科學家發(fā)現(xiàn)了一個在胚胎發(fā)育早期限制肢體生長的信號機制。發(fā)育過程中的許多步驟取決于胚胎的信號中心分泌的生化物質(zhì),。科學家對于信號中心是如何形成的已經(jīng)有了較多的了解,，但是對于一個特定的發(fā)育階段完成后，停止生長信號的機制了解得不多,。Paul J. Scherz 和同事描述了一個保證肢體停止生長的機制,。它涉及一個后肢芽的細胞生長的自然障礙，肢芽是肢體的胚胎生長中心,。當肢芽長到比分泌的生長因子的范圍大些時,，生長信號被肢體生長的自然限制機制所中斷。

The Limb Bud Shh-Fgf Feedback Loop Is Terminated by Expansion of Former ZPA Cells
Vertebrate limb outgrowth is driven by a positive feedback loop involving Sonic Hedgehog (Shh), Gremlin, and Fgf4. By overexpressing individual components of the loop at a time after these genes are normally down-regulated in chicken embryos, we found that Shh no longer maintains Gremlin in the posterior limb. Shh-expressing cells and their descendants cannot express Gremlin. The proliferation of these descendants forms a barrier separating the Shh signal from Gremlin-expressing cells, which breaks down the Shh-Fgf4 loop and thereby affects limb size and provides a mechanism explaining regulative properties of the limb bud.

Fig. 1. The Shh-Fgf feedback loop is disrupted between Shh and Gremlin. (A and B) An Fgf4 bead (A) maintains Shh expression in chick limbs at E7 (stage 28), unlike controls (B). (C to F) Ectopic Gremlin viral expression maintains both Fgf4 (C) and Shh (E) expression at E7 (stage 28) in chick limbs at higher levels than controls [(D) and (F)]. (G to J) A Shh bead implanted into the posterior chick limb is unable to maintain either Gremlin (G) or Fgf4 (I) expression at E7 (stage 28) as in controls [(H) and (J)]. (K) A Shh bead maintains ectopic Gremlin expression in the anterior at E7 (stage 28). The diagram on the bottom right shows the step of the feedback loop being tested in each experiment.

Fig. 2. Shh descendants are unable to express Gremlin. (A to C) Double in situ hybridizations for Gremlin (purple) and Shh (brown). At E4 (stage 23) in the chick, the domains of Gremlin and Shh expression are close together (A), but by E5.5 (stage 26) a gap has opened between them (B). The situation is similar in the mouse at E11.5 (C). (D) Shh descendants detected by ß-galactosidase staining of a Shh::CRE;R26R mouse at E11.5 closely resemble the domain of posterior Gremlin exclusion. (E to J) An in situ hybridization in mouse limbs for Gremlin [(E) and (F)], ß-galactosidase staining for Shh descendants [(G) and (H)] and the merged images [(I) and (J)]. At E11.5, Gremlin expression is contiguous but nonoverlapping with Shh descendants [(E), (G), and (I)]. Non-Shh descendants that are mixed into the Shh descendant domain express Gremlin, showing Gremlin repression is cell autonomous [(F), (H), and (J)].

Fig. 3. The Shh-Fgf4 signaling loop is prolonged by the removal of Shh descendants, leading to size regulation. (A to H) By removing a wedge of Shh descendants in E5 (stage 25) chick limbs and stapling the ZPA still expressing Shh to the anterior cells, Shh (A), Gremlin (C), and Fgf4 [(E) and (G)] expression is maintained compared with control limbs, [(B), (D), (F), and (H)] showing that the expansion of Shh descendants blocks the Shh-Fgf4 signaling loop. (G) and (H) are higher magnifications of (E) and (F), respectively. (I to M) Limbs in which Shh descendants are removed and the ZPA stapled to anterior cells show size regulation (I) to become comparable in anterior-posterior and proximal-distal patterning and size to contralateral control limbs (J). Limbs were injected with two adjacent dots of DiI (arrows) (K), and then either the Shh descendants were removed and the remaining tissue stapled (L) or the DiI-injected limbs were left unoperated (M). The cells in between the dots expand in the operated limbs, showing that Shh-expressing cells continue to proliferate until the distance between Shh and Gremlin expression lies outside the range of Shh diffusion again, thereby giving size regulation. (N to P) Ectopic expression of Gremlin in the posterior limb bud to E10 (stage 36) with the use of a virus, detected with an antibody, 3C2, against viral MA antigen (arrow) (P), gives rise to elongated growth of the posterior tissue (N) when compared with contralateral controls (O). Length of the ectopic growths were measured from the tip of the ectopic soft tissue to the proximal end of digit 4, which still forms cartilage.