| Abstract: |
Motility is a fundamental property of many cells. Sperm rely on a combination of rotational movements of their heads and flagellar beating for propulsion, enabling them to swim and fulfill their primary purpose of fertilizing the egg. The rotation of the human sperm head has been a subject of study for decades. However, optical illusions inherent to traditional microscopy and image processing have led to discrepancies in the observed direction of rotation. This lack of consensus has hampered our comprehension of its functional role in sperm swimming and fertilization.
The human sperm head is translucent and has a planar axial symmetrical morphology around its rotating axis, similar to a flattened ellipsoid, making it prone to optical illusions that can hinder accurate detection of the directionality of rotation. As such, the sperm head is subject to perception bistability, the spontaneous switching between two or more interpretations of an image under continuous viewing, which can further disguise the true direction of rotation in translucent objects. This optical illusion causes spinning translucent objects to appear to oscillate back and forth, or spin with a switchable and thus undefined direction.
One of the challenges in detecting the rotation of the human sperm head is that a microscope inherently acquires images in a single focal plane (2D). This cell, moving at high speeds, can generate optical illusions due to its flagellum beating up to 30 times per second, causing constant movement of the head out of the focal plane. Thus, acquiring the complete three-dimensional image scene at adequate periodic time intervals (3D+t, or 4D) is essential to resolve the sperm movements.
Our group has developed the necessary technology to answer this crucial question and challenge this paradigm. This technology is based on principles of bright-field microscopy, where the microscope objective is set to vibrate at frequencies up to 100 Hz, while a high-speed camera can capture 8000 images per second in a volume with a height of up to 20 microns (50 focal planes separated by 0.4 microns). In this captured volume, we can reconstruct the complete cell structure with high precision, allowing analysis of its detailed movements in time.
In a recent research (Corkidi et al, 2023), we have introduced an image analysis strategy for measuring the true direction of the spermatozoon head rotation which is based on the contrast inversion produced by the spherical aberration of microscope objectives depending on the focal plane observed. Tracking this effect as the head rotates enables the determination of its rotation direction. This certainty in the direction of the sperm head rotation is necessary to allow progress in our understanding of how the flagella moves in 4D and how the cell swims to achieve fertilization, providing a reference for future research in the field.
Corkidi et al, 2023. J Cell Sci 15 2023; doi: https://doi.org/10.1242/jcs.261306. |