为了成功抵达卵子，精子要先学会如何“换挡”？｜科学60秒

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2024-01-07 14:01

精子的“饥饿游戏”

获胜的是……@Gerd Altmann from Pixabay

逆流游泳本身就不是一件容易的事，如果再加上身处的液体像玉米糖浆一样黏稠，简直是雪上加霜。然而，这差不多就是哺乳动物精子在争夺卵子时所面临的处境。

澳大利亚莫纳什大学（Monash University）的机械工程师礼萨·努斯拉蒂（Reza Nosrati）表示：“对于精子来说，这是一场苦征恶战，就像一次非常艰难的马拉松。”但精子不会因逆流而退却，也不会因黏性而逃避。

努斯拉蒂团队发现，必须通过女性生殖道的生理“大回旋”这一点，实际上有助于精子以最佳效率游动，并可能引导它们到达最终的目的地。相关结果发表在《细胞报告-物理科学》（Cell Reports Physical Science）杂志上。

女性生殖道是一个非常复杂的环境，当精子向上游游动时，它们会穿过密度和流速各不相同的分泌物。第一个障碍来自子宫颈，由于子宫颈细胞会分泌黏液，子宫颈入口处实际上“波涛汹涌”，这种黏液流动有助于清除有害的致病菌，而这些细菌并不擅长游泳。

努斯拉蒂说：“它就像一道屏障和过滤装置，让环境变得有利于受精。”

精子一旦到达输卵管，也就是通常而言受精的地方，它们面临的不仅是更黏稠的液体，还有一条道走到黑的曲折之路。但是，这些变化可能不会阻碍精子的前进，而是充当提示或线索，为最聪明的精子提供助力。

努斯拉蒂：“这是否是一种机制，可以使更优质、更聪明的精子调整自己的游泳行为，从而获得相对于生殖道中数百万其他精子细胞的竞争优势？”

为了找到答案，努斯拉蒂团队对公牛精子进行了测试。之所以使用公牛精子，是因为它的形态、形状和行为与人类精子非常相似。为了避免观察大量精子的活动，他们在单细胞水平上研究公牛精子的行为——在改变黏度和流体速度时观察同一个细胞。同时，他们把重点放在鞭毛上，因为鞭毛是推动精子前进的主要结构。

他们建造了一个“精子竞技场”，或者用工程师的话说，一个微流体通道。

莫纳什大学机械工程研究员法林·亚兹丹帕拉斯特（Farin Yazdan Parast）说：“针对不同的黏度，每个微流体通道有三个不同的入口，因此我们可以将精子暴露于不同的黏度条件下，此外我们还设置了不同的流速来模拟女性生殖道中的液体状况。”

为了确保精子不会游出她的观察视野，亚兹丹帕拉斯特将精子头部拴在“竞技场”的地板上，这样一来……[查看全文]

These Researchers Put Sperm Through a Kind of 'Hunger Games'

Karen Hopkin: This is Scientific American's Science, Quickly. I'm Karen Hopkin.

Swimming against a current can be tough. But imagine having to do it in a fluid with the consistency of corn syrup. That's more or less the challenge faced by mammalian sperm as they race to reach an egg.

Reza Nosrati: That's such a hard race for sperm. It's like a very tough marathon.

Hopkin: Reza Nosrati is a mechanical engineer at Monash University in Melbourne, Australia. He says that sperm are not deterred by tide or averted by viscosity.

In fact, Reza and his team have found that having to navigate the physiological slalom of the female reproductive tract actually helps sperm swim with optimal efficiency and may guide them to their desired destination. The results appear in the journal Cell Reports Physical Science.

Nosrati: When you look at the female reproductive tract, I think it's been known for such a long time that it's a very complicated environment.

Hopkin: As sperm swim upstream, they make their way through secretions that vary in thickness and flow rate. One of the first hurdles comes at the cervix, the gateway to the uterus.

Nosrati: Right at the port of entry to the cervix, there is a very strong flow because of mucous secretions by the cells within the cervix.

Hopkin: That current helps to flush out any harmful, disease-causing bacteria, which are not known to be strong swimmers.

Nosrati: It's kind of like a barrier and a filter to keep the environment essentially favorable for fertilization.

Hopkin: And once sperm get to the fallopian tube, where fertilization typically takes place, they face not only fluids that are gooier but a course whose undulating curves lead to blind alleys...

Nosrati: ...and to the increasing level of geometrical confinement that the sperm need to leverage to find their way forward.

Hopkin: But rather than hampering the swimmers' progress, maybe these changes serve as cues—or clues—that give the cleverest cells a leg (or in this case a flagellum) up.

Nosrati: Would that be a mechanism that enables sperm of better quality, or smarter cells, to adjust their swimming behavior in order to gain some competitive advantage over millions of other sperm cells that are swimming in the reproductive tract?

Hopkin: To find out, Reza and the gang put bull sperm to the test.

Nosrati: We use bull sperm because it's—in terms of its morphology and shape and beating behavior is quite similar to human sperm.

Hopkin: But they didn't want to look at sperm by the handful.

Nosrati: We wanted to study these behaviors at the single-cell level—looking at the same cell while we're replacing the flow, while we're changing the viscosity.

Hopkin: And they wanted to focus on the flagellum because it's that whiplike tail that propels a sperm forward.

Nosrati: Now, how that flagellum beating and how the movement of that structure changes in response to changing flow conditions was completely ignored.

Hopkin: So the researchers built a sperm-sized testing arena. Or in engineer speak...

Farin Yazdan Parast: We designed a microfluidic channel.

Hopkin: Farin Yazdan Parast is a research fellow in mechanical engineering at Monash University. She says each microfluidic channel...

Yazdan Parast: ...had three different inlets for different viscosities so we can expose sperm to different viscosities, and also we have different flow rates to also mimic the fluid condition in the female reproductive tract.

Hopkin: To make sure the sperm didn't swim out of her microscopic field of view, Farin tethered their heads to the chamber floor, which...[full transcript]

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论文信息

Yazdan Parast, F., Gaikwad, A. S., Prabhakar, R., O’Bryan, M. K. and Nosrati, R. (2023) The cooperative impact of flow and viscosity on sperm flagellar energetics in biomimetic environments. Cell Reports Physical Science, 4(11), 101646

DOI: 10.1016/j.xcrp.2023.101646

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