The role of sonic hedgehog and bone morphogenetic proteins in the development of the vertebrate midbrain
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During development of the nervous system, signals from specialized organizing centers generate distinct cell types. The signaling molecule, Sonic Hedgehog (SHH) is expressed by the floor plate (FP) and is sufficient to specify the ventral midbrain pattern. In the spinal cord, Bone Morphogenetic Proteins (BMPs) expressed in the roof plate (RP) specify dorsal cell-fates. The attenuation of BMP signaling is required for SHHmediated patterning of the ventral hindbrain and spinal cord, while BMP signaling is required in conjunction with SHH for ventral forebrain patterning. This thesis will focus on the function of SHH and BMPs in the midbrain by examining the molecules ability to pattern and regulate development. Midbrains of Shh[superscript -/-] mice were examined. Some ventral cell fates are specified in the Shh[superscript -/-] mouse in a Ptc1 and Gli1 independent manner. Ventral midbrain induction was observed to be Hh-independent by the existence of a Pax7-negative ventral midbrain territory before embryonic day 9. Interestingly, dorsal markers are not uniformly altered and increased cell death was seen in Shh[superscript -/-] dorsal midbrains. These results suggest specific regulation of dorsal patterning by Shh, rather than a simple deregulation. Several BMPs and their antagonists are expressed in a spatial and temporal manner in the midbrain. Expression of BMPs is seen in the RP, and rostral FP (rFP), which also expresses SHH. BMP signaling was manipulated using in vivo electroporation. NOGGIN misexpression resulted in a loss of RP and a reduction of dorsal cell-fates that was preceded by cell-shape changes, delamination of cells into the lumen and their elimination. This was accompanied by a reduction and alteration of midbrain size and shape. BMP blockade changed N-Cadherin distribution and perturbed pseudostratified morphology of the neurepithelium. Ventrally, BMP blockade resulted in a decrease of proliferation, while increasing differentiation, Notch signaling molecules at the rFP and medial FP markers. However ventral midbrain cell-fates were correctly specified. Notch-Delta signaling was examined in the Mib[superscript -/-] mouse. Different regulation of cell-fates was observed in the midbrain and spinal cord. Mib[superscript -/-] midbrains lacked a mature lateral FP, however ventral cell-fates are specified. Mib[superscript -/-] spinal cords lose Shh expression and several ventral cell-fates.