An ultrastructural and cytochemical study of the nucleolus-DNA body complex of Amphiporus lactifloreus oocytes.

The components of the nucleolus-DNA body complex of oocytes from the hoplonemertean, Amphiporus lactifloreus, were studied by electron microscopy and cytochemistry. In the early stages of meiotic prophase I, the nucleus of female germ cells was characterized by a large heterochromatic body. From this body a polymorphic nucleolar apparatus, the nucleolus-DNA body complex, subsequently arose. This nucleolar complex was made up of spherical components termed nucleolar spherulae, which contained fibrillar electron-opaque material with cortical 20 nm dense granules, and characteristic ribbon-shaped components termed nucleolar lamellae also with dense granules. The latter components originated from the former by a process of self-peeling of the spherula cortex. A high level of transcriptional activity was located in the nucleolar spherulae which also contributed with the nucleolar lamellae to the formation of many secondary nucleoli and granular bodies. By ultrastructural cytochemistry a number of DNA-containing structures were visualized in nucleolar areas: first, cytochemically well-characterized DNA of partly decondensed chromatin was demonstrated in the thick cortex of spherulae and the inner part of lamellae arising from spherula cortex; second, more dispersed chromatin fibers, related with the cortical component, were revealed in the core of nucleolar spherulae, where ribonucleoproteins were also seen. A thin layer of ribonucleoproteins was also detected in the edge of both spherulae and lamellae of the nucleolar complex. These findings, which show that the nucleolar complex of previtellogenic oocytes of hoplonemerteans is basically dual, are discussed in terms of two synchronous functions: a) the start of the transcription in the nucleolus, b) dispersal of the intranuclear rDNA as a prerequisite to subsequent nucleolar transcription activity.