Innovation in microscopes lets UCLA lab make new findings about sperm movement
On the left, the pattern of movement for a sperm with an X chromosome, which will create a female fetus. On the right, a slightly different pattern of movement for a sperm with a Y chromosome, which will create a male fetus. (Courtesy of Mustafa Ugur Daloglu et. al./Nature Publishing Group)
By Joseph Ong
Nov. 13, 2018 12:37 a.m.
UCLA researchers determined sperm carrying a Y chromosome swim differently than sperm carrying an X chromosome.
In a study published October, Aydogan Ozcan, professor of electrical and computer engineering, and his lab constructed a microscope that allows for the 3D tracking of small moving cells like sperm cells. Using the microscope, they found that bull sperm carrying a Y chromosome swam in a helical pattern more frequently than sperm carrying an X chromosome.
In mammals, a sperm cell carrying a Y chromosome will develop into a male upon fertilization, while a sperm cell carrying an X chromosome will develop into a female.
About 8 percent of observed X-chromosome-carrying sperm traveled in a helical pattern, significantly less than the approximately 21 percent observed for Y-chromosome-carrying sperm. Of the roughly 500 sperm cells observed, this was the only swimming pattern where X- and Y-carrying sperm cells swam differently. Both X- and Y-carrying sperm swam in ribbon or slither patterns at roughly the same rates.
The average speeds of X- and Y-carrying sperm were the same, said Mustafa Daloglu, the lead author on the study and a graduate student in the Ozcan lab. This diverged from previous theories that X-carrying sperm swim slower because X chromosomes are heavier than Y chromosomes, he added.
“There was a misconception that a heavier (X) chromosome would make them swim slower. That turned out to not be the case,” Daloglu said.
Both Ozcan and Daloglu said they did not know what caused the difference in the swimming pattern of X- and Y-carrying sperm.
The microscope uses two green LED lights to illuminate a small sample chamber containing the sperm, Daloglu said. An image sensor, like the one used in phone cameras, then captured the movement of the sperm cells as they swam, Ozcan said.
Traditional microscopes can easily track an object in two dimensions but lack the ability to easily track an object’s depth, according to the study. Other imaging systems studying sperm movement tended to confine sperm into a small area for easier imaging. This limitation may have restricted the movement of the sperm, Ozcan said. The microscope used in this study had a large sample volume, so the sperm could swim freely, he added.
The microscope took 300 images over one second to track the position of the head and tail of the sperm, said Bryan Chong, a third-year bioengineering student. Using the information from both LED lights allowed the researchers to triangulate the position of the sperm and track it in 3D, he added.
Unlike traditional microscopes, the microscope used in the study did not use a lens to refocus light, said Ozcan, but captured it with the same technology as a cellphone camera. This means that the microscope is fairly inexpensive and less bulky than traditional light microscopes, he added.
Ozcan said that he has founded a company to use mobile devices like this one to assess the quality of sperm for both animal breeding and human health. The device can be used to identify sperm that have difficulty moving, which may mean they are unhealthy and contribute to infertility according to the National Institutes of Health.
“(The microscope) has huge implications for field testing of animal sperm (and) for animal breeding applications,” Ozcan said.
He added that he is working toward developing a test to assess the fertility of human sperm as well. He said that since the microscope is relatively inexpensive, he hopes the microscope can be used in fertility clinics or even for home use.