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 isknown 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 andosteocytes. This review explores the osteoblast-to-osteocyte transformation during intramembranous ossiﬁcation from both morphological and molecular perspectives. We investigate how these data supportﬁve 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. Those osteoblasts on the bone surface that aredestined for burial and destined to become osteocytes slow down matrix production compared to neighbouring osteoblasts, which continue to produce bone matrix. That is, cells that continue to producematrix actively bury cells producing less or no new bone matrix (passive burial). We summarize which morphological and molecular changes could be used as characters (or markers) to follow the transformation process.
Keywords: osteoblast; osteocyte; osteogenesis; intramembranous ossiﬁcation; preosteoblast; osteoid-osteocyte; preosteocyte