P-154 Structural changes of the human sperm tail during the early post-fertilization
Authors: Kai Y , Iseki H , Matsui T , Yamashita N 
Published: July 2024

Abstract

Study question
What happens to the sperm tails that are no longer needed after the goal of fertilization has been achieved?

Summary answer
The sperm tail lost its fibrous sheath and changed to a coiled morphology with a whisker root-like structure at the tip during the early post-fertilization.

What is known already
The sperm tail has been thought to play only a role in allowing the head and neck to reach the oocyte. The flagellum is found in the mammalian sperm tail and consists of a middle, main, and terminal part. It also contains an axon composed of two central microtubules surrounded by nine peripheral microtubule doublets (9d + 2s microtubule pattern) surrounded by an accessory structure called the fibrous sheath (FS), but few studies have been conducted on the post-fertilization fate of the mammalian sperm tail.

Study design, size, duration
To investigate sperm tail structure from the pronuclear stage to the second cleavage, we used abnormally fertilized oocytes derived from either conventional in vitro fertilization (cIVF, n = 41) or intracytoplasmic sperm injection (ICSI, n = 11). In this study, abnormally fertilized oocytes were collected between March 2022 and November 2023 and subjected to immunofluorescence analysis and field emission scanning electron microscopy (FE-SEM) analysis.

Participants/materials, setting, methods
Under ethical approval, abnormally fertilized oocytes were donated by infertile couples undergoing cIVF or ICSI cycles at the Yamashita Shonan Yume Clinic in Japan. Informed consent was obtained from all participants. Immunofluorescence assay was performed to analyze sperm tail morphology using antibodies against H3K27me3, α-tubulin, and AKAP4. FE-SEM analysis was performed to detect the 9d + 2s structure characteristic of the sperm tail axon.

Main results and the role of chance
Tri-pronuclear zygotes derived from conventional in vitro fertilization (cIVF-derived 3PN) had two male pronuclei, whereas those derived from intracytoplasmic sperm injection (ICSI-derived 3PN) had one. Each zygote had the same number of coiled microtubule structures as the male pronucleus, and the 9d + 2s structure characteristic of the axon was identified, suggesting a sperm tail. No difference in sperm tail shape was observed between ICSI-derived 3PN and cIVF-derived 3PN. AKAP4, which is specifically localized in the FS associated with the principal piece of the flagellum, was observed in sperm tails attached to the zona pellucida but not in those in the oocyte, suggesting that the FS of the flagellum is lost before the pronuclear stage, leaving the microtubules of the axonemes exposed. The sperm tail was attached to one of the spindle poles during the first cleavage of abnormally fertilized oocytes, but no differences in the structure of the spindle pole with or without the sperm tail were observed. The tip of the sperm tail was branched like a “whisker root”, suggesting that the sperm tail was degraded from the end, but the degree of tip branching did not differ from the pronuclear stage to the second cleavage.

Limitations, reasons for caution
Normal fertilized oocytes were not used in this study because they are used for clinical treatment. The cell biological significance of sperm tail branching is currently unknown and requires further experimentation.

Wider implications of the findings
These findings shed light on the processes that occur during human fertilization, suggest new roles for the sperm tail, and may have implications for assisted fertilization approaches in the future.

Trial registration number
not applicable