The apical ectodermal ridge is a timer for generating distal limb progenitors

Abstract

The apical ectodermal ridge (AER) is a transient embryonic structure essential for the induction, patterning and outgrowth of the vertebrate limb. However, the mechanism of AER function in limb skeletal patterning has remained unclear. In this study, we genetically ablated the AER by conditionally removing FGFR2 function and found that distal limb development failed in mutant mice. We showed that FGFR2 promotes survival of AER cells and interacts with Wnt/␤-catenin signaling during AER maintenance. Interestingly, cell proliferation and survival were not significantly reduced in the distal mesenchyme of mutant limb buds. We established Hoxa13 expression as an early marker of distal limb progenitors and discovered a dynamic morphogenetic process of distal limb development. We found that premature AER loss in mutant limb buds delayed generation of autopod progenitors, which in turn failed to reach a threshold number required to form a normal autopod. Taken together, we have uncovered a novel mechanism, whereby the AER regulates the number of autopod progenitors by determining the onset of their generation.

Development of the Endochondral Skeleton

“…Hoxa13 and Hoxd13 have been shown genetically to control mesenchymal condensation in the autopod (Box 1) of the mouse (Fromental-Ramain et al 1996;Stadler et al 2001). Hoxa13-and Hoxd13-deficient embryos display defects in cell-cell adhesion mediated through Eph-ephrin signaling molecules that are involved in selfsorting of undifferentiated mesenchyme into skeletal precursors (Wada et al 1998;Lu et al 2008). Specifically, in the Hoxa13 -/ -autopod, the mesenchyme expressed a reduced level of EphA7 that normally demarcates the digit primordia, whereas ephrin A3, a high-affinity ligand for the EphA7 receptor normally marking the periphery of presumptive digits, was expressed diffusively in the autopod mesenchyme in the mutant embryo (Stadler et al 2001).…”

Mechanisms of digit formation: Human malformation syndromes tell the story

“…A failure in AER signaling leading to a collapse in the progenitor cell pool causes severe limb malformations such as the group of ectrodactyly syndromes also called split-hand/foot malformations (reviewed e.g., in Duijf et al, 2003). The distal progenitors will subsequently, under the influence of different signaling cues, differentiate to the lineages found in the growing limb, namely, the dermis in the most superficial layers and cartilage in the center, with the soft connective tissues arising in between Searls, 1973, 1974;Pearse et al, 2007;Lu et al, 2008). …”

Signaling Cascades Governing Cdc42-Mediated Chondrogenic Differentiation and Mensenchymal Condensation

“…Upregulation of cell adhesion proteins such as N-cadherin is a hallmark of condensing cells, whereas loss of cell adhesion molecules is able to abolish condensation (Delise and Tuan 2002;Bobick et al 2009). The molecular mechanisms governing condensation are not fully understood, though several genes have been implicated in this process such as bone morphogenic proteins (BMPs) and SRY (sex determining region Y)-box 9 (Sox9) (Fromental-Ramain et al 1996;Wada et al 1998;Lu et al 2008). Conditional inactivationof Sox9 in limb mesenchymal progenitors leads to the absence of condensations, while simultaneous deletion of Bmp2 and Bmp4 in limb mesenchymes leads to the loss of zeugopod elements and to defective joint articulations (Reversade et al 2005).…”