Structure and postnatal development of photoreceptors and their synapses in the retina of the tree shrew (Tupaia belangeri).

The "all cone" retina of the tree shrew (Tupaia belangeri) was examined in the adult and early postnatal stages by light and electron microscopy. Rods are not as rare as previously thought, but make up about 4% of the photoreceptors. They are relatively short and narrow cells, which stain (toluidine blue) more intensively and lie more proximal than cones. Among the cones three morphological varieties could be distinguished. Most cones stain lightly but have a light or a dark giant mitochondrion in their inner segment; a third type stains darker but occurs only rarely. All cones possess extensive radial processes ("lateral fins") around the basal part of their inner segments. Such fins are well known from reptiles and birds, but have only once been described in a mammal (gray squirrel). The maturation of the retina in Tupaia belangeri proceeds centrifugally, i.e., from the vitreal to the scleral side, as in most mammals. A few synapses are already present at birth in the outer and inner plexiform layers, but seem to be more advanced in the latter. Such early synapses are small and have only few synaptic vesicles; they appear almost mature by day 14. The light-sensitive outer segments develop last. The first disks are seen by day 10, but regular membrane stacks are only present by day 18. Thus, it seems that the retina is functional when the young first open their eyes, which occurs around day 18.