Breaking egg barrier: A sperm story

Sperm doesn’t shift into high gear in mammals just to show off, new research shows. It originally needed that extra speed to break the egg barrier.

Later on, evolution enabled sperm to use its souped-up swimming to navigate tricky reproductive pathways even before reaching the egg.

That is the finding of a new study led by Jean-Ju Chung, an assistant professor of cellular and molecular physiology at the Yale School of Medicine. The study appears in the journal Cells.

In her previous work, Chung has looked at the molecular structures and mobility of sperm in placental mammals such as mice and humans. Placental mammals are distinguished by the presence of a placenta that sustains the fetus during development.

Chung and her team have established that sperm tails are lined with channels that take in calcium to help them swim through the female reproductive tract. The navigation for this is controlled by a quartet of subunits in the channel complex located in each sperm’s tail.

Visual comparison of capacitated and uncapacitated sperm.
Researchers analyzed hyperactivated motility in the sperm of gray short-tailed opossum. The top image shows sperm cells before hyperactivated motility has been activated; in the bottom the sperm demonstrate the heightened swimming movement.

In the new study, first author Jae Yeon Hwang, a postdoctoral fellow in Chung’s lab, reached further back into the evolution of mammals by investigating sperm activity in gray short-tailed opossum, a marsupial species. Chung’s team found that in these animals, hyperactivated motility – a heightened swimming movement – is also triggered by a calcium-mediated mechanism, just as it is in placental mammals. However, this high-gear motility seems to be used mainly for egg penetration rather than navigation into the fertilization site.

Noting that marsupials and placental mammals diverged from a common ancestor 160 million years ago, and that marsupial physiology has a less complex female reproductive tract than other placental mammals, the researchers suggest hyperactivated motility in sperm developed first as a way to penetrate egg barriers, and only later was used for navigating the female reproductive tract.

“It is awe-inspiring to see how male and female systems have delicately adapted to each other in order to successfully reproduce,” Chung said.

Sperm motility is a key parameter in selecting sperm for in vitro fertilization procedures. Motile sperm has a lower frequency of DNA damage.

Günter Wagner, the Alison Richard Professor of Ecology and Evolutionary Biology at Yale, and Jamie Maziarz, a research associate in Wagner’s lab on Yale’s West Campus, are co-authors of the study.

The research was funded by the Yale School of Medicine and the National Institute of Child Health and Human Development.

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