[Electron microscopic study of the endopolyploid nuclei of the giant cells of the rat trophoblast. I. The ultrastructure of polytene chromosomes and their activity products at different stages of the endoreplicative cycle].

The fine structure of the rat trophoblast giant cell nuclei (on the 12--14th days of the foetus development) has been studied at the two stages of endomitotic cycle--endointerphase and endoprophase. In the endointerphasic nuclei, no large accumulations of condensed chromatin occur, the peripheral chromatin is poorly seen, thin chromatin threads are evenly distributed over the karyoplasm, with their rather clear orientation from the nucleolus towards the nuclear envelope. Among decondensed chromonemes and in association with them, a lot of interchromatinous granules are seen, perichromatinous granules being of a rare occurrence; sometimes small nucleolus-like structures are met. In the endoprophasic nuclei, chromatinous elements are seen as large blocks of condensed chromatin, having various shapes, often elongated, with uneven outlines and zones of decondenced chromatin between them. The peripheral and preinucleolar chromatin is clearly identified. Chromatin blocks near the nucleolus are similar in their size and arrangement to discs of dipterian polytene chromosomes, the interdisc spaces being like the puff zones. The latter are made of loosely arranged fibres 10--20 nm thick and granules 20--30 nm in diametre; part of the puff zones is of fibrillar structure. In the nuclei with condensed chromatin, nucleolus-like fibrillo-granular bodies (1--1.5 mkm) are recognized, in addition to large masses of condensed chromatin with numerous perichromatinuous granules inside and large masses having a comb-honey structure. The revealed morphological differences in structures, formed on endointerphase chromosomes and polytene chromosomes of the endoprophase, may point to the various nature of the products of gene activity produced at different periods of the nuclear cycle.