Figure 3: Positional value of chick leg-polarizing region cells is irreversibly specified.
From: Sonic hedgehog-expressing cells in the developing limb measure time by an intrinsic cell cycle clock
(a–c) HH27 leg-polarizing region grafted to posterior of HH20 host-polarizing region (a) give rise to digit IV (b,c, n=3/4; one case no digit from graft). (d–f) HH27 leg-polarizing region grafted to posterior of HH18/19 leg-polarizing region treated with cyclopamine (d) gives rise to digit IV (e,f, n=3/4 one case limb truncated). (g–i) HH27 leg-polarizing region grafted to anterior margin of HH21 leg treated with cyclopamine (g) give rise to digit IV *, a duplicate digit I* from host (h,i, n=2/3, one case no duplicated host digits, note extra digit IV* (i) is out of plane with the other digits and appears posterior although it is an anterior digit. (j–l) HH27 leg-polarizing regions grafted to anterior margins of HH21 wing treated with cyclopamine (j) give rise to digit IV* (k,l n=2/2). All scale bars, 1 mm.
