Fate and Form in the Mammalian Intestine

Abstract

To maintain homeostasis throughout adult life, organs must coordinate the proliferation, differentiation, and spatial organization of their constituent cells. In actively cycling tissues like the gut, tissue-specific stem cells routinely divide to replace old and damaged cells. Crucially, at steady state these cells must be as likely to produce differentiated progeny as they are to self-renew – all while maintaining the physical integrity of the tissue, even as cells are constantly replaced. How do multicellular systems couple and control cell fate (Chapter 1) and tissue form? Ultimately the size and shape of a self-renewing tissue depends on the number of stem cells it contains. In the adult mammalian intestine, the number of stem cells is thought to reflect the number of niches available to house them. What determines the abundance of these stem cell zones (SCZs), and what sets the size of each one? To answer these questions, I develop live imaging methods that enable the study of cell and tissue dynamics in intestinal epithelial organoids (Chapter 2). Intestinal organoids have the remarkable capacity to self-organize SCZs even in the absence of a structured external niche. SCZs in vitro appear to maintain a typical size over time that is quite similar to SCZ size in vivo. Is this evidence that the epithelium encodes its own homeostatic program, or is something very different going on under the hood? Here I show that hydraulic oscillations – rather than size control intrinsic to the epithelium – drive SCZ fission and give rise to the typical SCZ size in intestinal organoids (Chapter 3). I demonstrate that it is possible to control SCZ number simply by tuning ion channel activity to trigger or prevent inflation of the organoid lumen. This mechanism of pressure-driven patterning in the intestinal epithelium may occur more generally in disease and development.