What part of the body is affected by major depressive disorder?

Neurodegeneration: There is evidence that in recurrent major depression, astraglial cells, the cells in the brain that support neurons and encapsulate synapses suffer. As this occurs over time, they can wither and perish resulting functionally in a decreased ability of the brain to process information/ adapt. Physically this can be seen on pet/spect/fmri scans. Effective treatment appears to slow/stop the process.