ULTRASTRUCTURAL STUDY ON NUCLEAR-CYTOPLASMIC RELATIONSHIPS IN OOCYTES OF AFRICAN LUNGFISH PROTOPETERUS AETHIOPICUS .I. NUCLEOLO-CYTOPLASMIC PATHWAYS

Growing oocytes of Protopterus, like those of some amphibians and teleosts, show an impressive development of the nucleolar apparatus. Numerous nucleolus-like bodies establish close spatial relationships with the nuclear envelope by extending pedicels and streams of finely dispersed material towards the inner membrane. At such contact points, gaps in the perinuclear cistern are more frequent than elsewhere along the nuclear boundary. Expansion of the outer nuclear membrane gives rise to blebs, with or without visible content, and these become pinched off to form small vesicles in the perinuclear cytoplasm. Small, electron dense aggregates, indistinguishable from nucleolar material occur on both sides of the nuclear envelope opposite to each other, some being connected by a slender portion of the same material within a nuclear pore. Such accumulations are interpreted as detached parts of nucleolar bodies in transit to cytoplasmic sites where they presumably participate in the biogenesis of ribosomes. At the height of nucleolar emission, nucleoplasm and perinuclear cytoplasm are so rich in small electron dense particles that they are almost indistinguishable from each other. At this stage of massive transport, the route provided by the nuclear pores seems to be insufficient and another, more spacious, gateway may be in operation. The latter involves direct passage of material across the nuclear membranes preferentially where these form blebs. This view is supported not only by the overt spatial relationships between nucleolar pedicels and blebs, but by the occurrence within perinuclear lacunae and blebs of particles that seem to be derived from nucleolar bodies. Furthermore, frequent interruptions in the nuclear membranes preferentially located where they expand into outpocketings suggest that at these sites temporary gateways may exist in the living cell that permit easy access of intranuclear components to the cytoplasm. © 1969 Springer-Verlag.