Coupling glial numbers and axonal patterns.

The control of the rate of cell division enables cells to respond to signals from other cells and this promotes the emergence of order as cell mass increases during growth. Glial cell proliferation is coupled to axon guidance, and the sequential deployment of glial cells in constrained numbers enables the sequential sorting out of axons into appropriate trajectories through time.(1) This is achieved by the neuron-dependent regulation of glial division at the G(1) phase. Early on, Prospero plays a key role controlling the G(1) phase and it enables the glia to proliferate in response to neurons. Later, Prospero maintains subsets of glia in G(1) arrest, retaining mitotic potential, whereas non-Prospero glia terminally differentiate. Only this population of Prospero quiescent precursors can overproliferate when neurons are eliminated, inducing a repair response. It is compelling to investigate whether the vertebrate homologue Prox1 may enable the repair response of vertebrate glia.