The research team, working at the Hong Kong University of Science and Technology, evaluated four transcranial approaches in transgenic mice whose microglia — the brain’s resident immune cells — glow with green fluorescent protein. The thinned-skull window removes only the outer bone layer, leaving a delicate membrane roughly 20 to 50 micrometers thick. The PoRTS window polishes and reinforces that thinned bone with transparent cyanoacrylate cement, extending the usable field of view to three by three millimeters. The optical clearing window leaves the skull intact and instead strips out lipids and collagens chemically while matching the bone’s refractive index, rendering it transparent. Finally, the team introduced a hybrid they call the thinned-clearing window, which thins the skull first and then applies the clearing agents, dramatically accelerating a process that can otherwise take more than an hour in older animals.

That hybrid design proved to be the standout performer. In aged mice, blood vessels become embedded in the spongy inner bone, and clearing an intact skull around them is slow and often incomplete. By thinning the outer layer first, the researchers cut clearing time from roughly an hour to about ten minutes and achieved a 1.7-fold boost in fluorescence signal compared with the thinned-skull window alone. High-resolution imaging depth also improved significantly, from roughly 120 to 150 micrometers under the other windows to about 200 micrometers under the thinned-clearing window — enough to resolve the finest branches of microglial processes that blur away under conventional optics.

To understand precisely why image quality collapses with depth, the team turned to adaptive optics. Using a wavefront-sensor-based system that measures optical distortion directly from fluorescent guide stars — individual microglia at depths of 50, 150, and 250 micrometers — they quantified wavefront errors across all window types. Surprisingly, the root-mean-square and peak-to-valley aberration metrics showed no significant differences between windows, even though the thinned-clearing window delivered brighter, deeper images. The likely explanation is that the extra improvement comes from reduced scattering, which wavefront sensors are relatively insensitive to. What the measurements did reveal is that aberrations climb steeply with depth, because the focused laser beam sweeps through a wider cone of bone deeper in the tissue, accumulating distortion along the way.

The team then deployed a state-of-the-art adaptive optics two-photon microscope, known as the ALPHA-FSS system, which senses the distorted light field directly from fluorescent cells and compensates for it using a spatial light modulator. Through a thinned-clearing window, this system resolved the finest microglial processes at depths beyond 440 micrometers — more than double the reach of standard two-photon imaging through the same window. The improvement was consistent across all four window types, underscoring that adaptive optics is becoming essential equipment for anyone hoping to image the cortex at cellular resolution through intact bone.

But the most consequential discovery came from simply watching the windows age. Over 21 days of longitudinal imaging, fluorescence signals decayed rapidly in every window type, and the decay coincided with a growing layer of bone appearing on the inner surface of the skull. Third-harmonic generation imaging confirmed the diagnosis: mature osteocytes were visible in the original bone, and newly formed osteocytes appeared beneath it, marking fresh woven bone growing from the endosteum — the inner bone surface that no surgical preparation touches. Because the outer periosteum is removed during window preparation, the regenerative machinery of the bone is unleashed from the inside, driven by pathways such as Wnt and BMP signaling released from the stressed dura mater. Even re-thinning or re-clearing the window failed to restore the original signal, because this immature woven bone has optical properties that resist the clearing chemistry.

Recognizing bone regrowth as the fundamental bottleneck, the researchers pursued a counterintuitive strategy: exploiting a notorious side effect of glucocorticoid hormones, which suppress bone formation by inhibiting osteoblasts and promoting bone resorption. Applying an off-the-shelf dexamethasone ointment directly to the thinned skull held regrowth at bay for over four weeks, keeping fluorescence signals stable while untreated windows degraded within a week. Hydrocortisone ointment produced a similar effect. Yet glucocorticoids are small molecules that penetrate the meninges and diffuse into brain tissue, and the team observed that the commercial ointments displaced microglia and altered their morphology in the superficial cortex — a reminder that the brain’s immune sentinels are exquisitely sensitive to pharmacological interference.

