Fig. 5. HrPEM mRNA concentrates in the ER-rich CAB region. (A-E) Merged images of ER (red) and HrPEM (green) in cortices isolated from Halocynthia 8-cell-stage blastomeres. Areas where ER and HrPEM mRNA are perfectly colocalized appear yellow. (A) Quartet of cortices isolated from an 8-cell embryo attached by the posterior pole to a coverslip. Cortices of vegetal-posterior (vp) blastomeres are rich in cER and in HrPEM in and around the CAB (CAB arrowhead). Animal-posterior (ap) blastomeres have a cER network that lacks HrPEM. (B,C) Views of the CAB and of the cER network that surround the CAB. C is a higher magnification of the boxed region (edge of CAB) in B. In C, large patches of HrPEM mRNA localize on the cER network in the immediate vicinity of the CAB (CAB arrowhead), shown by arrows. The cER network away from the CAB (arrowheads in C) displays only occasional and small mRNA-rich patches. (D,E) These merged images show that the ER network, which is normally compacted in the CAB, has stretched away from the attached cortex under the force of shear. E is a higher magnification view of the boxed region in D. Large patches of HrPEM mRNAs are detected both on and inbetween stretched ER tubes (arrowheads show green RNA-rich patch). (F,G) Nonmerged images. Maximum magnification of confocal images of the inside of the CAB showing colocalization of ER [ER is labelled with DiIC(16)3 in F, and HrPEM mRNA labelled by Alexa green 488 in G] in some regions of the CAB (arrows). In other regions, differences in the relative intensity of mRNA and ER labelling can be seen.