No abstract

Vertebrates develop organs and appendages in a proportionally coordinated manner, and animals that regenerate them do so to the same dimensions as the original structures. Coordinated proportional growth involves controlled regulation between allometric and isometric growth programs, but it is unclear what executes this control. We show that calcineurin inhibition results in continued allometric outgrowth of regenerating fins beyond their original dimensions. Calcineurin inhibition also maintains allometric growth of juvenile fins and induces it in adult fins. Furthermore, calcineurin activity is low when the regeneration rate is highest, and its activity increases as the rate decreases. Growth measurements and morphometric analysis of proximodistal asymmetry indicate that calcineurin inhibition shifts fin regeneration from a distal growth program to a proximal program. This shift is associated with the promotion of retinoic acid signaling. Thus, we identified a calcineurin-mediated mechanism that operates as a molecular switch between position-associated isometric and allometric growth programs.

Summary Cell differentiation typically occurs with concomitant shape transitions to enable specialized functions. To adopt a different shape, cells need to change the mechanical properties of their surface. However, whether cell surface mechanics control the process of differentiation has been relatively unexplored. Here we show that membrane mechanics gate exit from naive pluripotency of mouse embryonic stem cells. By measuring membrane tension during early differentiation, we find that naive stem cells release their plasma membrane from the underlying actin cortex when transitioning to a primed state. By mechanically tethering the plasma membrane to the cortex by enhancing Ezrin activity or expressing a synthetic signaling-inert linker, we demonstrate that preventing this detachment forces stem cells to retain their naive pluripotent identity. We thus identify a decrease in membrane-to-cortex attachment as a new cell-intrinsic mechanism that is essential for stem cells to exit pluripotency.

We developed a method for visualizing tissues from multicellular organisms using cryo-electron tomography. Our protocol involves vitrifying samples with high-pressure freezing, thinning them with cryo-FIB-SEM (focused-ion-beam scanning electron microscopy) and applying fiducial gold markers under cryogenic conditions to the lamellae post-milling. We applied this protocol to acquire tomograms of vitrified Caenorhabditis elegans embryos and worms, which showed the intracellular organization of selected tissues at particular developmental stages in otherwise intact specimens.

Diatoms are eukaryotic, unicellular algae encased within siliceous cell walls (frustules), which are precisely reproduced generation by generation. The production of this nanostructured silica is under genetic control and the isolation of specific gene products (the proteins silaffins, silacidins) guiding the biomineralization processes, and which are necessary to produce the frustules, has already been described. Under silicon starvation, the amount of silacidins present in the cell walls of Thalassiosira pseudonana increases relative to other proteins. Natsilacidins, the native and highly phosphorylated silacidins are enormously effective in silica precipitation whereas silacidin A', the nonphosphorylated form, is not. This indicates an important role for natsilacidins in the survival of diatoms under silicic acid depleted conditions.