A fatty acid found in fish is critical to turning dysfunctional
round-headed sperm into strong swimmers with cone-shaped heads packed with
egg-opening proteins, a new study finds.
Docosahexnoic acid, also called DHA, is an important omega-3 fatty acid involved
in eye and brain development; recent studies in mice have implicated it in male
fertility as well. "DHA is high in the testes and brain, but until this it
was not well understood what it does in these tissues," study researcher
Manabu Nakamura, of the University of Illinois at Urbana-Champaign, told
LiveScience. "About three years ago we created the knockout mouse, which
isn't able to make its own DHA, and we learned that DHA is really essential for
sperm formation." In the new study, Nakamura and colleagues figured out
why DHA is so critical to healthy sperm.
Like other omega-3s, DHA is found in fish, especially cold-water oceanic
fish, and algae. For fish-fearing wannabe fathers, seafood and algae aren't our
only source of the nutrient; your body can also make DHA from other omega-3
acids.
Sterile swimmers
In their past work, Nakamura and colleagues studied mice without a
DHA-synthesizing enzyme, finding that if these mice also didn't get DHA in
their diet, the males were infertile. Fertility returned when their diet was
supplemented with the fatty acid.
Following these findings, in the new study the team looked at how sperm
develops in DHA-deficient mice. The researchers determined that DHA plays a
role in the formation of a structure called the acrosome on the head of the
sperm. The acrosome is a pointy caplike structure containing enzymes that break
through the egg's outer layers, enabling the sperm to fertilize it.
"The acrosome on top of this cone head is gigantic, it is a large
sack containing lots of enzymes," Nakamura said. "When the sperm
meets the egg, the acrosome bursts and it releases enzymes and helps the sperm
penetrate into the egg." The acrosome forms when many little bubbles of
membrane (called vesicles) fuse together inside the sperm-to-be. These bubbles
hold the enzymes the sperm needs to penetrate and fertilize the egg. When they
come together they all fuse into one long sheet of membrane in the front of the
sperm, forming a cap, the acrosome.
Without DHA, this membrane fusion doesn't happen. If the vesicles don't
fuse, the acrosome doesn't get made and sperm maturation halts. At this point,
the researchers only see sperm with round heads, not the cone-shaped heads of
healthy sperm.
DHA deficiency
Because humans and other mammals are able to make their own DHA from
other fatty acids, DHA deficiency isn't very common. But, if that
DHA-synthesizing enzyme is defective, it could lead to problems with
infertility. Low blood levels of DHA have been linked to decreased fertility in
the past; a DHA-rich diet could clear up these infertility problems.
"As long as this endogenous [within the body] system is working
fine, humans can synthesize enough DHA in their bodies if they have the
precursor," Nakamura said. "But some groups [of people] may have a
decreased ability to synthesize DHA. In this case the dietary supplements may
help." In the long term, the acrosome could be a target for a male birth
control pill, if the acrosome formation could be switched on and off, but the
researchers aren't studying that yet. They are, however, looking to other parts
of the body to see how the DHA deficiency affects brain and eye function. It
could act in very similar ways, by facilitating vesicle fusion, in other parts
of the body. The study is published in the October 2011 issue of the journal
Biology of Reproduction.
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