Spatial organization and genetic information in brain development.

In the course of brain development neurons acquire qualitative and quantitative biochemical and morphological properties which depend on the position of the cells within the nervous system. In the dimensions tangential to multilayered cell sheets mechanisms contributing to spatial order include induction by adjacent tissue as well as internal generation of morphogenetic fields (presumably by reactions involving autocatalysis and lateral inhibition). In the dimensions across the sheet cells of different types are produced in one layer and sort into another layer, guided presumably by contact mediated cell interaction. Positional and directional cues encoded in the developing brain are essentially involved in axonal guidance and the formation of neuronal connections. In mammals and man, the number of neurons and their connections in the brain is much higher than the number of genes. This is possible because there are repetitive neuronal circuits in the brain, and there is topographic order of connections between different brain areas. For instance, few quantitatively graded markers would suffice for specifying the projection of one area of the nervous system onto another, generating spatial order for a large number of fibers while requiring only a limited amount of genetic information. Higher brain functions, such as learning and memory, may logically require only a neural network consisting of repetitive subunits. On the other hand, it is an evolutionary advantage for an organism to be endowed, from the outset, with a pattern of neural connections which is subtly and quantitatively tuned for efficiency in dealing with the environment, while remaining flexible for change and adaptation in the course of learning.(ABSTRACT TRUNCATED AT 250 WORDS)