Diverse contribution of Col2a1-expressing cells to the craniofacial skeletal cell lineages.

OBJECTIVES: Craniofacial skeletal development requires deliberate coordination of two distinct mechanisms of endochondral and intramembranous ossification. Col2a1-expressing cells encompass growth-associated skeletal progenitors in endochondral bones of the limb. The objective of this study was to determine the contribution of Col2a1-expressing cells to the craniofacial skeletal cell lineages. We hypothesize that Col2a1-expressing progenitors significantly contribute to various modes of ossification associated with the craniofacial development.
METHODS: Cellular fates of Col2a1-expressing cells were studied based on a cre-loxP system using a Col2a1-cre transgene and an R26R-tdTomato reporter allele. We analysed three distinct locations of the craniofacial skeletal complex representing unique ossification mechanisms: the cranial base, the calvaria and the mandibular condyle.
RESULTS: Col2a1-cre consistently marked a majority of skeletal cells in the cranial base. Interestingly, Col2a1-cre also marked a large number of osteoblasts and suture mesenchymal cells in the calvaria, in addition to chondrocytes in the underlying transient cartilage. In the mandibular condyle, Col2a1-cre marked chondrocytes and osteoblasts only during the growth phase.
CONCLUSIONS: Col2a1 is expressed by progenitors of the skeletal lineage in canonical endochondral bone formation occurring in the cranial base. In contrast, other ossification mechanisms of the craniofacial complex utilize Col2a1-expressing cells in a different manner, whereby Col2a1 may be expressed in more differentiated or transient cell types of the skeletal lineage.