The Cardinal Edge


The development of tetrapod upper limbs shares an evolutionary origin and has been adapted and specialized for different functions for different species, such as flight in birds, swimming and balance in sea mammals, and coordination and grabbing objects in humans. The basis of tetrapod limb development has common developmental patterns, starting with the formation of the limb bud via Sonic hedgehog (Shh) signaling, where later developmental steps are modified for specialized functions. This review covers the basic developmental patterns of mammalian tetrapod development seen in humans, beginning with the formation of the limb bud, to the axis development of the limb bud, segmentation of the limb bud, then to cartilage formation in limb bud segments, digit ray formation, and lastly digit elongation and segmentation. Tetrapod limb development is a big focus in developmental biology, since there are many limb malformations caused by many different factors, such as over-expression of Sonic hedgehog (Shh) or under-expression. Understanding the formation of limbs help shed light on why limb malformations occur and future implications may be determining ways to prevent such malformations from occurring.