Limb development
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Limb development in vertebrates is an area of active research in both developmental and evolutionary biology, with much of the latter work focused on the transition from fin to limb.
9-week human fetus from ectopic pregnancy
The Turing reaction-diffusion mechanism illustrates the complex chemical interactions involved in developmental pattern formation. "A" activates itself and "B", while "B" inhibits "A". The model depicts a slowly diffusing activator's (A) interaction with a rapidly diffusing inhibitor (B). The reaction-diffusion system is responsible for the characteristic patterning of the autopod, zeugopod, and stylopod in limb development.
Programmed cell death
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Programmed cell death is the death of a cell as a result of events inside of a cell, such as apoptosis or autophagy. PCD is carried out in a biological process, which usually confers advantage during an organism's lifecycle. For example, the differentiation of fingers and toes in a developing human embryo occurs because cells between the fingers apoptose; the result is that the digits are separate. PCD serves fundamental functions during both plant and animal tissue development.
Dying cells in the proliferate zone
Cell death in the peripheral vs central nervous system
A conserved apoptotic pathway in nematodes, mammals and fruitflies