Fig. 4. HrPEM and macho 1 mRNA localization on the cER network in eggs and zygotes. (A) Edge of a cortex isolated from an unfertilized egg, labelled to visualize the cER network (red, left panel), and the network of HrPEM (green, middle panel). The right panel shows an overlay of red and green images and the colocalization (yellow) of ER and mRNA on the cortex (arrowhead). Most ER tubes outside the cortex are devoid of HrPEM (er arrow). (B) Vegetal pole region (v) of cortex isolated from a fertilized egg 20 minutes after fertilization (this area corresponds to the densely labelled regions of isolated cortices seen in Fig. 2C,D. Microvillated region characteristic of the vegetal pole region is indicated (mv). (C) Edge of an unfertilized egg cortex treated with puromycin-KCl to detach particles from the cER. Only a weak signal of HrPEM remains, and partial colocalization (arrowheads) is shown. ER tubes outside the cortex are indicated (er arrow). (D) Edge of a fertilized egg cortex showing the continuity between the HrPEM-labelled cER network (upper part) and the nonlabelled cytoplasmic ER network projected outside the cortex during the shearing process (lower part). The arrows (er arrow) show the zone of transition between cER and cytoplasmic ER outside the cortex. (E) Colocalization of cER and HrPEM in the central region of an isolated unfertilized egg cortex. The yellow colour results from superposed green and red images as in panel A. (F) Edge of an unfertilized egg cortex showing the continuity between the cER network (arrowhead) and the cytoplasmic ER tubes projected outside the cortex (red). Note the lack of HrPEM signal outside the cortex. The arrows (er arrow) show the zone of transition between cER and cytoplasmic ER outside the cortex. (G) Edge of an isolated unfertilized egg cortex colabelled for ER (red signal) and macho 1 mRNA (green signal). Co-localization is seen on the cortex (yellow; arrowhead), but not outside the cortex (red, er arrow). (H) Fragment of isolated cortex exposed to hypo-osmotic shock showing colocalization (yellow) of ER and HrPEM. Most of the cER network is vesiculated (arrowheads).

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