The prevailing explanation for precise developmental regulation of neuron numbers is that neurons and postsynaptic targets are size matched by a process involving competition and neuronal cell death. Xeonpus laevis of different ploidy have the same body size, but differ in neuron size and number. These differences provide a way to test whether cell death matches neuron numbers to peripheral size. An alternate possibility, that total neuronal volume rather than neuron number is matched to peripheral size, will also be tested. Lumbar motor and sensory neuron population sizes (cell numbers and neuronal nuclear size) will be measured and compared with limb size (surface area, volume, and muscle fiber number). The measurements will be made for normally developing diploids, triploids and tetraploids at stages before, during, and after cell death, and show whether there is any correlation between either neuron number or total neuron volume and some measure of peripheral size. Additionally, hindlimb buds at an early developmental stage will be transplanted between donors and hosts of different ploidy. Measuring neuron populations that develop in the hosts will show whether limbs have intrinsic qualities that regulate cell death to produce neuron populations of appropriate numbers. This information is relevant to understanding how similarities and differences in neuron population size result from normal or abnormal development.
