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Journal of Experimental Biology 76,191-204 (1978)
Published by Company of Biologists 1978

Axonal Adaptations to Osmotic and Ionic Stress in an Invertebrate Osmoconformer (Mercierella Enigmatica Fauvel) : I. Ultrastructural and Electrophysiological Observations on Axonal Accessibility

H.LE B. SKAER ¹, J. E. TREHERNE ¹, and J. A. BENSON ²

¹ Department of Zoology, University of Cambridge, U.K.
² Department of Zoology, University of Cambridge, U.K.; Laboratory of Sensory Sciences, University of Hawaii at Manoa, 1993 East-West Road, Honolulu, Hawaii 96822

The giant axons in Mercierella are overlaid by narrow glial processes which provide an incomplete covering of the axonal surface. Where more complete covering occurs the intercellular clefts are not sealed by tight junctional complexes. Ionic lanthanum penetrates to the surfaces of axons from sea-water-adapted animals (in normal saline and during initial exposure to hyposmotic saline) and, also, to the surface of hyposmotically adapted axons. A relatively free intercellular access to the axon surfaces is also indicated by the rapid electrical responses of sea-water-adapted axons to hyposmotic dilution and of hyposmotically adapted axons to sodium-deficient saline. The giant axon possesses an unusual ultrastructural specialization: hemidesmosome-like structures (associated with the axon membrane) which are connected to a network of neuronlaments within the axon. Theoretical considerations suggest that these structures could enable the axons to withstand appreciable excesses in intracellular hydrostatic pressure resulting from osmotic imbalance during hyposmotic stress.

Note:

The Editor reports, with deep regret, the apparent passing of Mercierella enigmatica which it is now proposed becomes Ficopotamus enigmaticus (Fauvel) (ten Hove & Weerdenburg, 1978, Biol. Bull. 154, 96-120).