Electron tomographic analysis of cytokinesis in the brown alga Silvetia babingtonii (Fucales, Phaeophyceae).

In brown algae, membrane resources for the new cell partition during cytokinesis are mainly flat cisternae (FCs) and Golgi-derived vesicles. We used electron tomography coupled with rapid freezing/freeze substitution of zygotes to clarify the structure of transient membrane compartments during cytokinesis in Silvetia zygotes. After mitosis, an amorphous membranous structure, considered to be an FC intermediate was observed near endoplasmic reticulum clusters, lying between two daughter nuclei. FCs were arrayed at the cytokinetic plane, and a tubular membranous network was formed around them. This network might be formed by the consecutive fusion of spherical vesicles that are linked to the edges of FCs to form a membranous network (MN). At the initial stage of the formation of a membranous sac (MS) from the MN, the MS had flat and swollen parts, with the latter showing membranous tunnels. Coated pits were detected with high frequency at the swollen parts of the MS. This observation indicated that membranous tunnels disappeared by recycling of excess membrane via endocytosis, and the swollen part became flat. The MN appeared at the edges of the growing MS. MN and the MN-MS complex were observed along the cytokinetic plane in several spaces. The MS expanded by the incorporation of MN or other MS in its neighborhood. With the maturation of the new cell partition membrane, the thickness of the MS became constant and the membrane cavity disappeared. The changes in the surface area and volume of the transient membrane compartment during cytokinesis were analyzed from the tomographic data.