PATTERNS OF VERTEBRATE NEUROGENESIS AND THE PATHS OF VERTEBRATE EVOLUTION

Any substantial change in brain size requires a change in the number o f neurons and their supporting elements in the brain, which in turn re quires an alteration in either the rate, or the duration of neurogenes is. The schedule of neurogenesis is surprisingly stable in mammalian b rains, and increases in the duration of neurogenesis have predictable outcomes: late-generated structures become disproportionately large, T he olfactory bulb and associated limbic structures may deviate in some species from this general brain enlargement: in the rhesus monkey, re duction of limbic system size appears to be produced by an advance in the onset of terminal neurogenesis in limbic system structures. Not on ly neurogenesis but also many other features of neural maturation such as process extension and retraction, follow the same schedule with th e same predictability. Although the underlying order of event onset re mains the same for all of the mammals we have yet studied, changes in overall rate of neural maturation distinguish related subclasses, such as marsupial and placental mammals, and changes in duration of neurod evelopment distinguish species within subclasses. A substantial part o f the regularity of event sequence in neurogenesis can be related dire ctly to the two dimensions of the neuraxis ill a recently proposed pro someric segmentation of the forebrain [Rubenstein ct al., Science, 266 : 578, 1994]. Both the spatial and temporal organization of developmen t have been highly conserved in mammalian brain evolution, showing str ong constraint on the types of brain adaptations possible. The neural mechanisms for integrative behaviors may become localized to those loc ations that have enough plasticity in neuron number to support them.