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.
An examination of the topographic distribution of lipofuscin pigment granules with the light and electron microscope revealed either smaller and randomly "dispersed" or larger and more complex "clustered" pigment configurations in the cytoplasm of neurons in the dorsal ganglia and ventral spinal cord of 24-month old male mice. Qualitative comparisons revealed no major differences in shape, size, complexity, density, orientation, and cytologic distribution of the pigment bodies in motor and sensory neurons. In general, when the pigment granules were quite numerous within the 2 types of cells, they were smaller in size (∼lµ), had a dense homogeneous matrix with few bands or lamellae, and were uniformly distributed throughout the cytoplasm. In contrast, when the pigment configurations were less in number, they were usually larger in size (∼3µ), had a more complex internal banded structure, and appeared more localized within the cell. Examination of the bands revealed a repeating pattern of ∼70 A. The bands appeared to fuse, forming hexagonal arrays of linear densities intersecting at an angle of approximately 120° in some regions of the pigment bodies. Structural similarities suggested that the striated membranous bands may be composed of phospholipids.