Expression of OPN mRNA based on in situ hybridization with an anti-sense (a-g, n-q) and a sense (h-k, r-u) probe in mouse heads at E 12.5 (a,h), E14.5 (b,e,i), E17.5 (c,f,j) and P1 (d,g,k) and tibias at E 12.5 (l,r), E14.5 (m,s), E17.5 (n,p,t) and P1 (o,q,u). The reaction of OPN was localized in all reaction plates (arrowheads). e) Magnification of the squared region in (b); f) magnification of the squared region in (c); g) magnification of the squared region in (d); p) magnification of the squared region in (n); q) magnification of the squared region in (o). Mad, mandible; B T, bone trabecula-like matrices; MC, Meckel’s cartilage; TI, tibia; HC, hyaline cartilage. Scale bars: 100 µm. a-d, h-o, r-u) magnification: 10x; e-g, p,q) magnification: 40x.