Evolution of the nucleolar apparatus during oogenesis in Acipenseridae

ABSTRACTEvolution of the nucleoli has been followed during oogenesis in the Acipenserid fishes, Acipenser ruthenus (the sterlet) and A. güldenstädti (the sturgeon) using light and electron microscopes. In the ovaries of adults, the oogonial nuclei usually have a single nucleolus with an adjacent mass of paranucleolar fibrillar material. The cytoplasm of the oogonia contains two dense bodies peculiar only to gonocytes, one being electron dense and containing RNA and the other being electron-lucent and lacking RNA. Neither is surrounded by membrane. The fine structure of the electron-lucent body is identical to that of the paranucleolar material, while the RNA-containing body structurally resembles the nucleolus. A nuclear origin for both cytoplasmic bodies is likely.Leptotene-stage oocytes usually still have a single nucleolus. During zygotene, it is adjacent to the nuclear envelope and opposite to the chromosomes contracted in synizesis. At pachytene, a ‘cap’ of extrachromosomal chromatin is formed under the nuclear envelope and around the nucleolus. Bivalents also contact this cap. In early diplotene, the primary nucleolus still persists. The material of the cap is dispersed beneath the entire nuclear envelope in the form of granules of extra DNA; each granule then produces a peripheral (secondary) nucleolus. These become typical amphinucleoli with differentially developed granular parts, depending on the age of the nucleolus and the stage of meiosis. Their fibrillar parts always face the nuclear envelope. New peripheral nucleoli continue to form as long as granules of extra DNA persist under the nuclear envelope, i.e. approximately until vitellogenesis.In early vitellogenesis, the peripheral nucleoli become transformed, by re-distribution of their fine structural components, into circular threads trailing towards the centre of the nucleus. The axis of the thread consists of fibres and is coated with granules. In late vitellogenesis, the nucleoli round up and become vacuolized; they are then peripheral again.Proteinaceous RNA-lacking structures which are also produced in the nuclei during oogenesis in the Acipenseridae, are the ‘nuclear bodies’ and. ‘spheres’. The former are adjacent to peripheral nucleoli, the latter form on lampbrush chromosomes. Both are ultra-structurally alike. The loops of lampbrush chromosomes produce also RNP bodies (‘granules’ of Callan & Lloyd, 1960) which are ultrastructurally similar to nucleoli but lack segregation of granules from fibres into spatially distinct parts.The evolution of the nucleolar apparatus during oogenesis in the Acipenseridae closely resembles that in amphibians.