Elsevier

Brain Research

Volume 540, Issues 1–2, 1 February 1991, Pages 209-216
Brain Research

Microtubules have special physical associations with smooth endoplasmic reticula and mitochondria in axons

https://doi.org/10.1016/0006-8993(91)90509-TGet rights and content

Abstract

Ultrastructural morphometry was used to document the non-random spatial distributions of organelles within the compact myelinated region of avian oculomotor axons. These regions contain large numbers of loosely packed neurofilaments (NFs) (241/μm2) and only a relatively small number of microtubules (MTs) (4/μm2), mitochondria (0.6/μm2), and smooth endoplasmic reticulum (SER) (1.6/μm2). Random co-occurrences between the relatively sparsely distributed MTs, mitochondria, and SER are probably infrequent in these axons. The actual co-occurrences of MTs, mitochondria, and SER with MTs were counted and compared to the co-occurrences expected in a random Poisson distribution. At long distances (200 nm), the co-occurrences were random. At shorter distances (40 nm and less), MTs were still randomly associated with other MTs. However, at these shorter distances, the spatial associations of mitochondria with MTs and of SER with MTs were not random; such preferential stable associations may be produced by specific MT associated cross-bridging proteins. In axons, MTs tend to be clustered together, giving the appearance of MT bundles. We propose that the MT-MT bundling is an indirect result of MT concentration along the continuous intra-axonal SER network, to which the MTs are apparently tied directly by dynamic molecular cross-bridges.

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    *

    Current address: Integrated Microscopic Analysis Facility, Department of Pathology, University of South Carolina, Columbia, SC 29208, U.S.A.

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