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| In Living Color: A New Blueprint For Body Parts |
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| SciMed - Biology | |||
| TS-Si News Service | |||
| Friday, 27 July 2007 20:00 | |||
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New discoveries could change how scientists think about life in general.
 Drosophila is a genus of small flies that belong to the family Drosophilidae. Family members are often called fruit flies (but include vinegar flies, wine flies, pomace flies, grape flies, and picked fruit-flies). Drosophila melanogaster, in particular, has been a model research organism for nearly a century. The species has a relatively short short life span (about two weeks) and a genomic structure that has a number of parallels to human beings. While genetics led the way with initial uses for D. melanogaster, it has become an important investigative vehicle for developmental biology. Researchers study how a complex organism can arise from a relatively simple fertilised egg. There is a great deal of work being done on how various adult structures develop in the pupa (such as the compound eye, wings, legs and other organs). The drosophila egg is about half a millimeter long. • It takes roughly one day after fertilisation for the embryo to develop and hatch into a worm-like larva. • The larva eats and grows continuously, moulting one, two, and four days after hatching (1st, 2nd and 3rd instars). • After two days as a third instar larva, the larva moults one more time to form an immobile pupa. • Over the next four days, the body is completely remodelled to give the adult winged form, which then hatches from the pupal case and is fertile within about 12 hours. Timing is for 25°C; at 18°C, development takes twice as long. The metamorphosis of biology into a science offering numerically precise descriptions of nature took a leap forward with a Princeton team's elucidation of a key step in the development of fruit fly embryos. The research is a strong example of biologically relevant questions that can be addressed by theoretical physicists. The discoveries could change how scientists think about life in general.
Developing organisms are faced with the challenge of creating reproducible patterns in an inherently noisy system. The Princeton team has produced two papers that tackle the problem of pattern formation and reproducibility in the early stages of Drosophila embryonic development.
 "I think the prevailing view has been that cells accomplish all their functions using a complicated combination of mechanisms, each one of which is rather sloppy or noisy," said team member William Bialek. "This research, however, indicates that in the initial hours of a fly embryo's development, cells make decisions to become one part of the body or another by a process so precise that they must be close to counting every available signaling molecule they receive from the mother.”
The embryo is initially patterned by gradients of transcription factors that are read out by the embryo in order to activate the formation of more complex patterns. The authors of these papers address questions such as:
 The figure at the left is a two-hour-old Drosophila embryo showing the expression of the Bicoid protein. The protein forms a gradient with the highest expression at the anterior end of the embryo (figure left). Three hours into a fly embryo's development, it remains a single large cell with an unusual characteristic. Unlike other cells, which have a single nucleus, the embryo has thousands, each of which awaits a signal from the mother to form itself into a specialized cell. This signal arrives in the form of a Bicoid droplet that enters the embryo at one end, Like food coloring in water, it diffuses out molecule by molecule through the nuclei. The concentration decreases with distance.
They research team also explaina inconsistencies between the theoretical predictions to precision limits and those that have been measured experimentally.
The team's findings indicate that two neighboring nuclei can determine their different places and functions within the embryo accurately if the concentration of Bicoid between them varies by only about 10 percent -- a quantity that on the scale of the tiny embryo amounts to only a few molecules of Bicoid.
"This signaling requires a sensitivity approaching the limits set by basic physical principles," Bialek said. "Perhaps more important than the answers we have found so far, this work has led us to sharpen the kinds of questions we ask about living cells as we try to understand them with the same kind of mathematical precision that we understand the rest of the physical world."
The two papers constitute the Ph.D. research of first author Thomas Gregor, who is now pursuing postdoctoral work in the Department of Physics at the University of Tokyo. The overall project was a collaboration among three Princeton faculty members:
The first of two papers describes the sophisticated techniques required to make development process movies. The techniques could help scientists investigate a wide variety of biological systems.
Probing the Limits to Positional Information. Thomas Gregor, David W. Tank, Eric F. Wieschaus and William Bialek, Cell, 2007, 130:1:153-164.
Summary. "The reproducibility and precision of biological patterning is limited by the accuracy with which concentration profiles of morphogen molecules can be established and read out by their targets. We consider four measures of precision for the Bicoid morphogen in the Drosophila embryo: the concentration differences that distinguish neighboring cells, the limits set by the random arrival of Bicoid molecules at their targets (which depends on absolute concentration), the noise in readout of Bicoid by the activation of Hunchback, and the reproducibility of Bicoid concentration at corresponding positions in multiple embryos. We show, through a combination of different experiments, that all of these quantities are ∼10%. This agreement among different measures of accuracy indicates that the embryo is not faced with noisy input signals and readout mechanisms; rather, the system exerts precise control over absolute concentrations and responds reliably to small concentration differences, approaching the limits set by basic physical principles."
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In the second paper, the group poses a new question, never before asked by scientists studying embryos: How precisely can cells in the embryo read the blueprint? So precisely, the paper suggests, that a precious few molecules signaling a change can make a decisive difference. Stability and Nuclear Dynamics of the Bicoid Morphogen Gradient. Thomas Gregor, Eric F. Wieschaus, Alistair P. McGregor, William Bialek and David W. Tank. Cell, Volume 130, Issue 1, 13 July 2007, Pages 141-152.
Summary. Patterning in multicellular organisms results from spatial gradients in morphogen concentration, but the dynamics of these gradients remain largely unexplored. We characterize, through in vivo optical imaging, the development and stability of the Bicoid morphogen gradient in Drosophila embryos that express a Bicoid-eGFP fusion protein. The gradient is established rapidly (∼1 hr after fertilization), with nuclear Bicoid concentration rising and falling during mitosis. Interphase levels result from a rapid equilibrium between Bicoid uptake and removal. Initial interphase concentration in nuclei in successive cycles is constant (±10%), demonstrating a form of gradient stability, but it subsequently decays by approximately . Both direct photobleaching measurements and indirect estimates of Bicoid-eGFP diffusion constants (D ≤ 1 μm2/s) provide a consistent picture of Bicoid transport on short (∼min) time scales but challenge traditional models of long-range gradient formation. A new model is presented emphasizing the possible role of nuclear dynamics in shaping and scaling the gradient.
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| Last Updated on Tuesday, 23 October 2007 17:34 |
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