STRUCTURAL ORGANIZATION OF THE RETINA IN THE TREE SHREW (TUPAIA GLIS)

This investigation was undertaken to establish the gross and ultrastructural organization of the photoreceptors and retina in the Malayan tree shrew (Tupaia glis). Photographs of the fundus revealed no specialization or differentiation of a central foveal region. Histologic sections revealed a single row of relatively short and thick cones distributed uniformly throughout the retina. Electron micrographs of the retina indicated that the receptor outer segments are closely invested by pigment-filled epithelial processes and an amorphous interstitial material. The internal fine structure of the receptor outer segments revealed the characteristic stacks or arrays of bimembranous discs. The ellipsoid portions of the cone inner segments include tightly packed and extraordinarily large mitochondria. These mitochondria consist of unique patterns of concentric cristae arranged in highly ordered whorls of lamellar configurations. The cone synaptic pedicles contain a unique system of tubules not previously described in synaptic endings. Histologic sections indicated that only cone populations are located in the central region of the retina, whereas histologic, histochemical, and ultrastructural comparisons suggested that photoreceptors with some "rodtype" features are located more peripherally. The relatively small proportion of these rodtype receptors among the great preponderance of cone populations is in general accord with the tree shrew's diurnal habits as well as its great reliance on photopic vision and its visually guided behavior.