Sparse HUVECs undergo morphogenesis more rapidly in 3-dimensional collagen culture. HUVECs isolated from the same cord and grown as confluent or sparse monolayers were trypsinized and seeded in a 3-dimensional collagen gel. Photographs were taken at 0, 1, 3, and 15 hours using phase contrast optics. Data are representative of 3 experiments. For all images in Figure 6: original magnification, × 200.