Fig. 2. Morphology of the cumulus. The direction of the cumulus movement is shown by the largest arrows (A,B,D-F). (A) SEM image showing the surface view of a cumulus. The epithelium is bulged at the cumulus. (B) SEM image showing the inside view of a cumulus. Asterisks indicate CM cells, which extend lamellipodia-like processes (thick arrows). Note that germ disc epithelial cells extend thin, long cytoplasmic projections like cytonemes (thin arrows) from their basal side onto the surface of the CM cells. A single epithelial cell has a single projection. (C) SEM image showing the inside view of germ disc epithelial cells far from the cumulus in the same embryo as in B. No cytoneme-like projections are seen. (D,E) Laser scanning microscopy (LSM) images of a cumulus stained with phalloidin (green) and TOTO-3 (purple). A series of optical sections separated by intervals of 0.52 µm were obtained to cover the epithelial and mesenchymal layers of the cumulus. D and E were constructed by overlaying 16 and nine successive optical sections, respectively, selected from the data set. D focuses on the CM cells, and E on the interface between the CM cells and the germ disc epithelial cells. Lamellipodia-like processes abundant with F-actin ingress into the spaces between the lateral surfaces of the epithelial cells as indicated by arrows (a and b). (F) LSM image showing a sagittal section of a cumulus stained with phalloidin-fluorescein. Cytoplasmic projections extend from the basal side of the epithelial cells are seen (arrows). Scale bar: 10 µm in A-C; 20 µm in D-F.

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