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. 2017 Oct 31;8:1218. doi: 10.1038/s41467-017-01120-3

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 9 — Osr1-deficiency impairs the myogenic niche in developing limbs. a Decreased abundance of COL6 and FN (Fibronectin) in proximal (inserts 1 and 3) and dorsal (inserts 2 and 4) muscle primordia of Osr1-deficient E11.5 limbs. Boxed regions are shown in the numbered panels to the right and are magnifications of the area. b Reduced abundance of interstitial FN and disarrangement of COL14 in E13.5 Osr1 ^GCE/GCE limb muscles as compared to wild-type littermate controls. c Extracellular matrix (ECM) produced by Osr1^GCE/+ cells, but not ECM produced by Osr1^GCE/GCE cells supports myoblast proliferation (n = 3). d E11.5 Col6a1 ^−/− embryos show a decrease in myogenic progenitor numbers (n = 3). e Transwell assay shows a beneficial effect of E13.5 Osr1^GCE/+ embryonic FAPs (eFAPs) but not Osr1^GCE/GCE eFAPs on myogenesis. Transwell culture without eFAPs was used as control (n = 3). f Cxcr4 ^−/− embryos show muscle defects overlapping with Osr1^GCE/GCE embryos. g Sections of Cxcr4 ^+/+ and Cxcr4 ^−/− embryos show almost complete reduction of the biceps femoris anterior head and hypoplasia of the biceps femoris accessory head. h Decreased phosphorylated ERK (pERK) and (i) CyclinD1 in myogenic Lbx1⁺ cells in E11.5 Osr1 mutants (n = 3). j Schematic model of Osr1⁺ cell function. Osr1⁺ cells (green) produce chemokines as Cxcl12 and a specific extracellular matrix (ECM) supporting myoblast proliferation and survival involving pERK signaling. Cxcl12 in addition likely provides local migratory cues ensuring correct spatial distribution of myoblasts as a prerequisite for muscle patterning. k Schematic model of limb muscle connective tissue compartmentalization. Specific transcription factors (Osr1 and Tcf4 are shown as examples) are expressed regionally in the limb bud with areas of overlap creating a regionalized code that likely determines the composition of the local niche. Error bars represent s.e.m. T-test: *=p < 0.05; **=p < 0.01; ***=p < 0.001. N-numbers indicate biological replicates, i.e. samples from different specimen in d, h, i or independent assays in c, e