Micro-bio-fluidica: The role of fluid dynamics in embryonic development and other small biological structures.. Oreste Piro

Transcription

1 Micro-bio-fluidica: The role of fluid dynamics in embryonic development and other small biological structures.. Oreste Piro University of Balearic Islands, Palma, Mallorca, Spain

2 Colaborators: Julyan H. E. Cartwright Laboratorio de Estudios Cristalográficos, IACT (CSIC-UGR), Granada, Spain Nicolás Piro Institut de Ciéncias Fotóniques, Castelldefels, Barcelona, Spain Idan Tuval Departament of Applied Mathematics and Theoretical Physics, University of Cambridge

3 Introduction From the very act of fertilization and the subsequent early stages the development of an embryo involves the dynamical process in fluid flows. Genes, insufficient in numbers to specify every detail of the ontongeny of typical biological organisms, often act as big planners of the development process leaving many aspects to be performed in a self-controlled way by the laws of physics and chemistry. In this talk we will concentrate in some of the instances where genes leaves in the hands of fluid dynamics the performance of fundamental tasks in development.

4 Introduction Structures in a developing embryo are typically of microscopic size, the fluid is an aqueous solution, and flow velocities are at most of a fraction of a millimeter per second. Reynolds numbers Re = UL / ν of these flows, then, range from 10-1 to 10-6 (creeping flow regimes): Viscous forces overwhelm the inertial forces Motion stops when the driving stops, and reverses when the driving reverses (Swimming at low Re is very different to swimming at our familiar Re ranges). Mixing fluids at such regimes is very difficult.

5 A list of examples: Cartwright, Piro & Tuval HFSP Journal 3, 77 93, (2009),

6 Fluid dynamical basis of lateralization in developing vertebrates embryos. Recommended introductory reading: RIGHT HAND, LEFT HAND. (On the origins of asymmetry in brains, bodies, atoms and cultures) by Chris McManus

7 SITUS INVERSUS SITUS SOLITUS

8 Historical Background Dr. Thomas Watson (London, 1835) The organs exacted the appearance which the same viscera would present if seen reflected in a plane mirror London Medical Gazette.

9 Historical Background Dr. Thomas Watson (London, 1835) Huxley and De Beer (1934) The Elements of Experimental Embryology.

10 Historical Background Dr. Thomas Watson (London, 1835) Huxley and De Beer (1934) W. Layton (The Jackson Lab, 1976) -Mice with unusual genetic mutation: iv for inverted viscera. - Homozygotes % had situs inversus.

11 Historical Background Dr. Thomas Watson (London, 1835) Huxley and De Beer (1934) W. Layton (The Jackson Lab, 1976) B. Afzelius (University of Stockholm, 1976) Kartagener s Syndrom: -Bronchiectasis -Sinusitis -Men are infertile -Situs Inversus? Random location Left or Right? Follow the flow! Complex Systems 08, CRP Lavin, January 2008

12 Historical Background Dr. Thomas Watson (London, 1835) Huxley and De Beer (1934) W. Layton (The Jackson Lab, 1976) B. Afzelius (University of Stockholm, 1976) N. Brown and L. Wolpert (1990) - F-molecule.

13 Historical Background Dr. Thomas Watson (London, 1835) Huxley and De Beer (1934) W. Layton (The Jackson Lab, 1976) B. Afzelius (University of Stockholm, 1976) N. Brown and L. Wolpert (1990) M. Levin (Harvard University, 1995) - Hensen s node. - Sonic Hedgehog.

14 Historical Background Dr. Thomas Watson (London, 1835) Huxley and De Beer (1934) W. Layton (The Jackson Lab, 1976) B. Afzelius (University of Stockholm, 1976) N. Brown and L. Wolpert (1990) M. Levin (Harvard University, 1995) M. Brueckner (Yale University, 1997) What was wrong in the IV mouse? IV Gene.

