The mammals' main claim to fame--besides hair and nipples--is its bulging forebrain, or cerebral cortex, which controls aspects of thought and emotion. Now neurobiologists report in today's issue of the Proceedings of the National Academy of Sciences that they have discovered a process in the developing mouse embryo that experts suggest forges a close ancestral link between bird and mammal brains.
As an embryo blossoms into a complex organism, its simple, uniform cells change into specialized cell lines, such as blood or liver cells. This role-casting--called lineage restriction--has been best documented in transparent flatworms, in which the fate of each stem-cell line can be directly followed. But the process is less clear in mammals. Now, neurobiologist Chia-Yi Kuan and his colleagues at Yale University have traced lineage restriction in some cells in developing mouse brains. The researchers extracted from mouse embryos early versatile cells, called embryonic stem cells, and modified them by inserting a bacterial gene called lacZ. The gene expresses an enzyme, not normally found in mice, that can be stained blue. They injected the genetically tagged stem cells back into mouse embryos. After the mice were 5 months old, their brains were removed and stained to find the descendants of the original stem cells.
To their surprise, the researchers found that cells deep in the cortex had turned blue. Earlier work had suggested a different pattern of development: a radial pattern of genetically marked cells in the outermost parts of the cortex. So the Yale team's stained stem-cell descendants appeared to be programmed to develop mainly as deep cortex cells. The finding, says Yale team member Pasko Rakic, suggests that the two cell groups are different, so that at least two types of founder cells help build the cortex, with their fates sealed early in development. This study, Rakic says, lifts "a veil of secrecy from cortex evolution."
The findings also imply that mammal and bird brains may be fashioned in a similar way, says Harvey Karten, a developmental neurobiologist at the University of California, San Diego. Research by Karten and others has found that groups of neurons in bird brains arise from certain stem cells earmarked early in development. Although such lineage restriction hasn't been shown conclusively in birds yet, Karten speculates it is the simplest explanation for the organization of groups of neurons in the bird brain with biochemistry and neural connections that resemble portions of the mammalian cerebral cortex. According to Karten, the Yale team's work therefore implies that the primitive brain of an unknown ancestor common to mammals and birds must have developed in a similar way.