Human sperm tail- spinners

[madman]

Abstract

Sperms have attracted the attention of many researchers since it was discovered by Antonie van Leeuwenhoek in 1677. Though a small cell, it's every part has a complex structure and a different function to play in carrying life further. The sperm tail is the most complicated structure with more than 1000 proteins involved in its functioning. With the advent of advanced three-dimensional microscopes, many studies are still undergoing to understand the exact mechanism of sperm tail movement. Most recent studies have shown that sperms move by spinning rather than swimming. Furthermore, each small subunit of the tail including axonemal and peri-axonemal structures plays essential roles in sperm motility, capacitation, hyperactivation, fertilization.

Methodology: Relevant literature (from 1982 till 2020) on sperm tail anatomy, movement, and functions were searched from various English language full length and review articles using PUBMED, SCOPUS, or Google database.

Conclusion: There is still a lot needed to be discovered about human sperm tail movement and its role in male fertility. Sperm tail has complex anatomy with surrounding axoneme having 9+2 microtubules (9 outer doublets and one central doublet) arrangement along its entire length and additional peri-axonemal structures that all contribute to sperm motility and fertilization. In the future, various sperm tail proteins and their subunits can be used as markers of male fertility.





Introduction

Human sperms were first discovered in the year 1677 by Antonie van Leeuwenhoek and were called animalcules or little animals [1]. To date, sperm is one of those cells which have attracted the attention of several researchers and many studies are still going on to know its structure, movement, and functions. It plays a crucial role in carrying life further by supplying the male partner’s genetic material and proteins to the female oocyte at the time of fertilization [2]. A mature human spermatozoon consists of two main parts: a head with a nucleus and acrosome, and a tail for movement.





Anatomy of Sperm Tail

How does a sperm move?


Functions of different structural components of Mammalian sperm tail

1. Axoneme
a. Microtubules:
b. Radial Spokes:
c. Dynein arms:

2. Peri-axonemal structures
a. Outer Dense Fibers (ODFs):
b. Fibrous Sheath (FS):
c. Mitochondrial sheath (MS):






Conclusion

Hence, the sperm tail is a unique part of sperm with very complex anatomy. The movement of the sperm tails has attracted many researchers over more than 200 years and to date studies are going on to know the exact physiology of its movement. Most recent studies have revealed that sperm don’t swim, instead, they show a spinning movement. Furthermore, each part of the tail has a complex structure and contributes to male fertility. Mutation in genes encoding for various proteins associated with sperm tail can affect sperm motility, its capacitation, hyperactivation, and fertilizing capabilities, all leading to male infertility. Hence, in the future, a detailed study of each and every component of the sperm tail including axoneme and periaxonemal structures can be used in understanding and managing various sperm pathologies. Some of them can even be used as biomarkers of sperm health.

 

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*Human sperm is known to produce more than 20 swimming strokes in less than a second [18].






*The most recent studies of 2020, suggest that the sperms don’t move by simple swimming motion rather they show spinning movement. This has been suggested by observing sperms using 3-dimensional (3D) microscopes. It was found that every sperm moves by rotating around its own axis, and also around a middle axis, similar to a spinning top. During this movement, it produces an asymmetric wave which results in a one-sided stroke and an out-of-plane pulsating wave that causes the rolling of sperm flagellum, which appears as the equal beating of sperm tail on all sides in 3D microscopy. This rolling movement of the sperm tail hides the asymmetric beats in 2D microscopy and gives the impression that the sperm tail moves symmetrically from side to side, thus making the asymmetric beats invisible to date. Hence, with advanced 3D microscopy, it was proved that sperm uses asymmetry to create symmetry [21]. The sperms roll in a corkscrew manner or simply drill through the fluid to move forward. This spinning movement of human sperm is a complex process, where the spinning of sperm head results in rotation of sperm tail in the direction of swimming, thereby causing a drilling movement in the fluid with the production of helical waveforms. Furthermore, the changes in the chirality of moving waves produced by sperm tail, known as “perversion” in mathematics are very much similar to the spiraling of tendrils in plants and are very essential for the spinning movement of the head of the sperm [18].

 

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Figure 1: Detailed microscopic anatomy of different parts of the sperm tail.

 

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