Embryology

DISCUSSION: The apical ectodermal ridge lies directly over the fetal limb bud and directs longitudinal growth of the limb. Illustration A depicts proper limb development following the apical ectodermal ridge. Illustration B demonstrates transverse absence (also known as congenital amputation) following an insult to the apical ectodermal ridge. A vascular injury to the limb bud mesoderm or epithelial ectoderm may result in congenital deformity, but not a complete transverse absence of the limb.

DISCUSSION: For vertebrates, the axial skeleton derives from the somites, which are segmental units organized in pairs on both sides of the developing neural tube. Somites are formed in a rostro-to-caudal sequence by the epithelialization of mesenchymal cells at the rostral end of the presomitic mesoderm. The function of the Hox genes is to regulate somitization of the axial skeleton. It is generally accepted that a specific combination of Hox genes expressed at a particular somatic level determines the axial identity of the resulting structures. Iimura et al & Turnpenny et al both provide articles which highlight the role that Hox genes and other regulatory genes play in controlling cellular movements during gastrulation, therefore contributing to body formation.

DISCUSSION: The somites are mesodermal in origin; somites eventually form dermis (dermatome), skeletal muscle (myotome), sclerotome or cartilage, tendons, and endothelial cells. The sclerotome of the somites develops into the axial skeleton. The neural crest forms the peripheral nervous system, the lateral plate mesoderm becomes the dermis, and the dorsal myotome (epimere) becomes the dorsal muscles.

Illustration A depicts the sclerotome of the somites development into the vertebra. The nucleus pulposus forms from the notochord and the annulus fibrosus forms from the sclerotome.