Nuclear pore clusters and perinuclear material in midgut epithelial cells of Dytiscus marginalis (Insecta, Coleoptera).

In ultrathin sections and freeze-fractures in hexagonal nuclear pore arrangement is described in midgut epithelial cells of Dytiscus marginalis. The majority of the pores is concentrated in several cup-like indentations of the nuclear envelope and the pore center-to-center distance is about 130 to 140 nm. In addition, in the regions of the nuclear pore clusters, patches of cytoplasm containing paracrystalline structures of closely packed hexagons of 18 to 20 nm width are found sandwiched between the nuclear envelope and a "secondary" envelope. In such areas, structures are commonly observed that suggest transfer of material from the nucleus to the cytoplasm, sometimes in a dumb-bell-shaped state and more often as slender filaments which migrate across the nuclear pores.