Fig. 1.

Schematic representation detailing steps involved in intramembranous and endochondral ossification processes. Both processes are initiated by the condensation of mesenchymal progenitors. In intramembranous ossification, MSCs differentiate directly into osteoblasts and deposit bone without preceding cartilage formation. A subset of osteoblasts become osteocytes upon being embedded within the bone matrix. This process occurs mainly in the flat bones of the skull and clavicle. During endochondral ossification, MSCs differentiate into chondrocytes, which then form a growth plate and undergo hypertrophy. Hypertrophic chondrocytes mineralise their matrix (mineral deposits indicated in red) and undergo apoptosis, triggering the differentiation of perichondrial cells into osteoblasts. This process forms long bones. Abbreviations: MSCs, mesenchymal stem cells; RUNX2, runt-related transcription factor 2; OSX, Osterix; SOX9, SRY-box transcription factor 9 [Created with BioRender.com].