The cells of the developing human embryo all carry the same genetic material, but they specialize into more than 200 different cell types that perform specific functions. These different cell types develop in precise spatial patterns that form the body plan of the embryo. Decades of genetic and biochemical studies have revealed that cells communicate using secreted signaling molecules, and that the signaling pathways activated inside the cell by these molecules are essential for conveying positional information to each cell, so that it specializes correctly. While many of the components of these pathways have been discovered, how the processes of signal interpretation and specialization occur has remained obscure due to the difficulty of observing and perturbing embryonic development. This challenge is particularly acute for mammalian embryos that develop in utero. Human embryonic stem cells (hESCs) represent a potential solution to these challenges, as they are capable of following developmental programs in a culture dish, enabling elucidation of human development to a degree that is otherwise impossible. This project combines experimental studies of hESCs grown in particular spatial patterns and mathematical modeling to dissect how cells interpret signals and differentiate in spatial patterns during the earliest stages of development. Coupled with this research program is a spectrum of educational activities aimed at training the next generation of scientists to perform interdisciplinary, quantitative research in the biological sciences. Planned activities include outreach to K-12 students, education aimed at high school advanced placement teachers, and innovative undergraduate and graduate courses. Following successful introduction at Rice University, these courses will be widely distributed through the Coursera online platform.
Mechanistic studies of pattern formation during mammalian development are challenging due the difficulty of observing and manipulating the embryo in utero. This project takes advantage of a recently developed in vitro system in which patterns form in hESC colonies grown in controlled geometries using micropatterning technology. The patterns form from the combination of an inductive signal added to the culture medium (BMP4) as well as paracrine signals between cells that have been identified as Wnt and Activin-Nodal signals. The project will use this system to quantitatively deconstruct the mechanisms of early patterning in the mammalian embryo. First, similar micropatterning technology will be used to isolate extremely small colonies of hESCs (1-10 cells). Colonies with a single cell will be used to directly measure the response to the inductive signal without interference from neighboring cells. Colonies with two cells will be used to understand the response to the paracrine signals in a simplified system, while those with larger number of cells will be used to measure how cells integrate signals from multiple neighbors. Results from these small colonies will be used to create mathematical models capable of predicting patterning in larger colonies of thousands of cells that display the full spectrum of embryonic fates. These models will be validated and further developed by experiments using live-cell imaging reporters for the activity of the BMP, Wnt, and Nodal signaling pathways together with reporters for cell fates. These dynamic observations are only possible in the in vitro development system, and will be used to correlate the dynamics of signaling in each individual cell with the fate it ultimately adopts. Taken together, these results will provide unprecedented understanding of early mammalian development and reveal principles of paracrine signaling and in vitro tissue self-organization.
