Regulation of the proliferation of embryonic chick leptomeningeal cells in vitro by both contact inhibition and co‐culture with CNS neurons

Cultures of leptomeningeal cells (LMCs) isolated from the pia‐arachnoid membrane of 10‐day chick embryos were studied in order to elucidate the mechanisms which regulate the proliferation of these embryonic nonneuronal cells. Thymidine incorporation was quantified in LMC cultures of varying density and in co‐cultures containing both LMCs and neurons. The following results were obtained. First, proliferation of LMCs was very sensitive to contact inhibition. Second, proliferation was also inhibited by co‐culture of leptomeningeal cells with neurons isolated from either the cerebral hemispheres or optic lobes. Third, co‐culture with sympathetic neurons isolated from the peripheral nervous system did not inhibit LMC proliferation; thus, the inhibition resulting from co‐culture with neurons exhibits some regional specificity. Fourth, proliferation of the leptomeningeal cells was inhibited by addition of the sonicate of either cerebral neurons or LMCs but stimulated by addition of the sonicate of sympathetic neurons. The effect of addition of varying numbers of either intact or sonicated cerebral neurons on thymidine incorporation by cerebral glial cells was also examined. Addition of intact cerebral neurons stimulated cerebral glial proliferation at all concentrations. In contrast, addition of sonicate of cerebral neurons stimulated proliferation at low concentrations but inhibited it at high concentrations. Since addition of intact cerebral neurons inhibits the proliferation of leptomeningeal cells under the same conditions which stimulate the proliferation of cerebral glial cells, these neurons might play a complex role in orchestrating the proliferation of nonneuronal cells during development of the brain.