Three-dimensional constructed and semi-rotated movie of the spindle at the first cleavage of cIVF-derived 4PN. This video (http://movie-usa.glencoesoftware.com/video/10.1530/REP-23-0290/video-3) is available from the online version of the article at https://doi.org/10.1530/REP-23-0290.

The tip of the sperm tail branched like a ‘whisker root’ from the pronuclear stage to the second mitosis stage, suggesting that the conformational changes associated with the flagellum degradation occur from the terminal end. However, we did not observe differences in the degree of conformational change for the developmental stages analyzed (Fig. 1J and Supplementary Table 1). Although the function of the branching structure at the tip of the sperm tail is not known, a study was conducted early in the adoption of ICSI as an assisted reproductive technique in which the middle or tail of the sperm was cut off at the tip and injected into the oocyte (Chen et al. 1996). The results showed that both methods produced comparable clinical pregnancy outcomes. This suggests that the fibrous root-like structure observed at the sperm tip in this study is not essential for fertilization itself.