Fine structure of the nuclear apparatus of the marine psammobiotic ciliate Geleia orbisFauré-Fremiet (Karyorelictida)

Abstract

Summary The nuclear apparatus of G. orbis usually consists of two macronuclei and one micronucleus. The macronuclei have vesicular structure; each contains more than 100 small compact nucleoli displaying a fibrillar core and a granular cortex. The nucleoli are often clustered into aggregates of 10–12 units and arranged there along a spiral line. The macronuclear chromatin is almost fully decondensed and appears as 20 nm thick fibrils in the nuclear matrix; only a small amount of chromatin seems to be condensed in form of rare small bodies. The macronuclear matrix contains strands and septae of microfilamentous material which often encircle a number of closed compartments inside the nucleus. Some of these are filled with denser fibrillar material and contain RNP granules resembling interchromatinic ones ; they seem to be areas of transcribed chromatin. The macronuclei of some specimens display also single central spheres formed by densely packed sinuous 10–12 nm thick fibrils and containing both protein and RNA. The spheres are in close contact with many nucleoli. The macronuclear envelope has numerous pore complexes. Its outer side is coated with many tubular channels of the smooth endoplasmic reticulum, each channel beginning near a pore complex. The channels form a dense canalicular zone between the macronuclei and around the micronucleus. The micronucleus contains a single spongy mass of condensed chromatin which is coarsely particulate, with particle size about 30 nm. Crystalloids and associated dense bodies of unknown nature occur in the peripheral chromatin-free zone of the micronucleus. The micronuclear envelope displays frequent pore complexes.