Table 1.

Population Doubling Times of OPCs in Culture

Days in culture	Population doubling time
	PDGF	PDGF + bFGF
	h
8	27 ± 4 (306)	47 ± 4 (203)
18	43 ± 6 (332)	63 ± 3 (455)
90	40 ± 4 (119)	36 ± 5 (127)
330	45 ± 4 (335)	ND
390	45 ± 7 (143)	37 ± 3 (136)

Purified OPCs that had been in culture for various periods of time were cultured at clonal density (2,000 cells per T25 flask) in B-S media containing either PDGF or PDGF + bFGF for 8 d before the number of cells in each clone was counted. The number of clones analyzed is indicated in parentheses. Days in culture refers to the total time that the cells were in culture before replating at clonal density for the assay. The population doubling time was determined as in Materials and Methods. Data are shown as mean ± SEM.