Building a tough flagellum in human sperm (Introduction)

by David Turell , Monday, March 25, 2019, 00:33 (1868 days ago) @ David Turell

This article describes what a human sperm must overcome to fertilize an egg:

https://cosmosmagazine.com/mathematics/mathematicians-solve-the-mystery-of-human-sperm-...

" And now, our new research has discovered what gives human sperm the strength to succeed in the race to fertilise the egg – and it’s all to do with their tails.

"In the reproductive tract, sperm encounter a complex chemical and physical landscape. A sperm must escape the deadly acids of the vagina, penetrate the thick cervical mucus barrier – this is around a hundred times thicker than water – not get trapped in a cervical crypt that leads to nowhere, endure uterine contractions, to eventually find the tiny opening that leads to the oviducts – the tube through which an egg passes from an ovary. This is all while being attacked in the uterus by white blood cells.

"This all takes place before a sperm can penetrate another thick protective layer that surrounds the egg. All this may sound excessively difficult for the tiny sperm, but these barriers are there for protection – the uterus is designed to receive a baby, and the sperm could be an invading force carrying diseases.

"Our research has discovered that it is a reinforced outer layer which coats the tails of human sperm that gives them the strength to make the powerful rhythmic strokes needed to break through this jelly-like cervical mucus blockage.

"Sperm tails – or flagella – measure just the breadth of a hair in length and are incredibly complex. So to find out more about how they work, we used a virtual sperm model to compare the tails of sperm from humans and other mammals (which fertilise inside the body) with sperm from sea urchins (which fertilise in open water outside the body).

"We found that although the tails of sea urchin and human sperm share the same bendy inner core, it seems that the tails of sperm in mammals may have evolved a reinforcing outer layer. This outer layer, or “cape”, gives them the exact amount of extra strength and stability needed to overcome the thick fluid barrier they come up against in internal fertilisation.

***

"Sperm tails – or flagella – measure just the breadth of a hair in length and are incredibly complex. So to find out more about how they work, we used a virtual sperm model to compare the tails of sperm from humans and other mammals (which fertilise inside the body) with sperm from sea urchins (which fertilise in open water outside the body).

"We found that although the tails of sea urchin and human sperm share the same bendy inner core, it seems that the tails of sperm in mammals may have evolved a reinforcing outer layer. This outer layer, or “cape”, gives them the exact amount of extra strength and stability needed to overcome the thick fluid barrier they come up against in internal fertilisation.

***

"Of course, we don’t know which adaptation came first, the stronger sperm or the cervical mucus – or whether they co-evolved."

Comment: The authors comment just above is the key issue. Co-evolving by chance is impossible, but appearing together by design is a requirement. The point that the uterus has to have those protection is obvious.