Question

Upper Limb Bud Development Questions

a) Does the limb rotation bring the pre-axial border to the radial or ulnar side?

b) Does the post-axial border contain the more rostral or more caudal dermatomes?

c) How does rotation of the developing limb affect the location of the upper limb muscle compartments?

Answer 1

a) Pre-axial (cranial) border

• demarcates portion of the limb bud that lies cranial to the axis of the limb e.g radial aspect of upper limb and tibial aspect of lower limb
• marked out by the cephalic vein in the upper limb, and the great saphenous vein in the lower limb
• clinical relevance: pre-axial polydactyly involves first digit of the hand (radial side) or first digit of the foot (big toe)

b).the postaxial, backward toward the caudal end....

c). The limbs are at first directed backward nearly parallel to the long axis of the trunk, and each presents two surfaces and two borders. Of the surfaces, one—the future flexor surface of the limb—is directed ventrally; the other, the extensor surface, dorsally; one border, the preaxial, looks forward toward the cephalic end of the embryo, and the other, the postaxial, backward toward the caudal end. The lateral epicondyle of the humerus, the radius, and the thumb lie along the preaxial border of the upper limb; and the medial epicondyle of the femur, the tibia, and the great toe along the corresponding border of the lower limb. The preaxial part is derived from the anterior segments, the postaxial from the posterior segments of the limb-bud; and this explains, to a large extent, the innervation of the adult limb, the nerves of the more anterior segments being distributed along the preaxial (radial or tibial), and those of the more posterior along the postaxial (ulnar or fibular) border of the limb. The limbs next undergo a rotation or torsion through an angle of 90° around their long axes the rotation being effected almost entirely at the limb girdles. In the upper limb the rotation is outward and forward; in the lower limb, inward and backward. As a consequence of this rotation the preaxial (radial) border of the fore-limb is directed lateralward, and the preaxial (tibial) border of the hind-limb is directed medialward; thus the flexor surface of the fore-limb is turned forward, and that of the hind-limb backward