Microfibrils are not a structural substrate for matrix mineralization. (A) Illustrative von Kossa–stained WT cOb cultures in which Fbn1 or Fbn2 expression was silenced by RNAi with histograms on the immediate and far right showing the number of mineralized nodules and levels of indicated transcripts (n = 3), respectively. (B) Illustrative von Kossa–stained Fbn2^−/− cOb cultures in which Fbn1 was silenced with histograms on the immediate and far right, showing the number of mineralized nodules and levels of indicated transcripts, respectively (n = 3). Error bars indicate mean ± SD, and asterisks indicate statistically significant differences (P < 0.05).