Isolation and culture of bone-forming cells (osteoblasts) from human bone.

The most conspicuous function of the osteoblast is the formation of bone. During phases of active bone formation, osteoblasts synthesize bone matrix and prime it for subsequent mineralization. Active osteoblasts are plump, cuboidal cells rich in organelles involved in the synthesis and secretion of matrix proteins. Unlike fibroblasts, they are obviously polarized, secreting matrix onto the underlying bony substratum which consequently grows by apposition. Some osteoblasts are engulfed in matrix during bone formation and are entombed in lacunae. These cells are described as osteocytes and remain in the bone matrix in a state of low metabolic activity. At the completion of a phase of bone formation, those osteoblasts which avoided entombment in lacunae lose their prominent synthetic function and become inactive osteoblasts, otherwise known as bone-lining cells. In mature bone, lining cells cover most of the bone surfaces. Osteocytes and bone-lining cells should not be considered as inactive cells since they play a major role in the regulation of bone modeling and remodeling and in calcium homeostasis (1).