Understanding fertilization through intracytoplasmic sperm injection (ICSI)

Abstract

Since the establishment of in vitro fertilization, it became evident that almost half of the couples failed to achieve fertilization and this phenomenon was attributed to a male gamete dysfunction. The adoption of assisted fertilization techniques particularly ICSI has been able to alleviate male factor infertility by granting the consistent ability of a viable spermatozoon to activate an oocyte. Single sperm injection, by pinpointing the beginning of fertilization, has been an invaluable tool in clarifying the different aspects of early fertilization and syngamy. However, even with ICSI some couples fail to fertilize due to ooplasmic dysmaturity in relation to the achieved nuclear maturation marked by the extrusion of the first polar body. More uncommon are cases where the spermatozoa partially or completely lack the specific oocyte activating factor. In this work, we review the most relevant aspects of fertilization and its failure through assisted reproductive technologies. Attempts at diagnosing and treating clinical fertilization failure are described.

Keywords: Assisted oocyte activation; Calcium influx; Failed fertilization; ICSI; Oocyte activation; PLCζ; Sperm cytosolic factor.

Figure 1

Spermatozoa injected into oocytes either as a head only, separated head and tail, and tail only. Regardless of the sperm component injection, fertilization is achieved, however, extremely low in the group that was inseminated with a distinctly separated head and tail.

Figure 2

ICSI cycles including study group (blue) and paired control (red) as well as the gold standard (yellow). The time interval from hCG administration to oocyte pick up (OPU) was shorter than the gold standard (P < 0.001) (a). The time interval from hCG administration to decoronization (DEC) was also shorter in the study group in comparison to their own control (P < 0.01) and standard (P < 0.001) (b). Finally, a similar pattern in observed in the hCG administration to ICSI injection between the study being the shortest in comparison to their own control (P < 0.01) and standard (P < 0.001) (c).

Figure 3

Time interval between hCG administration and cumulus removal. An example of patients (n = 20) plotted according to the length of time from hCG administration to removal of the cumulus (DEC, decoronization). The patients in the study group (blue) had the shortest time interval (hCG → DEC) in comparison to their own cycle with fertilization (red) and the gold standard (yellow).

Figure 4

Transmission electron microscopy (TEM) of spermatozoa was performed to diagnose globozoospermia in a man.

Figure 5

Phospholipase C zeta (PLCζ) staining of spermatozoa where the central cell within the dotted box is positive (green fluorescence). The solid box indicated a sperm nucleus with PLCζ localized in the equatorial segment. DAPI was used to counterstain the nuclei.

Figure 6

Metaphase II oocytes treated with caged IP3 (a). The uncaging of IP3 through UV exposure enhanced calcium in-flux in one oocyte (red/pink) (b). The side graph depicts calcium oscillation observed at different time points following target release IP3 (c).