Morphology and transcriptional activity of mouse oocyte chromosomes

Abstract

Lampbrush chromosomes of growing amphibian oocytes carry thousands of lateral loops each of which consists of a chromatin fiber heavily encrusted with nascent ribonucleoprotein fibrils. These are believed to be responsible for the accumulation and maintenance of RNA transcripts found stored in the egg. In the case of mammalian oocytes, lampbrush chromosomes are most likely to occur during the major growth phase and also possibly during pachytene-early diplotene stages of meiosis. We have examined pachytene and early diplotene mouse oocyte chromosomes through the light microscope using sections of plastic-embedded material and air dried spreads stained with either silver nitrate or methyl green pyronin. Our results indicates that the projections radiating from the chromosomal axis are bundles of chromatin fibers rather than single fibers covered with an ribonucleoprotein matrix. These bundles may represent partially unfolded chromomeres. The axis itself could be partially dispersed revealing threads surrounding a fine linear element. -- Little is known about chromosome structure in growing mammalian oocytes, the stage when transcriptional activity is likely to be most rapid. In our preparations chromosomes at this stage appear as partially condensed fuzzy threads of relatively uniform width. In some cases, the fuzzy thread is seen to contain a dense linear core in the center. Thus, during the growth phase, the chromosomes retain a relatively condensed axis, a characteristic of meiotic chromosomes in general. RNA-containing material is found diffusely spread within the nucleus but not specifically associated with the chromosomes. Electron microscopic analysis of spread chromatin from growing oocytes demonstrates that most transcription units possess only one or two nascent ribonucleoprotein fibrils while a few have more. These and other published data indicate that mouse oocytes do not have true lampbrush chromosomes at any stage of their development.

The Histone Deacetylase Inhibitor Trichostatin A Induces Retrogressive Chromatin Decondensation in the Germinal Vesicle of Porcine Cumulus-enclosed Oocytes

“…It has been shown that changes in chromatin configuration in oocytes correlate with a decline in transcriptional activity [2,7,12]. This is consistent with the basic processes that are already well known in other cell types and in which acetylation of nucleosome core histones is essential in both chromatin remodeling and transcription.…”

“…During this period, chromatin in the germinal vesicle (GV) is not static: first, growing oocytes display very strong transcriptional activity [2][3][4][5][6][7]; second, chromatin configuration is greatly altered in the latter half of the growth period [2,[6][7][8][9]; and third, recent studies have proven the existence of growth-dependent regulation of gene imprinting [10]. In the second of these activities, chromatin configuration is categorized broadly into two kinds by the presence or absence of a condensed mass of chromatin around the nucleolus.…”

Ultrastructural cytochemistry of the chromosomes of growing oocytes ofPerinereis cultrifera(Nereidae, Polychaeta, Annelida) after fixation in situ

“…Indeed, at the same stage (vitellogenesis) amphibian lampbrush chromosomes no longer possess a proteinaceous axis as seen on spread preparations and on sections of germinal vesicles [16]. Diplotene chromosomes of growing oocytes of Perinereis seem to be more similar to those of oocytes from mammalian species in which chromosomal axes were reported [14,15,17,18]. However, the interpretation of the present results obtained by in situ examination of diplotene chromosomes of Perinereis raises some questions.…”

“…In mammalian spread chromosomes it is accepted that silver reveals proteinaceous axes [12]. In diplotene chromosomes, this was demonstrated not only in male Downloaded by [JAMES COOK UNIVERSITY] at 10:31 17 March 2015 293 meiosis [13] but also in growing oocytes [14,15]. Therefore, it is possible that the silver deposits observed in situ in germinal vesicles of oocytes of Perinereis reveal sections of the proteinaceous axial core of diplotene chromosomes.…”

Contributions of Electron Microscopic Spreading Preparations (“Miller Spreads”) to the Analysis of Chromosome Structure

“…Usually, however, nonnucleolar transcription units of a variety of cell types and species are characterized by relatively few transcript fibrils, with extended unoccupied chromatin regions, indicative of a low level of ongoing transcriptional activity (Figs. 6 a, 7 b, c; see also Miller and Bakken 1972;Kierszenbaum and Tres 1975;Laird and Choii 1976;Foe et al 1976;Amabis and Nair 1976;Oda et al 1977;Villard and Fakan 1978;McKnight et al 1979;Harper and Puvion-Dutilleul1979;Bakken 1979, 1980;Hughes et al 1979;Cotton et al 1980;Petrov et al 1980;Beyer et al 1980Beyer et al , 1981Scheer et al 1981;Greimers and Deltour 1981 ;Bachvarova et al 1982;Rattner et al 1982; Puvion-DutilleuI1983).…”