Dynamics of the transition from osteoblast to osteocyte

Abstract

Osteocytes are derived from osteoblasts and make up over 90% of the cells in bone. However, the mechanisms that control the differentiation of osteoblasts into osteocytes embedded in bone matrix are not well understood. With the recent developments of transgenic models for manipulating gene expression in osteocytes and of transgenic mice carrying lineage reporters for osteoblasts and osteocytes, unprecedented new insights are becoming possible. In this article we review recent advances, such as comparative gene and protein expression studies, that are delineating the changes in gene and protein expression that accompany osteocyte differentiation. We also review recent studies in which time-lapse dynamic imaging approaches have been used to visualize osteoblast and osteocyte populations within bone. These approaches reveal the key role of cell motility in bone cell function and highlight the dynamic nature of mineralized tissues. Changes in motile properties of the cell may be key in the transition from osteoblast to osteocyte, as reflected in the altered expression of many molecules involved in cytoskeletal function.

Figure 1

Schematic diagram illustrating the proposed transitional stages from osteoblast to mature osteocyte. During this process, the volume of the cell body and the number of cell organelles decreases. 1= proliferating preosteoblast, 2= preosteoblastic osteoblast, 3= osteoblast, 4= osteoblastic osteocyte (type I preosteocyte), 5= osteoid osteocyte (type II preosteocyte), 6= type III preosteocyte, 7= young osteocyte, 8= old osteocyte (Modified from Franz-Odendaal et al2)

Figure 2

Schematic diagram summarizing the motile properties of the cells at different stages during the transition from osteoblast to mature osteocyte and illustrating the changes in expression of key osteocyte marker genes at these different transitional stages.