APOPTOSIS REGULATES THE NUMBER OF SCHWANN-CELLS AT THE PREMYELINATINGSTAGE

At the premyelinating stage, the Schwann cells of peripheral nerves ar e able to recognize the axon, to arrange themselves along it in a nono verlapping manner, and finally to establish a one-to-one cell-axon rel ationship. The mechanism that regulates these processes is not known i n detail. We found the existence of a significant Schwann cell apoptos is in vivo of rat postnatal sciatic nerve, peaking around postnatal da y 3. More than 50% of the neonatal Schwann cells cultured in axon-free medium undergo a rapid apoptosis. The apoptosis can be suppressed by addition of survival factors such as Neu differentiation factors or by increasing the adhesion of Schwann cells to substratum, We suggest th at in neonatal nerves in vivo, Schwann cells are highly susceptible to apoptosis, but they are saved from death by contact with axons. The d ramatic increase in number of Schwann cells between postnatal day 0 an d 3 overcomes the number of axons available for them, Consequently the Schwann cells that fail to contact an axon undergo apoptosis, In conc lusion, the number of Schwann cells in the developing nerves is regula ted by the apoptosis and clearly depends on the survival signals from axons.