[Truncated abstract] Developmentally, the rodent retinocollicular projection is often thought of as a homogenous projection of retinal ganglion cell (RGC) axons, however the extensive period of RGC neurogenesis and sequential arrival of their axons into central targets such as the superior colliulus (SC) suggests otherwise. RGC axons are already present in the developing SC at embryonic (E) day 16.5-17. RGCs born on E15 have innervated the SC by birth, whereas axons derived from RGCs that are born last (E19) do not grow into the SC until postnatal (P) days 4-6 (Dallimore et al., 2002). These observations may go someway to explaining why, after SC lesions in rats at P2, there is greater growth distal to the lesion site compared to lesions made at P6 (Tan and Harvey, 1997b). It may be that the post lesion growth is simply de novo growth of axons from late-born RGCs rather than regeneration of pre-existing, injured axons. Early and late cohorts of growing RGC axons presumably encounter different developmental terrains as they grow from retina to central targets, possibly resulting in differences in developmental milestones and growth potentials. There may also be differences in guidance cues, further suggesting that gene expression in early vs late born RGCs may differ. To examine differences between early (E15) and late (E19) born RGCs during development, the time-course and extent of programmed RGC death in normal rat pups, and RGC death following the removal of target-derived trophic factors, was assessed. ... On the other hand, LCM captured GCL analysed for gene expression at P0 and P7 revealed decreases in AKT, Math5, Notch1, c-jun, DCC, Arginase-1 mRNA levels and a considerable decrease in GAP-43 expression. It is not surprising to see differences in gene expression between whole eye and the more specific GCL samples, as the cells in all layers of the retina have very different functions and different developmental profiles. It is important to note decreases in mRNA expression in the GCL for a number of the genes analysed at P0 and P7, reflecting cessation of RGC death and completion of axonal growth into central visual targets. I also examined at the protein level expression of DCC, Arginase1, c-Jun and Bcl-2 at birth (P0) in BrdU labeled RGCs born on E15 or E19. When comparing the percentage of double labelled cells compared to the total number of cells expressing each protein, Bcl-2, c-Jun and Arg1 were expressed more in E15 RGCs (22.90%, 72.71%, and 16.44% respectively in E15 RGCs, compared with 0.52%, 13.17% and 3.59% in E19 RGCs). In contrast, DCC was expressed more at birth in E19 RGCs (18.05% in E19 RGCs compared with 9.23% in E15 RGCs). This shows there is clearly a difference in the expression of proteins in the two cohorts of RGCs, which is consistent with PCR data and with their growth state as their axons encounter the changes in the newborn brain. The overall findings of this research suggest that seemingly homogenous populations of neurons are quite different in their developmental profile and in their response to injury. This work may provide new ways of determining better strategies for CNS repair and the most effective way of targeting cells for regeneration and survival.
| Identifer | oai:union.ndltd.org:ADTP/246436 |
| Date | January 2009 |
| Creators | Dallimore, Elizabeth Jane |
| Publisher | University of Western Australia. School of Anatomy and Human Biology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Elizabeth Jane Dallimore, http://www.itpo.uwa.edu.au/UWA-Computer-And-Software-Use-Regulations.html |
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