Fig. 1.

Imaging ¹⁵N enrichment (δ¹⁵N) in the cecum tissue and intestinal lumen 8 h after i.v. injection of ¹³C,¹⁵N threonine. A mosaic of 16 individual high-resolution NanoSIMS images is shown. ¹²C¹⁴N⁻ secondary ion intensity distribution images of the same area are shown to illustrate the structure of the tissue and lumen biomass. ¹⁵N hotspots are indicated by white arrows. The patchy distribution of ¹⁵N hotspots in the tissue is likely attributable to the heterogeneity in mucus release activity both within and between mucus cells (24, 25).