Collagen and myosin fibrils are endogenous harmonophores that both give rise to Second Harmonic Generation (SHG). By combining four polarization SHG images provided by a scanning microscope, we show that the orientation of the principal axis of the nonlinear susceptibility tensor chi(2) can be determined for each pixel of the image. The ratio rho = chi33/chi15 of the principal components of chi(2) of collagen and myosin was obtained with the same method, and found within the range 1.6-1.8 and 0.5-0.6 respectively. The orientation of the principal axis of chi(2) is shown to be correlated to the orientation of the fibrils themselves. This provides a straightforward method, which we call Orientation Field-Second Harmonic Microscopy (OF-SHM), to reconstruct orientation fields of fibrils at various scales and resolutions in different biological systems (from muscle sarcomere to the whole embryo).

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The adult vasculature is comprised of three distinct compartments: the arteries, which carry blood away from the heart and display a divergent flow pattern; the capillaries, where oxygen and nutrient delivery from blood to tissues, as well as metabolic waste removal, occurs; and the veins, which carry blood back to the heart and are characterized by a convergent flow pattern. These compartments are organized in series as regard to flow, which proceeds from the upstream arteries to the downstream veins through the capillaries. However, the spatial organization is more complex, as veins may often be found paralleling the arteries. The factors that control the morphogenesis of this hierarchically branched vascular network are not well characterized. Here, we explain how arteries exert a morphological control on the venous pattern. Indeed, during vertebrate development, the following transition may be observed in the spatial organization of the vascular system: veins first develop in series with the arteries, the arterial and venous territories being clearly distinct in space ͑cis-cis configuration͒. But after some time, new veins grow parallel to the existing arteries, and the arterial and venous territories become overlapped, with extensive and complex intercalation and interdigitation. Using physical arguments, backed up by experimental evidence ͑biological data from the literature and in situ optical and mechanical measurements of the chick embryo yolk-sac and midbrain developing vasculatures͒, we explain how such a transition is possible and why it may be expected with generality, as organisms grow. The origin of this transition lies in the remodeling of the capillary tissue in the vicinity of the growing arteries. This remodeling lays down a prepattern for further venous growth, parallel to the existing arterial pattern. Accounting for the influence of tissue growth, we show that this prepatterned path becomes favored as the body extends. As a consequence, a second flow route with veins paralleling the arteries ͑cis-trans configuration͒ emerges when the tissue extends. Between the cis-cis and cis-trans configurations, all configurations are in principle possible, and self-organization of the vessels contributes to determining their exact pattern. However, the global aspect depends on the size at which the growth stops and on the growth rate.

This Note presents experimental evidence that a hyperbolic tissue flow plays an important role in the establishment of the organization plan of vertebrates. We have followed the development of chicken embryos from the gastrula stage up to the moment when the body plan is recognizable. We have found that establishment of this plan occurs in the presence of a uniform tissue flow which at all stages presents a hyperbolic pattern. The flow is bidirectional in the antero-posterior direction, with a fixed point (stagnation point of the flow) which is a point of zero speed in all directions, in the reference frame of the egg. This stagnation point of the flow is located at the level of the presumptive yolk stalk of the chicken (analogous to the mammal navel). On either sides (left and right) of the body, the flow is also bidirectional. The antero-posterior bidirectionality and the left-right bidirectionality result in splitting of the embryo into four domains with vortex-like flow, with partial mirror symmetry between the left/right halves and top/bottom ones. The center of symmetry is the stagnation point. The broken symmetry of the flow is up-scaled in the adult animal. Areas with straightforward tissue movement are the ones where axial structures develop. The lateral domains with vortex-like flow colocalize with the future limb plates.ß 2011 Published by Elsevier Masson SAS on behalf of Acade ´mie des sciences. R E ´S U M E ĆetteNote pre ´sente un ensemble de preuves expe ´rimentales de ´montrant que la formation du plan d'organisation de l'embryon de poulet passe par un e ´coulement de tissu embryonnaire de nature hyperbolique. Il est sugge ´re ´que ce phe ´nome `ne est ge ´ne ´ral pour la formation des te ´trapodes. A `partir d'une configuration initialement 2D (« coque » ou « plaque » de tissu au sens me ´canique), un e ´coulement viscoe ´lastique de tissu, bidirectionnel dans le sens ante ´ro-poste ´rieur et bidirectionnel dans le sens gauchedroite forme l'embryon aviaire typique. Cet e ´coulement pre ´sente un point neutre, identifie ćomme un point de stagnation de l'e ´coulement. Ce point est rigoureusement situe ´a l'emplacement du pe ´dicule vitellin pre ´somptif des oiseaux (analogue du nombril des mammife `res). Autour du point de stagnation, l'e ´coulement pre ´sente une brisure de syme ´trie se traduisant par une syme ´trie miroir gauche-droite et haut-bas. Cette brisure de syme ´trie est transmise a `tous les stades ulte ´rieurs de de ´veloppement (principe de Curie),

It is getting increasingly evident that physical properties such as elastoviscoplastic properties of living materials are quite important for the process of tissue development, including regulation of genetic pathways. Measuring such properties in vivo is a complicated and challenging task. In this paper, we present an instrument, a scanning air puff tonometer, which is able to map point by point the viscoelastic properties of flat or gently curved soft materials. This instrument is an improved version of the air puff tonometer used by optometrists, with important modifications. The instrument allows one to obtain a direct insight into gradients of material properties in vivo. The instrument capabilities are demonstrated on substances with known elastoviscoplastic properties and several biological objects. On the basis of the results obtained, the role of the gradients of elastoviscoplastic properties is outlined for the process of angiogenesis, limb development, bacterial colonies expansion, etc. which is important for bridging the gaps in the theory of the tissue development and highlighting new possibilities for tissue engineering, based on a clarification of the role of physical features in developing biological material.