A human newborn's brain is uniquely impressionable, allowing social interactions and the environment to shape its development. But this malleability may come with a price, a new study finds. A comparison of juvenile chimpanzee and human brains suggests that differences in the development of myelin—the fatty sheath that surrounds nerve fibers—may contribute not only to our unusual adaptability, but also to our vulnerability to psychiatric diseases that start in early adulthood.
Research increasingly suggests that psychiatric illnesses like depression and schizophrenia may involve problems with the timing of neural signals, says Douglas Fields, a neuroscientist at the National Institutes of Health in Bethesda, Maryland, who was not involved in the study. The nerve fibers, or axons, that connect neurons are usually protected by myelin, which enhances the neural relay of information throughout the brain. "Myelin speeds transmission of information [by] at least 50 times," Fields says, "so it matters a great deal whether or not an axon becomes myelinated."
Humans start out with comparatively few myelinated axons as newborns. We experience a burst of myelin development during infancy that is followed by a long, slow growth of myelin that can last into our thirties, says Chet Sherwood, a neuroscientist at George Washington University in Washington, D.C., and a co-author of the new study. In contrast, other primates, such as macaques, start out with significantly more myelin at birth, but stop producing it by the time they reach sexual maturity. However, Sherwood says, "extraordinarily little data exists" on brain growth and the development of myelin in our closest genetic relatives, chimpanzees.
Such a study is not easy to conduct, however: A moratorium on chimp breeding has made young chimp brains hard to come by, Sherwood says. Any study of fetal or young chimps requires collecting the brains of animals that have died natural deaths. Despite these difficulties, lead author Daniel Miller, then a graduate student at George Washington University, and his colleagues obtained 20 brains from chimps that ranged in age from stillborns to 12-year-olds, largely from veterinary pathologists who were preserving the brains of chimpanzees for research.
The team treated the brain tissue with a stain that marks myelin and compared analogous parts of fetal, infant, and young chimpanzee brains to human brains at similar growth stages. The chimps had significantly more myelin than humans did, both in utero and at birth, they report online today in the Proceedings of the National Academy of Sciences. But rather than prolonging the development of myelin into mid-adulthood as humans do, chimps stop producing myelin when they hit sexual maturity at about 12 years old. The pattern in chimps is similar to that in macaques, suggesting that the pattern and rate of myelin growth in the human brain is unique, Sherwood says.
Fields agrees, noting that the new study "adds to the well-established and growing body of data showing that human brain development is more protracted than in other animals." That may allow more opportunity for the environment, rather than genes alone, to direct the brain's development, he says.
Opportunity could also be a source of risk. Many of the changes that occur in the human brain during adolescence—including disorders such as depression, bipolar disorder, and schizophrenia—may be associated with delayed myelination, Sherwood speculates. At the very least, he says, slow myelination in humans and the timing of the onset of these disorders is "an interesting coincidence."