Fig. 7. Models for the role for BMPRIA in mesoderm development. (A) In wild-type embryos, epiblast cells that are recruited to the anterior primitive streak (red arrow) or to the middle primitive streak (green arrow) are fated to form somites (red square) or LPM (green hexagon), respectively, at the late streak stage. (B,C) Mutant embryos. (B) In mutant embryos, epiblast cells recruited to the anterior primitive streak probably form somites as in wild-type embryos (red arrows and red square). However, many epiblast cells passing through the middle primitive streak (green arrow), which should form LPM, change their fate to form ectopic somites (yellow arrow and yellow square); LPM is consequently reduced (green arrow and hexagon). (C) Alternatively, some of the prospective PXM cells are abnormally recruited to the middle primitive streak (yellow arrow) and form ectopic somites (yellow square). LPM is reduced owing to loss of contributing cells (green hexagon) resulting from reduced BMP signaling.