Growth plate specific genes found using the unusual ossification of the metatarsal and pisiform

We utilize the substantial variation in the location of growth plates within the mammalian skeleton to identify genes specific to growth plate patterning. Third metatarsals (MT3) form only a single growth plate located at the distal end, and the pisiform (in non-human mammals) forms an active growth plate unlike the other carpals. Comparison between the growth plates of mouse distal MT3 and pisiform to generalized endochondral ossification in proximal MT3 and other carpals controls for the effects of age, systemic growth factors, and biomechanical environment. We identify numerous differentially expressed genes (DEGs) using RNA-seq in 4- and 9-day old mice. DEGs from the MT3 are disproportionately represented in gene ontology (GO) categories including limb development and growth and factors associated with the Fzd8-Ror2-Wnt5a pathway. The Pisiform-Carpal comparison includes GO terms associated with skeletal and cartilage development and abnormalities of the carpal bones. A limited set of DEGs shared by both the MT3 and Pisiform-Carpal datasets includes Wnt5a. Wnt5a is expressed in the perichondrium in both ends of the MT3 and the pisiform, but expression at the columnar/hypertrophic cell boundary is stronger in the distal MT3 and pisiform growth plates. The related Wnt10a gene is solely expressed in the bone collar of the distal MT3 and pisiform. This confirms previously identified roles for Wnts in regulating osteogenesis and cell polarity within the growth plate.