Formation, transport, and storage of ribonucleic acid containing structures in oocytes of Acheta domesticus (Orthoptera)

Structural organization of an active, chromosomal nucleolar organizer region (NOR) identified by light microscopy, and subsequent TEM and STEM electron microscopy

The three-dimensional arrangement of the chromatin components within the nucleolar organizer regions (NORs) from living oocyte nuclei was investigated. As a suitable cell system we chose vitellogenic oocytes of the orthopteran insect Acheta. This cell type is particularly attractive for analysis of nucleolar chromatin, since structural and functional aspects of NORs during early oogenic stages (including the association of NORs with amplified rDNA copies) are particularly well known (Lima-de-Faria 1974). In the course of the present study we first identified putative chromosomal NORs in isolated nuclei of mid-diplotene oocytes according to morphological characteristics using differential interference contrast (DIC) or phase contrast light microscopy. The presence of actively transcribing chromosomal NORs during this late stage of Acheta oogenesis obviously had been overlooked by previous investigators, probably due to difficulties of chromosome visualization. For a more detailed ultrastructural analysis, NORs were gently sedimented from opened nuclei and processed for sectioning using a modified "end-embedding" procedure (Mott and Callan 1975; Spring and Franke 1981). A small number of thick and thin sections could be made from individual NORs. Sections were analyzed by light microscopy, conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Whereas the structural connection of NORs to the chromosome axis and also the general arrangement of active nucleolar genes within the NOR complex could be seen with TEM, the visualization of individual nucleolar genes and the organization of transcription complexes was only possible using bright field STEM of thick sections at low temperature.

Nucleolus-like bodies in embryonal carcinoma cells of the embryoid bodies isolated from mouse teratocarcinoma

Undifferentiated embryonal carcinoma cells (EC cells) in the embryoid bodies isolated from mouse teratocarcinoma contained nucleolus-like bodies (NLBs) of smaller sizes in their cytoplasm (their sectional area averaged about 0.036 micrometers2). At the onset of EC cell differentiation, the average sectional area of NLBs significantly increased (about 0.107 micrometers2). When EC cells had differentiated into mesenchymal cells and endothelial cells of primitive blood vessels, NLBs decreased dramatically both in size and number. The possible role of NLBs in the differentiation process of EC cells is discussed.

Distribution of nuclear pores and perinuclear dense substances in spermatocytes I of some Oedionychina fleabeetles

The nuclear envelope of growing postpachytene spermatocyte I differs notably in structure between the fleabeetles Omophoita cyanipennis and Oedionychus bicolor. The former species shows a more conventional structure with an even and probably random distribution of nuclear pores, and a strongly electron-opaque layer of fibrogranular material (FM) separated from the outer nuclear membrane by an intermediate layer of about 40 nm thickness. Peripherally from the FM layer, a continuous corona of granular dense material (GM) is accumulated around the nucleus. In Oedionychus, all nuclear pores are clustered in "nuclear sieves", i.e., shallow, cup-like indentations of the nuclear envelope. The sieves are filled with an electron-opaque substance resembling the FM of the Omophoita spermatocytes. This substance is kept at a distance of about 40 nm from the outer nuclear membrane. GM is produced only at the sives, and is thus discontinuous. The sieves with their contents are called nuclear sieve complexes (NSC).

Frequencies of circular units of nucleolar DNA in oocytes of two insects, Acheta domesticus and dytiscus marginalis, and changes of nucleolar morphology during oogenesis

The organization of the extrachromosomal nucleolar material in oocytes of two insect species with different ovary types, the house cricket Acheta domesticus (panoistic ovary) and the water beetle Dytiscus marginalis (meroistic ovary), was studied with light and electron microscopic techniques. Stages early in oogenesis were compared with fully vitellogenic stages (mid-to-late diplotene). The arangement of the nucleolar material undergoes a marked change from a densely aggregated to a dispersed state. The latter was characterized by high transcriptional activity. In spread and positively stained preparations of isolated nucleolar material, a high frequency of small circular units of transcribed rDNA was observed and rings with small numbers (1--5) of pre-rRNA genes were predominant. The observations suggest that the 'extra DNA body' observed in early oogenic stages of both species represents a dense aggregate ofnumerous short circular units of nucleolar chromatin, with morphological subcomponents identifiable in ultrathin sections. These apparently remain in close association with the chromosomal nucleolar organizer(s). The observations further indicate that the individual small nucleolar subunit circles dissociate and are dispersed as actively transcribed rDNA units later in diplotene. The results are discussed in relation to principles of the ultrastructural organization of nucleoli in other cell types as well as in relation to possible mechanisms of gene amplification.

