Association of actin filaments with axonal microtubule tracts

Axoplasmic organelles move on actin as well as microtubules in vitro and ax ons contain a large amount of actin, but little is known about the organiza tion and distribution of actin filaments within the axon. Here we undertake to define the relationship of the microtubule bundles typically found in a xons to actin filaments by applying three microscopic techniques: laser-sca nning confocal microscopy of immuno-labeled squid axoplasm; electronmicrosc opy of conventionally prepared thin sections; and electronmicroscopy of tou ch preparations-a thin layer of axoplasm transferred to a specimen grid and negatively stained. Light microscopy shows that longitudinal actin filamen ts are abundant and usually coincide with longitudinal microtubule bundles. Electron microscopy shows that microfilaments are interwoven with the long itudinal bundles of microtubules. These bundles maintain their integrity wh en neurofilaments are extracted. Some, though not all microfilaments decora te with the S1 fragment of myosin, and some also act as nucleation sites fo r polymerization of exogenous actin, and hence are definitively identified as actin filaments. These actin filaments range in minimum length from 0.5 to 1.5 mu m with some at least as long as 3.5 mu m. We conclude that the mi crotubule-based tracks for fast organelle transport also include actin fila ments. These actin filaments are sufficiently long and abundant to be ancil lary or supportive of fast transport along microtubules within bundles, or to extend transport outside of the bundle. These actin filaments could also be essential for maintaining the structural integrity of the microtubule b undles.