Identification, visualization and clonal analysis of intestinal stem cells in fish

Abstract

Recently, a stochastic model of symmetrical stem cell division followed by neutral drift has been proposed for intestinal stem cells (ISCs). This division mode has been suggested to represent the predominant mode of stem cell progression in mammals. In contrast, stem cells in the retina of teleost fish show a preferential asymmetric division mode. To address whether the mode of stem cell division is following phylogenetic or ontogenetic routes we characterized and analysed the entire gastrointestinal system with a particular focus on ISCs in the teleost medaka (Oryzias latipes). We analysed the entire intestine in adult medaka by X-ray micro-computed tomography within the intact body and combined that analysis with the cellular and molecular composition of the intestinal system. This allowed correlating the 3D topography with functional domains of the intestinal system. Morphology and gene expression data consistently argue for a subdivision of the medaka intestine into a small and large intestine. Analysis of ISCs in proliferation assays and via genetically encoded lineage-tracing highlights a stem cell niche in the furrow between the long intestinal folds. This niche is functionally equivalent to the mammalian intestinal crypts and stem cells in this compartment are characterized by the expression of sox9, axin2 as well as lgr5, homologs of mammalian ISC markers, emphasizing the evolutionary conservation of the Wnt-pathway components in the stem cell niche of the intestine. Consistent with a preferential symmetric division mode of medaka ISCs, the stochastic, sparse initial labelling of ISCs ultimately resulted in extended labelled or unlabelled domains originating from single stem cells in the furrow niche contributing to both, homeostasis and growth. Thus different modes of stem cell division co-evolved within one organism, and in the absence of physical isolation in crypts, ISCs contribute to homeostatic growth.