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MORPHOLOGICAL EVIDENCE FOR THE PARTICIPATION OF MICROTUBULES IN AXONAL TRANSPORT *

Fine structural studies have shown a uniform pattern of neuronal organization, in that the complex intracellular environment of the perikaryon is separated from morphologically well-characterized and often distant synaptic foci, by a tract of axoplasm that is structurally simple in its variety of components and notably possesses longitudinally oriented neurofilaments and microtubules set in an electron-lucent matrix. Studies on living neurones have indicated that the axoplasm is not only secreted within the cell body and passed slowly outwards, but that it is, in addition, the site of the varied traffic in which structural and molecular components are translocated at a range of speeds in excess of that of bulk centrifugal flow. Intracellular translocation of particulates is a widespread cytoplasmic activity, and the conservatism of evolution and specialization emerging from studies on subcellular structure and function prompts a search for a common mechanism underlying this activity. Experimental evidence from neurones and other cell types suggests that a salient function of cytoplasmic microtubules is the control and/or mediation of intracellular transport. The premise of this account is that functional coupling between translocated particulates and any axoplasmic component responsible for the movement and preserved as an identifiable system by present preparative techniques is likely to involve close physical juxtaposition. Accordingly, the distribution of membrane-limited inclusions with respect to neurofilaments and microtubules has been examined in detail, in thin sections of central axons of ammocoete larvae of the lamprey (Petromyzon marinus), to augment previous studies by a number of investigators on the nervous system of other animals. Subjective examination of electron micrographs may leave open the possibility of apparent association of axoplasmic components by fortuitous pairing in a randomly dispersed system, and the distribution of axoplasmic mitochondria was selected for more quantitative analysis by comparison between data obtained from micrographs and appropriately constructed computed “model axons.” It is suggested that the axoplasm is morphologically more organized than is sometimes supposed, and that this should be taken into account in attempts to identify the structural basis of neuronal function and especially of the logistic details of the physiological rapport between the cell body and its extensions.