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Numerous biophysical hypotheses invoke tissue stiffness as a key parameter for shaping tissue during development and for influencing cell behaviours during disease progression. However, currently available methods are insufficient to test hypotheses that concern the physical properties of bulk tissues. Here we introduce, validate and apply a new 3D magnetic device that generates a uniform magnetic field gradient within a space that is sufficient to accommodate a vertebrate, organ-stage embryo under live conditions. The device allows for rapid, nontoxic measurement of the spatial variation of absolute elastic modulus and viscosity deep within mesenchymal tissues and within epithelia. By applying the device to map the spatiotemporal variation of viscoelastic properties within the early mouse limb bud, we identified an anteriorly biased mesodermal stiffness gradient along which cells move collectively to shape the early bud. Tissue stiffness corresponds to the nascent expression domain of fibronectin that isWnt5a-dependent. The findings challenge the notion thatWnt5aregulates cell movements by chemotaxis, and raises the possibility thatWnt5amodifies the tissue microenvironment to promote durotaxisin vivo. Importantly, the ability to precisely measure tissue stiffness in 3D has the potential to instigate and refine mechanisms of development and disease progression.

Functional annotation of mutations that cause human limb anomalies is enabled by basic developmental studies. In this study, we focus on the prepatterning stage of limb development and discuss a recent model that proposes anterior and posterior domains of the early limb bud generate two halves of the future skeleton. By comparing phenotypes in humans with those in model organisms, we evaluate whether this prepatterning concept helps to annotate human disease alleles. WIREs Dev Biol 2017, 6:e270. doi: 10.1002/wdev.270 For further resources related to this article, please visit the WIREs website.

The establishment of trophectoderm (TE) manifests as the formation of epithelium, and is dependent on many structural and regulatory components that are commonly found and function in many epithelial tissues. However, the mechanism of TE formation is currently not well understood. Prickle1 (Pk1), a core component of the planar cell polarity (PCP) pathway, is essential for epiblast polarization before gastrulation, yet the roles of Pk family members in early mouse embryogenesis are obscure. Here we found that Pk2−/− embryos died at E3.0–3.5 without forming the blastocyst cavity and not maintained epithelial integrity of TE. These phenotypes were due to loss of the apical–basal (AB) polarity that underlies the asymmetric redistribution of microtubule networks and proper accumulation of AB polarity components on each membrane during compaction. In addition, we found GTP-bound active form of nuclear RhoA was decreased in Pk2−/− embryos during compaction. We further show that the first cell fate decision was disrupted in Pk2−/− embryos. Interestingly, Pk2 localized to the nucleus from the 2-cell to around the 16-cell stage despite its cytoplasmic function previously reported. Inhibiting farnesylation blocked Pk2's nuclear localization and disrupted AB cell polarity, suggesting that Pk2 farnesylation is essential for its nuclear localization and function. The cell polarity phenotype was efficiently rescued by nuclear but not cytoplasmic Pk2, demonstrating the nuclear localization of Pk2 is critical for its function.

Mutant mice deficient in the fibroblast growth factor 10 (Fgf10) gene exhibit an eye-open phenotype at birth. It has previously been shown that FGF10 has a dual role in proliferation and migration during the early and later stages of eyelid development, respectively. To verify the role of FGF10 during eyelid closure, explant culture of Fgf10-null eyelid anlagen was performed, by which it was examined whether or not exogenous FGF10 could rescue the expression of activin betaB and transforming growth factor alpha, known to be required for eyelid closure. We found that the expression of these genes was markedly induced while that of Shh or Ptch1, Ptch2 was not. We also observed the distribution of filamentous actin (F-actin) after FGF10 application in the mutant eyelid explant, finding that the FGF10 protein induced F-actin accumulation. We further examined filopodia of the eyelid leading edge cells, finding the length of the filopodia was significantly reduced in the mutant. These results verify that FGF10 promotes eyelid closure through activating activin and TGFalpha-EGFR signaling.

The development of avian cutaneous appendages, feathers and scales, is known to arise from the epithelial–mesenchymal interaction. Here we show that FGF10 is associated with this developmental process as an early signal from mesenchymal cells underlying nascent cutaneous placodes. Expression of Fgf10 was detected in the mesenchymal cells underneath the developing placodes. Forced expression of Fgf10 in the femoral skin suppressed expression of Shh and a zinc finger gene snail-related (cSnR), while induced expression of Bmp2 in the interbud region, resulting in thickening of the epidermal layer. Furthermore, forced expression of Fgf10 in the foot skin caused marked ingrowings of the epidermis. The cells in the epidermal ingrowings expressed β-catenin, proliferating cell nuclear antigen, and an epidermal stem cell marker p63. These results support the idea that FGF10 is a mesenchymally derived stimulator of epidermal development through crosstalk with bone morphogenetic protein (BMP), β-catenin, and other signaling pathways.

The development of the eyelid requires coordinated cellular processes of proliferation, cell shape changes, migration and cell death. Mutant mice deficient in the fibroblast growth factor 10 ( Fgf10 ) gene exhibit open-eyelids at birth. To elucidate the roles of FGF10 during eyelid formation, we examined the expression pattern of Fgf10 during eyelid formation and the phenotype of Fgf10- null eyelids in detail. Fgf10 is expressed by mesenchymal cells just beneath the protruding epidermal cells of the nascent eyelid. However, Fgf10 -null epithelial cells running though the eyelid groove do not exhibit typical cuboid shape or sufficient proliferation. Furthermore, peridermal clumps are not maintained on the eyelid leading edge, and epithelial extension does not occur. At the cellular level, the accumulation of actin fibers is not observed in the mutant epithelial leading edge. The expression of activin/inhibin βB ( Act β B/Inhbb ) and transforming growth factor α ( Tgfa ), previously reported to be crucial for eyelid development, is down-regulated in the mutant leading edge, while the onset of sonic hedgehog ( Shh ) expression is delayed on the mutant eyelid margin. Explant cultures of mouse eyelid primordia shows that the open-eyelid phenotype of the mutant is reduced by exogenous FGF10 protein, and that the expression of Act β B and Tgfa is ectopically induced in the thickened eyelid epithelium by the FGF10 protein. These results indicate a dual role of FGF10 in mouse eyelid development, for both proliferation and coordinated migration of eyelid epithelial cells by reorganization of the cytoskeleton, through the regulation of activin, TGFα and SHH signaling.