Autoradiographic tracing procedures have been used to study the organization of retinogeniculate axons in seven primates, i.e., four species of New World monkeys, one species of Old World monkeys and two species of prosimians. These data suggest that the basic primated pattern of geniculate lamination consists of two parvocellular layers, two magnocellular layers, and two poorly developed and highly variable superficial (S) layers which are ventrally lacoted. Ocular input to each member of each of the three pairs differs. In the macaque, the squirrel, and the saki monkey, the parvocellular layers subdivide and interdigitate into four leaflets so as to give the appearance of four parvocellular “layers”. These leaflets are much less extensive in the owl and marmoset monkeys. In some individual macaque monkeys, there is further splitting of the parvocellular leaflets into subleaflets, giving the appearance of six parvocellular “layers”. The prosimians (galago and slow loris) have two additional layers that are not found in pithecoid primates, and only one superficial layer is apparent. The two additional layers are termed “koniocellular” since they consist of very small cells. Finally, New and Old World monkeys have both ipsilateral and contralateral retinal input to the interlaminar zones.



We conclude that the basic pattern of lateral geniculate organization is six layers, but not the traditional six. Prosimians have evolved two additional layers, the koniocellular layers, and have possibly lost one superficial layer. Both New World and Old World monkeys have elaborated the parvocellular layers by forming leaflets to varying extents. With the possible exception of the single S layer in prosimians, layers form pairs that are similar in cell types, but different in ocular input.

Patterns of retinal terminations and laminar organization of the lateral geniculate nucleus of primates.

The laminar structure of the dorsal lateral geniculate nucleus was studied in carnivores of the weasel, raccoon and fox families. All have the same basic pattern of laminar organization in their lateral geniculate nuclei. Laminae A and C receive a contralateral projection from the retina and laminae A1 and C1 receive an ipsilateral projection. It is not known whether lamina C2 receives a retinal projection.

There are some variations to the basic pattern of laminar organization. Thus, in the mink and weasel laminae A and A1 are each subdivided into a pair of separate but similar leaflets. In the ferret and coatimundi these laminae are only partly subdivided. In the skunk, raccoon, ringtailed cat and fox there is only one lamina A and one lamina A1 in each lateral geniculate nucleus. There is some variation of the C laminae also. The C laminae are best displayed in the fox, where all three laminae — C, C1 and C2 — can be recognized in Nissl sections. In other carnivores, for example the mink and ferret, the C laminae cannot be distinguished from each other in normal brains and are described on the basis of the degeneration that is seen in Nauta and Fink-Heimer sections after a unilateral enucleation. In the raccoon lamina C can be identified but lamina C1 may be missing.

The perigeniculate nucleus is variable in structure in carnivores. In the weasel and raccoon family the perigeniculate nucleus is a thick layer containing many large cells while in the cat and fox, it is a thin and rather diffuse group of cells. The perigeniculate nucleus may receive a direct retinal projection but if so, it is only a very sparse one.

Lamination of the dorsal lateral geniculate nucleus in carnivores of the weasel (Mustelidae), Raccoon (Procyonidae) and Fox (Canidae) families

Chapter I - The thalamus of primates

The effects of monocular lid-suture deprivation on development were evaluated by measurement of cell sizes in the lateral geniculate nuclei of six deprived and three normally reared galagos. In all animals autoradiographic demonstration of the retino-thalamic projections from one eye was used to define the lamination and distinguish the monocular from the binocular segment of the nucleus. Our results indicate that monocular deprivation significantly affects cell growth in both the binocular and monocular geniculate segments, with the greater change occurring in the binocular segment, suggesting that both visual experience and binocular competitive interactions influence geniculate cell growth in these primates. In animals forced to use their deprived eye for 2 months or more by reverse suturing, disparity of cell sizes is reduced in the monocular segment, while differences in binocular segment cell sizes are maintained. Our results also show that monocular deprivation with or without later reverse suture has an unequal influence on different geniculate layers, such that cells in laminae 4 and 5 are not as severely affected as the remaining layers. This differential influence could relate either to the unique pattern of projection of these layers to cortex or to functional differences between layers.

A. Krishnamurti

Papers

Observations on the lamination of the lateral geniculate body in some primates