Elsevier

Experimental Cell Research

Volume 317, Issue 19, 15 November 2011, Pages 2719-2724
Experimental Cell Research

Review Article
Stem cell self-renewal in intestinal crypt

https://doi.org/10.1016/j.yexcr.2011.07.010Get rights and content

Abstract

As a rapidly cycling tissue capable of fast repair and regeneration, the intestinal epithelium has emerged as a favored model system to explore the principles of adult stem cell biology. However, until recently, the identity and characteristics of the stem cell population in both the small intestine and colon has remained the subject of debate. Recent studies based on targeted lineage tracing strategies, combined with the development of an organotypic culture system, have identified the crypt base columnar cell as the intestinal stem cell, and have unveiled the strategy by which the balance between proliferation and differentiation is maintained. These results show that intestinal stem cells operate in a dynamic environment in which frequent and stochastic stem cell loss is compensated by the proliferation of neighboring stem cells. We review the basis of these experimental findings and the insights they offer into the mechanisms of homeostatic stem cell regulation.

Introduction

In adult, stem cells are characterized by their ability to generate multiple differentiated cell types while maintaining their capacity for long-term self-renewal [1], [2]. The intestinal epithelium involves a single layer of cells that line the intestinal tract. In normal homeostasis, the specialized differentiated cells types that orchestrate the uptake of nutrients into the body are routinely and rapidly turned over. To sustain this high rate of loss, new cells are continuously generated by a population of multipotent stem cells, which reside at the base of the intestinal crypt. Beginning with the early pioneering research of Leblond and coworkers in the late 1940s [3], efforts have been made to define the intestinal stem cell compartment, and to identify the regulatory factors that ensure long-term self-renewal.

In vertebrates, the intestinal tract is comprised of two anatomically and functionally distinct segments, the small intestine and colon (for a review, see Ref. [4]). Although both involve an outer layer of smooth muscle, a central layer of connective tissue (stroma), and an inner absorptive epithelial lining (mucosa), the organization of the tissue in the small intestine and colon is tailored to their respective function. To maximize the surface area available for absorption, the lining of the small intestine has numerous finger-like protrusions, known as villi, which project into the lumen. The villi lie in close association with invaginations called crypts of Lieberkühn. By contrast, the surface of the colonic mucosa is essentially flat, with multiple crypts that penetrate deep into the underlying submucosa.

The intestinal epithelium is characterized by four main lineages: columnar cells (enterocytes in the small bowel and colonocytes in the colon), mucin-secreting (goblet) cells, endocrine cells, and Paneth cells [5]. The columnar cells, which are the most abundant, are polarized with a basal nucleus and an apical brush border, and perform both an absorptive and secretory role. Goblet cells contain mucigen granules, which are discharged onto the surface as intestinal mucus, and serve to protect and lubricate the mucosa. Endocrine cells, which are found throughout the intestinal epithelium, produce peptide hormones, which are secreted basally in an endocrine or paracrine manner. Finally, Paneth cells, which are largely confined to the small intestine, contain secretory granules, and express a number of proteins, including lysozyme. These cells help to maintain a sterile environment in the crypt and, as we will discuss below, have an important role in regulating the stem cell compartment.

The lining of the intestinal epithelium is columnar, involving only a single cell layer, and in mouse is completely renewed every 5 days or so throughout life. This high rate of self-renewal is sustained by a sub-population of adult stem cells that reside at the crypt base [6]. The stem cells give rise to transit amplifying (TA) cells, which expand rapidly through multiple rounds of cell division as they move in coherent migration streams along the inner surface of the crypt. TA cells progressively differentiate and, in small intestine, emerge onto the villus as mature, functional epithelial cells. They continue migrating along this epithelial conveyer belt until they reach the villus tip, where they die and are shed into the lumen. By contrast, Paneth cells migrate down toward the crypt base where they persist for around 6–8 weeks [7].

Although the cellular composition and organization of the small intestine and colon was established in early studies, the nature, location, and multiplicity of the stem cell population has long been the subject of debate. Even as late as the 1980s, the question of whether the different cell types of the intestinal epithelium are derived from a single multipotent stem cell (termed the “Unitarian hypothesis” by Leblond [8]), or from a cohort of committed progenitors remained open. However, by tracking genetic inheritance patterns introduced at random into single crypt cells via somatic mutation [9], [10], [11], [12], the validity of the Unitarian hypothesis was confirmed.

In early studies, it was postulated that cycling, yet DNA label-retaining, cells at position + 4 (directly above the Paneth cell compartment) represent the crypt stem cell population [13]. Others placed emphasis on a larger population of immature crypt base progenitors that lie intercalated between Paneth cells at the crypt base spanning cells positions + 1 to + 4 [7], [8]– termed the “stem cell zone”. Lately, by developing a gene-specific inducible genetic labeling system, researchers were able to show that cells positive for the expression of the leucine-rich G protein-coupled receptor 5 (Lgr5) were capable both of long-term self-renewal, and of generating all mature differentiated cell types [14]. Similar data were obtained using a CD133-based [15] and Bmi1-based [16] lineage tracing strategy. The expression of high levels of Lgr5 in crypt base progenitors throughout the Paneth cell-rich zone, together with the reported co-expression of CD133 and Lgr5 [15], undermines the significance of the + 4 position. Moreover, even though Bmi1 expression is reportedly strongest at cell position + 4, since sorted Lgr5hi cells express the highest levels of Bmi1[17], [18], it seems likely that Lgr5 and Bmi1 mark overlapping, if not identical, cell populations.

The identification of Lgr5 as a putative intestinal stem cell marker was further supported by subsequent studies that showed that individual Lgr5hi crypt-based cells had the remarkable capacity to generate entire crypt and villus-like structures (organoids) in culture [19]. In both colon and small intestine, Lgr5 expression is largely restricted to crypt-base columnar cells. In the small intestine, around 14 crypt-base progenitors form a network of small plate-like cells that lie intercalated between much larger Paneth cells at the bottom of each crypt (Fig. 1a). But how do these crypt base stem cells maintain balance between proliferation and differentiation?

Section snippets

Lineage tracing

Recently, two lineage tracing strategies have been designed, which provide new insight into the mechanisms controlling intestinal stem cell self-renewal. In the first, an inducible genetic labeling system based on a ubiquitous promoter was used to mark a representative population of cells in the small intestine and colon of adult mice [20]. Following the transient expansion and subsequent loss of TA cell-derived clones, labeled clones become restricted to those derived from the intestinal stem

Theory

To understand the origin and nature of the scaling behavior, and gain insight into the possible mechanisms of stem cell regulation, it is helpful to consider a simplified caricature of the clonal dynamics that nevertheless captures in full the basic underlying process. If we suppose that stem cell competence is regulated by proximity to the crypt base, access to limited niche space is sufficient to ensure the long-term survival of the stem cell population and, as we will see below, is

Discussion

The coincidence of experiment with theory shows that intestinal crypt is maintained by a single, equipotent, stem cell population in which the frequent and stochastic loss of cells is compensated by the symmetric division of neighbors, leading to neutral drift clone dynamics. These findings suggest that, even if intestinal stem cell division can involve spindle orientation, or the asymmetric segregation of DNA, as reported in the recent literature [26], [27], it does not confer a long-term bias

Acknowledgments

BDS and HC are grateful to Carlos Garcia-Lopez, Allon Klein, Hugo Snippert, and Doug Winton for illuminating discussions.

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