Assisted fertilization and embryonic axis formation in higher primates

Abstract

In naturally fertilized embryos of various organisms, the spermatozoon provides a localized cue to initiate early embryonic patterning. In mice, the sperm entry point (SEP) may reorient the first cleavage division, which separates the zygote into two halves that follow distinct fates. However, it is unknown whether the mechanical injection of spermatozoa into an oocyte by intracytoplasmic sperm injection (ICSI), a technique commonly used in human assisted reproduction, possesses such a role. Rhesus macaque embryos fertilized by ICSI were examined in order to determine the consequences of placing the spermatozoon at specific positions in the ooplasm and whether this can provide new information about patterning in mammalian eggs. The SEP specified by the injected spermatozoa was most often localized near the first cleavage plane and was mainly distributed along the boundary zone that separates the embryonic and abembryonic parts of the monkey blastocyst. Moreover, the ICSI data, when compared with naturally fertilized mouse embryos, showed a similar outcome in terms of cleavage axes and first embryonic axis specification. As there are no studies to date regarding sperm entry in human oocytes and its influence on embryonic development, this investigation using the rhesus macaque as a clinical model is noteworthy.

Effect of sperm entry on blastocyst development after in vitro fertilization and intracytoplasmic sperm injection — mouse model

“…1). These results are consistent with previous studies in ICSI-derived monkey embryos [38] and mouse embryos fertilized in vivo [19,36,39]. Most importantly, the presented data suggest that the relationship established between an egg and a sperm in early embryo development is not altered by the method of spermatozoon introduction.…”

“…Since the position of the incoming sperm has been linked to the embryonic-abembryonic axis formation in the blastocyst [36][37][38][39], we investigated this relationship in eggs where sperm entered the cytoplasm in different ways. We used the SEP provided by NM, IVF, or ICSI as a point of reference during preimplantation embryo development.…”

Fertilization and Cleavage Axes Differ In Primates Conceived By Conventional (IVF) Versus Intracytoplasmic Sperm Injection (ICSI)

“…Cleavage progression is mostly toward the animal pole (2 nd PB) in a meridional pattern. Cortical rotation translocates both the polar bodies and the MIP into the furrow region, consistent with a previous report describing the sperm injection site as localized near the first cleavage plane after cell division 30 .…”

“…Notwithstanding excellent investigations conducted on the best-studied mammalian model, the mouse, crucial information is scarce regarding early fertilization events influencing axis formation in non-rodent mammals 30,60–62 . Controversy surrounds the evidence indicating that developmental information encoded within oocytes and early zygotes determines axis development of the blastocyst; strengths and limitations of the various experimental designs have been discussed 17 .…”

“…Other TLVM human studies describe transient fertilization cone assembly (albeit at low frequency), pronuclei migration rates of 50 µm/hr, and varying juxtapositioned pronuclei within the zygote’s cytoplasm, but how these observations influence overall embryo quality is not yet known 41 . Ironically, in the rhesus monkey, an animal model closely aligned to human fertilization events 13 , fluorescent bead marking of ICSI sperm injection sites suggests that the sperm entry point (SEP) oriented the first cleavage plane and later aligned along the embryonic-abembryonic boundary in the blastocyst, as described for mice 30 .…”