Effect of CSF on the in vitro proliferation of cortical cells. Cortical cell cultures from E17 (red) or E19 (yellow) Wistar rat brains were incubated in either supplemented Neurobasal medium (control) or 100% CSF from E18, E19, E20 or E21 fetal brains and proliferation assessed by luminescence assay. The luminescence of control cell cultures at the point of plating out the assays was also measured. Data are presented as fold of control (day 0 luminescence value). Data are means ± S.E.M. of a minimum of three experiments each performed in triplicate. *** P < 0.001, *P < 0.05 compared to day 0 as determined by t-test analysis.