Development of the Limbs

The limbs develop as a result of permissive and instructive interactions between epithelium and mesenchyme which are initiated at specified times and places along the lateral body wall. These cell lines continue to proliferate at defined positions along the embryonic axis, giving rise to local thickenings that soon develop into limb buds (McQueen and Towers 2020, Zuniga and Zeller 2020). The earliest cell lineages of the lateral body wall that will give rise to the upper and lower limb girdles are significantly different. Cell populations destined for pectoral girdle development arise initially from primary neurulation processes (XR Ch 14) and are found within the head-neck transition zone, a region where derivatives of the pharyngeal arches (innervated by cranial nerves) and of somites (innervated by spinal nerves), coexist at the same rostrocaudal levels (XR Ch 18 old 17). Here, the mesenchymal populations contribute to the head, neck, pectoral girdle and trunk. Cell populations destined for pelvic girdle development arise from both primary neurulation (caudal part of neural plate) and secondary neurulation processes, where neural tissues as well as somites arise from the midline caudal eminence, and a neural plate is not formed (XR Ch 14, XR Ch 19 old 18). Here, the mesenchymal populations contribute to the trunk, pelvic girdle and associated structures. For details of the early specification of pectoral and pelvic girdle and their different gene profiles, see Young et al (2019).

Limb bud development shows meristematic growth where distal populations form different limb profiles and regions in a proximodistal sequence. A broad plate forms at the distal tip and digital rays develop within the plate marking the position of the forming digits. The digits later separate and become tipped with nails (XR Ch new number 15). Fig. 20.1 old 19.1 shows the main stages in the development of a human upper limb; the stages in the development of the lower limb are essentially similar. Both upper and lower limb buds protrude laterally from the trunk, encased in an ectodermal epithelium rimmed by a longitudinal ridge of columnar epithelial cells called the apical ectodermal ridge. The ectoderm will ultimately form the epidermis (XR Ch 15 dev skin). Beneath the epithelium, a mixed population of mesenchymal cells derived from the proliferating epithelial somatopleuric lateral plate (Ch. 12) undergoes restriction to develop into upper or lower limbs before the limb buds are visible. These cells give rise to the connective tissues, bones, cartilages, ligaments, tendons, muscle attachments and their encasing layers, loose connective tissue and dermis of each limb. Limb muscles originate from paraxial mesenchyme cells that migrate from the somites. This cell population also contributes to some of the endothelial cells that form an extensive vascular network in the early limb bud. Motor and sensory nerves and their associated Schwann cells, together with melanocytes destined for the skin, migrate into a developing limb somewhat later. Motor neurons are derived from the neural plate, and sensory neurons, Schwann cells and melanocytes are derived from the neural crest.

Much of what is known about the development of the limbs is derived from early experimental studies on laboratory-based amniote embryos. Production of chimeric embryos (chick-quail and mouse-chick) illustrated the reciprocity of tissue interactions between these species (Wessells 1977, Ribatti 2019). The evolutionarily conserved nature of limb development has been confirmed by the demonstration of the spatiotemporal expression of the requisite genes in laboratory animal models, and by their identification in human embryonic tissue and in in vitro studies using iHPSCs. However, as the developmental timeline of development (heterochrony) is significantly different between human, chick and mouse (Fig 8.2) spatiotemporal similarities in gene expression between human and laboratory animal species should be treated with appropriate caution (Keyte and Smith 2014, Dobreva et al 2021).