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J. Cell Biol.,
Volume 139, Number 3, November 3, 1997 817-829
Max-Planck-Institut für Entwicklungsbiologie, D-72076 Tübingen, Germany
Localization of maternally provided RNAs
during oogenesis is required for formation of the antero-posterior axis of the Drosophila embryo. Here we
describe a subcellular structure in nurse cells and oocytes which may function as an intracellular compartment for assembly and transport of maternal products
involved in RNA localization. This structure, which we
have termed "sponge body," consists of ER-like cisternae, embedded in an amorphous electron-dense mass.
It lacks a surrounding membrane and is frequently associated with mitochondria. The sponge bodies are not
identical to the Golgi complexes. We suggest that the
sponge bodies are homologous to the mitochondrial
cloud in Xenopus oocytes, a granulo-fibrillar structure
that contains RNAs involved in patterning of the embryo.
Exuperantia protein, the earliest factor known to be
required for the localization of bicoid mRNA to the anterior pole of the Drosophila oocyte, is highly enriched
in the sponge bodies but not an essential structural component of these. RNA staining indicates that sponge
bodies contain RNA. However, neither the intensity of
this staining nor the accumulation of Exuperantia in
the sponge bodies is dependent on the amount of bicoid mRNA present in the ovaries. Sponge bodies surround nuage, a possible polar granule precursor. Microtubules and microfilaments are not present in sponge
bodies, although transport of the sponge bodies through the cells is implied by their presence in cytoplasmic bridges. We propose that the sponge bodies are
structures that, by assembly and transport of included
molecules or associated structures, are involved in localization of mRNAs in Drosophila oocytes.
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