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Murine small intestinal crypt development is initiated during the first postnatal week. Soon after formation, overall increases in the number of crypts occurs through a bifurcating process called crypt fission, which is believed to be driven by developmental increases in the number of intestinal stem cells (ISCs). Recent evidence suggests that a heterogeneous population of ISCs exists within the adult intestine. Actively cycling ISCs are labeled by Lgr5, Ascl2 and Olfm4; whereas slowly cycling or quiescent ISC are marked by Bmi1 and mTert. The goal of this study was to correlate the expression of these markers with indirect measures of ISC expansion during development, including quantification of crypt fission and side population (SP) sorting. Significant changes were observed in the percent of crypt fission and SP cells consistent with ISC expansion between postnatal day 14 and 21. Quantitative real-time polymerase chain reaction (RT-PCR) for the various ISC marker mRNAs demonstrated divergent patterns of expression. mTert surged earliest, during the first week of life as crypts are initially being formed, whereas Lgr5 and Bmi1 peaked on day 14. Olfm4 and Ascl2 had variable expression patterns. To assess the number and location of Lgr5-expressing cells during this period, histologic sections from intestines of Lgr5-EGFP mice were subjected to quantitative analysis. There was attenuated Lgr5-EGFP expression at birth and through the first week of life. Once crypts were formed, the overall number and percent of Lgr5-EGFP positive cells per crypt remain stable throughout development and into adulthood. These data were supported by Lgr5 in situ hybridization in wild-type mice. We conclude that heterogeneous populations of ISCs are expanding as measured by SP sorting and mRNA expression at distinct developmental time points.

The epithelial cell lining of the gastrointestinal tract is the most rapidly proliferating tissue in the body. The constant state of renewal of differentiated epithelial cells is sustained by a continual supply of progeny from multipotent progenitors that originate from stem cells located within the intestinal crypts. In addition to supporting normal epithelial homeostasis, intestinal stem cells (ISC) are thought to play an important role in the rapid expansion of the gut during development, tissue regeneration following injury or surgical loss, and malignancy. Because of the lack of specific ISC markers required to isolate and characterize these cells, our current knowledge of the biology of ISC results largely from indirect measures of their behavior published during the past 40 years. The recent description of several potential ISC markers and the use of transgenic mice, both as a tool to lineage trace and to isolate specific cells expressing these markers, have provided a tremendous advancement to our current understanding of these cells. This brief review provides a general historical overview of our understanding of ISC and the tools available to study their behavior in the context of normal and pathological conditions, as well as potential future clinical applications that may result from this exciting area of research.