Abstract
Locations of a distinctive mode of trans-endocytosis involving dendrites, axons, and glia were quantified through serial section electron microscopy. Short vesicular or long vermiform evaginations emerged from dendrites and axons and were engulfed by presynaptic or neighboring axons, astrocytes, and, surprisingly, a growth cone to form double-membrane structures called spinules.
In total, 254 spinules were evaluated in 326 m3 of stratum radiatum in area CA1 of mature rat hippocampus. Spinules emerged from spine heads (62 %), necks (24 %), axons (13 %), dendritic shafts (1 %), or nonsynaptic protrusions (under 1 %) and invaginated into axons (90 %), astrocytic processes (8 %), or a growth cone (1 %).
Coated pits occurred on the engulfing membrane at the tips of most spinules (69 %), and double-membrane structures occurred freely in axonal and astrocytic cytoplasm, suggesting trans-endocytosis. Spinule locations differed among mushroom and thin spines.