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Development

Towards the end of the fourth week the limbs begin to develop from limb buds made up of mesenchyme (somatic mesoderm) covered with surface ectoderm. The apical ectodermal ridge at the tip of each limb bud induces the mesenchyme beneath it to elongate.  At the end of each limb the hand or foot first develops as a single flat outgrowth, then programmed death of selective cells (apoptosis) causes it to divide into distinct digits.

Movement of Limbs

Initially the limbs develop high on the trunk where they are supplied by the ventral rami of adjacent spinal nerves.  Spinal roots C5 – T1 supply the upper limb bud and L2 – S3 supply the lower limb bud. During weeks six through eight the limbs descend to their adult height taking their nerve supply with them.  To attain adult anatomical position, the upper and lower limbs rotate in opposite directions and to different degrees, with the result that the adult elbow points posteriorly and the adult knee points anteriorly.

Skeletal Elements

Cartilaginous bones begin to develop from chondrification centers early in the fifth week.  Ossification of the long bones (osteogenesis) begins from primary ossification centers, which appear in the middle of the long bones in the seventh week.  Ossification of the carpal (wrist) bones does not begin until approximately the first year after birth.  The skeletal muscle of the limbs is derived from myotomal cells that migrate into the limbs, followed by the branches of their associated spinal nerves.  

Clinical Correlations

Limb Malformations

Amelia is complete absence of one or more limbs. Phocomelia is a defect wherein the upper portion of a limb is absent or poorly developed, so that the hand or foot attaches directly to the body by a short, flipperlike stump. These defects are often due to a failure of inductive signaling factors, and may inherit in a Mendelian fashion.

Malformations of Hands and Feet

Syndactyly is congenital anomaly characterized by two or more fused fingers or toes.  Macrodactyly (megadactyly) is enlargement of one or more digits.  Polydactyly is a condition wherein there are extra digits, whereas in ectrodactyly there are fewer than normal.

Clubfoot (talipes equinovarus)

Clubfoot is a common foot malformation (1/5,000 infants) characterized by abnormal positions of the foot (e.g., inverted).  Some cases result from compression of the infant in the uterus (e.g., with oligohydramnios)

Achondroplasia

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