15 Historical Background Dr. Thomas Watson (London, 1835) Huxley and De Beer (1934) W. Layton (The Jackson Lab, 1976) B. Afzelius (University of Stockholm, 1976) N. Brown and L. Wolpert (1990) M. Levin (Harvard University, 1995) M. Brueckner (Yale University, 1997) N. Hirokawa (University of Tokio, 1999) - Kinesins monocilium in the node. - Rotating clockwise Leftward Fluid Flow

16 2-3 µm 50 µm Rotate clockwise at 10 Hz Re = ωa /ν Creeping flow conditions

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19 Left or Right? Follow the flow! Complex Systems 08, CRP Lavin, January 2008

20 In Nature 418, (2002), Nonaka et al. published the letter Determination of left-right patterning of the mouse embryo by artificial nodal flow. Their concluding sentences read Of the questions that remain unanswered, one of the most important concerns the mechanism by which the vectorial flow is generated by the rotational movement of the cilia. Hydrodynamic considerations may provide some insight: it will be essential to characterize precisely the movement and morphology of the cilia, the shape of the node and the hydrodynamic properties of the nodal flow. In the same issue, Claudio Stern commented upon the Nonaka et al. letter in an article, Fluid flow and broken symmetry, in which he noted that the nodal flow highlights the significance of biomechanical phenomena in generating biological pattern. The research was also highlighted in Nature Science Update that week, in which John Whitfield Embryos grow with the flow noted that Finding out how flow drives form is the million-dollar question. Left or Right? Follow the flow! Complex Systems 08, CRP Lavin, January 2008

21 How does the clockwise motion of tens of monocilia in a configuration that does not know what is left can drive a leftward flow in the node? And, if the observed flow is leftward, how is the fluid recirculating within the node, as it must, since the node is a closed structure? Finally, how does the nodal flow lead to left-right symmetrybreaking in the embryo? These questions legitimally belong, we thought, to the realm of physics and fluid mechanics.

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23 L x u= 8πr 3 -Single Vortex. (Rotlet Model) - Shape. - Tilt.

24 Inv genetic abnormality 100 % Situs inversus Normal mice Views of the xy plane for the tilt angle varying between 0 and 24 : 0 (a), 8 (b), 16 (c), and 24 (d).

25 Experimental verification Okada & al. Cell, 121, , 2005

26 Experimental verification Okada & al. Cell, 121, , 2005

27 Experimental verification (Nonaka et al. PLOSS 2005) Average tilt 22,5 degrees.

28 How the information on the symmetry broken by the nodal flow is transmitted to the embryo? Chemosensing or Mechanosensing? Mixing in Creeping flows: - Advection by the flow: τ a=l/v - Molecular Diffusion: τ d=l2/d Pe = vl / D 5<Pe<250 1<τa<5 s 25<τd<250 s

29 Morphogen transport by Advection-diffusion?

30 Realistic simulation of the nodal flow.

31 Realistic simulation of the nodal flow.

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33 Morphogen transport by NVP s. Nodal vesicular parcels: large (0.3 5 μm in diameter), membranesheathed structures that contain multiple lipophilic granules. They are released from nodal cells into the nodal fluid, transported leftward by the nodal flow, and are eventually fragmented when they come into contact with rotating monocilia or the walls of the node. Trajectories of NVPs across the node Release of an NVP Fragmentation of an NVP

34 Morphogen transport by NVP s. Artistic view of the process

35 NVP s transported as passive scalars. Assuming that NVP s break, by any mechanism, in the vicinity of walls and cilia, we ask what is the expcted statistics of breakdowns across the space

36 Breakdown of NVPs However: Mechanical forces seem to be insufficient to break the NVP membrane. Possibilities: Unknown biochemical mechanisms triggered by the proximity of the NVP to either the walls or the cilia. Unknown subtleties in the behavior of membranes transported by Stokes flows with shear... Essentially an open question.

37 Conclusions A sophisticated way in which nature use fluid flows to code developmental information has been basically understood. Left-right asymmetry in the body plan ultimately determined by the chirality of amino acids determining the structure of proteins that conform the molecular motors and their rotation. Interrogation: Why does nature take care to break the symmetry in a given direction, rather than leaving things to chance and allowing half the population to have situs inversus?