Question

Describe the development of the hands and feet. Be specific about the timing and appearance of the limbs and digits.

Answer 1

Formation of limbs:

The bones of the limbs including the bones of the shoulder and the pelvic girdles are formed from the mesenchyme of the limb buds. They are all formed by endochondral ossification.

Limb buds are paddle-shaped outgrowths that arise from the side of the wall of the embryo at the beginning of the second month (5th week) of intrauterine life. Each bud is a mass of mesenchyme covered by ectoderm.

The mesenchyme of the limb buds are derived from the lateral plate mesoderm. The mesenchyme gives rise to bones, connective tissue, and some blood vessels. The muscles of the limbs are derived from the myotomes of somites that migrate into the limbs.

The forelimb bud appear little earlier than hindlimb buds. As each forelimb bud grows, it becomes subdivided by constrictions into arm, forearm, and hand (during the 6th week of IUL). The hand itself soon shows the outline of digits. The interdigital areas show cell death because of which digit separate from each other. The similar changes occur in hindlimb also.

While limb buds are growing, the mesenchymal cells in the buds form cartilaginous models, which subsequently ossify to form the bones of the limbs.

The limb buds are at first directed towards forward and laterally from the body of the embryo. Each bud has a preaxial border and a postaxial border. The thumb and great toe are formed on the preaxial border.

The radius is the preaxial bone of the forearm. In the later part of development, the forelimbs are adducted to the side of the body. The original ventral surface forms the anterior surface of the arm, forearm, and hand. Thus, the upper limb rotates 90 degrees laterally such that, the extensor surface forms the posterior surface of the upper limb and the thumb lies laterally. The rotation takes place during the seventh week of intrauterine life.

In the case of the lower limb, the tibia is the preaxial bone of the leg. Adduction of the limb is accompanied by medial rotation with the result that the great toe and tibia come to lie on the medial side. The original ventral surface of the limb is represented by the inguinal region, the medial side of the lower part of the thigh, the popliteal surface of the knee, the back of the leg, and the sole of the foot. Thus, the lower limb rotates 90 degrees medially such that, the extensor muscles lie on the anterior surface of the lower limbs and the great toe lies medially.