How do osteoblasts become osteocytes

If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells: the osteogenic cell. These osteogenic cells are undifferentiated with high mitotic activity; they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. When they differentiate, they develop into osteoblasts.

The dynamic nature of bone means that new tissue is constantly formed, while old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast, which is found on bone surfaces, is multinucleated, and originates from monocytes and macrophages two types of white blood cells rather than from osteogenic cells.

Osteoclasts continually break down old bone while osteoblasts continually form new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant, but subtle, reshaping of bone. Learning Objectives Distinguish among the four cell types in bone.

Key Points Osteogenic cells are the only bone cells that divide. Osteogenic cells differentiate and develop into osteoblasts which, in turn, are responsible for forming new bones.

Osteoblasts synthesize and secrete a collagen matrix and calcium salts. During osteogenesis, osteoblasts lay down osteoid and transform into osteocytes embedded in mineralized bone matrix.

Despite the fact that osteocytes are the most abundant cellular component of bone, little is known about the process of osteoblast-to-osteocyte transformation. What is known is that osteoblasts undergo a number of changes during this transformation, yet retain their connections to preosteoblasts and osteocytes. This review explores the osteoblast-to-osteocyte transformation during intramembranous ossification from both morphological and molecular perspectives. We investigate how these data support five schemes that describe how an osteoblast could become entrapped in the bone matrix in mammals and suggest one of the five scenarios that best fits as a model.