Figure 1: The chick limb-polarizing region has an intrinsic cell cycle clock.
From: Sonic hedgehog-expressing cells in the developing limb measure time by an intrinsic cell cycle clock
(a–n) HH20 wing- (a) and leg-polarizing regions (h) grafted with pins to posterior and anterior of HH24 wing- and leg-polarizing regions express Shh after 32 h (arrows—b,e,i,l, n=8/8), endogenous Shh undetectable (asterisks—b,e,i,l). Wings with posterior grafts normal (c,d, n=2/2), wings with anterior grafts have an additional digit 1* (f,g, n=2/2), legs with posterior leg grafts gave rise to digit IV (j,k, n=3/3) and anterior leg grafts to digit IV* and induce a digit I* (m,n, n=2/2). (o–p) Cell cycle parameters of wing- (o) and leg-polarizing regions (p)—bars indicate s.e.m. (q–s) HH20-polarizing regions grafted in place of HH24 wing- and leg-polarizing regions (q). After 24 h, 61 and 65% of cells in grafted wing (n=10, r) and leg (n=11, s) polarizing regions, respectively in G1 compared with 79 and 76% of cells in equal numbers of contralateral limb bud-polarizing regions. In both experiments there is a significant difference in G1 numbers between host and donor polarizing regions (Pearson’s χ2 test—P<0.05) consistent with graft behaving intrinsically. All scale bars, 1 mm.
