Figure 8.

The effect of surface geometry on the fate of 128 osteoblasts. A cell-based model is shown representing bone formation on three different surface geometries: (a) a concave segment of a 200 μm diameter osteon in cortical bone, (b) a flat surface in cortical or cancellous bone, and (c) the convex surface of a 100 μm diameter trabecula in cancellous bone. In each case, a set of 128 osteoblasts is shown at t =0 as ∼ 12.5 μ cuboidal cells covering an appropriate segment of the bone surface. Below, the same surface and the new bone formed by this set of 128 osteoblasts are shown. In each case, some fraction of cells must become embedded in the matrix as osteocytes to maintain an appropriate osteocyte density and some osteoblasts must be retained as lining cells covering the remaining surface. Cells that are not required as osteocytes or lining cells are presumed to be lost through apoptosis. The probability of osteoblast transit into the osteocyte or lining cell population, and the probability of apoptosis are dependent on surface geometry, the density of osteocytes in the matrix, and the thickness of the new bone that is formed at the site.