In this work, we quantify the mechanical properties of the extra-cellular matrix (ECM) in live zebrafish using Brillouin microscopy. Optimization of the imaging conditions and parameters, combined with careful spectral analysis, allows us to resolve the thin ECM and distinguish its Brillouin frequency shift, a proxy for mechanical properties, from the surrounding tissue. High-resolution mechanical mapping further enables the direct measurement of the thickness of the ECM label-free and in-vivo. We find the ECM to be ~500 nm thick, and in very good agreement with electron microscopy quantification. Our results open the door for future studies that aim to investigate the role of ECM mechanics for zebrafish morphogenesis and axis elongation.

Interactions between the immune and central nervous systems strongly influence brain health. Although the blood-brain barrier restricts this crosstalk, we now know that meningeal gateways through brain border tissues facilitate intersystem communication. Cerebrospinal fluid, which interfaces with the glymphatic system and thereby drains the brain’s interstitial and perivascular spaces, facilitates outward signaling beyond the blood-brain barrier. Here, we report that cerebrospinal fluid can exit into the skull bone marrow. Fluorescent tracers injected into the cisterna magna of mice migrate along perivascular spaces of dural blood vessels and then travel through hundreds of sub-millimeter skull channels into the calvarial marrow. During meningitis, bacteria hijack this route to invade the skull’s hematopoietic niches and initiate cranial hematopoiesis ahead of remote tibial sites. Because skull channels also directly provide leukocytes to meninges, the privileged sampling of brain-derived danger signals in cerebrospinal fluid by regional marrow may have broad implications for inflammatory neurological disorders.

Microdroplets and microbeads were produced with nanometer size precision by observing the optical resonances and used as optical barcodes.