The solution was dose control. By delivering dexamethasone through a drug-loaded sponge at a concentration tenfold lower than the commercial ointment — 0.0135 percent — the researchers still suppressed skull regrowth for 28 days while leaving microglial morphology in the superficial cortex essentially unchanged, with more than 70 percent of cells remaining stationary at the surface and no significant displacement below 60 micrometers. The team then went further, replacing the sponge with a transparent hydrogel made of hyaluronic acid methacrylate, loaded with the same moderate dexamethasone dose and sealed under a coverslip. The hydrogel window maintained high optical transparency, inhibited skull thickening for more than four weeks, and — because the drug released slowly — produced no detectable changes in microglial shape or distribution. The main limitation is dehydration: the hydrogel shrinks within a few days and must be refreshed every three to four days, a problem the authors expect future anti-dehydration hydrogel formulations to solve.

The study also delivered a practical warning about sealing materials. When thinned-skull windows were sealed with silicone gel, a common choice in the field, the researchers observed a rapid influx of inflammatory myeloid cells into the dura within days, scattering light and destroying image quality. Windows sealed with a removable UV gel showed no such collapse, suggesting that inadequate sealing allows infection-driven inflammation that had previously been mistaken for an inherent limitation of the technique. For laboratories choosing among window designs, the message is that sealant choice can matter as much as surgical skill.

Together, the findings give the neuroscience community something it has lacked: a quantitative map of how long each transcranial window lasts, how deep it can see, and what actually degrades it over time. Skull regrowth, it turns out, is not a nuisance specific to one preparation but a universal response that accelerates after the first week and cannot be reversed by re-treating the window. Suppressing it with carefully dosed glucocorticoids, delivered through an optically transparent hydrogel, extends high-quality imaging from days to weeks — a window of time long enough to track disease progression, therapeutic effects, and the slow choreography of the brain’s immune cells in health and disease. The authors caution that no concentration of dexamethasone eliminated microglial effects entirely, and that future work should seek agents that silence bone regrowth while remaining confined to the skull. For now, the combination of the thinned-clearing window, adaptive optics, and drug-loaded hydrogels offers one of the clearest, longest-lasting views yet into the living brain.

Subject of Research: Optical transcranial windows for long-term multiphoton brain imaging in living mice, including skull regrowth suppression with glucocorticoid-loaded hydrogels

Article Title: Optical Windows for Transcranial Brain Imaging in Living Mice: Skull Thinning, Clearing, and Beyond

Article References: Fu, Y., Yan, G., She, Z., He, Y., Liu, K., & Qu, J. (2026). Optical Windows for Transcranial Brain Imaging in Living Mice: Skull Thinning, Clearing, and Beyond. Advanced Science, 13(52), Article e76237. https://doi.org/10.1002/advs.76237

Image Credits: AI Generated

DOI: 10.1002/advs.76237

Keywords: transcranial window, multiphoton microscopy, skull regrowth, optical clearing, adaptive optics, thinned-skull window, dexamethasone, hydrogel, microglia, in vivo brain imaging, two-photon fluorescence, wavefront aberration

The research team, led by investigators at the University of Minnesota, developed a pipeline called TME-CARTographer, or TME-CART, which integrates multiphoton microscopy of living tumor tissue with graph theory, behavioral analysis, and interpretable deep learning. Rather than studying T cells in simplified culture dishes, the scientists imaged therapeutic T cells navigating intact slices of autochthonous pancreatic tumors from the KPC mouse model, a genetically engineered system that faithfully recapitulates human pancreatic cancer, including its dense fibrotic stroma and abundant immunosuppressive myeloid cells. Second harmonic generation imaging revealed the fibrillar collagen network while fluorescent reporters labeled carcinoma cells, CD11b-positive myeloid cells, and the T cells themselves, allowing the team to track every moving player across four dimensions of space and time.

