Plasmalemmal vesicles (caveolae) of fibrous astrocytes of the cat optic nerve

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Abstract
Plasmalemmal vesicles (caveolae) are described in fibrous astrocytes of the cat optic nerve. In thin sections, astrocytic caveolae appear as flask‐shaped invaginations of the plasma membrane with a 60‐nm lumen that communicates with the extracellular space through a 35‐nm stoma. Occasional bilobed caveolae occur. The caveolae extend approximately 85 nm into the astrocytic cytoplasm and are often embedded within a granular or filamentous ectoplasmic substance connected by 4–8‐nm filament bridges to underlying bundles of 10‐nm glial filaments. In freeze‐fracture replicas, the caveolar stomata appear as dimples on the P face and as craters on the E face, often arranged in hexagonal or linear arrays and spaced at a center‐to‐center distance of 110–130 nm. The caveolar membrane is apparently particle‐free. Fibrous astrocytes related to the connective‐tissue septa of the optic nerve show different densities of caveolae on different areas of their plasma membranes. Plasma membranes apposing a basal lamina have few caveolae, whereas membranes not apposed to the basal lamina but to other astrocytic membranes have up to 17 caveolae/μm2. Caveolae also occur on astrocytic plasma membranes apposed to myelin sheaths. Possible functions of the astrocytic caveolae are discussed in the light of plasmalemmal properties of other types of caveolae‐bearing cells.