Intranuclear inclusions in the developing neurons of the rat cuneate nuclei.

The ultrastructure of intranuclear rodlets, microtubules, fibrillar lattices and membranous inclusions found in the developing cuneate nuclei of rats is described. Rodlets, ranging in diameter from 96--312 nm and in length from 1--2 micrometer, are made up of tightly packed straight filaments measuring 5--8 nm in diameter. Microtubules with a diameter of 26 nm are clustered together. Fibrillar lattices are made up of fibrils with a diameter of 9 nm arranged in layers or sets. Two to nine sets make up a lattice, with a maximum width of 68 nm, in which the adjacent sets are arranged at an angle to each other. Rodlets and fibrillar lattices occur in 6.8% of the neurons. Membranous inclusions, reported here for the first time in normal neurons, are of 2 types: small vesicles of 0.1--0.6 micrometer and large vacuoles measuring 1--2 micrometer. Both types are bounded by either a single or a double membrane and generally have an electron lucent content. Membranous inclusions occur in 25.3% of the neurons. Changes in the frequency of occurrence of the various intranuclear inclusions in the course of postnatal development are also reported.