Ultrastructure of the periosteum from membrane bone.

Specimens of skull and palate were taken from 7 beagle dogs after perfusion and the periosteum examined by light and electron microscopy. Three zones were evident in the periosteum, differing in terms of the proportion of cells, fibres and matrix. Zone I consisted of osteoblasts adjacent to the bone surface and a supraosteoblast layer of smaller, compact cells, Zone II was a relatively translucent zone with numerous capillaries and Zone III consisted of cells intermingled with collagen fibrils. Quantification of the relative volumes of tissue components using stereology indicated significant differences between the three zones in each bony site, but not between the two sites. Measurement of numerical density, surface density, profile cross-sectional area, cell volume and cell surface/volume ratio of fibroblast-like cells revealed marked differences between these cells in Zone I and the other two zones; it is possible that the fibroblast-like cells seen in Zone I represent osteoprogenitor cells. Zone II represents the classical cambial layer and contains the majority of the vascular elements present in the periosteum. Zone III contains large volumes of fibroblasts and collagen fibrils and corresponds to the classical fibrous layer. The similarity of this zonal organisation in different regions suggests that periosteum has a consistent structure in membrane bone.