Differential frequency of P-cells and I-cells in magnocellular and parvocellular laminae of monkey lateral geniculate nucleus. An ultrastructural study.

Acute retrograde ultrastructural changes resulting from complete removal of areas 17, 18 and 19 were used to develop criteria for identification of principal or projective neurons (P-cells) and local interneurons (I-cells) in the dorsal lateral geniculate nucleus (LGNd) of monkeys. Four and six days after axotomy, marked chromatolytic alterations and diminution of rough endoplasmic reticulum were noted in 89 examined neurons of medium or large size, with rich cytoplasmic matrix, large mitochondria, and exhibiting only postsynaptic sites on the perikarya and dendrites. Seventeen other neurons showing no signs of degeneration had a pale matrix, small dense mitochondria and both postsynaptic and presynaptic sites on their perikarya and dendrites. It was concluded that the former group represented P-cells and the latter was characteristic of I-cells. The morphologic criteria derived from this study were applied to the quantitative analysis of neuronal populations in separate magnocellular and parvocellular laminae. The findings indicate that the latter contain 4.4% of I-cells whereas the former have 15.6% of this neuronal category. The dissimilarity suggests the more preeminent role of I-cells in the magnocellular laminae where they could maintain the transient responses of Y-cells which are known to be the predominant population among the P-cells of these laminae.