“An embryo first begins to develop limbs at about 4 weeks gestation. The limbs first form as buds that protrude from the ventral wall, consisting of a mesenchymal core and covered by ectoderm. The core is formed from a layer of the somatic lateral plate mesoderm.

At approximately six weeks of in-utero development, the framework for embryos upper limbs is almost completely developed. The limbs develop from an area of thickened ectoderm that forms what is known as the “”apical ridge.”” Through a series of intercellular signaling mechanisms, this ridge induces the surrounding mesenchymal tissue to remain in an undifferentiated state. However, in addition to maintaining differential potential, the apical ectodermal ridge also causes rapid cellular proliferation and hence lengthening of the developing limb, known as the “”progress zone.”” As the pluripotent cells move distally from the origin, the milieu of growth factors change in such a way that the cells began to differentiate into the finite structures of the limb.

As the limb bud takes form, a series of circumferential constrictions cause segmentation and eventually flattening of the most distal portions, initiating the formation of the hand. Obliteration of the cells in between segments of the apical ectodermal ridge, now forming digital rays, physically separates the fingers. Prior to this selective tissue destruction, the hands have a characteristic “”webbed”” appearance.”
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