Structure and organization of membrane organelles along distal microtubule segments in growth cones

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Abstract
Advance and stabilization of organelle‐rich cytoplasm within the neuronal growth cone is coupled to axon elongation (Goldberg and Burmeister, 1986; Aletta and Greene, 1988), and this involves forward movement of organelles from the growth cone base along distinct tracks toward the leading edge. Membrane‐bound organelles that advance first within the growth cone often make transient excursions toward the leading edge, and at the light microscope level these leading organelles appear to colocalize with distal microtubule (MX) segments (Dailey and Bridgman, 1989). We have used electron microscopy (EM) to identify the membranous organelles adjacent to distal MT segments, and to examine their structural interactions with MTs. In both glutaraldehyde‐fixed and rapid frozen whole‐mount growth cones, attenuated endoplasmic reticulum (ER)‐like membrane elements were the most common organelle type located adjacent to distal MX segments. These ER‐like membrane elements coursed roughly parallel to MTs and frequently terminated within an electron‐dense bulb at the MX tip. Blind‐ended membrane tubes, dense‐core vesicles, clear vesicles, and vacuoles were also found adjacent to distal MX segments. Quantitative analyses of organelle‐MT associations suggest that elements of the ER‐like membrane system may frequently advance ahead of other membrane‐bound organelles. Freeze‐etch EM revealed crossbridging structures between MTs and membranous organelles, which is consistent with the idea that advance of leading membrane organelles into the growth cone periphery is mediated by microtubule‐based motor transport mechanisms. The results suggest that distal microtubule segments serve as transport elements for advance of membrane organelles into more peripheral growth cone regions, and together MTs and ER‐like membrane organelles may initiate the conversion of dynamic F‐actin‐rich cytoplasm to more stable organelle‐rich cytoplasm (i.e., axoplasm).