Anatomy and Cell Biology
Ph.D.: 1977, University of Pittsburgh, Pittsburgh
Postdoctoral: 1979, Harvard Medical School, Boston
Postdoctoral: 1981, University of California, San Diego
Animals that do not form tissues and organs have about the same number of genes as humans — i.e., their genomes are about the same size. Despite this fact, mammals and birds are millions of times more complex than primitive marine animals. If the information needed to make organs and tissues is not directly encoded in DNA where does it reside?
We hypothesize that tissue and organ formation is largely an emergent process and that the “missing” information resides in a complex biomechanical code, which embryologists and bioengineers are only beginning to understand. Our empirical data point to emergent mechanisms that rely on: 1) tissue-scale motion, 2) modulation of the biomechanical (material) properties of embryonic tissues, and 3) subsequent morphogenetic bending and folding based on the aforesaid tissue material properties. In other words, the critical morphogenetic displacements and folding events — which characterize all amniotes embryos — are manifested at the tissue scale (> 0.1mm) and are emergent, not hard-wired.
To test our hypothesis, we use experimental perturbations, time-lapse imaging and computational approaches to examine live quail embryos. We record de novo tissue and organ morphogenesis using multi-color fluorescence microscopy — one channel(s) for cell markers and the other channel(s) for extracellular matrix (ECM) fibers. We then compare the wide-field motion of ECM filaments to the motion of individual cells using a variety of statistical and engineering algorithms. Our data are clear and consistent — in early bird embryos we find little difference between the movements of individual cells versus the adjacent ECM. The spatial scale we monitor spans from one micrometer up to one centimeter, four orders of magnitude.
Thousands of recordings and computational analyses of early organ formation and gastrulation, all support our hypothesis. In general, about 75-85% of cellular motion in early bird embryos is not via individual cell “migration”, but occurs as a result of tissue displacement (cells+ECM). Perhaps the best example, is work by our colleagues. Their in vivo data prove that folding of the right and left heart forming regions toward the midline is driven by tissue level deformations — with little evidence of individual cell “migration” (Aleksandrova, Rongish et al, Developmental Biology, 363(2):348-361, 2012). Recent, experimental work using tissue explants and microsurgery suggests the presence of latent morphogenetic force generating regions, which may drive large-scale deformation (> 0.1mm) during gastrulation and early mesodermal morphogenesis. For example, tissue plugs removed from the avian “organizer” (Hensen’s node) produce elongated structures consistent with notochordal tissue, but only when cultured under permissible mechanical conditions.
Early tissue and organ formation can be analogized to the formation of a hornet’s nest, which is a well-known example of a complex morphogenetic system. There is no genomic information or cell regulatory code that contains the “blueprints” for the construction of a “new” hornet’s nest. The nest architecture arises from the actions of thousands of hornets following simple instinctive rules. No biologist, and no hornet, can predict the location and exact shape of a given nest. Most importantly — the nest building process cannot be understood by the study of individual hornets or their sub-unit parts (eyes, legs, cells, proteins, genes). We argue that a similar situation exists during organ formation in amniote embryos.
A compelling example of an emergent tissue pattern is found in our work with the embryonic mouse allantois. Normally the allantois forms the umbilical cord containing two major blood vessels. However, when cultured on a flat surface the allantoic tissue forms a primary capillary network organized into small hexagonal arrays. This “whole-tissue” adaption to a new mechanical state proves that formation of a hexagonal micro-vascular pattern is an emergent process, and not genetically determined. In this case the mechanical state of the allantoic tissue regulated its vascular morphogenetic fate toward formation of planar polygonal vessels.
We conclude that in amniote embryos a complex emergent biomechanical code largely governs early tissue formation and organogenesis.
Charles D. Little, PhD