Figure 4

Internalization of the proto-nucleus. A) Schematic showing that the nuclear membrane is a single membrane due to the membrane-lined nuclear pores. B1) Initial stage before proto-cytoplasmic cell has begun to surround proto-nucleus cell. B2) Intermediate stage at which proto-cytoplasmic cell has largely surrounded proto-nucleus cell. B3) Full internalization; proto-nucleus is now a nucleus; the two cells are now a single cell that possesses a nucleus. These are one-dimensional slices through the cells; in three dimensions the proto-cytoplasm cell surrounds the proto-nucleus cell on all sides. Filled arrows in B1-B3 represent non-respiratory (e.g., Na/K ATPase) pump activity. These pumps pump Na and Ca out of the cytoplasm into the external medium or lumen, and K out of the external medium or lumen into the cytoplasm. As the proto-cytoplasm cell surrounds the proto-nucleus cell, these pumps would thus automatically (i.e., without any change in their orientation in the membrane) work so as to maintain an external-like (high Na and Ca, low K) ionic environment in the lumen separating the cells. “L” shaped lines represent cell anchoring proteins; lines are grey because it is possible the nanotubes alone could provide the physical support for internalization. Nanotubes are represented by the grey connections between the cells. Because these drawings are slices through the cell, it appears that the nanotubes separate the lumen into compartments. However, because these are tubes, the lumen is actually continuous (see Figure 5B2).