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Abstract
Axis formation is one of the most important events occurring at the beginning of animal development. In the ascidian egg, the antero-posterior axis is established at this time owing to a dynamic cytoplasmic movement called cytoplasmic and cortical reorganisation. During this movement, mitochondria, endoplasmic reticulum (ER), and maternal mRNAs (postplasmic/PEM RNAs) are translocated to the future posterior side. Although accumulating evidence indicates the crucial roles played by the asymmetrical localisation of these organelles and the translational regulation of postplasmic/PEM RNAs, the organisation of ER has not been described in sufficient detail to date owing to technical difficulties. In this study, we developed three different multiple staining protocols for visualising the ER in combination with mitochondria, microtubules, or mRNAs in whole-mount specimens. We defined the internally expanded “dense ER” using these protocols and described cisterna-like structures of the dense ER using focused ion beam-scanning electron microscopy. Most importantly, we described the dynamic changes in the colocalisation of postplasmic/PEM mRNAs and dense ER; for example, macho-1 mRNA was detached and excluded from the dense ER during the second phase of ooplasmic movements. These detailed descriptions of the association between maternal mRNA and ER can provide clues for understanding the translational regulation mechanisms underlying axis determination during ascidian early embryogenesis.
Highlights
The morphogenesis of animal embryos begins with the formation of body axes, namely the antero-posterior and dorsoventral axes
Emergence of dense endoplasmic reticulum (ER) in the myoplasm and translocation to the posterior side To re-examine the translocation of ER during the second phase of movement, fertilised eggs were injected with carbocyanine dye (DiIC18(3)) and observed using time-lapse confocal imaging
The background staining of the ER covered the entire egg cytoplasm, a densely stained ER mass was clearly identified within the myoplasm
Summary
The morphogenesis of animal embryos begins with the formation of body axes, namely the antero-posterior and dorsoventral axes. 30–50 min after fertilisation, the sperm aster migrates towards the posterior pole and to the centre of the egg to fuse with the female nucleus (Roegiers et al 1999; Sardet et al 2007) This movement of the sperm aster contributes to the translocation of the myoplasm from the vegetal pole to the future posterior pole (second phase; Sawada and Schatten 1989; Chiba et al 1999; Roegiers et al 1999). During these movements, the cortical region of the myoplasm contains the ER, designated as cortical ER (cER) by Sardet et al (1992), and the inner region is occupied by mitochondria-rich cytoplasm (MRC)
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