Effect of prosencephalectomy on the connections of the inferior olivary nucleus in fetal rhesus monkeys

Abstract

The structural organization and fiber distribution of the inferior olivary complex were examined in six fetal rhesus monkeys which had been killed between 143 and 147 days of gestation. About 70 days before killing them, four fetuses had undergone removal of the prosencephalon which also resulted in damage of the rostral half of the mesencephalon. The other two fetuses served as normal controls. The brain stems were serially sectioned transversely and parasagittally and were stained with a silver reduction method. Both the position and configuration of the main divisions of the inferior olivary complex were similar in the normal and experimental fetuses, an indication that such structures as the corticospinal tract probably affect only slightly the structural organization of the inferior olivary nucleus. The main effects of prosencephalectomy on the inferior olivary complex were the loss of the cortico-olivary fibers and central tegmental fasciculus which made it possible to visualize better the distribution of the spinal afferents. The whole principal nucleus and the rostral half of the medial accessory nucleus were almost completely devoid of any afferents. Spinal fibers terminated in the caudal two-thirds of the dorsal accessory nucleus and on the medial portion of the caudal third of the medial accessory nucleus, including the dorsal cap. However, the density of afferents within this part of the nucleus was not as great as in normal controls, probably because other fibers also project to this region. In addition, the reticulo-olivary fibers were normally distributed in the rostral third of the dorsal accessory nucleus. Efferent fibers originated from all parts of the olivary complex in the experimental animals, more so from the dorsal accessory and from the caudomedial portion of the medial accessory nuclei; in normal fetuses. most efferents originated from the principal nucleus.