Fig. 3. Neurite extension on FN depends on RPTP and matrix rigidity. (A,B) Primary neurons isolated from the hippocampi of RPTP^+/+ and RPTP^–/– mice were plated on FN-coated polyacrylamide gels of decreasing rigidities, incubated for 48 hours, fixed and visualized using phalloidin. Anti-Tau immunofluorescence was used as axonal marker. Typical (A) stage 2 and (B) stage 3 neurons are shown. Bars, 10 µm. (C,D) Control neurons showed faster differentiation on soft than on rigid substrates; RPTP^–/– neurons, however, showed a high level of differentiation irrespective of matrix rigidity (C). Similarly, control neurons extended longer neurites on soft than on rigid surfaces, whereas RPTP^–/– neurons extended long neurites on both soft and rigid surfaces (D). Results shown in C and D (mean ± s.e.) were statistically significant (P<0.01). (E,F) Phalloidin staining of RPTP^–/– and RPTP^+/+ neurons. Neurites of RPTP^–/– neurons appeared more meandering than the neurites of control neurons. Bars, 15 µm.