Limb Development

Limb buds appear at the end of the fourth week (upper limbs first, followed 1-2 days later by lower limbs). The core of the limb bud is lateral plate mesoderm (forming bone and connective tissue) covered by a layer of ectoderm. The Apical Ectodermal Ridge (AER) at the distal tip maintains a population of rapidly proliferating cells (the progress zone), ensuring proximodistal growth (humerus to phalanges). The Zone of Polarizing Activity (ZPA), located at the posterior border, secretes Sonic Hedgehog (SHH) to determine the anterior-posterior axis (thumb vs. little finger). During the 6th week, the distal limb buds flatten to form hand and footplates. Digits are formed through apoptosis of the intervening tissue in the notches. By the 7th week, limbs undergo rotation: upper limbs rotate 90 degrees laterally (extensors on lateral/posterior side), while lower limbs rotate 90 degrees medially (extensors on anterior side). This explains the adult dermatome patterns.

Hox genes (Homeobox) determine the position of the limbs along the body axis and the identity of the bones (e.g., Hox 11 for the radius/ulna). Myoblasts from the somites migrate into the limb buds to form the muscles. Bone formation occurs via endochondral ossification, where a cartilage model is replaced by bone. The first sign of chondrification appears in the 6th week. Ectodermal Wnt-7a expression specifies the dorsal surface (e.g., extensor muscles, nails).

Limb defects affect roughly 6 per 10,000 live births. Polydactyly (extra digits) and Syndactyly (fused digits) are common. Syndactyly results from failure of apoptosis. Meromelia (partial absence) or amelia (total absence) of limbs can occur due to vascular insults or teratogens like thalidomide. Clubfoot (Talipes equinovarus) may be related to restricted intrauterine movement (oligohydramnios). Understanding limb rotation is essential for interpreting the helical arrangement of dermatomes on the legs.

Prenatal ultrasound at 18-20 weeks can identify most major limb defects and skeletal dysplasias. Postnatal X-rays are used to assess bone structure and ossification centers.

Syndactyly and polydactyly are treated with plastic surgery, usually in early childhood. Clubfoot is managed with the Ponseti method (serial casting and bracing). Major limb absence may require prosthetics and physiotherapy.