The dictyate nucleus of the growing mouse oocyte exhibits, besides the nucleolus, 3 ultrastructurally distinct types of smaller formed entities referred to as extranucleolar bodies. The extranucleolar bodies of the first type (fibrillogranular bodies) consist of intermingled masses made up of closely arranged convoluted fibrils, 6-10 nm in width, interspersed with electrondense granules approximately 15 nm in diameter. The extranucleolar bodies of the second type (fibrillar bodies) are composed of an entanglement of loosely arranged convoluted fibrils 4-10 nm in diameter; such bodies are also characterized by the presence within their mass of irregularly shaped widely scattered islands of varying sizes made up of densely packed fibrillar material exhibiting a somewhat greater electron opacity. The extranucleolar bodies of the third type (coiled bodies) appear to consist of an aggregate of highly contorted threads, the thickness of which varies from 20 to 35 nm; the threads themselves are composed of bundles of fibrils 5 nm thick which are irregularly twisted along their axis.

An attempt is made to interpret these morphological findings in the light of current knowledge concerning the architectural and functional organization of the oocyte nucleus in general during the protracted dictyate stage of meiotic prophase. Our observations would be consistent with the view that the various types of extranucleolar bodies are morphological expression - like the puffs of the polytene and the loops or spheroids of the lampbrush chromosomes - of differential gene activity on the part of the localized regions of the chromosomes during oocyte growth.

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