Laboratory of Epithelial Morphogenesis
Morphogenesis is the process that generates the shape of tissues and organs. Our lab investigates the molecular and cellular basis of epithelial tissue morphogenesis to better understand basic mechanisms of animal development. Anomalies in epithelial morphogenesis underlie a range of congenital diseases such as spina bifida, cystic kidneys and vascular aneurysm, and acquired diseases such as cancer. The hallmark of these diseases is the disruption of the normal morphology of tissues and organs that impairs their function. Therefore understanding the basic mechanisms provides the foundation for understanding numerous disease processes.
The motivation for our work
The morphogenesis of tissues and organs is controlled primarily by coordinated changes in cell shape, rearrangements of cell-cell contacts, cell proliferation and cell death (Figure 1, Video 1). To understand the underlying mechanisms it is necessary to decompose the relative contribution of each of these cell behaviors to morphogenesis, elucidate the forces and force-generating proteins regulating these behaviors, and the mechanisms that regulate force generation in space and time. The convergence of recent advances in imaging technologies, genetically encoded fluorescent proteins, computational image analysis and mathematical modeling opened up new opportunities to address these questions that until recently were intractable.
Figure 1. Coordinated changes in cell behaviors remodel tissue shape. During development, coordinated changes in cell shape, rearrangements of cell-cell contacts, cell proliferation and cell death lead to global changes in tissue morphology. Our lab employs genetic analysis, live imaging and quantitative image analysis to elucidate the molecular and cellular basis of epithelial morphogenesis. Shown are coordinated cell behaviors that can lead to tissue elongation.
The model systems we use
The imaginal discs of fruit fly Drosophila are established models for the analysis of epithelial development because of their simplicity and accessibility to experimental analysis. The imaginal discs give rise to adult appendages (e.g. legs, wings, eyes). They are set aside from the surface ectoderm during embryonic development. During larval stages, secreted morphogens, that emanate from localized signaling centers, promote proliferation of disc epithelial cells and determine the cellular identities of the various parts of developing appendages. During post-larval stages the imaginal discs undergo extensive structural remodeling to generate the final shape of adult appendages. Our lab investigates the mechanisms that regulate the narrowing and elongation of the leg from a flat disc to a hollow cylinder (Figure 2), and the generation of the regular hexagonal lattice of the fly retina by cell shape changes and cell death (Figure 3).
Figure 2: Morphogenesis of the leg imaginal disc is a model for epithelial elongation. Epithelial elongation is a conserved process that alters the proportions of epithelial sheets and tubes. It restructures the early embryo, and tubular epithelia such as the neural tube, the lungs airways, the kidney collecting system, and the collecting ducts of secretory organs. The different parts of the leg are specified in the leg imaginal disc (left). During post-larval stages, the epithelium of the leg imaginal disc telescopes out and during this process it narrows and elongates to form the adult appendage (right). Our lab employs the leg imaginal disc as a model to investigate the molecular and cellular basis of tissue elongation and the regulation of the process by extracellular signals.
Figure 3: The apical epithelium of fly retinal is a model for epithelial remodeling by coordinated cell shape change and cell death. The fly eye is composed of ~800 ommatidia. Shown is a single ommatidium at 28 and 40h after puparium formation (APF). Each ommatidial unit is composed of a core of eight photoreceptors (not shown in the image) capped by four cone cells and surrounded by two large semi-circular 1° cells. A single file of lattice cells (LCs) surrounds each ommatidial unit. At early stages the LCs are isometric but as development proceeds the 2° LCs narrow and elongate to form the edges of the lattice, the 3° LCs compact to form every other corner of the lattice, while sensory bristles cells occupy remaining corners. During this process superfluous LCs die and delaminate from the epithelium. Our lab employs the eye imaginal disc to investigate how mechanical forces generated by contractile and protrusive cytoskeletal proteins regulate cell shape changes and cell death, and how extracelluar signals and polarity proteins regulate mechanical force generation in space and time.
The approaches we use
The genes and molecular mechanisms that regulate epithelial morphogenesis are only partially characterized. Therefore, a major goal is to identify novel genes that participate in tissue morphogenesis and elucidate their mechanism of function. Genetic screens carried out by many labs including our own identified many genes affecting leg and eye morphogenesis. We investigate the mechanism of function of a subset of these genes, in particular those involved in protrusive and contractile force generation and in cell-cell adhesion. Our work takes advantage of a range of tools developed over the years by the fly community to examine the loss- and gain-of-function phenotypes and the in vivo localization of almost any protein encoded in Drosophila. To complement these capabilities, our lab develops computational image analysis tools to segment and track epithelial cell in time-lapse movies obtained by 4D (3D + time) confocal microscopy (Figure 4). We employ the extracted motion data of cell centroids and vertices (the geometric points were three or more cells meet) to infer the mechanical forces driving tissue remodeling (Video 1).
Figure 4: Computational image analysis of epithelial morphogenesis. Epithelial morphogenesis involves cell intercalation, cell shape changes, cell division and cell death in tissues that consist of hundreds to thousands of cells. The quantitative analyses of these behaviors at a system level require computational tools to identify cells and follow their behavior. Our lab develops computational tools to follow cell behavior in time-lapse movies. We recently developed TTT, a computational pipeline designed to (A-B) segment apical outlines of epithelial cells in 3D, (C-D) track the motion of cell centroids, and detect (E) mitosis and (F) apoptosis.
Video 1: Analysis of tissue mechanics using non-invasive approaches. Vertices are geometric points in epithelial tissues where three or more cells meet. Remodeling of epithelial tissues can be described by the displacement, loss or creation of new vertices. Inverse cellular vertex models operate on the motion of epithelial cell vertices to infer the forces that deform epithelial tissues during morphogenesis (e.g. tensions along cell-cell contacts, pressures in the cells, stress fields acting on tissue domains). To employ these methods to estimate spatiotemporal patterns of mechanical parameters driving epithelial tissue remodeling, we embarked on a new effort to directly track the motion of cell vertices. Shown are tracked vertices in a time-lapse video of the apical epithelium of the fly retina during delamination of superfluous lattice cells and cell shape changes. This process generates the regular hexagonal lattice of the fly retina.
The Bottom Line
The processes and mechanisms that regulate epithelial morphogenesis are evolutionarily conserved. Therefore insights gained from our studies on imaginal disc morphogenesis have the power to explain related processes in human development. To understand epithelial morphogenesis at a system level we develop computational tools to understand how mechanical forces are generated and how they coordinate at the tissue level to affect cellular and tissue level behaviors.
- The role of contractile and protrusive proteins in leg elongation
- The spatial cues that regulate the recruitment and activation of contractile and protrusive proteins during leg elongation
- The role of contractile and protrusive proteins in cell delamination and cell shape changes of lattice cells in the fly retina
- The role of cell polarity proteins in controlling the localization and activity of contractile and protrusive proteins during eye morphogenesis.
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