Research Projects:
Cytoskeletal control and physical mechanics of convergent extension and axis elongation.
The goal of this project is to identify biomechanical mechanisms that regulate cell shape and drive mediolateral cell behaviors, establish
passive tissue properties such as stiffness as well as active processes that generate forces of extension, and
how passive mechanics and active force generating processes are coordinated within the frog mbryo.
We use an established toolkit consisting of three elements: 1) the aquatic frog Xenopus laevis for direct
modulation of protein function and gene expression; 2) high resolution confocal microscopy to visualize cell
behaviors, cytoskeletal dynamics, and tissue architecture, and 3) biophysical and bioengineering tools to measure force generation by individual cells and coordinated by tissues, and determination of mechanical properties such as the viscoelastic properties of embryonic tissues. This toolkit is allowing us to understand how actomyosin dynamics drive cell
shape changes, generate traction forces, establish passive tissue properties such as stiffness, active force
production by convergence and extension, and how passive mechanics and active forces shape a vertebrate
embryo.
Cell and tissue mechanics of mesenchymal sheet migration, spreading, and engulfment.
During morphogenesis mesenchymal cells migrate, rearrange, and change shape to remodel the embryo. Mesenchymal cells may operate as lone-agents or may coordinate their behaviors with thousands of other cells. Coordinated movements of mesenchymal cells in the early embryo construct the vertebrate body plan and populate the tissues in preparation for later rounds of organogenesis. Many genes and many cells participate in these events but there is little experimental evidence on how genes and cells generate physical forces and integrate biochemical pathways to drive mechanical movements within embryos. To study these processes we take a generalist approach to this problem: applying tools from biophysics, engineering, cell biology, and embryology such biomechanical testing, high-resolution confocal time-lapse microscopy, computer simulation, and microsurgical techniques. Microsurgery is used to isolate small pieces of embryonic frog tissue exposing the operation of a single morphogenetic "machines". Isolated frog tissues continue to behave as if they were still within the embryo and undergo many aspects of organogenesis. Once isolated from the embryo tissue can be studied within an experimentally controlled microenvironment. By altering the function of proteins within the tissue, or creating mixed populations of cells we can assess the role of cues within that microenvironment and how these cues modulate intracellular processes to guide a cell's behavior. Use of a consistent experimental framework will allow these studies to focus on the coupling between the mesenchymal cellular microenvironment and the physical events driving mesenchymal morphogenesis.
Molecular and mechanical mechanisms that control cell contraction.
We have developed laser-activation, nano-perfusion, and electrical-stimulation to externally trigger epithelial contraction in a highly repeatable way. All methods produce nearly identical epithelial contractions that stimulate F-actin remodeling within several minutes, however, each method can be used for its unique strengths to investigate molecular pathways that trigger and transduce signaling stimuli (nano-perfusion and laser-activation) as well as the physical mechanics of contraction (electrical-stimulation and laser-activation). This quantitative biomechanical analysis will let us understand the multi-scale mechanics of epithelial morphogenesis, from the molecular trigger and control, to the micro-mechanics of cells and tissues, to the macro-scale interactions of force and tissue stiffness.
The regulation of cell polarity and tissue movements that shape the heart forming region.
Previous studies in mouse, chicken, zebrafish, and frog indicate that cardiac precursor cell movement toward the ventral midline is required for normal formation of the vertebrate heart. Disruption of cardiac precursor cell motility or the establishment of cell polarity in these cells results in severe heart defects in zebrafish and mouse that may underlie some of the genetic basis of Chronic Heart Disorders (CHD) in humans. This project combines modern approaches to visualize tissue architecture and cell movements with classical embryological approaches using grafting and explanation techniques to challenge hypotheses on the role of cell migration and cell polarity in cardiac precursor cell movements. The unobstructed view of the heart forming region of the Xenopus embryo and the ease of microsurgery on amphibian embryos provides a unique model to understand the cell and tissue interactions required to lay out the heart field prior to formation of the heart tube.