Phagocytes of the transected feline optic nerve: an ultrastructural study

Following eye enucleation, there is a spatio-temporal pattern of degeneration along the length of the feline optic nerve, and this depends on the proximity of the segment or zone to the transected end. Thus, the rate of axonal degeneration depends on how close it is to the point of excision of the eye. In the proximal and medial segments, degeneration is rapid and the phagocytes originate from extrinsic haematogenous cells, which have invaded the optic nerve from surrounding meningeal blood vessels and capillaries. These phagocytes appear initially around blood vessels and then progressively spread to the parenchyma. In the distal segments, where Wallerian degeneration has occurred, the corresponding process is relatively slow and the phagocytes originate intrinsically. In contrast, these phagocytes are first seen in the parenchyma and then later spread to the blood vessels. Ultrastructurally, these phagocytes share fine features specifically identified with neuroglial cells such as microglia, oligodendrocytes but not astrocytes. For example, these phagocytes are rich in organelles such as mitochondria, ribosomes, microtubules, and rough and smooth endoplasmic reticulum including the Golgi apparatus. Cytoplasmic inclusions such as dense laminar bodies, lipid droplets, lysosomes and lipofuscin granules are also present. Intermediate filaments, seen only in astrocytes, are not found in these phagocytes. Although the study has established that the phagocytes are intrinsic and neuroglial in origin, their true identity is still unresolved. Secondarily, the study has shown that oligodendrocytes continue to survive and remain viable in an axon-free environment. The question about their new role in Wallerian degeneration remains.