In mouse embryos, the ventral ectoderm of the emerging forelimb at E9.5 (embryonic day 9.5) already appears thicker in comparison to the dorsal ectoderm and it corresponds to the early AER. By E10, this thickening is more noticeable since the epithelium now consists of two layers and becomes confined to the ventral-distal margin of the bud although it is not detectable in living specimens using light microscope or by scanning electron microscopy (SEM). Between E10.5-11, a linear and compact AER with a polystratified epithelial structure (3-4 layers) has formed and positioned itself at the distal dorso-ventral boundary of the bud. After reaching its maximum height, the AER in mouse limb buds flattens and eventually become indistinguishable from the dorsal and ventral ectoderm. The structure of the human AER is similar to the mouse AER.