People have been looking at microscope slides of the brain almost since there were microscopes (Alzheimer’s paper on his disease came out in 1906). Amazingly, something new has just been found [ Proc. Natl. Acad. Sci. vol. 111 pp. 9633 - 9638 '14 ]
To a first approximation, the axon of a neuron is the long process which carries impulses to other neurons far away. They have always been considered to be quite delicate (particularly in the brain itself, in the limbs they are sheathed in tough connective tissue). After an axon is severed in the limbs, all sorts of hell breaks lose. The part of the axon no longer in contact with the neuron degenerates (Wallerian degeneration), and the neuron cell body still attached to the remaining axon, changes markedly (central chromatolysis). At least the axons making up peripheral nerves do grow back (but maddeningly slowly). In the brain, they don’t, yet another reason neurologic disease is so devastating. Huge research efforts have been made to find out why. All sorts of proteins have been found which hinder axonal regrowth in the brain (and the spinal cord). Hopefully, at some point blocking them will lead to treatment.
THe PNAS paper found that axons in the optic nerve of the mouse (which arise from neurons in the retina) burp out mitochondria. Large protrusions form containing mitochondria which are then shed, somehow leaving the remaining axon intact (remarkable when you think of it). Once shed the decaying mitochondria are found in the cells supporting the axons (astrocytes). Naturally, the authors made up a horrible name to describe the process and sound impressive (transmitophagy).
This probably occurs elsewhere in the brain, because accumulation of degrading mitochondria along nerve processes in the superficial layers of the cerebral cortex (the gray matter on the surface of the brain) have been seen. People are sure to start looking for this everywhere in the brain, and perhaps outside as well.
Where else does sort of thing this occur? In the fertilized egg, that’s where. Sperm mitochondria are activated in the egg (which is why you get your mitochondria from mommy).