Fig. 1. Analysis of En1 expression during calvarial bone development between E11.5 and P1, as recapitulated by X-gal staining of En1^lki/+ mice. (A,B) Calvarial En1 expression within the craniofacial mesenchyme (Aa-Ac, white arrows) at E11.5 (Aa,Ba), E12.5 (Ab) and E13.5 (Ac,Bb). Expression of En1 during overt calvarial bone formation (E15.5-P1) is prominent along the osteogenic fronts (arrowhead) and developing sutures (black arrows) (Ad-Af); it is expressed by ectocranial periosteal osteoblasts (Ac, Af, asterisks; Bc,Bd, arrow) as well as by the endosteal osteoblasts and osteocytes of the frontal bone trabeculae (Bc,Bd, arrowheads). In the interfrontal suture, En1 is expressed in the mesenchyme (Bc,Be, arrow; Be'), but not in osteoprogenitors populating the frontal bone margins (Be, arrowheads). By contrast, En1 is expressed by osteoprogenitors within the coronal suture (Bf, arrowheads; Bf'). p, parietal bone; f, frontal bone; e, eye. Scale bars: 2 mm in A; 1 mm in Ba-Bc; 0.1 mm in insets in Ba,Bb, and in Bd-Bf.