The first major finding concerns collagen, the structural protein that dominates the desmoplastic stroma of pancreatic tumors. The team discovered that collagen fibers act as high-affinity microscopic highways. T cells traveling through the tumor overwhelmingly remain colocalized with collagen-rich regions at every time point measured, and even in carcinoma-dense zones lacking prominent collagen signal, T cells were largely absent. Aligned fibers promote rapid, directional, almost ballistic migration, but this guidance comes at a steep price. The researchers quantified a phenomenon they call migration anisotropy, showing that once a T cell engages with the fiber network, deviation from the fiber axis becomes physically unfavorable. Using nanopatterned substrates that mimic tumor collagen spacing, they measured a median migration anisotropy coefficient of 0.32, indicating a strong directional bias parallel to the underlying texture.

To translate this behavior into a spatial map, the team built an algorithm called MechanoTrack, which computes mechanoconductance, the mathematical inverse of mechanoresistance, for every pixel of the tumor terrain. The resulting topology resembles a landscape of ridges and valleys: T cells preferentially travel along high-conductance ridges corresponding to collagen fibers and rarely descend into low-conductance valleys where carcinoma cells reside. Critically, the analysis showed that T cells predominantly engaged in mono-sampling, exploring only one mechanoconductance region rather than cross-sampling between high and low regions. Once a T cell commits to the collagen network, it effectively becomes trapped on that path, gliding past or around its targets instead of seeking them out. This creates what the authors term physical immunosuppression, immune exclusion zones dictated purely by the geometry of the extracellular matrix.

Collagen, however, tells only half the story. The team found that CD11b-positive myeloid cells, comprising tumor-associated macrophages and myeloid-derived suppressor cells that together account for more than ninety-five percent of CD11b-positive cells in these tumors, colocalize with collagen fibers at a striking rate exceeding ninety-four percent. These myeloid cells migrate ten to twenty times more slowly than T cells, which suggests they function as nearly immobile roadblocks stationed along the collagen highways. When the researchers introduced mesothelin-specific engineered T cells, a therapeutic T cell receptor that prolongs survival in this model, they observed that the cells remained confined within collagen-myeloid-rich territories and rarely dispersed through the tumor volume over time.

At the single-cell level, the team categorized four distinct T cell behaviors: migration, sensing with protrusive probing, repulsion after contact with myeloid cells, and sequestration, in which the T cell stops moving entirely and rounds up. Their PhenoTrack algorithm, which classifies behavior from velocity, circularity, and colocalization data across time, revealed that embedding myeloid cells within three-dimensional collagen matrices dramatically shifted the behavioral balance. Migration events fell while sequestration events surged, confirming that immunosuppressive myeloid cells not only chemically impair T cell function but can physically halt effective movement through direct contact. Graph-theoretic modeling and Monte Carlo simulations reinforced the picture: treating the collagen network as a weighted graph showed that myeloid-laden fibers fragment the network, reduce path availability from seventy-six percent under simulated myeloid depletion to twenty-five percent in controls, and force T cells into tortuous detours measured as the ratio between actual path length and straight-line distance.

The therapeutic implications of these encounters were tested directly. Blocking major histocompatibility class I presentation on myeloid cells had modest effects, but immune checkpoint blockade against PD-1 significantly increased the number of migrating T cells and relieved myeloid sequestration, indicating that PD-1 and PD-L1 signaling at the contact interface between T cells and myeloid cells is a potent suppressive mechanism. The team then trained an eleven-layer deep neural network on a twenty-three-dimensional feature space extracted from the imaging data. The models achieved testing accuracies above ninety-two percent with area under the receiver operating characteristic curves exceeding 0.97, and post hoc explanation methods, including SHAP, LIME, and partial dependence plots, ranked collagen signal, distance to collagen, distance to myeloid cells, and mechanoresistance among the most influential drivers of T cell suppression.

The interpretability analysis yielded surprises that conventional statistics would likely have missed. Partial dependence plots revealed nonlinear, biphasic relationships between mechanoresistance and T cell behavior, and two-variable plots showed that the combination of effective collagen distance with myeloid proximity or T cell acceleration produced the largest shifts in model predictions, exposing synergistic interactions between matrix architecture, cellular neighborhood, and mechanical force exertion. Perhaps most compelling, the deep learning framework accurately predicted how immunosuppression would change following myeloid depletion. When mice were treated with a CCR2 inhibitor for two weeks, residual myeloid cells correlated positively with local T cell suppression, while more complete depletion produced far less suppression. In tumor slices treated with liposomal clodronate, near-uniform myeloid depletion allowed mesothelin-specific T cells to disperse throughout imaged tumor volumes, spend significantly more time in non-suppressed states, and substantially improve tumor sampling as confirmed by entropy-based dispersity analysis.

