ECS-facing membrane patches — all crops

Each card is one representative cell's extracellular-space (ECS)-facing membrane from a FIB-SEM crop, comparing Chemical fixation vs Rapid HPF (high-pressure freezing). Per crop the cell with the most ECS-facing surface is shown.

• Mesh. The cell segmentation is surfaced by marching cubes and smoothed with a physical-scale Gaussian (σ ≈ 1.5×voxel). Surfaces are coarsened to 16 nm voxels for the web (manuscript metrics use finer/native resolution). Only membrane facing ECS is kept (vertices within ~1 voxel of extracellular space); cell–cell contacts and crop-boundary cut faces are removed.
• Signed curvature (1/nm). Mean curvature from the cotangent Laplacian — a strictly local 1-ring quantity (depends only on each vertex and its immediate mesh neighbours, no smoothing scale). Sign is set by the outward normal: red = convex (membrane bulges into ECS, e.g. microvilli); blue = concave (invagination). The numeric value H is the reciprocal of a radius: the local membrane "fits" a sphere of radius R = 1/|H|. Worked examples — H = 0.005 nm−1 ↔ R = 200 nm (gentle membrane curvature); 0.02 nm−1 ↔ 50 nm (microvillus shaft); 0.05 nm−1 ↔ 20 nm (sharp microvillus tip); 0.10 nm−1 ↔ 10 nm (very sharp spike). The colour limits are per‑crop — the colourbar saturates at the 90th percentile of |H| in this particular patch.
• Protrusion / indentation (nm). Per-vertex signed displacement of the actual surface from a Gaussian-smoothed reference of the same surface (σ ≈ 60 nm). Units are nanometres, not 1/nm: the value is literally "how far this vertex sits outside (red) or inside (blue) its ~60 nm smoothed neighbourhood." Scale-aware where curvature isn't — a gentle 100 nm-tall ridge shows large positive protrusion but low curvature; a 5 nm bump on flat membrane shows high curvature but tiny protrusion.
• When the two agree vs. disagree. Curvature answers "is the surface bent here, and which way?" — protrusion answers "does this point stick out (or in) compared to its ~60 nm neighbourhood?". On a microvillus brush both light up together. On a slowly undulating surface only protrusion fires — the bends are too gentle for curvature to read.
• Contact gap (nm). Distance from each membrane point to the nearest neighbouring cell. Dark/purple = tight apposition (close cell–cell contact); yellow = open extracellular space. The in-volume distance transform can't see neighbours that lie just outside the crop, so any vertex whose reading exceeds its distance to the crop face is dropped from the gap panel — the silhouette goes missing rather than getting painted with a fake "far" value. The bd-clip badge on each card reports the fraction of patch vertices dropped.