Head rotation in human spermatozoa is essential for different swimming modes and fertilisation, as it links the molecular workings of the flagellar beat with sperm motion in three-dimensional (3D) space over time. Determining the direction of head rotation has been hindered by the symmetry and translucent nature of the sperm head, and by the fast 3D motion driven by the helical flagellar beat. Analysis has been mostly restricted to two-dimensional (2D) single focal plane image analysis, which enables tracking of head centre position but not tracking of head rotation. Despite the conserved helical beating of the human sperm flagellum, human sperm head rotation has been reported to be uni- or bi-directional, and even to intermittently change direction in a given cell. Here, we directly measure the head rotation of freely swimming human sperm using multi-plane 4D (3D+t) microscopy and show that: (1) 2D microscopy is unable to distinguish head rotation direction in human spermatozoa; (2) head rotation direction in non-capacitating and capacitating solutions, for both aqueous and viscous media, is counterclockwise (CCW), as seen from head to tail, in all rotating spermatozoa, regardless of the experimental conditions; and (3) head rotation is suppressed in 36% of spermatozoa swimming in non-capacitating viscous medium, although CCW rotation is recovered after incubation in capacitating conditions within the same viscous medium, possibly unveiling an unexplored aspect of the essential need of capacitation for fertilisation. Our observations show that the CCW head rotation in human sperm is conserved. It constitutes a robust and persistent helical driving mechanism that influences sperm navigation in 3D space over time, and thus is of critical importance in cell motility, propulsion of flagellated microorganisms, sperm motility assessments, human reproduction research, and self-organisation of flagellar beating patterns and swimming in 3D space.

中文翻译：

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人类精子在四个维度自由游动时以保守方向旋转。

人类精子的头部旋转对于不同的游泳模式和受精至关重要，因为它将鞭毛节拍的分子运作与精子在三维 (3D) 空间中随时间的运动联系起来。精子头部的对称性和半透明性质以及螺旋鞭毛驱动的快速 3D 运动阻碍了头部旋转方向的确定。分析主要局限于二维 (2D) 单焦平面图像分析，它可以跟踪头部中心位置，但不能跟踪头部旋转。尽管人类精子鞭毛具有保守的螺旋跳动，但据报道人类精子头的旋转是单向或双向的，甚至在给定细胞中间歇性地改变方向。在这里，我们使用多平面4D（3D+t）显微镜直接测量自由游动的人类精子的头部旋转，结果表明：（1）2D显微镜无法区分人类精子的头部旋转方向；(2) 在非获能和获能溶液中，无论是水性介质还是粘性介质，从头到尾看，在所有旋转的精子中，无论实验条件如何，头部旋转方向都是逆时针(CCW)；(3) 在非获能粘性介质中游动的精子中，36% 的头部旋转受到抑制，尽管在相同粘性介质中的获能条件下孵育后逆时针旋转恢复，这可能揭示了受精获能基本需求的一个尚未探索的方面。我们的观察表明，人类精子的 CCW 头部旋转是保守的。它构成了一种强大而持久的螺旋驱动机制，随着时间的推移影响精子在 3D 空间中的导航，因此对于细胞运动、有鞭毛微生物的推进、精子运动评估、人类生殖研究和鞭毛跳动模式的自组织至关重要并在 3D 空间中游泳。