'Knowing where a gene is expressed in the brain can provide powerful clues about what its role is', said Dr Ed Lein, of the Allen Institute for Brain Science. 'This means that we have a blueprint for human development: an understanding of the crucial pieces necessary for the brain to form in a normal, healthy way, and a powerful way to investigate what goes wrong in disease'.
To produce their map of gene expression, called the BrainSpan Atlas, the scientists used tissue from four fetal brains preserved between 15 and 21 weeks after conception. They sliced each into about 3,000 ultra-thin sections, isolated RNA and looked at the activity level of about 20,000 genes.
The researchers point to autism as a disorder with strong links to early brain development where the information they have unearthed may lead to greater understanding. They present some early investigations into the condition alongside other preliminary studies in the journal Nature.
Dr Lein says the team looked for 'a hub of genetic action' that could be linked to autism, and ultimately found it among 'newly generated excitatory neurons in the cortex, the area of the brain that is responsible for many of the cognitive features affected in autism such as social behaviour'.
The finding is also consistent with an imbalance between excitatory and inhibitory signalling suggested in previous autism studies.
Dr Thomas Insel, director of the National Institute of Mental Health in the USA, who was not involved in the study, said the BrainSpan Atlas 'is already transforming the way scientists approach human brain development and neurodevelopmental disorders like autism and schizophrenia. Although the many genes associated with autism and schizophrenia don't show a clear relationship to each other in the adult brain, the BrainSpan Atlas reveals how these diverse genes are connected in the prenatal brain'.