocysts showed an altered ZP outer surface, comparable to that of immature/atretic oocytes. Such alterations could reflect sub-optimal culture conditions and/or could be related to blastocyst degenerative processes. The blastocyst ZP surface ultrastructure was unaffected by the fertilization technique (cIVF or ICSI). These data suggest that blastocyst survival in vitro is related to ZP ultrastructure maintenance.