Fig. 4. Netrin is a chemorepellent for purified chick spinal cord oligodendrocyte precursors in chemotaxis assays. (A) Ethidium bromide labeled migrating oligodendrocyte precursors on the lower surface of the filter after 20 hours in control conditions. (B) The number of migrating cells decreased when netrin 1 was added to the lower well, suggesting that cells are repelled by netrin 1. (C) The number of migrating cells increased when PDGF was added to the lower well, suggesting that cells are attracted by PDGF. (D) Boyden chamber assays. Two chambers are separated by a filter, cells are placed in the top chamber and putative chemotactic molecules placed in the upper or lower chamber. Migrating cells are counted on the lower surface of the filter. (E) Quantitation of the migration. Addition of 100 ng/ml netrin 1 to the lower chamber inhibited oligodendrocyte precursor migration, whereas addition of 20 ng/ml PDGF promoted migration (Armstrong et al., 1990). The chemoattraction of PDGF was blocked by addition of netrin 1 to the lower chamber in a dose-dependent manner. (F) Addition of netrin 1 to the upper chamber increased the number of oligodendrocyte precursors migrating through the filter. Scale bar in A: 100 µm for A-C.