| Popis: |
Localising a sound on the horizontal plane is critical for animal survival. Binaural animals, including birds and mammals, locate sounds by calculating the time difference of the sound arriving at each ear, or the interaural time difference (ITD). In the chick, the auditory hindbrain (AHB) is the neuronal circuit responsible for calculating ITD. The AHB resides in the dorsolateral region of rhombomere 5 and consists of 3 neuronal nuclei; the nucleus angularis, nucleus magnocellularis (nM) and the nucleus laminaris (nL). The structure and intricate circuitry of the AHB is critical for its function, but difficulties arise in studying the development of this neuronal circuit as there are no known markers for the AHB before E7, when the generation and migration of these neurons is largely complete. Within rhombomere 5 of the chick hindbrain, there are also 4 distinct motor nuclei, which occupy the ventral region of the hindbrain. These are the abducens, accessory abducens and the dorsal and ventral extents of the facial motor nucleus. The correct segregation of these nuclei is dependent on differential type II cadherin expression between each motor nuclei. In this thesis, homeodomain protein expression in the developing hindbrain is characterised. The data presented suggests that the developmental origin of the AHB is the Dbx2 progenitor domain of the ventricular zone, and Dbx2 is identified as a nuclear marker for the neurons of the nM and nL. Perturbations to homeodomain protein expression at the time of AHB neuron generation, results in the failure of the AHB to express fgf8 and n-cadherin mRNA. These genetic manipulations, and the resultant loss of FGF8 expression, cause disruptions to motor nuclei segregation, possibly via the modulation of cadherin expression. A model is proposed whereby the AHB may act as a signalling centre for regulating the development of other neuronal nuclei within the hindbrain. |
