If these interpretations of our results be true, then the question arises not only where and how the viral substance migrates, but also where and how the mature virus is formed and released. Information is available from our studies of two of the tumor viruses (8,16) and one virus in tissue culture (20). In the following discussion I will illustrate only the formation of the Gross leukemia agent in the mouse thymus.

Intranuclear forms of the virus, similar to the nucleoidal form of polyoma (2-5), or the membrane-bound form of herpes (14, 15), have not been observed in the leukemic thymus. In general, the nuclei show only the usual distribution of nuclear granules and chromatin. However, in two instances, particles, or granules, distinctly different from the rest of the nucleoplasm have been seen in nuclear areas of little density. These particles are of considerable density and are usually held together by a homogeneous material in large aggregates or clumps. Further, the individual particles near the edge of a clump appear to be more closely associated with nucleolar material than with other nuclear elements (figs. 5, 6, and 7).

Definitive cytoplasmic forms of the leukemia virus have not been observed. As noted, inclusion bodies (fig. 9) containing ordered arrays of fine filaments or dense spots have been seen. Further, threads of material similar to the modified mitochondrial cristae are seen scattered throughout the cytoplasm (fig. 9), but more often close to the cell border. In all preparations of the leukemic thymus, ordered aggregates of fine filaments, which possess an axial period and side brush-like filaments, are observed at the cell border, usually in association with developing particles (figs. 11 through 13).

The definitive leukemia virus forms only at the cell border. Its formation appears to involve both the peripheral filamentous material in the cytoplasm and the modified plasma membrane (figs. 13 through 16). The components of the plasma membrane form twisted filaments also at the point of virus formation. These filaments have a period and structure similar to the peripheral cytoplasmic filaments (figs. 13, 15, and 16). Thus the initial step in the virus formation is a combination of filaments of the cytoplasm and the cell border, which appear as a dense thickening and protrusion of the cell surface. Increase in size of the virus is brought about by increased aggregation of the filaments into larger, spiralling threads. These apparently first form the outer limits of the virus, and then, by close packing, form an ordered array within the central portion of the particle (fig. 17). Among the threads occurs a matrix material, similar to that in the cytoplasm, surrounding the peripheral filaments. While still attached to the cell surface the virus shows reorganization of material such that a distinct nucleoidal area is apparent, surrounded by incomplete threads giving the appearance of an outer coat three to four membranes thick (figs. 13 through 20). As the virus particle leaves the cell border, there is an apparent condensation and fusion of the filamentous material to give the final form of nucleoid, the various components being separated by a matrix material of little density (fig. 21).

Sections have been cut in planes parallel with and within the cell surface, as well as secantial and normal to the surface. Thus it has been possible to follow the sequence of events in the particle formation from different views. By this method it has also been determined that the cell surface is highly irregular (fig. 14). From the determination of the cell contour and the method of virus formation it is concluded that the definitive virus forms only at the cell border, and that previously described virus-containing vacuoles in the cytoplasm are probably depressions in the virus-forming cell surface.

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All figures are of mice tumor cells. Those infected with polyoma virus are from a parotid tumor and those with leukemia are from a thymus lymphoma.

Figure 1. Nucleus (nu) of polyoma tumor cell showing relation of nucleolus (ncl) to fine filaments (f), with which virus particles (v) seem to be associated. Adjoining cytoplasm (cy) and nuclear membrane (nm) are seen in upper part. x 60,000.

Nucleus (nu) of polyoma tumor cell showing relation of nucleolus

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Figure 2. Nucleus (nu) with patch of viruses (v) in polyoma tumor cell. Nuclear membrane (nm), part of the plasma membrane (pin), and cytoplasm (cy) containing mitochondria (mi) and "inclusion bodies" (ib), as described in text, are seen. X 60,000.

Figure 3. Portion of cell of polyoma tumor showing viruses (v) in nucleus (nu) and nucleolar filaments (upper left). Cytoplasm (cy) is limited by plasma membrane (pm) and contains mitochondrion (mi). X 60,000.

Figure 4. Highly magnified intranuclear particles as seen in figures 2 and 3. Arrows point to fine twisted filaments of the internal virus structure. These seem to represent sections of the total ball-of-yarn-structure of virus nucleoid. X 300,000.

Nucleus (nu) with patch of virusesPortion of cell of polyoma tumor showing virusesHighly magnified intranuclear particles as seen in figures 2 and 3

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Figure 5. Portion of leukemia tumor cell showing aggregates of dense intranuclear particles (dp). Individual particles seem to be associated with nucleolar material (ncl). Nuclear membrane (nm) between nucleus (nu) and cytoplasm (cy), and portion of large "inclusion body" (ib) are indicated. X 120,000.

Figure 6. Cell similar to that of figure 5. Intranuclear particles (dp) appear to be related to filamentous material (f). Nuclear membrane (nm) located at right. X 120,000.

Figure 7. Portion of leukemia tumor cell. Nucleoplasm close to nuclear membrane (nm) discloses aggregates of filamentous material (f) and rows of tiny dense particles connected by fine filaments (arrow). X 120,000.

Portion of leukemia tumor cellCell similar to that of figure 5Nucleoplasm close to nuclear membrane

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Figure 8. Cytoplasmic area of polyoma tumor cell containing normal mitochondria (mi) and transforming mitochondria with densely outlined membranes possessing dense (mij) or faint (mi») matrix but no cristae. Inclusion bodies (ib, ibs) are limited by a double membrane and show matrices of varying density and structure, within which dense particles may be found (ib2). Aggregates of viruses (v) also seen, both in remainder of "inclusion body" (upper right) and freed from disrupted inclusion body (ib,) in which double membrane is observed (lower left). RNP granules appear throughout cytoplasm. X 60,000.

Cytoplasmic area of polyoma tumor cell

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Figure 9. Portion of cytoplasm of leukemia tumor cell showing various "inclusion bodies" (ib), some of which are limited by distinct double membranes. In their matrices, arrays of fine filaments and rows or aggregates of tiny dense particles (p). and occasionally large and very dense particles (pj), are seen. At upper part is a presumably transforming mitochondrion (tmi), within which are observed arrays of filaments and dense tiny particles, and uniform, regularly arranged vesicles. The latter are presumed to represent fragmentation stages of mitochondrial cristae. In center another body shows filaments (fi) possessing a very fine axial period. Fragments of such types of mitochondria (arrows), lipide clumps (1), and RNP granules are found throughout cytoplasm. X 120,000.

Portion of cytoplasm of leukemia tumor cell showing various inclusion bodies

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Figure 10. Detail of inclusion body (ib), in cytoplasm of polyoma tumor cell, showing crystalline array of viruses (v). At right, mitochondria (mi) and a portion of nucleus (nu) are seen. X 60,000.

Figure 11. Portion of leukemia tumor cell showing, at border, plasma membrane (pm) and peripheral cytoplasm composed of fine filamentous material (f). Note strand of apparently same material (f1, f2) through cytoplasmic matrix, down to nuclear membrane region (nm). Mitochondrion (mi), numerous free or aggregated RNP granules, and rough profiles of the endoplasmic reticulum (er) are also seen. X 120,000.

Detail of inclusion bodyPortion of leukemia tumor cell showing