Interchromatin granules

Thiry, M. (1995) Behavior of interchromatin granules during the cell cycle. Fur. J. Cell. Biol. 68(1), 14-24. 

Current textbook descriptions of nuclear structure typically describe interphase nuclear organization as divided into heterochromatin and euchromatin compartments, where the heterochromatin corresponds to densely, heavy metal-stained masses, frequently adjacent to the nuclear envelope or nucleolus, and the euchromatin corresponds to the lighter stained, more finely textured regions lying between the heterochromatin and other identifiable, subnuclear compartments, such as the nucleolus or interchromatin granule clusters. 

Fig. 11. A. Typical aspect of a cluster of interchromatin granules in normal rat liver nucleus. Glutaraldehyde>Epon, EDTA method chr, bleached chromatin ->, perichromatin granules fibrils ig interchromatin granules. X42,300. B. Cluster of interchromatin granules in the nucleus of an adrenal cortex cell from normal rat. Glutaraldehyde-Epon, EDTA method. The granules are irregularly shaped and have a striking tendency to form chainlike aggregates. small rods may be found,...

The question of the D-RNA-containing particles in mammalian cells has been extensively studied by Swift (1959) and by Monneron and Bernhard (1969). The latter elaborated a new, very effective method for selective staining of ribonucleoprotein structures (uranyl staining and EDTA differentiation). A number of ribonucleoproteins have been observed granules and fibrils in nucleoli, perichromatin granules, interchromatin granules, and perichromatin fibrils (Fig. 11). 

Thus interchromatin granules very probably correspond to the complexes of D-RNA with informofers. 8ometimes the connection of interchromatin granules with chromatin masses through the so-called perichromatin fibrils may be found. Perichromatin fibrils are 30 A in diameter and have the same sensitivity to enzyme action as interchromatin granules. 

Some support for this idea may be found in electron microscopic observations. Usually the structures described as D-RNP are observed directly in the area of active RNA synthesis in the chromosomes. Thus the Beermann particles at first appear in Balbiani rings engaged in the production of D-RNA. The perichromatin and interchromatin granules are also sometimes observed in contact with chromosomal masses, with which they are connected by 30A strands. Finally, the helical structures described in Amoeba, which are presumably also D-RNP, grow from a central axis that probably corresponds to a DNP strand. Of course, these observations are indirect, but they are quite compatible with the view that the ribonucleoprotein complex is formed immediately after the synthesis of D-RNA, even before completion of the RNA chain. 

This process has not yet been studied at all, and the mechanism of the movement of macromolecules from chromosomes to the cytoplasm is quite unclear. The only useful information is based on electron microscopic observations. One can observe particles presumably containing D-RNA, such as Beerman granules or perichromatin granules, interchromatin granules, and helices in different parts of the nucleus. They may be bound to chromatin threads or lie freely in the nuclear sap or even be in contact with the nuclear membrane. These pictures may reflect the movement of the particles from chromatin to the nuclear membrane (Beerman and Bahr, 1954 Stevens and Swift, 1966, Monneron and Bernhard, 1969). As all these types of particles are of ribonucleoprotein nature and have many properties similar to those of isolated nuclear D-RNP, it is possible to suggest that D-RNA moves to the nuclear membrane in a complex with informofers. 

Electron microscopic observations. As was pointed out above, electron microscopy allows one to follow the particles presumably containing D-RNP to the vicinity of the nuclear membrane. After the particles reach the membrane pores they are elongated and cross the membrane through the pores in the form of strands or rods 30 A to 100 to 200 A in width. This is true for perichromatin particles, interchromatin granules, and Beermann particles (Beermann and Bahr, 1954 Stevens and Swift, 1966 Monneron and Bernhard, 1969). Sometimes these rods may be followed on the outer side of the nuclear membrane or even into the cytoplasm, but they are very quickly lost from view in the cytoplasm. Reconstitution of the original structures does not take place. 

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