Abstract:We have examined the development of retinal projections in a diminutive polyprotodont marsupial, the fat-tailed dunnart, Sminthopsis crassicaudata. Here, we document the most immature mammalian visual system at birth described to date. At postnatal day (P) 0, the retinal ganglion cell layer has yet to form, and axons have not entered the optic stalk. By P4, the retinal ganglion cell layer could be distinguished at the posterior pole, and the front of growing axons extended one-third the length of the optic sta… Show more
“…No additional animals were killed. Data were obtained from Tables 1^5 of Robinson and Dreher (1990), Table 2 from Finlay and Darlington (1995), Tables 1^3 of Ashwell et al (1996), data reported in Dunlop et al (1997), Table 1 of Darlington et al (1999), as well as from the general literature where noted. Data for the cortical events in this study refer to posterior (presumptive visual) cortex.…”
Section: Data Used To Construct the Comparative Mammalian Modelmentioning
“…No additional animals were killed. Data were obtained from Tables 1^5 of Robinson and Dreher (1990), Table 2 from Finlay and Darlington (1995), Tables 1^3 of Ashwell et al (1996), data reported in Dunlop et al (1997), Table 1 of Darlington et al (1999), as well as from the general literature where noted. Data for the cortical events in this study refer to posterior (presumptive visual) cortex.…”
Section: Data Used To Construct the Comparative Mammalian Modelmentioning
“…In grey squirrels the AOS nuclei are already innervated at birth (Cusick and Kaas, 1982), although these retinal projections undergo redistribution during postnatal development. Studies in the marsupial fat-tailed dunnart (Dunlop et al, 1997) have shown that the development of visual pathways in general and of accessory optic projections in particular, occurs entirely postnatally in this species: retinal projections don't reach the accessory optic nuclei until P15. This makes the dunnart a suitable species for studies on the development of the visual system.…”
The accessory optic system (AOS) is formed by a series of terminal nuclei receiving direct visual information from the retina via one or more accessory optic tracts. In addition to the retinal input, derived from ganglion cells that characteristically have large receptive fields, are direction-selective, and have a preference for slow moving stimuli, there are now well-characterized afferent connections with a key pretectal nucleus (nucleus of the optic tract) and the ventral lateral geniculate nucleus. The efferent connections of the AOS are robust, targeting brainstem and other structures in support of visual-oculomotor events such as optokinetic nystagmus and visual-vestibular interaction. This chapter reviews the newer experimental findings while including older data concerning the structural and functional organization of the AOS. We then consider the ontogeny and phylogeny of the AOS and include a discussion of similarities and differences in the anatomical organization of the AOS in nonmammalian and mammalian species. This is followed by sections dealing with retinal and cerebral cortical afferents to the AOS nuclei, interneuronal connections of AOS neurons, and the efferents of the AOS nuclei. We conclude with a section on Functional Considerations dealing with the issues of the response properties of AOS neurons, lesion and metabolic studies, and the AOS and spatial cognition. The accessory optic system (AOS) is formed by a series of terminal nuclei receiving direct visual information from the retina via one or more accessory optic tracts. In addition to the retinal input, derived from ganglion cells that characteristically have large receptive fields, are direction-selective and have a preference for slow moving stimuli, there are now well characterized afferent connections with a key pretectal nucleus (nucleus of the optic tract) and the ventral lateral geniculate nucleus. The efferent connections of the AOS are robust, targeting brainstem and other structures in support of visual-oculomotor events such as optokinetic nystagmus and visual-vestibular interaction. The present chapter reviews the newer experimental findings while including older data concerning the structural and functional organization of the AOS. We then consider the ontogeny and phylogeny of the AOS and include a discussion of similarities and differences in the anatomical organization of the AOS in nonmammalian and mammalian species. This is followed by sections dealing with retinal and cerebral cortical afferents to the AOS nuclei, interneuronal connections of AOS neurons, and the efferents of the AOS nuclei. We conclude with a section on Functional Considerations dealing with the issues of the response properties of AOS neurons, lesion and metabolic studies, and the AOS and spatial cognition.
“…Their lack of a pouch and their fetal‐like nature at birth circumvents the need for in utero experimental procedures that would be required on more conventional mammalian model systems such as mice, rats, rabbits, ferrets, or cats. Thus, their immaturity at birth and their protracted period of postnatal development make marsupials, like Monodelphis , excellent models for the study of visual system development and plasticity 17–22…”
Section: Monodelphis Domestica: a Novel In Vivo Model System To Invesmentioning
Transplantation of neural stem/progenitor cells has been proposed as a novel approach for the replacement and repair of damaged CNS tissues. We have evaluated the influence of the host cellular microenvironment upon the survival, differentiation, and integration of neural progenitor cells transplanted into the CNS. Using this approach, we have investigated the fate of neural progenitor cells in vivo following transplantation into the developing mammalian eye. Murine brain progenitor cells (mBPCs) isolated from neonatal mice expressing the green fluorescent protein (GFP) transgene were transplanted into the eyes of Brazilian opossums (Monodelphis domestica). Monodelphis pups are born in an extremely immature, fetal-like state. The eyes of neonatal pups provide a fetal-like environment in which to study cellular interactions between host tissues and transplanted neural progenitor cells. mBPCs were transplanted by intraocular injection in hosts ranging in age from 5 days postnatal to adult. The transplanted cells were easily identified because of their GFP fluorescence. Extensive survival, differentiation, and morphological integration of mBPCs within the host tissue was observed. We found that the younger retinas provided a more supportive environment for the morphological integration of the transplanted mBPCs. Cells with morphologies characteristic of specific retinal cell types were observed. Moreover, some transplanted mBPCs were labeled with antibodies characteristic of specific neural/retinal phenotypes. These results suggest that the host environment strongly influences progenitor cell differentiation and that transplantation of neural progenitor cells may be a useful approach aimed at treating degeneration and pathology of the CNS.
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