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Curr Biol
1998 Oct 08;820:1117-20. doi: 10.1016/s0960-9822(98)70466-x.
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Localization of mitochondrial large ribosomal RNA in germ plasm of Xenopus embryos.
Kobayashi S
,
Amikura R
,
Mukai M
.
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In Xenopus, factors with the ability to establish the germ line are localized in the vegetal pole cytoplasm, or germ plasm, of the early embryo [1-3]. The germ plasm of Xenopus, and of many other animal species including Drosophila, contains electron-dense germinal granules which may be essential for germ-line formation [4-5]. Several components of the germinal granules have so far been identified in Drosophila [6-10]. One of these is mitochondrial large ribosomal RNA (mtlrRNA), which is present in the germinal granules (polar granules) during the cleavage stage until the formation of the germ-line progenitors or pole cells [8-9]. MtlrRNA has been identified as a factor that induces pole cells in embryos that have been sterilized by ultraviolet radiation [11]. The reduction of mtlrRNA in germ plasm by injecting anti-mtlrRNA ribozymes into embryos leads to the inability of these embryos to form pole cells [12]. These observations clearly show that mtlrRNA is essential for pole cell formation in Drosophila. Here, we report that mtlrRNA is enriched in germ plasm of Xenopus embryos from the four-cell stage to the blastula. Furthermore, our electron microscopic studies show that this mtlrRNA is present in the germinal granules during these stages. Thus, mtlrRNA is a common component of germinal granules in Drosophila and Xenopus, suggesting that the mtlrRNA has a role in germ-line development across phylogenetic boundaries.
Figure 1.
Distribution of mtlrRNA in the vegetal region of early embryos. Whole-mount embryos at the (a,b) two-cell stage, (c,d) early four-cell stage, (e,f) late four-cell stage, (g,h) eight-cell stage, (i,j) blastula stage, and (k,l) gastrula stage were hybridized in situ with an antisense mtlrRNA probe. Vegetal views of the embryos are shown in (a–j); (b,d,f,h,j) are higher-magnification views of the vegetal pole regions; (k) is a section through a gastrula embryo hybridized with the mtlrRNA probe and stained with Mallory–Heidenhain solution. In (k,I), the yolk granules are intensely stained. A cell indicated by an arrow in (k) is enlarged in (I); its nucleus, the arrowhead in (l), is surrounded by yolk-free cytoplasm, or germ plasm. Embryos in (a,c,e,g,i) and those in (b,d,f,h,j) are shown at the same magnification. Scale bars represent 1 mm in (i,k), 200 μm in (j), and 50 μm in (l).
Figure 2.
Distribution of mtlrRNA in germ plasm. (a) Micrograph of the vegetal pole region of a late four-cell stage embryo hybridized with an antisense mtlrRNA probe. The mtlrRNA signal was detected by immunogold labeling and silver enhancement (see Materials and methods). Arrows show the mtlrRNA signal in patches of germ plasm. (b) An electron micrograph of the germ plasm shown in (a). A section was cut through the germ plasm where the mtlrRNA signal was concentrated. Note that a large number of mitochondria and electron-dense granules are observed in the yolk-free cytoplasm. (c–e) Higher-magnification views of sections through the germ plasm of (c) two-cell stage, (d) late four-cell stage, and (e) blastula stage embryos hybridized with an antisense mtlrRNA probe. The arrowhead in (c) indicates germinal granules. Arrows in (d) and (e) point to the mtlrRNA signal in germinal granules. The label m indicates mitochondria; scale bars represent 5 μm in (b) and 0.5 μm in (e).
