Structural analysis of ILV formation. (A and A’) Tomographic slice (A) and corresponding tomographic reconstruction (A’) of a wild-type MVE from a root cell. (B and B’) Tomographic slice (B) and tomographic reconstruction (B’) of the MVE depicted in A and A’, showing a detail of the concatenated network of ILV buds connected through narrow membrane bridges. (C–F) Tomographic slices from plant MVEs showing examples of concatenated ILV bud networks connected to the endosomal limiting membrane. (G) Tomographic slices showing intermediates in the process of ILV bud concatenation. Shallow invaginations corresponding with early stages in ILV bud formation are marked by asterisks; “a” indicates the first ILV to form, and “b” the second one. (H) Model of ILV bud concatenation based on the images depicted in G. (I–I”) Tomographic slice (I) and corresponding tomographic reconstruction (I’ and I”) of three ILV buds connected by lateral membrane bridges. Red arrowheads indicate interconnecting membrane bridges. Bars: (A and A’) 50 nm; (B–I”) 20 nm.