Abstract
Pore-linked filaments were visualized in spreads of anuran spermatocyte nuclei using transmission electron microscope. We used Odontophrynus diplo and tetraploid species having the tetraploid frogs reduced metabolic activities. The filaments with 20-40 nm width are connected to a ring component of the nuclear pore complex with 90-120 nm and extend up to 1 microm (or more) into the nucleus. The filaments are curved and connect single or neighboring pores. The intranuclear filaments are associated with chromatin fibers and related to RNP particles of 20-25 nm and spheroidal structures of 0.5 microm, with variations. The aggregates of several neighboring pores with the filaments are more commonly observed in 4n nuclei. We concluded that the intranuclear filaments may correspond to the fibrillar network described in Xenopus oocyte nucleus being probably related to RNA transport. The molecular basis of this RNA remains elusive. Nevertheless, the morphological aspects of the spheroidal structures indicate they could correspond to nucleolar chromatin or to nucleolus-derived structures. We also speculate whether the complex aggregates of neighboring pores with intranuclear filaments may correspond to pore clustering previously described in these tetraploid animals using freeze-etching experiments.
Highlights
The importance of the study of nuclear pore complexes (NPCs) and of intranuclear filaments in nucleocytoplasm transport has been predicted to have many potential applications for gene expression control and antiviral therapies
NPCs are structures embedded in the nuclear envelope (NE) that mediate the transport of macromolecules and particles between nucleus and cytoplasm during interphase
First transmission electron microscope (TEM) studies in amphibian oocyte nuclei reported filaments attached to the nucleoplasmic face of the NPC (Afzelius 1955) that may be baskets, as commented
Summary
The importance of the study of nuclear pore complexes (NPCs) and of intranuclear filaments in nucleocytoplasm transport has been predicted to have many potential applications for gene expression control and antiviral therapies. The cytoplasmic ring with 125 nm in diameter is decorated with eight 30 to 50 nm long kinked filaments. The nuclear ring with 90 nm is capped with a basket-like assembly built from eight thin filaments with 8-10 nm diameter and 50 to 100 nm long. These filaments join at their distal ends in a smaller terminal ring of 30 to 50 nm diameter. First transmission electron microscope (TEM) studies in amphibian oocyte nuclei reported filaments attached to the nucleoplasmic face of the NPC (Afzelius 1955) that may be baskets, as commented TEM experiments for NE spreads of Xenopus laevis oocytes showed that the baskets are insensitive to RNase treatment, its structural integrity depending on the presence of divalent cations as well as on the use of 1% tannic acid (Jarnik and Aebi 1991)
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