While it is proposed that perivitelline threads play an important role in cross-linking the cumulus cells and oocyte during maturation, the mechanism underlying such a role remains unclear. Perivitelline threads (black arrows) were observed at the adhesive region between the membrane of ooplasm and the ZP, and cytoplasmic fragments were clearly formed from a cleavage furrow by perivitelline threads at the first cleavage. Perivitelline threads of normally-fertilized human oocytes at the first cleavageĪll the embryos (I, II, and III) had an adhesive region (white arrow) between the membrane of ooplasm and the zona pellucida (ZP). Additionally, based on time-lapse observations of ICSI embryos obtained from assisted reproductive technology treatments conducted in our clinic, perivitelline threads were confirmed to be involved in forming cytoplasmic fragments at the first cleavage (Fig. A study in 2017 observed perivitelline threads in more than 50% of cleavage-stage human embryos using time-lapse imaging, and the rate of cytoplasmic fragmentation (at the first cleavage) was significantly decreased in embryos without perivitelline threads. Meanwhile, studies have investigated the formation of cytoplasmic fragments at the early stage of development. In these cases, patients have a high risk of having no embryos transferred or cryopreserved at the end of treatment, and no solution has been found so far. Most of the embryos with severe fragmentation do not show signs of compaction and are arrested before the blastocyst stage. In our clinic, approximately 600 cycles of in vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI) are performed annually, and there are a number of cases that show severe cytoplasmic fragmentation at early embryonic stages, regardless of maternal age. In recent years, there has been an increasing number of infertile couples who hope for pregnancy and visit IVF clinics, mainly because of the trend towards late marriage in Japan.
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