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Limb Development in the Human Embryo
A description of early human limb development
Between 4-6 weeks in development, limbs begin to emerge in the human embryo. Clearly visible on 6 week-old embryos are the limb buds that have been induced by the expression of Fibroblast Growth Factor, Hox 4, and Sonic hedgehog among other regulatory molecules. The expression of these molecules punctuate a complex choreography in four dimensions. The molecules that induce the development of limb buds in humans are the same molecules that initiate the development of wing buds in chickens, and other winged amniots (animals that develop within an amniotic sack). At first, Hox gene expression initiates the development of all limb buds. These buds are then differentiated by defining the proximal and distal ends of the bud through the expression of a Fibroblast Growth Factor. Hox genes are then expressed again, but this time they help to differentiate some gross characteristics of development, such as differentiating fins from other limbs and the development of fingers and toes. In the human, the rule of colinearity is found in Hox expression as it is in the fly with the bithorax complex, as the more proximal parts of limbs (such as the scapula and the humerus) are on one end of the Hox gene cluster while the proximal parts of limbs (such as the digits) are on the other. This is a great example of how repetition in biology helps to determine forms and functions.
Similarities in the choreography of the development lead to an overall similarity in the anatomical structures of tetrapod limbs; they are all composed of the similar three sections: stylopod, zeugopod and autopod. These homologies should not surprise those who follow developmental biology, as many genes that initiate regulatory processes are highly conserved amongst all animals, and anatomical structures are often conserved across phylum and class.
What is needed is an account of biological change that helps to understand the flexibilities inherent in morpho-genic fields as potentials for expression induced by the heterogeneous spaces and times important in the development of bodies. This would also be an account that would pay attention to the different types of chemical competencies expressed by molecules at different types of moments. Not all moments are the same in genetic development, some moments carry greater impact by directing specific types of interactions. For those of us interested in contemplating the importance of the diversity of living things, these moments matter.