Bone is a complex tissue that contains at least four different cell types of the osteoblastic lineage. (1 ) Active osteoblast-a plump, polarized, cuboidal cell rich in organelles involved in the synthesis and secretion of matrix proteins. (2 ) Osteocyte-an osteoblast with low metabolic activity that has been engulfed in matrix during bone formation and entombed in lacunae. (3 ) Bone-lining cell-osteoblasts that have avoided entombment in lacunae and lose their prominent synthetic function; these cells cover most of the bone surfaces in mature bone. (4 ) Preosteoblast-a fibroblastic proliferative cell with osteogenic capacity. In addition, bone contains cells of a distinct lineage, the osteoclast (reviewed in Ch. 4).
