Anatomical structure that is an aggregation of similar cells from which cartilages and bones form, and from which chondrogenesis and osteogenesis are initiated during repair and/or regeneration. (Hall and Miyake 1995). [ PSPUB:0000170 http://purl.obolibrary.org/obo/go/references/0000034 VSAO:0000006 http://dx.plos.org/10.1371/journal.pone.0051070 ]