Fig. 2. Tubule formation as a function of time. Matrigel cultures were initiated in serum free medium supplemented with 5 µg/ml insulin, 5 µg/ml transferrin, 5x10^–8 M hydrocortisone and 10 ng/ml EGF, and then examined under an inverted microscope. (a) A microscope field after 1 day in culture. The same microscope field after (b) 2 days and (c) 4 days in culture. Bar in a, 50 µm (same scale for b and c).