Ultrastructure of developing germ cells in the fetal human testis

Electron microscopic studies of the testis were performed on 12 human embryos and fetuses between 9 and 30 weeks post conceptionem. According to their ultrastructural features, the fetal germ cells could be divided into the following three stages of differentiation: 1) gonocytes, 2) intermediate cells, and 3) fetal spermatogonia. Sertoli cells were present among the germ cells in all the testes studied. The gonocytes showed the highest nucleo-cytoplasmic ratio. Their round nucleus contained a centrally located, prominent nucleolus. The cytoplasm displayed a well developed Golgi apparatus, lipid droplets and parallel arrays of short cisternae of the rough surfaced endoplasmic reticulum (rER). Microfilaments were numerous, particularly just beneath the cell membrane. The intermediate cells were found to extend several cytoplasmic processes and to contain a moderate number of long, branched and/or widened rER cisterna which were frequently connected to the perinuclear cisterna. Intermediate cells were often connected to one another by intercellular cytoplasmic bridges. The fetal spermatogonia also displayed cytoplasmic bridges. These cells showed the lowest nucleo-cytoplasmic ratio and more condensed nuclear chromatin. The mitochondria were situated close to the nucleus. Many of them were connected by a cementing substance. Lipid droplets and rER cisternae were rare in these cells. Infoldings of the inner nuclear membrane were often present in the gonocytes and in the intermediate cells, but were rarely observed in the fetal spermatogonia. Glycogen particles, polyribosomes, and chromatoid bodies ("nuage") were present in all the three germ cell types. With the maturation of the fetus, the number of gonocytes was found to decrease, whereas the number of fetal spermatogonia increased. The Sertoli cells also changed their ultrastructure, showing an increase in the number of rER cisternae, as well as of microfilaments, lipid droplets, and secondary lysosomes.

Localization of ribosomal DNA within oocytes of the house cricket, Acheta domesticus (Orthoptera: Gryllidae)

A large DNA-containing body is present in addition to the chromosomes in oocytes of the house cricket Acheta domesticus. Large masses of nucleolar material accumulate at the periphery of the DNA body during the diplotene stage of meiotic prophase I. RNA-DNA hybridization analysis demonstrates that the genes which code for 18S and 28S ribosomal RNA are amplified in the ovary. In situ hybridization indicates that the amplified genes are localized within the DNA body of early prophase cells. As the cells proceed through diplotene the DNA which hybridizes with ribosomal RNA is gradually incorporated into the developing nucleolar mass.

[Autoradiographic studies on the oosome in the oocyte ofPimpla turionellae L. (Hymenoptera)]

During the longitudinal growth of the oocytes ofPimpla the voluminous oosome area has considerable amounts of newly synthetized proteins (= 1st stage of oosome formation). Short time incubations result in labelling the oosome area as distinctly as the plasm in the nurse cells, in spite of the considerable distance between nurse cells and oosome area. Thus there is no evidence of trophocytic proteins being transported towards the oosome. Consequently during the 1st stage of oosome formation more proteins are synthetized in the oosome area than in the ooplasm.All over the oocyte, oocytic and extra-oocytic components are adding to the propagation of ooplasm (intussusceptive growth). During the 1st stage of its formation, the oosome could possibly contain an additional system for appositional growth within the posterior part of the oocyte, or/and the proteins synthetized herein could play a part in the synthesis of fats. Further questions in connection with these preliminary hypotheses can be answered by more refined methods only. During the oocytic stages following the degeneration of the nurse cell chamber (= 2nd stage of oosome formation), the ever more ball-shaped oosome accumulates trophocytic RNA, which is present all over the rest of the egg space as well. This stable RNA is meant to serve either for the somatic embryogenesis or the germ line.Further autoradiographic observations of the ovarioles essentially confirm known facts concerning the extra-oocytic supply of the oocyte as stated for meroistic ovarioles. This includes the supply with stable and unstable RNA from the nurse cell chamber and proteins from haemolymph for vitellogenesis as well as nurse cell plasm rich in RNA, available during the degeneration of the nurse cell chamber.