Fig. 2. Differentiation of secondary chondrocytes. (A-L) e11, (M-O) e13, (P-R) e14. (A,D,G,J,M,P) Alcian Blue/Chlorantine Fast Red stained controls. (A) SCs forming on the anterior and posterior (arrowhead) aspect of the QJ. (B,C) Adjacent sections to A, showing that Col2 (B) is upregulated as SCs form, and that Col10 (C) upregulation is delayed. (E,F) Adjacent sections to D, showing Ihh expression (E) in the same domain as Col10 (F). (H,I) Adjacent sections to G. (H) Bmp7 expression is observed throughout the periosteum, and (I) Gdf5 expression is restricted to the perichondrium of the Q; neither reveal a change in the germinal region. (K,L) Adjacent sections to J, which show that Sox9 (K) expression is confined to the SCs of the QJ, and that Frzb (L) expression is upregulated by SCs. (N,O) Adjacent sections to M, showing opn expression (N) in osteocytes; ocn expression (O) in osteoblasts adjacent to the PO precedes opn expression. (P,Q,R) At e14, chondroclast activity is evident (asterisk in P), and opn expression (Q) is additionally upregulated by late SCs (yellow arrowhead); ocn expression (R) remains specific for osteoblasts. oC, osteocytes; oB, osteoblasts. Scale bars: 100 µm.