zebrafish; midbrain; hindbrain; her5; transgenic; neurogenesis; brain development
Tallafuss, A. (2003) The induction and early development of the midbrain-hindbrain in the embryonic zebrafish. Ph.D. Thesis. .
The midbrain-hindbrain domain (MH) is an important region of the vertebrate embryonic brain. Indeed, in contrast to other brain domains, it is not formed by segmentation processes, but responds to an organizer activity located at the midbrain-hindbrain boundary (MHB). Thus it is of great interest to understand the factors and mechanisms underlying MH formation. The present work uses the zebrafish system to add important information on the poorly understood steps of early MH development. These results are summarized below: (i) It is known that MH development is regulated by planar information, however the upstream factors regulating the expression of the known MH markers her5, pax2.1, wnt1 and fgf8 are not known. We identified the zebrafish Btd-related factor Bts1 as a specific regulator of pax2.1 and its dependent genetic cascade (pax5, eng3). Because of the crucial function of Bts1 in MH induction, we analysed the regulation of its own expression. We demonstrate that the induction of bts1 expression likely depends on FGF and Wnt signaling. Thus, we identified one cascade of MH induction through planar signaling. (ii) Anterior neural plate development is thought to respond to vertical signaling. So far, the precise role of vertical signals in MH development remains unclear. Using a combination of experimental manipulations in mutant lines affected in non-neural tissues, we unravelled the inhibitory influence of a long-range signal, emanating from the prechordal plate, on the refinement of a neural cluster at the forebrain-midbrain boundary. These results give evidence that vertical signals have a precise role in MH development. (iii) The earliest gene selectively expressed in the prospective MH is her5. Using a reporter approach in zebrafish transgenics, we identified her5 regulatory elements. Further, the PACher::egfp transgenic line allows tracing cells of the entire presumptive MH from MH induction onwards. Using this transgenic line in mutant contexts, we demons