Axons in the Chick Embryo Follow Soft Pathways Through Developing Somite Segments.

During patterning of the peripheral nervous system, motor axons grow sequentially out of the neural tube in a segmented fashion to ensure functional integration of the motor roots between the surrounding cartilage and bones of the developing vertebrae. This segmented outgrowth is regulated by the intrinsic properties of each segment (somite) adjacent to the neural tube, and in particular by chemical repulsive guidance cues expressed in the posterior half. Yet, knockout models for such repulsive cues still display initial segmentation of outgrowing motor axons, suggesting the existence of additional, yet unknown regulatory mechanisms of axon growth segmentation. As neuronal growth is not only regulated by chemical but also by mechanical signals, we here characterized the mechanical environment of outgrowing motor axons. Using atomic force microscopy-based indentation measurements on chick embryo somite strips, we identified stiffness gradients in each segment, which precedes motor axon growth. Axon growth was restricted to the anterior, softer tissue, which showed lower cell body densities than the repulsive stiffer posterior parts at later stages. As tissue stiffness is known to regulate axon growth during development, our results suggest that motor axons also respond to periodic stiffness gradients imposed by the intrinsic mechanical properties of somites.

Author Correction: Niche stiffness underlies the ageing of central nervous system progenitor cells.

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

Niche stiffness underlies the ageing of central nervous system progenitor cells.

Ageing causes a decline in tissue regeneration owing to a loss of function of adult stem cell and progenitor cell populations1. One example is the deterioration of the regenerative capacity of the widespread and abundant population of central nervous system (CNS) multipotent stem cells known as oligodendrocyte progenitor cells (OPCs)2. A relatively overlooked potential source of this loss of function is the stem cell 'niche'-a set of cell-extrinsic cues that include chemical and mechanical signals3,4. Here we show that the OPC microenvironment stiffens with age, and that this mechanical change is sufficient to cause age-related loss of function of OPCs. Using biological and synthetic scaffolds to mimic the stiffness of young brains, we find that isolated aged OPCs cultured on these scaffolds are molecularly and functionally rejuvenated. When we disrupt mechanical signalling, the proliferation and differentiation rates of OPCs are increased. We identify the mechanoresponsive ion channel PIEZO1 as a key mediator of OPC mechanical signalling. Inhibiting PIEZO1 overrides mechanical signals in vivo and allows OPCs to maintain activity in the ageing CNS. We also show that PIEZO1 is important in regulating cell number during CNS development. Thus we show that tissue stiffness is a crucial regulator of ageing in OPCs, and provide insights into how the function of adult stem and progenitor cells changes with age. Our findings could be important not only for the development of regenerative therapies, but also for understanding the ageing process itself.

Effect of vital dyes on human corneal endothelium and elasticity of Descemet's membrane.

The purpose of this study was to evaluate the effects of vital dyes on human Descemet's membranes (DMs) and endothelia. DMs of 25 human cadaveric corneas with research consent were treated with dyes routinely used in Descemet membrane endothelial keratoplasty (DMEK), 0.05% Trypan blue (TB) or a combination of 0.15% Trypan blue, 0.025% Brilliant blue and 4% Polyethylene glycol (commercial name Membrane Blue Dual; MB). The effects of these two dyes on (i) endothelial cell viability, (ii) DM mechanical properties as assessed by atomic force microscopy, and iii) qualitative DM dye retention were tested for two varying exposure times (one or four minutes). No significant differences in cell toxicity were observed between treatments with TB and MB at the two different exposure times (P = 0.21). Further, both dyes led to a significant increase in DM stiffness: exposure to TB and MB for one minute increased the apparent elastic modulus of the DM by 11.2% (P = 8*10-3) and 17.7%, respectively (P = 4*10-6). A four-minute exposure led to an increase of 8.6% for TB (P = 0.004) and 13.6% for MB (P = 0.03). Finally, at 25 minutes, the dye retention of the DM was considerably better for MB compared to TB. Taken together, a one-minute exposure to MB was found to improve DM visibility compared to TB, with a significant increase in DM stiffness and without detrimental effects on endothelial cell viability. The use of MB could therefore improve (i) visibility of the DM scroll, and (ii) intraoperative unfolding, enhancing the probability of successful DMEK surgery.

The role of cell body density in ruminant retina mechanics assessed by atomic force and Brillouin microscopy.

