Organogensis

VI.1 COORDINATORS
VI.2 PARTICIPANTS
VI.3 SUMMARY
VI.4 INTRODUCTION
VI.5 SPECIFIC AIMS
VI.6 BACKGROUND AND SIGNIFICANCE

• VI.6.i General Background to Organogenesis
• VI.6.ii Interplay of Physical and Genetic Mechanisms in Organogenesis
  • VI.6.ii.a Cell Adhesion
  • VI.6.ii.b Chemotaxis and Haptotaxis
  • VI.6.ii.c Epithelial and Mesenchymal Tissues
  • VI.6.ii.d Epithelia and Cell Polarity
  • VI.6.ii.e Mesenchyme and Extracellular Matrix
  • VI.6.ii.f Mesenchymal Condensation
  • VI.6.ii.g Cell Excitability
  • VI.6.ii.h Reaction-Diffusion Mechanisms
  • VI.6.ii.i Differentiation
  • VI.6.ii.j Summary of Basic Mechanisms of Pattern Formation and Morphogenesis
  • VI.6.ii.k Neuronal Guidance, Fasciculation and Defasciculation
• VI.6.iii Background to Specific Experimental Systems
  • VI.6.iii.a Gastrulation
  • VI.6.iii.b Limb Development
  • VI.6.iii.c Innervation
    • VI.6.iii.c.1 Skeleto-muscular innervation
    • VI.6.iii.c.2 Retinotectal innervation
  • VI.6.iii.d Cardiovascular Development
    • VI.6.iii.d.1 Early vertebrate heart development
    • VI.6.iii.d.2 Trabeculation

VI.7 THEORETICAL FRAMEWORK
VI.8 PRELIMINARY RESULTS
VI.9 RESEARCH DESIGN AND METHODS
VI.10 RELATIONSHIP TO CYTOSKELETON (PROJECT 2) AND BIOLOGICAL NETWORKS (PROJECT 1)
VI.11 TIMELINE

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VI.6.iii.c Innervation:
VI.6.iii.c.1 Skeleto-muscular innervation:

Muscle cells migrate from the somites and make limb-type specific muscles that depend entirely on the developing skeleton, particular on the surface connective tissue on each cartilage element. The source of the somitic muscle cells (with regard to axial level) makes absolutely no difference; if we put the somites that would have made wing muscles opposite the leg bud, they will make leg muscles (Christ et al., 1977; Chevallier et al. 1977).

However, although the muscle cells are originally naive, once they form muscles the incoming nerve cell fibers seek them out and only innervate the appropriate muscle; the muscles are now the dominant carrier of information. Thus if the spinal cord is inverted so that the nerve fibers that should go into the wing are now closer to the leg, they will navigate across the whole embryo to get to the wing muscles, even though they pass through "wrong" tissues in transit (Lance-Jones, 1988). This selectivity must involve both chemotaxis (globally) and differential adhesion (locally).

We will investigate how these same skeletal elements influence neural pathfinding in the avian limb. Nerve fiber growth in the presence of extracellular chemotactic guidance cues will model this innervation.

Another important question is how a proper topographic map forms between the motor neurons and the various muscle sites that require innervation. Our initial simulations show that, though bundling and debundling can occur in a topographically ordered way so that neighboring neurons come to innervate neighboring target cells, the mapping is weak. For example, random axon movements, which are needed for bundling in the absence of axon-derived chemoattractants, tend to disturb this pattern (Goodhill, 1997,1998). E.g. gradients of receptor density on axons that are complementary to spatially varying fibronectin production, guides mesenchymal condensation.

Long-range diffusible molecules have been strongly implicated in vertebrate neural network formation. These diffusible factors act as both guidance cues at the extracellular level and as morphogens at the intracellular level, turning on or off genetic switches for the transcription of proteins such as fibronectin.

The nervous system expresses numerous members of the cadherin family (Yagi and Takeichi, 2000). Cadherins are well positioned to be important regulators of growth cone migration, fasciculation, defasciculation, and target cell recognition during development. Cadherins are necessary for morphoregulatory processes during development, and regulate retinotectal projection (Inoue and Sanes, 1997). We will use zebrafish to study cadherin function and regulation. We can easily manipulate protein expression levels experimentally and genetically to test simulation predictions. Cadherins may provide a combinatorial code for the wiring of circuits in the nervous system (Redies and Takeichi, 1996) and may control connectivity within the retina, retinofugal projections, and tectofugal projections (Bixby and Harris, 1991; Matsunaga et al., 1988a 1988b; Redies, 1997). We will experimentally manipulate cadherin expression and function, and examine the consequences on neuronal circuits in the visual system. In particular, we will examine the effects of cadherin downregulation on optic nerve fasciculation and defasciculation.

VI.6.iii.c.2 Retinotectal innervation:

Defects in the projection of the retina to the optic tectum lead to loss of vision even when the retina, nerve, and optic tectum function normally. During retinal development, undifferentiated neuroepithelial precursor cells segregate and differentiate, forming laminae of different retinal cells (Cepko et al., 1996). Synaptic connections form within the retina which regulate processing of visual information (Dowling, 1970). Ganglion cells extend axons from the retina along specific pathways to visual centers of the brain, particularly the optic tectum (Burrill and Easter, 1995; Burrill and Easter, 1994; Sanes and Yamagata, 1999). During this migration, ganglion cell axons must make many decisions to arrive at the dorsal tectum (Mueller, 1999). Fascicles are maintained during migration of the optic nerve, and axons defasciculate specifically at retinorecipient targets within the brain.

For many years, zebrafish have served as a model for retinotectal projection (Burrill and Easter, 1994; Kaethner and Stuermer, 1992; Stuermer, 1988; Stuermer et al., 1990), supplying a detailed background literature that describes key anatomical features and processes in visual system development. The nervous system of a fish simple allowing a clear view of developmental events. The low cost and simplicity of raising large numbers of staged embryos facilitates experiments. Sophisticated techniques for large-scale genetic screening for mutants in development are available for zebrafish (Driever et al.,1996; Haffter et al., 1996), e.g. on regeneration-competent vs. incompetent spinal cord which elucidated the Notch-1 signaling pathway and involvement of the gene Sonic hedgehog (Shh) in cord regeneration (Chernoff et al., 2001).