Nuclear envelope inclusions demonstrated by freeze-fracture.

Inclusions in the perinuclear space of the nuclear envelope of human diploid (MRC-5) fibroblasts, limpet (Patella vulgata) haemocytes, and yeast (Saccharomyces cerevisiae) as observed with the freeze-fracture technique are described. The significance of these inclusions is discussed and it is tentatively concluded that they represent vesicles engaged in transporting macromolecules between nucleus and cytoplasm. Although the inclusions were infrequently observed, their demonstration in mammalian, invertebrate and lower eukaryotic cell types raises the possibility that this form of nucleocytoplasmic exchange may potentially be adopted under appropriate circumstances by the eukaryotic cell in general.