MORPHOLOGY AND TRANSCRIPTIONAL ACTIVITY OF MOUSE OOCYTE CHROMOSOMES

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. Pachytene and early diplotene mouse oocyte chromosomes were examined through the light microscope using sections of plastic-embedded material and air dried spreads stained with either AgNO3 or methyl green pyronin. The projections radiating from the chromosomal axis are evidently bundles of chromatin fibers rather than single fibers covered with a 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. 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. 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. EM analysis of spread chromatin from growing oocytes demonstrates that most transcription units possess only 1 or 2 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.