In Vivo Imaging Reveals Existence of Crypt Fission and Fusion in Adult Mouse Intestine

Abstract

The intestinal epithelium is a repetitive sheet of crypt and villus units with stem cells at the bottom of the crypts. During postnatal development, crypts multiply via fission, generating 2 daughter crypts from 1 parental crypt. In the adult intestine, crypt fission is observed at a low frequency. Using intravital microscopy in Lgr5^{EGFP-Ires-CreERT2} mice, we monitored individual crypt dynamics over multiple days with single-cell resolution. We discovered the existence of crypt fusion, an almost exact reverse phenomenon of crypt fission, in which 2 crypts fuse into 1 daughter crypt. Examining 819 crypts in 4 mice, we found that 3.5% ± 0.6% of all crypts were in the process of fission, whereas 4.1 ± 0.9% of all crypts were undergoing crypt fusion. As counteracting processes, crypt fission and fusion could regulate crypt numbers during the lifetime of a mouse. Identifying the mechanisms that regulate rates of crypt fission and fusion could provide insights into intestinal adaptation to altered environmental conditions and disease pathogenesis.

Keywords: Development; Homeostasis; Regeneration; Renewal.

Figure 1

Crypt dynamics includes both crypt fission and crypt fusion. (A) Crypt fusion event where 2 separate crypts (day 0) merged into 1 crypt (day 4). Overview panels (middle) show crypt pattern to confirm crypt identity. Outer panels show enlargement of imaging planes at center and border of stem cell zone. (B) Representative examples of crypt fusion and (C) fission where a single event is followed over 5 consecutive days by IVM. Arrows point at separate lumen. Dotted lines indicate outlines of Lgr5^EGFP+ (green) crypts and lumen. Cartoons illustrate process at beginning (left) and end (right). Scale bars: 20 μm.

Figure 2

Fusion events of 2 independent crypts. (A) IVM images of 5 consecutive days reveals crypt fusion between tdTomato⁺ (red) and non-labelled crypt. Cartoons illustrate process at beginning (left) and end (right). Dotted lines indicate outlines of Lgr5^EGFP+ (green) crypts and lumen. Asterisks indicate competing cells within neutral drift. Scale bar: 20 μm. (B) Quantification of fission and fusion events in fixed whole mount samples of proximal and distal intestine (819 tdTomato⁺ crypts, 4 mice). Symbols indicate individual mice. Representative examples of crypt fission (left) and fusion (right) are depicted below. (C) Schematic representation of crypt cycle that includes crypt fission and fusion.

Supplementary Figure 1

Dimensions of small intestine during aging. (A) Frequency of all crypts with a bifurcation phenotype in mice of different ages, counted in whole mounts of distal intestine. (B) Length and (C) width of small intestine from mice of different ages. (D) Crypt density, measured as average distance between centers of neighboring crypts, in mice of different ages.

Supplementary Figure 2

Discrimination between crypt fission and fusion. (A) Representative image of whole mount sample of a field of Lgr5^EGFP+ crypts (green) and a single tdTomato⁺ crypt (red). Scale bar: 100 μm. (B) Schematic diagram illustrating the possible processes (fission and fusion) that can yield the 2 distribution patterns of tdTomato⁺ cells over 2 branches of an ‘intermediate’ 8-shaped crypt. Number of observations of α and β pattern are indicated. Crypt fission contributes equally to both labeling patterns. Because α pattern is not observed, we deduced that crypt fusion is responsible for the 33 scored events of the β pattern. (C) Images of all 33 observed fusion events. Scale bar: 20 μm.