Abbreviations used in this paper: Agc1, aggrecan; ATF6, activating transcription factor 6; BiP, immunoglobulin heavy chain binding protein; BSP, bone sialoprotein; Col I, type I collagen; CREBH, cAMP-responsive element binding protein H; E, embryonic day; ERSS, ER stress signaling; IHC, immunohistochemistry; Ihh, Indian hedgehog; MMP, matrix metalloproteinase; OASIS, old astrocyte specifi cally induced substance; PECAM-1, platelet/endothelial cell adhesion molecule 1; Ptc-1, Patched-1; PTHrP, parathyroid hormone-related peptide; S1P, site-1 protease; SCAP, SREBP cleavage-activating protein; SREBP, sterol regulatory element binding protein; WT, wild type.The online version of this article contains supplemental material.