Spontaneous mutation rate

The relationships among stem cells, crypts, and villi in the small intestine of mice as determined by mutation tagging

The number of stem cells that maintain a crypt of the small intestine is uncertain. Although the number of stem cells per crypt had been thought to be 10-20, current estimates indicate a much smaller number, possibly 1 stem cell per crypt. We report here that in SWR and C57BL/6 X SWR F1 mice, the results of mutation tagging are inconsistent with the existence of more than one stem cell per crypt. Mutations at the Dlb-1 locus mark the progeny of a single epithelial stem cell in the small intestine and reveal the size of the mutant clone. The epithelial cells are produced in the crypts and migrate up the villi to be sloughed off at the tip so that a mutant clone takes the form of a ribbon on the villus. Because the size of the mutant clones must be inversely proportional to the number of stem cells contributing to the villus, this provides a means of counting the stem cells (ethylnitrosourea was used to induce mutations in the intestine). In the duodenum the ribbons average 0.099 +/- 0.004 villus circumferences, indicating that there are 1/0.099 = 10.2 stem cells per villus. Because there are 10.5 +/- 0.67 crypts/villus, the estimate gives 10.2/10.5 = 0.97 +/- 0.1 stem cells/crypt. Because each crypt must have at least one stem cell, the vast majority can have only one stem cell and very few may have two or more.

The induction of dominant somatic mutations at the Dlb-1 locus

In the small intestine of heterozygous mice (Dlb-1b/Dlb-1a), the Dlb-1b allele results in a stainable epithelium. The mutation or loss of the dominant Dlb-1b allele in a stem cell results in a non-staining ribbon of cells on a villus of the small intestine. To determine if dominant mutations resulting in the gain of staining--the induction of a Dlb-1b-like allele--could also be detected, we examined Dlb-1a homozygous mice (SWR) 2 weeks after a single treatment with 250 mg/kg ethylnitrosourea. Mutations to the dominant allele should appear as brown ribbons on unstained villi. Such ribbons were observed in the treated group but not in controls. The mutant frequency was low compared to the frequency of Dlb-1a-like mutations reported at the Dlb-1b allele in heterozygous mice.