The authors emphasize that TME-CART is built around generic biophysical and behavioral features rather than pancreatic-specific biology, meaning the platform accepts standard multiphoton or confocal inputs and should apply to any desmoplastic solid tumor, including cancers of the breast, prostate, ovary, lung, and colon. From a translational standpoint, the work clarifies why T cell therapies have struggled in fibrotic tumors and argues for combination strategies that simultaneously disrupt the collagen architecture, deplete or reprogram suppressive myeloid cells, and engineer T cells that are physically optimized for navigation through dense tissue. The dual obstacle of fibrotic highways lined with cellular roadblocks is not an insurmountable one, the study suggests, but defeating it will require treating the tumor microenvironment as an interconnected mechanical and immunological system rather than a collection of independent barriers. With the analysis pipeline and source code publicly available, the team anticipates that TME-CART will serve as a discovery and screening tool for designing the next generation of cell-based immunotherapies for solid tumors.

Beyond its immediate findings, the study addresses a long-standing debate in pancreatic cancer biology about whether collagen should be viewed as friend or foe. Earlier work had suggested that dense stroma might, in some contexts, restrain tumor progression, complicating efforts to simply destroy fibrotic tissue. The present findings reconcile this tension by showing that collagen’s effects on immunity are spatially organized: the same fibers that structure the tumor also channel immune cells along paths that bypass malignant cells, meaning stroma-targeting strategies must consider not just how much collagen is present but how it is aligned and where myeloid cells are positioned along it.

The choice of imaging modality was central to the work. Multiphoton microscopy allows deeper penetration into living tissue than conventional confocal approaches while causing less photodamage, and second harmonic generation provides label-free visualization of fibrillar collagen, so the matrix architecture can be quantified without altering it. Capturing these dynamics in ex vivo tumor slices preserved the native stromal architecture that two-dimensional cultures cannot reproduce, which is precisely where prior studies of T cell migration have fallen short.

The engineered T cells used in the model recognize mesothelin, an antigen frequently expressed in pancreatic tumors, and had previously been shown to prolong survival without eliminating disease. The new analysis explains that partial success mechanistically: the cells infiltrate better than endogenous T cells but remain confined to matrix-defined corridors, leaving substantial tumor volume unsampled. This reframes the engineering challenge for next-generation cell therapies, suggesting that motility, persistence, and resistance to checkpoint-mediated arrest deserve the same design attention as antigen specificity.

More broadly, the work exemplifies a growing movement in cancer biology toward interpretable machine learning, where predictive models are paired with explanation tools so that biologists can extract testable hypotheses rather than opaque accuracy statistics. By validating its predictions with pharmacologic myeloid depletion, the platform demonstrates a closed loop of prediction and experimental confirmation that could accelerate combination therapy testing across desmoplastic malignancies.

Subject of Research: Spatiotemporal analysis of fibrotic and myeloid-mediated immunosuppression of therapeutic T cells in live pancreatic ductal adenocarcinoma

Article Title: Decoding spatiotemporal fibrotic and cellular immunosuppression of therapeutic T cells in live pancreatic ductal adenocarcinoma

Article References: Qian, G., Zhang, H., Stromnes, I. M., Eliceiri, K. W., & Provenzano, P. P. (2026). Decoding spatiotemporal fibrotic and cellular immunosuppression of therapeutic T cells in live pancreatic ductal adenocarcinoma. Molecular Systems Biology. https://doi.org/10.1038/s44320-026-00243-4

Image Credits: AI Generated

DOI: 10.1038/s44320-026-00243-4

Keywords: pancreatic cancer, T cells, tumor microenvironment, collagen, myeloid cells, deep learning, multiphoton microscopy, immunotherapy, TME-CART, immune exclusion, migration anisotropy, checkpoint blockade