VIS-A-VIS CELLS AND THE PRIMING OF BONE-FORMATION

Bone formation throughout skeletal growth and remodeling always entail s deposition of new bone onto a pre-existing mineralized surface. In c ontrast, the initial deposition of bone in development requires the fo rmation, ex novo, of the first mineralized structure in a nonmineraliz ed tissue. We investigated the cellular events associated with this in itial bone formation, with specific reference to the respective role o f cartilage and bone cells in bones which form via a cartilage model, The cellular architecture of initial osteogenic sites was investigated by light, confocal, and electron microscopy (EM) in the membranous os sification of fetal calvarial bones (not forming via a cartilage model ) and in the membranous ossification of the bony collars of endochondr al bones. Bone sialoprotein (BSP), which is expressed during early pha ses of bone deposition and has been proposed to be involved in the con trol of both mineral formation and bone cell-matrix interactions, was used as a marker of initial bone formation, We found that at all sites , BSP-producing cells (as identified by intracellular immunoreactivity ) are arranged in a characteristic vis-g-vis (face to face) pattern pr ior to the appearance of the first mineralizing BSP-immunoreactive ext racellular matrix, In perichondral osteogenesis, the vis-a-vis pattern comprises osteoblasts differentiating fr-om the perichondrium/periost eum and early hypertrophic chondrocytes located at the lateral aspects of the rudiment. By EM, the first mineral and the first BSP-immunorea ctive sites coincide temporally and spatially in the extracellular mat rix at the boundary between cartilage and periosteum, We further showe d that in an in vitro avian model of chondrocyte differentiation in vi tro to osteoblast-like cells, early hypertrophic chondrocytes replated as adherent cells turned on the expression of high levels of BSP in c onjunction with the switch to collagen type I synthesis and matrix min eralization, We propose a model for the priming of bone deposition, i, e,, the formation of the first bone structure, in which the architectu ral layout of cells competent to deposit a mineralizing matrix (the vi s-g-vis pattern) determines the polarized deposition of bone. For bone s forming via a cartilage model, the priming of bone deposition involv es and requires cells that differentiate from early hypertrophic chond rocytes.