Nerve growth factor, for example causes peripheral neuroblasts to undergo one or two rounds of replication, and then to become postmitotic and differentiate into neurons. The resulting neurons, in some cases, may depend on the presence of NGF for their survival. Wounding the monolayer allows them to re-enter the cell cycle. Tumor cells, by contrast, will grow to a multilayer.
Phenomenon turns out to be due to local depletion of growth factors.
High levels of serum, or flowing medium, permits fibroblasts to grow beyond a monolayer. Some trace compound is missing from tissue culture medium, and is eventually depleted. Down regulation of growth factor receptors defines a cell lifetime that is culture specific.