The number of pore complexes per nucleus was determined for a wide variety of cultured cells selected for their variable DNA content over a range of 1-5,600. The pore number was compared to DNA content, nuclear surface area, and nuclear volume. Values for pore frequency (pores/square micrometer) were relatively constant in the species studied. When the pore to DNA ratio was plotted against the DNA content, there was a remarkable correlation which decreased exponentially for the cells of vertebrate origin. Exceptions were the heteroploid mammalian ceils which had the same ratio as the diploid mammalian cells despite higher DNA content.The results are interpreted to mean that neither the nuclear surface, the nuclear volume, nor the DNA content alone determines the pore number of the nucleus, but rather an as yet undetermined combination of different factors. The surface and the volume of vertebrate nuclei do not decrease with decreasing DNA content below a given value. The following speculation is suggested to account for the anomalous size changes of the nucleus relative to DNA content in vertebrates. Species with small DNA complements have a relatively large proportion of active chromatin which determines the limits of the physical parameters of the nucleus. The amount of active chromatin may be the same for at least the vertebrates with low DNA content. At high DNA content, the nuclear parameters may be determined by the relatively high proportion of inactive condensed chromatin which increases the nuclear surface and volume.The two membranes of the nuclear envelope are interrupted by nuclear pore complexes, the structure and possible function of which have been the subject of several reviews (9-13, 19, 20, 32, 35). These pore complexes could control the exchange of macromolecules between the two major compartments of the cell either by a change in total number of pores or by a selective filter mechanism. Feldherr (8) determined that gold particle size rather than pore frequency seemed to influence the exchange rate. Paine et al. (29) found that the pore complex has a patent radius of 45 and, therefore, allows free exchange of small molecules. No selective filter function other than pore size has been demonstrated up to now. An increase in the nuclear pore number, however, could be brought about experimentally in lymphocytes that had been stimulated with phytohemag- 748THE JOURNAL OF CELL BIOLOGY-VOLUME 73, 1977" pages 748-760 on August 29, 2018 jcb.rupress.org Downloaded from http://doi.org/10. 1083/jcb.73.3.748 Published Online: 1 June, 1977 glutinin (PHA) (23,24), indicating a change in nuclear pore number during a change in metabolic activity. Also, the total pore number per nucleus doubled from a plateau in the G1 phase to the end of the cell cycle in HeLa cells, and the porevolume ratio stayed equal during this time (24). The question which arose from these observations is whether there is any relationship between the amount of DNA and the nuclear pore number and whether the volume or the sur...

Permissive and nonpermissive simian virus 40 (SV40)-infected cells were ultrastructurally analyzed. Viral particles were found in the cytoplasm, rough endoplasmic reticulum, nuclear envelope, lysosomes, and mitochondria. Upon entering the cell the virion obtains a tight membrane envelope. It seems to be either released from the envelope upon fusion with other membranes of the cell or aggregated into tubular membrane specializations upon fusion with other membrane-enveloped particles. Reconstructed morphological sequences and the finding of SV40 in different spaces of the cell suggest that entry of SV40 into the different compartments and eventually into the site of replication is facilitated by its capacity for being enveloped by a variety of membranes (notably the cell membrane and the nuclear membrane) and the sequential fusion and fission of these membranes.

Electron microscopy examination of Rous sarcoma virus-transformed chicken embryo cells revealed membrane-free nucleoids resembling type A oncornavirus. These particles were not detected in noninfected cells, nor did they accumulate in excess under conditions of glucosamine block in virus-transformed cultures.