Over the past decades, researchers have been trying to find the stem cells responsible for the development and homeostasis of the mammary gland. However, because of the lack of stem cell markers, the mammary stem cell has remained elusive. Since stem cells are long-lived and able to self-renew, these cells can accumulate mutations and may therefore be at the origin of mammary tumor development. In this thesis, we utilize intravital and three-dimensional (3D) microscopy methods in combination with marker-free lineage tracing to study mammary stem cells in their native environment. Using these tools, we aim to uncover the location and dynamics of these stem cells during mammary gland development, homeostasis, tumor initiation and progression. Moreover, we translate the findings from our murine models to patient-derived models of breast cancer. In chapter 1 we provide an overview of the recent advances in intravital and live cell microscopy that have led to the elucidation of stem cell behavior in both healthy and diseased epithelium. In chapter 2 we identify the mammary stem cells that drive the process of branching morphogenesis of the pubertal mammary gland. We define the location, number, and dynamics of these pubertal mammary stem cells, and describe how these cells together drive the development of the mammary gland. In chapter 3 we further investigate the process of branching morphogenesis and develop a model that describes both the complexity and heterogeneity of the branched structure of the mammary gland. In chapter 4 we propose a refined theory of stemness as a stochastic competition for niche space. With this model we predict the number of stem cells and stem cell survival probability. In chapter 5 we investigate how the adult mammary gland is renewed throughout adult life. We demonstrate that homeostatic turnover in the adult mammary gland is driven by three unipotent stem cell populations that are scattered throughout the ductal epithelium. Each stem cell drives tissue turnover in the immediate ductal compartment, leading to cohesive clonal fields that can span multiple ducts and branch points. In chapter 6 we reveal the quantitative dynamics of tumor growth in two murine models of mammary carcinoma. Using longitudinal intravital microscopy, marker-free lineage tracing, and Monte-Carlo modelling, we describe the key features that discriminate between equipotent and hierarchical tumor growth. In tumors following an equipotent model, the majority of cancer cells contribute to tumor growth. In the hierarchical model the bulk of proliferation and cell death can be uncoupled from tumor growth. In chapter 7 we construct a living biobank of human pre-invasive and invasive breast tumors. With 3D whole-gland microscopy, we visualize the process of invasive transformation of these patient-derived xenografts, and reveal two different growth patterns that correlated with invasive progression. In chapter 8 we explore the consequences of field clonalization in tumor initiation in the human breast. We find evidence that field clonalization leads to field cancerization, when a mammary stem cell acquires a tumorigenic mutation. As a consequence, parts of the epithelial mammary tree are predisposed to tumor formation.
|Award date||07 May 2020|
|Publication status||Published - 07 May 2020|