Dealy C N, Kosher R A
Department of Anatomy, University of Connecticut Health Center, Farmington, Connecticut, 06030, USA.
Dev Biol. 1996 Jul 10;177(1):291-9. doi: 10.1006/dbio.1996.0163.
Acquisition of limb-forming ability by discrete regions of the lateral plate of the chick embryo is dependent on a medial-lateral inductive signaling cascade moving sequentially from the area of Hensen's node to the somitic mesoderm, the intermediate mesoderm, and then to the prospective limb-forming regions of the lateral plate. IGF-I and insulin are expressed by medial tissues as they are influencing the prospective limb-forming regions of the lateral plate. Here we report that IGF-I and insulin, but not FGF-2 or FGF-4, induce the formation of limb bud-like structures in vitro from prospective limb regions before they have acquired the ability to form limbs independent of medial tissues, and also induce the formation of limb bud-like structures from the prospective flank. The limb bud-like structures induced by IGF-I and insulin possess a thickened cap of ectoderm along their distal tips that resembles the apical ectodermal ridge (AER) and this thickened distal apical ectoderm expresses the AER-characteristic homeobox-containing gene Msx-2. Like in normal limb buds, a population of highly proliferating cells which express the homeobox-containing gene Msx-1 are localized in the mesoderm directly subjacent to the thickened AER-like structures induced by IGF-I and insulin. However, the limb bud-like structures induced by IGF-I and insulin do not express sonic hedgehog, which encodes a secreted signaling molecule that has been implicated in regulating the anteroposterior patterning of the developing limb bud. IGF-I- and insulin-treated prospective limb explants give rise to rudimentary limbs containing identifiable skeletal elements when grafted into the coelom or to somites of host embryos. Overall, these results suggest that IGF-I and insulin may be endogenous signals produced by medial tissues that are involved in conferring limb-forming ability to the lateral plate and may promote the initial outgrowth of limb buds and possibly induce the AER. However, other signals are necessary to promote the expression of genes such as sonic hedgehog that regulate the patterning of the developing limb.
鸡胚侧板离散区域获得肢体形成能力依赖于从亨森结区域依次向体节中胚层、中间中胚层，然后到侧板预期肢体形成区域移动的中-外侧诱导信号级联。当内侧组织影响侧板预期肢体形成区域时，IGF-I和胰岛素在内侧组织中表达。在此我们报告，IGF-I和胰岛素而非FGF-2或FGF-4，能在体外从前体肢区域诱导形成肢体芽样结构，此时它们尚未获得独立于内侧组织形成肢体的能力，并且还能从前体侧腹区域诱导形成肢体芽样结构。由IGF-I和胰岛素诱导形成的肢体芽样结构在其远端顶端具有增厚的外胚层帽，类似于顶端外胚层嵴（AER），并且这种增厚的远端顶端外胚层表达含AER特征性同源框的基因Msx-２。与正常肢体芽一样，一群表达含同源框基因Msx-1的高度增殖细胞定位于由IGF-I和胰岛素诱导形成的类似AER结构下方的中胚层中。然而，由IGF-I和胰岛素诱导形成的肢体芽样结构不表达音猬因子，该因子编码一种分泌信号分子，参与调节发育中肢体芽的前后模式形成。当移植到宿主胚胎的体腔或体节中时，经IGF-I和胰岛素处理的前体肢外植体可产生含有可识别骨骼成分的雏形肢体。总体而言，这些结果表明，IGF-I和胰岛素可能是内侧组织产生的内源性信号，参与赋予侧板肢体形成能力，并可能促进肢体芽的初始生长，还可能诱导AER形成。然而，需要其他信号来促进诸如音猬因子等调节发育中肢体模式形成的基因的表达。
