Think of a developing brain the way you might think of constructing a house. First you need the physical structure—the foundation, floors, support beams, a roof—and then you add plumbing, ductwork, and wiring.
The cell division that happens in a developing fetus’s brain, particularly in the first trimester of pregnancy, is the structural stuff.
But brain development isn’t done after the first trimester, or even when a baby’s born, says Forrest Gulden, a scientist in Yale University’s department of neuroscience. “A lot of what’s happening throughout infancy, childhood, and adolescence is basically building the wiring.”
For researchers focused on how the Zika virus affects the human brain, one of the central questions has to do with timing: When is Zika most dangerous, and why?
Zika is unlike some viruses linked to birth defects—like rubella—which become far less risky to a fetus after the first trimester. Though especially dangerous early in pregnancy, Zika can continue to cause bad outcomes for fetuses well into the third trimester of pregnancy.
Since the brain is still developing even when a baby is born, how dangerous is a Zika infection to a newborn, or to a toddler?
This isn’t an easy question to answer because researchers are only just beginning to understand the mechanics of the virus. In very early brain development, for instance, Zika can cause the birth defect microcephaly—in which babies are born with abnormally small heads—because the virus diverts a key protein necessary for neural cell division, a crucial part of building the structure of the brain. The diversion happens as part of an immune response, as the body tries to fight off infection, but the result is that neural progenitor cells, key building blocks of the brain, don’t divide to form the brain’s structure as they otherwise would. That’s according to research by a team of researchers at Yale, who published their findings in the journal Cell Reports last month.