To what degree do different neuronal groups in the brain depend on one another during development? One approach to this question is to remove discrete neuronal populations at the time of their formation and then ascertain the effects on remaining systems. Drugs that disrupt mitosis, such as methylazoxymethanol acetate (MAM) and cytosine arabinoside, destroy dividing cells and can be used to ablate select neuronal populations depending on the timing of treatment. Other chemicals, irradiation and virus infections have also been used to study brain development in this way. Experiments in which portions of cerebral cortex or cerebellum were ablated with cytotoxins suggest that the quantitative aspects of axonal arbors of aminergic projections are programmed early in development in a manner that is relatively independent of neurons in the terminal field.
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