Chapter 12: Limbs

Limbs begins with the emergence of limb buds from the lateral body wall during the fourth week, driven by a mesenchymal core from the lateral plate mesoderm which forms the bones and connective tissue, while the musculature is derived from somatic cells that migrate into the limb. A critical regulator of this proximodistal growth is the Apical Ectodermal Ridge (AER), a thickening of ectoderm that secretes fibroblast growth factors (FGFs) to maintain the adjacent mesenchyme in a rapidly proliferating, undifferentiated state known as the progress zone. The summary explains how the limbs develop in a proximal-to-distal sequence—stylopod, zeugopod, and autopod—and describes the essential role of apoptosis (programmed cell death) in separating the handplates and footplates into distinct digits. Significant attention is given to the molecular regulation of limb patterning, including the role of HOX genes in positioning limbs along the craniocaudal axis, TBX5 and TBX4 in specifying forelimbs and hind limbs respectively, and the Zone of Polarizing Activity (ZPA) which utilizes Sonic Hedgehog (SHH) signaling to establish the anteroposterior axis. The text also outlines the dorsoventral patterning controlled by WNT7a and LMX1. Furthermore, the chapter covers the rotational changes during the seventh week where upper limbs rotate laterally and lower limbs rotate medially, as well as the innervation patterns derived from ventral primary rami. Finally, the summary addresses clinical correlates and congenital anomalies, ranging from drug-induced defects like those caused by thalidomide (amelia, phocomelia) to genetic conditions such as Holt-Oram syndrome, osteogenesis imperfecta, and syndactyly, alongside mechanical disruptions like amniotic bands.