SUMMARYThe tailbud is the posterior leading edge of the growing vertebrate embryo and consists of motile progenitors of the axial skeleton, musculature and spinal cord. We measure the 3D cell flow field of the zebrafish tailbud and identify changes in tissue fluidity revealed by reductions in the coherence of cell motion without alteration of cell velocities. We find a directed posterior flow wherein the polarization between individual cell motion is high, reflecting ordered collective migration. At the posterior tip of the tailbud, this flow makes sharp bilateral turns facilitated by extensive cell mixing due to increased directional variability of individual cell motions. Inhibition of Wnt or Fgf signaling or cadherin 2 function reduces the coherence of the flow but has different consequences for trunk and tail extension. Modeling and additional data analyses suggest that the balance between the coherence and rate of cell flow determines whether body elongation is linear or whether congestion forms within the flow and the body axis becomes contorted.

Regeneration of several organs involves adaptive reprogramming of progenitors, however, the intrinsic capacity of the developing brain to replenish lost cells remains largely unknown. In this study, we discovered that the developing cerebellum has unappreciated progenitor plasticity, since it undergoes near full growth and functional recovery following acute depletion of granule cells, the most plentiful neuron population in the brain. We demonstrate that following postnatal ablation of granule cell progenitors, Nestin-expressing progenitors (NEPs) specified during mid-embryogenesis to produce astroglia and interneurons, switch their fate and generate granule neurons in mice. Moreover, Hedgehog-signaling in two NEP populations is crucial not only for the compensatory replenishment of granule neurons but also to scale interneuron and astrocyte numbers. Thus we provide insights into the mechanisms underlying robustness of circuit formation in the cerebellum, and speculate that adaptive reprogramming of progenitors in other brain regions plays a greater role than appreciated in developmental regeneration.

Summary The diverse morphologies of animal tissues are underlain by different configurations of adherent cells and extracellular matrix (ECM). Here, we elucidate a cross-scale mechanism for tissue assembly and ECM remodeling involving Cadherin 2, the ECM protein Fibronectin and its receptor Integrin α5. Fluorescence crosscorrelation spectroscopy within the zebrafish paraxial mesoderm mesenchyme reveals a physical association between Integrin α5 on adjacent cell membranes. This Integrin-Integrin complex correlates with conformationally inactive Integrin. Cadherin 2 stabilizes both the Integrin association and inactive Integrin conformation. Thus, Integrin repression within the adherent mesenchymal interior of the tissue biases Fibronectin fibrillogenesis to the tissue surface lacking cell-cell adhesions. Along nascent somite boundaries, Cadherin 2 levels decrease becoming anti-correlated with levels of Integrin α5. Simultaneously, Integrin α5 clusters and adopts the active conformation and then commences ECM assembly. This cross-scale regulation of Integrin activation organizes a stereotypic pattern of ECM necessary for vertebrate body elongation and segmentation.

Summary During embryonic development and tissue homeostasis, cells produce and remodel the extracellular matrix (ECM). The ECM maintains tissue integrity and can serve as a substrate for cell migration. Integrin α5 (Itgα5) and αV (ItgαV) are the α subunits of the Integrins most responsible both for cell adhesion to the ECM protein Fibronectin (FN) and for Fibronectin matrix fibrillogenesis [1, 2]. We perform a systems level analysis of cell motion in the zebrafish tailbud during trunk elongation in the presence and absence of normal cell-Fibronectin interactions. Integrin α5 and Integrin αV have well-described roles in cell migration in vitro. However, we find that concomitant loss of integrin α5 and integrin αV leads to a trunk elongation defect without substantive alteration of cell migration. Tissue-specific transgenic rescue experiments suggest that the Fibronectin matrix on the surface of the paraxial mesoderm is required for body elongation via its role in defining tissue mechanics and inter-tissue adhesion.

A curious feature of organ and organoid morphogenesis is that in certain cases, spatial oscillations in the thickness of the growing “film” are out of phase with the deformation of the slower-growing “substrate,” while in other cases, the oscillations are in phase. The former cannot be explained by elastic bilayer instability, and contradict the notion that there is a universal mechanism by which brains, intestines, teeth, and other organs develop surface wrinkles and folds. Inspired by the microstructure of the embryonic cerebellum, we develop a new model of 2D morphogenesis in which system-spanning elastic fibers endow the organ with a preferred radius, while a separate fiber network resides in the otherwise fluidlike film at the outer edge of the organ and resists thickness gradients thereof. The tendency of the film to uniformly thicken or thin is described via a “growth potential.” Several features of cerebellum, +blebbistatin organoid, and retinal fovea morphogenesis, including out-of-phase behavior and a film thickness amplitude that is comparable to the radius amplitude, are readily explained by our simple analytical model, as may be an observed scale invariance in the number of folds in the cerebellum. We also study a nonlinear variant of the model, propose further biological and bioinspired applications, and address how our model is and is not unique to the developing nervous system.