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Physical forces drive the movement of tissues within the early embryo. Classical and modern approaches have been used to infer and, in rare cases, measure mechanical properties and the location and magnitude of forces within embryos. Elongation of the dorsal axis is a crucial event in early vertebrate development, yet the mechanics of dorsal tissues in driving embryonic elongation that later support neural tube closure and formation of the central nervous system is not known. Among vertebrates, amphibian embryos allow complex physical manipulation of embryonic tissues that are required to measure the mechanical properties of tissues. In this paper, we measure the stiffness of dorsal isolate explants of frog (Xenopus laevis) from gastrulation to neurulation and find dorsal tissues stiffen from less than 20 Pascal (Pa) to over 80 Pa. By iteratively removing tissues from these explants, we find paraxial somitic mesoderm is nearly twice as stiff as either the notochord or neural plate, and at least 10-fold stiffer than the endoderm. Stiffness measurements from explants with reduced fibronectin fibril assembly or disrupted actomyosin contractility suggest that it is the state of the actomyosin cell cortex rather than accumulating fibronectin that controls tissue stiffness in early amphibian embryos.
Fig. 3. Mechanics of paraxial-medial and paraxial-lateral tissues.(A) Schematic diagram of explants carrying complete sets of paraxial tissues (lateral-medial-medial-lateral or LMML) and those carrying only paraxial lateral tissues (lateral-lateral or LL) of the dorsal axis. (B) Fibronectin fibril localization in transverse sections of LMML and LL explants shows the location of paraxial mesoderm outlined by fibronectin. (C) A view of XmyoD expression through the endodermal face of explants reveals a distinct boundary between paraxial-medial (XmyoD expressing) and paraxial-lateral (non-expressing) tissues of control dorsal isolates and LMML explants. XmyoD is absent from most of the length of LL explants that can occasionally express a small `wedge' of XmyoD in their most posterior third (arrowhead). All explants are positioned so that anterior is at the top of the panel. (D) Frames and explant lengths over time from a single representative time-lapse (n=6) demonstrate that LMML and LL explants elongate to the same degree as control explants. (E) The stiffness of LL and MM isolates show that MM explants are significantly more stiff than LL explants. *P<0.05; **P<0.01.
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