Fig. 4. BrdU labelling and the origin of secondary chondrocytes at e14. Counterstains: Toluidine Blue (A); (B-G,I) Alcian Blue/Chlorantine Fast Red. BrdU labelled cells are black. (A) Pulse and immediate fix shows cycling cells in the germinal region, and a relative absence of BrdU label within the SCs. (B) Contralateral side to A, with a 24-hour chase following pulse. Doublets found within the SC domain are highlighted with red circles. (B') Higher magnification of image in B. (C) BrdU labelled cells at the boundary between the germinal region and the SCs. (D) Contralateral side to C shows labelled doublets after a 24-hour chase (circled). (D') Higher magnification of image in D. (E) BrdU labelling at 16 hours after explant and immediate fixation labels cells in the germinal region. (F) Contralateral side to E after a 24-hour chase shows no labelled chondrocytes in the SC domain. (G) BrdU label at 0 hour. (H) Adjacent section to G, which shows Col10 expression (yellow) superimposed on, and contained within, Col2 expression (red). Bracket indicates delay in Col10 upregulation. (I) 24-hour chase on contralateral side to G,H results in labelled doublets. (J) Adjacent section to I, showing that the Col10 domain has expanded to coincide with Col2 domain, black arrowhead. Scale bars: 100 µm.