Comparative studies on the astrocytic reaction in the lesioned central nervous system of different vertebrates

The morphology of the optic nerve of fish, amphibians, reptiles, birds and mammals, with special reference to the membrane architecture of the astrocytes, is described by means of ultrathin sections and freeze-fracture replicas. The reaction patterns of the astrocytes in response lesions of the optic nerve are discussed with respect to the regenerative capacity of the central nervous system (CNS). Astrocytic membranes in the optic nerve of the goldfish and Rana esculenta do not reveal orthogonal arrays of particles (OAP), in contrast to those of the optic nerve of the turtle, lizard, chicken, pigeon and rat. Reactive astrocytes in the crushed optic nerve of goldfish establish tunnel-like structures that are in close contact with the regenerating axons. During the regeneration period the distribution of intramembranous particles (IMP) in the astrocytic membrane is not uniform, in contrast to the situation found after complete regeneration and in the untreated optic nerve. Reactive astrocytes in degenerating optic nerves of chicken and rat form a glial scar. The density of the OAP decreases in reactive astrocytes of adult chicken and rats, whereas the density increases during the degeneration period in the optic nerve of the postnatal rat.

Astrocytes devoid of OAP are only found in regions of the vertebrate CNS capable of regeneration. In contrast, the inability to regenerate coincides with the appearance of OAP. Therefore, it is suspected that the presence of OAP in astrocytic membranes is at least one of several factors involved in the abortive regeneration of nerve fibers.