Bone morphogenetic proteins and zebrafish inner ear development.

This thesis describes the mRNA expression patterns of the Bone Morphogenetic
Proteins (BMPs), downstream members of the BMP signal pathway, BMP antagonists
and candidate target genes in the developing inner ear of wild type zebrafish. The crista
Bmp expression pattern is conserved between four vertebrate species. However, unlike
in chick, mouse and Xenopus laevis none of the hmps examined are macula markers in
zebrafish.
This thesis identifies sources of Bmp signalling (the cristae, the endolymphatic duct
(ED) and the semicircular canals (SCC)) and possible sites of Bmp action (the cristae,
posterior macula, SCC and the mesenchyme around the ED). It also provides the first
description of the early stages of ED development, a structure only recently described at
later stages in the zebrafish (8dpf), and two mRNA markers of this structure (bmp4 and
dachA).
In analysis of zebrafish mutants with defective cristae, the presence of cristae correlated
with the expression of the hmps and msxc, a putative Bmp target. This suggests the
Bmps are required to form cristae and express msxc. Gain and loss of function studies
have also supported a role for the Bmps in the development of the posterior macula and
SCc.
Ectopic hBMP4 protein was applied to the otic vesicle via protein-coated beads. This
inhibited the development of the posterior macula and SCC. However, these hBMP4
beads were not sufficient to induce the expression of ectopic msxc, generate ectopic
cristae or rescue crista development in mutants. Beads coated in a BMP antagonist did
not affect the development of endogenous cristae or the expression of endogenous msxc.
Rescued swirl (bmp2b) mutant adult zebrafish exhibit a balance defect. Early stages of
inner ear development in rescued embryos were found to progress normally up until
7dpf. However, it is not clear when the rescuing mRNA or protein degrades, and work
done by others in the lab has shown that Bmp2b is required at later stages to form adult
SCc. The ectopic hBMP4 experiments suggest that moderating levels of Bmp
signalling may be required for normal development of the SCC at early stages.