Cells in the central nervous system (CNS) respond to the stiffness of their environment. CNS tissue is mechanically highly heterogeneous, thus providing motile cells with region-specific mechanical signals. While CNS mechanics has been measured with a variety of techniques, reported values of tissue stiffness vary greatly, and the morphological structures underlying spatial changes in tissue stiffness remain poorly understood. We here exploited two complementary techniques, contact-based atomic force microscopy and contact-free Brillouin microscopy, to determine the mechanical properties of ruminant retinae, which are built up by different tissue layers. As in all vertebrate retinae, layers of high cell body densities ('nuclear layers') alternate with layers of low cell body densities ('plexiform layers'). Different tissue layers varied significantly in their mechanical properties, with the photoreceptor layer being the stiffest region of the retina, and the inner plexiform layer belonging to the softest regions. As both techniques yielded similar results, our measurements allowed us to calibrate the Brillouin microscopy measurements and convert the Brillouin shift into a quantitative assessment of elastic tissue stiffness with optical resolution. Similar as in the mouse spinal cord and the developing Xenopus brain, we found a strong correlation between nuclear densities and tissue stiffness. Hence, the cellular composition of retinae appears to strongly contribute to local tissue stiffness, and Brillouin microscopy shows a great potential for the application in vivo to measure the mechanical properties of transparent tissues.

The soft mechanical signature of glial scars in the central nervous system.

Injury to the central nervous system (CNS) alters the molecular and cellular composition of neural tissue and leads to glial scarring, which inhibits the regrowth of damaged axons. Mammalian glial scars supposedly form a chemical and mechanical barrier to neuronal regeneration. While tremendous effort has been devoted to identifying molecular characteristics of the scar, very little is known about its mechanical properties. Here we characterize spatiotemporal changes of the elastic stiffness of the injured rat neocortex and spinal cord at 1.5 and three weeks post-injury using atomic force microscopy. In contrast to scars in other mammalian tissues, CNS tissue significantly softens after injury. Expression levels of glial intermediate filaments (GFAP, vimentin) and extracellular matrix components (laminin, collagen IV) correlate with tissue softening. As tissue stiffness is a regulator of neuronal growth, our results may help to understand why mammalian neurons do not regenerate after injury.

Interkinetic nuclear migration is centrosome independent and ensures apical cell division to maintain tissue integrity.

Pseudostratified epithelia are widespread during animal development and feature elongated cells whose nuclei adopt various positions along the apicobasal cell axis. Before mitosis, nuclei migrate toward the apical surface, and subsequent divisions occur apically. So far, the exact purpose of this nuclear migration remained elusive. One hypothesis was that apical migration ensures that nuclei and centrosomes meet for mitosis. We here demonstrate that in zebrafish neuroepithelia apical nuclear migration occurs independently of centrosome position or integrity. It is a highly reproducible phenomenon linked to the cell cycle via CDK1 activity. We propose that the robustness of bringing nuclei apically for mitosis ensures that cells are capable of reintegrating into the epithelium after division. Nonapical divisions lead to cell delamination and formation of cell clusters that subsequently interfere with neuronal layering. Therefore, positioning divisions apically in pseudostratified neuroepithelia could serve to safeguard epithelial integrity and enable proper proliferation and maturation.

Mitotic position and morphology of committed precursor cells in the zebrafish retina adapt to architectural changes upon tissue maturation.

The development of complex neuronal tissues like the vertebrate retina requires the tight orchestration of cell proliferation and differentiation. Although the complexity of transcription factors and signaling pathways involved in retinogenesis has been studied extensively, the influence of tissue maturation itself has not yet been systematically explored. Here, we present a quantitative analysis of mitotic events during zebrafish retinogenesis that reveals three types of committed neuronal precursors in addition to the previously known apical progenitors. The identified precursor types present at distinct developmental stages and exhibit different mitotic location (apical versus nonapical), cleavage plane orientation, and morphology. Interestingly, the emergence of nonapically dividing committed bipolar cell precursors can be linked to an increase in apical crowding caused by the developing photoreceptor cell layer. Furthermore, genetic interference with neuronal subset specification induces ectopic divisions of committed precursors, underlining the finding that progressing morphogenesis can effect precursor division position.

Ectopic expression of single transcription factors directs differentiation of a medaka spermatogonial cell line.

The capability to form all cell types of the body is a unique feature of stem cells. However, many questions remain concerning the mechanisms regulating differentiation potential. The derivation of spermatogonial cell lines (SGs) from mouse and human, which can differentiate across germ-layer borders, suggested male germ cells as a potential stem cell source in addition to embryonic stem cells. Here, we present a differentiation system using an SG of the vertebrate model organism Oryzias latipes (medaka). We report differentiation of this cell line into 4 different ectodermal and mesodermal somatic cell types. In addition to differentiation into adipocytes by retinoic acid treatment, we demonstrate for the first time that directed differentiation of an SG can be induced by ectopic expression of single transcription factors, completely independent of culture conditions. Transient transfection with mitf-m, a transcription factor that has been shown to induce differentiation into melanocytes in medaka embryonic stem cells, resulted in the formation of the same cell type in spermatogonia. Similarly, the formation of neuron-like cells and matrix-depositing osteoblasts was induced by ectopic expression of mash1 and cbfa1, respectively. Interestingly, we found that the expression of all mentioned fate-inducing transcription factors leads to recapitulation of the temporal pattern of marker gene expression known from in vivo studies.