Perichondrium phenotype and border function are regulated by Ext1 and heparan sulfate in developing long bones: A mechanism likely deranged in Hereditary Multiple Exostoses

► Conditional deletion of Ext1 in perichondrium causes ectopic cartilage formation. ► Interfering with heparan sulfate function increases chondrogenic potential in vitro. ► Responsiveness to signaling factors such as BMP2 can increase after HS interference. ► Ext genes and heparan sulfate are needed to maintain the perichondrium phenotype.