Matrix vesicles in embryonic chick bone: considerations of their identification, number, distribution, and possible effects on calcification of extracellular matrices.

Bone tissue from normal 5- to 21-day-old embryonic chicks has been examined by transmission electron microscopy to identify extracellular matrix vesicles, their number, and distribution at beginning stages of tissue mineralization during early osteogenesis. Principal tissue treatment was fixation in glutaraldehyde and osmium, followed by staining with uranyl and lead salts. Some specimens were decalcified with EDTA and stained. In embryonic bone tissue prepared by these methods, it was rather difficult to identify structural organelles conclusively, but along the proximal and distal diaphyses of tibiae and femurs from chicks 5-12 days old, matrix vesicles were occasionally observed in the extracellular tissue spaces. Some vesicles appeared to contain or to be associated with mineral particles. Very few vesicles were found in these same regions in older chicks or along the periosteal surfaces of the long bones in chicks of any age examined. The vast majority of the extracellular mineral particles was associated with collagen fibrils in all chick bone tissues at all ages. The presence of only small numbers of vesicles, restricted to specific regions and ages of bone tissue undergoing active mineralization, argues against an obligatory requirement for matrix vesicles in the calcification of extracellular bone matrices.