Charles D. Little, PhD
Anatomy and Cell Biology
Ph.D.: 1977, University of Pittsburgh, Pittsburgh
Postdoctoral: 1979, Harvard Medical School, Boston
Postdoctoral: 1981, University of California, San Diego
The Importance of Emergent Biophysical Patterns during Formation of Tissues and Organs
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.
- Rupp, P.A., Czirók, A. and Little, C.D. AlphaV Beta3 Integrin dependent endothelial cell dynamics, in vivo. Development 131, 2887-2897, 2004.
- Zamir, E.A., Czirók, A., Cui, C., Little, C.D., and Rongish, B.J. Mesodermal Cell Displacements during Avian Gastrulation are due to Individual Cell-Autonomous and Convective Tissue Movements. Proc. Nat. Acad. Sci, USA, 103:52, 19806-19811, 2006. Selected as a “Must Read” article by the Faculty of 1000, Biology.
- Czirok, A., Zamir, E.A., Filla, M.B., Little, C.D., and Rongish, B.J. Extracellular matrix macro-assembly dynamics in early vertebrate embryos. In: Current Topics in Developmental Biology, Vol.3; pp237-258, 2006Czirok, A., Zamir, E. A., Szabo, A. and Little, C.D.; Multicellular sprouting during vasculogenesis, In: Current Topics Developmental Biology, Multiscale Modeling of Developmental Systems, 81:269-289, 2007.
- Perryn, E.D., Czirók, A. and Little, C.D. Vascular sprout formation entails tissue deformations and VE-cadherin dependent cell-autonomous motility, Dev. Biol. 313:545–555, 2008. NIH-PMC Ms #38746; doi:10.1016/j.ydbio.2007.10.036
- Zamir, E.A., Rongish, B.J and Little, C.D., The ECM moves during primitive streak formation—computation of ECM versus cellular motion. PLoS, Biology, 6(10): 2163-2171, 2008. doi:10.1371/journal.pbio.0060247. Selected as a Highlighted Article by PLoS, Biology(doi:10.1371/journal.pbio.0060268). Selected by the Faculty of 1000 as a “Must Read” article (http://www.f1000biology.com/article/id/1124585/evaluation).
- Rupp, P.A., Visconti, R.P., Czirok, A., Cheresh, D.A., and Little, C.D. Matrix Metalloproteinase 2-Integrin avb3 Binding Is Required for Mesenchymal Cell Invasive Activity but Not Epithelial Locomotion: A Computational Time-Lapse Study. Mol. Biol. Cell, 19:5529 –5540, 2008: doi10.1091/mbc.E07-05-0480. Selected as a highlighted InCytes Article by MBC: (http://www.molbiolcell.org/cgi/issue_pdf/incytesfrommbc_pdf/19/12).
- Cheng, C., Little, C.D., Rongish, B.J. Rotation of organizer tissue confers left-right asymmetry, Anat. Rec. 2009 292(4):557-561; PMID: 19301278
- Bénazéraf, B., Francois, P., Baker, R., Little, C.D. and Pourquié, O. A random cell motility gradient downstream of FGF controls elongation of amniote embryos. Nature, 466:248-252, 2010; doi:10.1038/nature091512010.
- Sato Y, Poynter G, Huss D, Filla MB, Czirok A, Rongish, B.J., Little, C.D, Fraser, S.E., and Lansford, R. Dynamic analysis of vascular morphogenesis using transgenic quail embryos. PLoS ONE 5(9), 2010: e12674. doi:10.1371/journal.pone.0012674
- Aufschnaiter, R., Zamir, E.A., Little, C.D., Özbek. S., Münder, S., David, C.N., Sarras, M.P., Zhang, X. In vivo imaging of basement membrane movement: ECM patterning shapes Hydra polyps. J.Cell Science, 124: 4027-4038, 2011. doi: 10.1242/jcs.087239
- Loganathan, R., Potetz, B.R., Rongish, B.J., Little, C.D. Spatial Anisotropies and Temporal Fluctuations in Extracellular Matrix Network Texture during Early Embryogenesis. PLoS, One, 7(5): 2012, e38266. doi:10.1371/journal.pone.0038266
- Cui C, Filla MB, Jones EAV, Lansford R, Cheuvront TJ., Al-Roubaie, S., Rongish, BJ. and Little, CD. Embryogenesis of the First Circulating Endothelial Cells. PLoS ONE 8(5) 2013: e60841.doi:10.1371/journal.pone.0060841. Faculty of 1000 PrimeRecommended. http://f1000.com/prime/718434232?bd=1&ui=27410
- Czirok, C. Rongish, B.j., and Little, C.D., Extracellular Matrix Dynamics in Early Development, In: Extracellular Matrix in Development, Biology of Extracellular Matrix, D.W. DeSimone and R.P. Mecham (eds.), doi: 10.1007/978-3-642-35935-4_2, Springer-Verlag Berlin Heidelberg 2013.
- Aleksandrova, A., Rongish, B.J., Little, C.D., and Czirok, A. Active cell and ECM movements during development. Methods in Molecular Biology, Ed: Celeste Nelson, Humana Press. 2014.
- Loganathan, R. Little, CD, Joshi, P., Filla, MB, Cheuvront, TJ, Lansford, R, Rongish. Mapping Morphogenetic Movements in Amniote Embryos. Accepted for publication Organogenesis, September 2014.