Arrest of cell division is crucial for cells to enter a program of terminal differentiation. In the developing organ or a differentiating tissue, growth arrest defines roughly the size of the cellular population that is further committed to become a domain of differentiated cells. Eventually, the balance between the number of cell divisions and the extent of subsequent programmed cell death determines the final size of a domain, a tissue, or an organ (for review, see Bryant and Simpson 1984; Raff 1992, 1996). Mitogenesis, cytostasis, and survival of neuronal cells can be induced and maintained by the same or by different growth or trophic factors. The signaling pathways that coordinate proliferation, growth arrest, and survival of cells and groups of cells in developing organisms are not known, but they probably involve as yet undetermined inter- and intra-cellular second messenger molecules.