The distribution of crossed and uncrossed optic fibers in the different layers of the lateral geniculate nucleus in the tree shrew (Tupaia glis)

Classes of axons and their distribution in the optic nerve of the tree shrew (Tupaia belangeri)

BACKGROUND

It has been suggested that retinal ganglion cells (RGCs) of Tupaia can be subdivided into three classes that correspond to the X, Y, and W classes in the cat. Estimates of these classes as determined by electrophysiological experiments and by histological studies of the retina are at variance. Because the RGC classes differ in axon diameter, this parameter could serve as a reliable criterion for the evaluation of RGC classes and their proportions.

METHODS

An electronmicroscopic analysis of four optic nerves was carried out. The density of axons and their diameters was recorded from cross sections in the middle of the nerves. Based on a theoretical model, axons were classified according to the three known RGC classes. In addition, we investigated how axons of different size are distributed within the nerve.

RESULTS

On average, the total number of axons is 570,000. More than 99% of axons are myelinated. Axon diameters can be 0.2-3.6 microns, and fiber diameters can be 0.3-4.6 microns. The frequency distributions of axon and fiber diameter are positively skewed and multimodal. Our analysis revealed three distinct axon diameter groups with the following mean axon diameters and proportions: the small or S group: 0.55 micron, 70%; the medium-sized or M group: 0.88 micron, 20%; the large of L group: 1.39 microns, 10%. Density and axon diameter plots produced consistent patterns, according to which axons of different sizes were distributed in the optic nerve. Thick axons are located dorsotemporally and centrally in the nerve. The average diameter value decreases toward the periphery. The smallest values are found at the ventronasal border of the nerve.

CONCLUSIONS

The observed axon diameter groups probably arise from functionally distinct RGC types. There is evidence that these groups correspond to functional W, X, and Y RGC classes. Our study also provides the first evidence of the existence of a topographic order of fibers within the nerve of the tree shrew.

Ultrastructure of the developing tree shrew lateral geniculate nucleus

We examined the ultrastructural development of the lateral geniculate nucleus (LGN) in postnatal tree shrews to distinguish which features, if any, show a correlation with the development of cell layers. Our data indicate that synaptogenesis has begun at birth (PO) which is prior to the development of cell layers. At postnatal day 8 (P8), when laminar borders can be distinguished, the majority of synaptic profiles are still immature. Although some mature synaptic profiles can be identified at P8 and especially at P15, complex synaptic arrangements characteristic of adults are absent at that time. Growth cones are present at all 3 ages but are less prevalent at P8 and P15 than at birth and appear to be present in slightly higher concentrations in the interlaminar spaces. These results suggest that LGN cell layer formation does not correlate with the beginning of synaptogenesis but that it may correlate with an increase in growth cones in the interlaminar spaces.

Rod receptors in the retina of Tupaia belangeri

The retina of Tupaia belangeri was studied by transmission electron microscopy. - Among a vast majority of cones a few typical rods have been found as a distinctly different class of photoreceptors. The outer segments of the rods measure 10 microns X 1.5 microns versus 6 microns X 2 microns in the cones. The repeating unit of the rod discs is 18 nm versus 22 nm in the cone discs. Rod discs tend to lose their continuity with the ciliary membrane, while cone discs do not. Rods lack the highly specialized giant mitochondria of the cone inner segment of Tupaia. The rods of Tupaia show other distinctive characters of mammalian rods as well. However, the rods of Tupaia are relatively short and do not develop a rod fiber.

Neuronal and synaptic organization of the lateral geniculate nucleus of the tree shrew, Tupaia glis

The ultrastructural study of the lateral geniculate nucleus (LGN) of the tree shrew (Tupaia glis) revealed two types of neurons: (1) a large thalamocortical relay cell (TCR), which may bear cilia, and (2) a small Golgi type-II interneuron (IN) with an invaginated nucleus. The narrow rim of pale cytoplasm of the IN contains fewer lysosomes and fewer Nissl bodies than the cytoplasm of the TCR. The IN perikarya, which in some cases establish somatosomatic contacts, frequently contain flattened or pleomorphic synaptic vesicles. The ratio of TCR to IN is 3:1. Three types of axon terminals were observed in the LGN. Two of them contain round synaptic vesicles but differ in size. The large RL boutons undergo dark degeneration after enucleation; they are the terminals of retino-geniculate fibers. The smaller RS boutons show dark degeneration after ablation of the visual cortex; they are the terminals of the cortico-geniculate fibers. The third type of bouton (F1) does not degenerate after either intervention. The boutons of this type are filled with flattened vesicles and are believed to be intrageniculate terminals. F2-profiles were interpreted as presynaptic dendrites of the IN. The characteristic synaptic glomeruli found in the LGN contain in their center an optic terminal. These optic terminals establish synaptic contacts with dendrites or spine-like dendritic protrusions of TCRs as well as with presynaptic dendrites. Synaptic triads were also seen. The distribution of the individual types of synaptic contacts in layers 3 and 4 were determined. Layer 4 contains only one third of the retino-geniculate synapses and of the synaptic contacts of F1-terminals.