Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A7: From Egg to Adult: Patterning and Morphogenesis in Animal Development |
Hide Abstracts |
Sponsoring Units: DBP Chair: Boris Shraiman, UCSB Room: LACC 408B |
Monday, March 21, 2005 8:00AM - 8:36AM |
A7.00001: How to Make a Neurocrystal: Modeling the developmental patterning of the fly's retina Invited Speaker: Animals' ability to create the complex patterns found in many organisms is an enduring source of wonder and a topic that has long drawn the interest of scientists of all stripes. Famously, it was an attempt to model developmental patterning that led to the discovery of the Turing instability. Here, we study one of the most remarkable and best-characterized examples of such pattern formation, the development of the fruit fly's compound eye. In the fly larva, a front of differentiation moves across the sheet of tissue that will become the adult retina. It leaves behind it a striking hexagonal array of cells marked by high levels of the protein Atonal. It has previously been noted that a standard activator-inhibitor model might explain this process [Meinhardt, 1992], but only recently has the basic genetic logic governing photoreceptor specification been deciphered [e.g. Frankfort and Mardon, 2002]. We build on these advances with the first model of retinal patterning based on experimentally verified interactions. Surprisingly, we conclude that a Turing-instability-based mechanism alone cannot reproduce the observed behavior. Instead, we propose that the pattern is generated primarily by a novel ``epitaxial'' process in which, as the front progresses, each newly-created row of unit cells acts as a template for the next one. A clear prediction of this model is that if the communication between successive rows is broken, even transiently, a striped pattern will appear. Preliminary experimental tests suggest that just such a phenomenon occurs in some mutants. Related patterning processes have been observed in systems as diverse as chick feather buds and vertebrate retinal ganglion cells [Pichaud, Treisman, and Desplan, 2001]; our model may thus describe an evolutionarily conserved module. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 9:12AM |
A7.00002: Axis definition during Hydra regeneration Invited Speaker: Hydra may recover even from dissociation into single cells. During such a reformation process, Hydra cells first form a hollow ball made of a cell-bilayer, subsequently the developmental isotropy is broken and an axis is established. The animal then reforms according to this axis. We show that a temperature gradient of about 1\r{ }C across the embryo determines the axis but not the orientation of the developing animal. A change in morphogenetic inflation-contraction cycles of the Hydra cell ball coincides with irreversible axis establishment, accompanied by a change in tissue elasticity and WNT expression. We suggest that a modulation of cell adhesion or internal pressure lock the axis during development, therefore corroborating the recently advanced hypothesis of a link between cell-adhesion regulation and the WNT cascade. Quantitative analysis of the early, Hydra specific gene ks1 reveals scaling of the expression-pattern size distribution. A plausible interpretation is, that transient collective cell-differentiation-fluctuations with increasing magnitude break the symmetry of the Hydra cell-ball; they establish the axis irreversibly upon short-range WNT cascade activation. Our interpretation suggests why Hydra regeneration starts with a hollow cell ball and how only five to ten organizer cells may convey their existence to the 10000 others. [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:48AM |
A7.00003: Force Regulation in Tissue Mechanics Invited Speaker: We have investigated tissue mechanics in live fly embryos perturbed by a UV microbeam and imaged with confocal microscopy. The actin cytoskeletons of these transgenic flies are labeled with green fluorescent protein to provide contrast without compromising biological function. We concentrate on dorsal closure, a model system for development and wound healing, to identify connections between forces, genetics, and morphogenesis. Dorsal closure is proving to be an attractive system for research in biological physics since key cell boundaries lie in a plane and exhibit multiple symmetries, which facilitates modeling. We find that four spatially and temporally coordinated processes are responsible for the dynamics of dorsal closure. The bulk of progress is driven by contractility in supracellular ``purse strings'' and in the amnioserosa, whereas adhesion-medicated zipping coordinates the forces produced by the purse strings. When the UV microbeam was used to block adhesion mediated zipping, altered dynamics preserve closure, attributed to an upregulation of the force produced by the remaining amnioserosa. In addition, the modeling of wild type and mutant phenotypes is predictive; although closure in myospheroid mutants ultimately fails when the cell sheets rip themselves apart, our analysis indicates that $\beta _{ps}$--integrin has an earlier, important role in zipping. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:24AM |
A7.00004: Topology and Robustness in the Drosophila Segment Polarity Network Invited Speaker: Previous work by von Dassow {\it et al.} demonstrated the robustness of a mathematical model of the genetic interactions that define the polarity of {\it Drosophila} embryo segments. I showed that this robustness is due to the positive feedback of gene products on their own expression. This topological feature of the network allows individual cells in the model segment to adopt different stable expression states (bistability) corresponding to different cell types in the segment polarity pattern. A positive feedback loop will only yield multiple stable states when the parameters that describe it satisfy a particular inequality. By testing which random parameter sets satisfy these inequalities, I show that bistability is necessary to form the segment polarity pattern and serves as a strong predictor of which parameter sets will succeed in forming the pattern. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700