Morphology and distribution of retinal ganglion cells projecting to different layers of the dorsal lateral geniculate nucleus in normal and Siamese cats.

Electrophoretic injections of horseradish peroxidase were made into physiologically characterized sites within the different layers of the dorsal lateral geniculate nuclei (LGNd) of normal and Siamese cats. The histochemical procedures used stained the cell bodies, dendrites, and axons of retrogradely labeled ganglion cells. In both normal and Siamese cats, only alpha and beta ganglion cells are labeled by injections restricted to the A laminae. In normal cats, the alpha/beta ratios (number of labeled alpha cells/number of labeled alpha + beta cells) resulting from injections into lamina A increase from about 0.045 at 0.5 mm from the area centralis to about 0.12 in the far periphery. The alpha/beta ratios observed outside of the area centralis in normal cats following injections into different parts of lamina A1 were lower at each eccentricity than those resulting from injections into corresponding parts of lamina A. Also, the cell bodies and dendritic fields of alpha and beta cells projecting to lamina A1 are somewhat larger than those projecting to corresponding parts of lamina A. Outside of the area centralis, the relative numbers of alpha and beta cells projecting to Siamese lamina A are normal. However, alpha cells comprise an abnormally small proportion of ganglion cells projecting to the normal segments of Siamese lamina A1 and an abnormally large proportion of cells projecting to the abnormal segments of lamina A1. In Siamese cats, alpha and beta cells projecting to lamina A1 are distributed continuously throughout virtually all of the ipsilateral and contralateral temporal retinas. Since large parts of the ipsilateral and contralateral hemifields are not represented in Siamese lamina A1, it seems that some of the retinal afferents to this lamina are being suppressed. Injections into the C laminae of the LGNd show that the same morphological classes of ganglion cells project to these laminae in normal and Siamese cats. The classes projecting to the contralateral as C laminae (laminae C and C2) include alpha, beta, gamma, and epsilon as well as two other groups of cells referred to g1 and g2 cells, gamma, epsilon, g1, and g2 cells project to the ipsilateral C lamina (lamina C1). In siamese, but not in normal cats, examples of all of these types are found far into the contralateral temporal retina following injections involving lamina C1. This indicates that all classes projecting to the ipsilateral C lamina misproject in Siamese cats.