Figure 9.

Ultrastructure of the proliferation zone. TEM micrographs of sections through the proliferation zone in the lateral soma cluster. A: Center of proliferation zone at low magnification reveals the presence of three distinct cell types: type A (PZ-A), type B (PZ-B) and type C cells (PZ-C). Note that layers of electron-dense processes of cell body glia (arrows) only partially separate cells from each other. B: Four neighboring type B cells (PZ-B). Type B cells are not surrounded by processes of cell body glia. They form irregularly shaped, convoluted processes that interdigitate with processes of other type B cells. The inner nuclear membrane is distinctly thicker than the outer one. C,D: Two examples of type A cells (PZ-A) at higher magnification. Type A cells have an irregularly shaped nucleus with large areas of diffuse heterochromatin. Note the distinctly different appearance of type B (PZ-B) and type C (PZ-C) cells in the surround (arrows, layers of electron-dense processes of cell body glia). E: Type A cell (PZ-A) in telophase of mitosis. Condensed daughter nuclei (stars) oppose each other. Thin cell membranes separate the two daughter cells (arrowhead; arrows indicate layers of electron-dense processes of cell body glia). F,G: Two examples of type B cells (PZ-B). Type B cells have a thicker rim of cytoplasm and a more regularly shaped nucleus than type A cells. Their inner nuclear membrane is distinctly thicker and more electron dense than the outer one. H: Example of a type C cell. Type C cells (PZ-C) are larger and have a more spherical nucleus than type A and type B cells. Note that a distinctive gap separates both nuclear membranes and that the cell is completely surrounded by a layer of electron-dense processes of cell body glia (arrows). Scale bars = 5 μm in A; 1 μm in B; 5 μm in D (applies to C–H).