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First published online May 16, 2007
doi: 10.1242/10.1242/dev.02850

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Regulation of ventral midbrain patterning by Hedgehog signaling

Roy D. Bayly¹, Minhtran Ngo^2,*, Galina V. Aglyamova² and Seema Agarwala^1,2,3,

¹ Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712-0248, USA.
² Section of Neurobiology and University of Texas at Austin, Austin, TX 78712-0248, USA.
³ Institute for Neuroscience, University of Texas at Austin, Austin, TX 78712-0248, USA.

Author for correspondence (e-mail: agarwala{at}mail.utexas.edu)

Accepted 13 March 2007

In the developing ventral midbrain, the signaling molecule sonic hedgehog (SHH) is sufficient to specify a striped pattern of cell fates (midbrain arcs). Here, we asked whether and precisely how hedgehog (HH) signaling might be necessary for ventral midbrain patterning. By blocking HH signaling by in ovo misexpression of Ptc1^loop2, we show that HH signaling is necessary and can act directly at a distance to specify midbrain cell fates. Ventral midbrain progenitors extinguish their dependence upon HH in a spatiotemporally complex manner, completing cell-fate specification at the periphery by Hamburger and Hamilton stage 13. Thus, patterning at the lateral periphery of the ventral midbrain is accomplished early, when the midbrain is small and the HH signal needs to travel relatively short distances (approximately 30 cell diameters). Interestingly, single-cell injections demonstrate that patterning in the midbrain occurs within the context of cortex-like radial columns of cells that can share HH blockade and are cytoplasmically connected by gap junctions. HH blockade results in increased cell scatter, disrupting the spatial coherence of the midbrain arc pattern. Finally, HH signaling is required for the integrity and the signaling properties of the boundaries of the midbrain (e.g. the midbrain-hindbrain boundary, the dorsoventral boundary), its perturbations resulting in abnormal cell mixing across `leaky' borders.

Key words: Midbrain-hindbrain boundary, Motor and dopaminergic neurons, Morphogen, Cell affinities, Size regulation, Midbrain radial columns, Chick

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