The Questions of Developmental Biology

Thus, although the mesenchyme cells induce and sustain the AER and determine the type
of limb to be formed, the AER is responsible for the sustained outgrowth and development of the
limb (Zwilling 1955; Saunders et al. 1957; Saunders 1972; Krabbenhoft and Fallon 1989).
The AER keeps the mesenchyme cells directly beneath it in a state of mitotic proliferation and
prevents them from forming cartilage. Hurle and co-workers (1989) found that if they cut away a
small portion of the AER in a region that would normally fall between the digits of the chick leg,
an extra digit emerged at that place* (Figure 16.9).