Lack of orthogonal particle assemblies and presence of tight junctions in astrocytes of the goldfish (Carassius auratus). A freeze-fracture study.

The cytoplasmic membranes of astrocytes in the optic nerve of the goldfish were investigated by means of freeze-fracture techniques. Astrocytes of normal and regenerating optic nerves did not differ in the fine structure of plasma membranes. Emphasis is placed on the following results: Astrocytic membranes of fish do not reveal the orthogonal particle assemblies that are believed to be generally characteristic for astrocytes in the white matter. Astrocytes reveal extensive membrane areas occupied by tight junctions, which to date have not been described as characteristic astrocytic structures. These junctions are astro-astrocytic and are frequently intercalated by gap junctions. Desmosomes are another characteristic type of astro-astrocytic junction. By means of freeze-fracture replicas it can be demonstrated that they occur in more or less close association with tight and gap junctions. Caveolae are also seen in the astrocytic membranes of fish: their density and distribution show distinct variations. Caveolae occur at the interface between astrocytes and the interstitial space, between astrocytes and myelin sheaths, and in astrocytic processes. It is suggested that the differences between the astrocytic membranes of fish and mammals reflect different physiological functions. They are discussed in relation to the problem of neuronal-glial interrelationships and the behavior of astrocytes during fiber regeneration in the CNS.