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P. G. McMenamin; The Unusual Arrangement of Retinal Vessels in Marsupials: Why Do They Travel in Pairs?. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1224.
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© ARVO (1962-2015); The Authors (2016-present)
In the few marsupial species studied to date that possess a retinal vasculature, the arterial and venous segments of retinal vessels, down to the smallest calibre capillaries, occur in pairs. This unique pattern of vasculature in the retina has been described in only a few marsupial species. The aim of the present study was to confirm this pattern in a few other available non-eutherian mammalian species (marsupials and monotremes) to determine if this is a phenomenon widespread within this animal group.
The retinae and posterior segment from marsupials and a monotreme were compared with the usual retinal vascular patterns in eutherian mammals. Species studied included a monotreme, the echidna (Tachyglossus aculeatus) and a range of Australian marsupials, the honey possum (Tarispedidae rostratus), fat-tailed dunnart [Sminthopsis crassicaudata], numbat [Myrmecobius fasciatus], grey bellied dunnart [Sminthopsis griseoventer], broad footed marsupial mouse (Antechinus godgmani) and the North American opossum (Didephis virginiana). Eyes were fixed in gluteraldehyde or paraformaldehyde and retinae were embedded in resin for light and electron microscopic analysis.
Examination revealed that in those species with retinal vessels (fat-tailed dunnart, numbat, marsupial mouse, opossum) the pattern of vessels differs from the conventional plexus-like arrangement of eutherian mammalian retinal vasculature (i.e. anastomotic networks of capillaries between arterioles and venules). By contrast in marsupials retinal vessels always occur in closely related pairs with the arteriolar limb usually situated on the vitread aspect. Vessels penetrate the retina and branch to form layers of paired capillaries in the nerve fiber layer, the inner nuclear layer and the outer plexiform layer. The capillaries which form hairpin end loops display classical morphological features of CNS capillaries. The lining endothelium is supported by a distinct basal lamina that splits to envelop pericytes. The latter which although abundant, are invariably interposed between the two vessels that form each vascular unit.
The phylogenetic relations of this vascular pattern which is found in marsupial retina and CNS and in the CNS of other classes of non-mammalian vertebrates suggest that retinal vascularisation evolved independently in marsupial and eutherian mammals and that the former have evolved from a more primitive vascular pattern in a common ancestor. The functional significance of the paired vessels is as poorly understood as the anatomy.
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