Freeze-fracture morphology of nuclear pockets.

Nuclear pockets (NP) are found in numerous human tumours and in certain non-neoplastic cells. This study concerns the structure of NP in cells from two malignant rhabdoid tumours, one embryonal rhabdomyosarcoma, two centroblastic/centrocytic lymphomas, one centrocytic lymphoma and one Ki-1 lymphoma, as well as in normal neutrophils. Structures were noted in freeze-fracture replicas that were interpreted as corresponding to the NP seen in ultrathin sections and were classified into four types. A lack of nuclear pores was common to all types. In addition, intramembranous particles were either absent or very scanty on both the E- and the P-faces of areas of the nuclear membrane involved in pocket formation. It can be concluded from the lack of nuclear pores that no interchange between the nucleus and cytoplasm takes place in these areas. The reason for the lack of intramembranous particles is not known. It is suggested that the nuclear lamina (intermediate filaments of the nuclear skeleton) is not in contact with the nuclear membrane here.