Fig. 4. Cre-mediated inactivation of the Bmpr gene in the limb resulted in severe abnormalities in the hindlimb and more subtle defects in the forelimb. Although variability existed in the hindlimb phenotype, it was typically quite severe. (A) The most severe phenotype resulted in complete agenesis of the hindlimb in approximately one-fifth of the mutants (8/42 hindlimbs), as depicted in this P0 neonate (arrow). (B) Forelimb development was comparatively normal, although malformations, including partial polydactyly (arrow) and dysplastic digits (data not shown), were detected. (C) A representative malformation of the hindlimb, including a partial duplication of the distal region of the digit (arrowhead) and hematoma at the tip of the digits (arrow). The proximal skeletal elements of the digit (metatarsals and proximal phalanges) were typically reduced in number with the majority of the hindlimbs containing two (11/42) or three (13/42) digits. (D) A cleared whole-mount preparation of the forelimb of a Bmpr mutant was stained with Alizarin Red and Alcian Blue to visualize the bone structure. This panel demonstrates a partial polydactylous digit composed of a distal phalange, including the nail. (E) Forelimb from opposite side of animal depicted in D. (F) A mutant skeletal preparation demonstrates a reduction in digit number associated with syndactyly and a partial duplication (arrowhead) of the digits more distally. (G) Skeletal preparation of a rare polydactylous hindlimb demonstrating four proximal metatarsals, but seven distal phalanges. (H) A transverse section through a normal hindlimb at the level of the digits reveals the typical dorsal/ventral organization of the musculature and tendons. Arrows indicate the flexor digitorum profundus tendon in two of the digits. (I) A similar section through a mutant hindlimb demonstrated a double dorsal phenotype. The flexor digitorum profundus tendon did not display the prominent phenotype observed in the normal animal (asterisks), and the musculature formed an overall mirror-image symmetry unlike the obviously polarized structure of the normal animal. The hindlimb depicted in this panel is comparatively normal when compared to the distribution of mutant hindlimb phenotypes observed in the mutants. However, this mutant hindlimb still demonstrated a partial duplication of the digit, which is displaced ventrally (arrowhead) compared with the other digits. (J) Lateral view of normal hindlimb skeletal prep from a normal P10 mouse, Arrow indicates the sesamoid process, which is a ventral structure. The inset is a higher magnification view of the sesamoid process. (K) Lateral view of mutant P10 hindlimb demonstrating lack of sesamoid process. (L) A higher magnification view of normal digit transverse section indicated by right arrow in H. (M) A higher magnification view of the mutant digit for comparison to L (area indicated by right asterisk in I).