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An electron microscope study of goldfish Mauthner cells is reported.(1) The cell is covered by a synaptic bed approximately 5 micro thick containing unusual amounts of extracellular matrix material in which synapses and clear glia processes are implanted. The preterminal synaptic neurites are closely invested by an interwoven layer of filament-containing satellite cell processes. The axoplasm of the club endings contains oriented mitochondria, neurofilaments, neurotubules, and relatively few synaptic vesicles. That of the boutons terminaux contains many unoriented mitochondria and is packed with synaptic vesicles and some glycogen but no neurofilaments or neurotubules. The bare axons of club endings are surrounded by a moderately abundant layer of matrix material. The synaptic membrane complex (SMC) in cross-section shows segments of closure of the synaptic cleft approximately 0.2 to 0.5 micro long. These alternate with desmosome-like regions of about the same length in which the gap widens to approximately 150 A and contains a condensed central stratum of dense material. Here, there are also accumulations of dense material in pre- and postsynaptic neuroplasm. The boutons show no such differentiation and the extracellular matrix is largely excluded around them. The axon cap is a dense neuropil of interwoven neural and glial elements free of myelin. It is covered by a closely packed layer of glia cells. The findings are interpreted as suggestive of electrical transmission in the club endings.

Observations additional to those previously reported (34) on boutons terminaux and club endings on Mauthner cell lateral dendrites, primarily as seen in sections of permanganate-fixed material, are described. Certain new findings on OSO(4)-fixed endings are also included. The boutons terminaux are closely packed in the synaptic bed with approximately 100 to 150 A gaps between their contiguous unit membranes and a few interspersed glial extensions. Their synaptic membrane complexes (SMC) appear as pairs of unit membranes separated by approximately 100 to 150A clefts. They contain many vesicles and unoriented mitochondria, but no neurofilaments. The club endings after KMnO(4) fixation are, as after OSO(4) fixation (34), again seen surrounded by a layer of extracellular matrix material. These endings contain relatively few synaptic vesicles, a few unit membrane limited tubules approximately 300 A in diameter, and mitochondria oriented perpendicular to the SMC. Neurotubules and neurofilaments are not clearly seen. These components are also virtually absent in the Mauthner cytoplasm. No ribosomes are seen in the KMnO(4)-fixed material. The unit membranes of the SMC of club endings show up clearly in essentially the same junctional relations described after formalin-OSO(4) fixation (34). In addition, the synaptic discs in transverse section show a central beading repeating at a period of approximately A associated with scalloping of the cytoplasmic surfaces. In oblique views, dense lines are seen repeating at a period of approximately 90 A. In frontal views a hexagonal array of close-packed polygonal facets is seen. These repeat at a period of approximately 95 A. Each has a central dense spot <25 A in diameter. Similar subunits are seen in the unit membranes of synaptic vesicles.

Schmidt-Lanterman clefts in frog sciatic nerves have been studied in thin sections by electron microscopy utilizing permanganate fixation and araldite embedding. It is shown that they are shearing defects in myelin in which the lamellae are separated widely at the major dense lines. Each lamella consisting of two apposed Schwann cell unit membranes approximately 75 A across traverses the cleft intact. The unit membranes composing each lamella sometimes are slightly ( approximately 50 to 100 A) separated in the clefts. The layers between the lamellae contain membranous structures which may be components of the endoplasmic reticulum. These layers are continuous with the outer layer of Schwann cytoplasm and the thin and inconstant cytoplasmic layer next to the axon (Mauthner's sheath). Each of these layers in perfect clefts constitutes a long helical pathway through the myelin from the axon. One of these is connected with Schwann cytoplasm and the other directly with the outside. A type of cross-sectional shearing defect, not hitherto recognized, is described and shown to be a kind of Schmidt-Lanterman cleft. Incomplete clefts are seen and interpreted as representing stages in a dynamic process whereby the myelin lamellae may be constantly separating and coming together again in life.

Adult chameleon myelinated peripheral nerve fibers have been studied with the electron microscope in thin sections. The outer lamella of the myelin sheath has been found to be connected as a double membrane to the surface of the Schwann cell. The inner lamella is connected as a similar double membrane with the double axon-Schwann membrane. The relations of these double connecting membranes suggest that the layered myelin structure is composed of a double membrane which is closely wound about the axon as a helix. These findings support the new theory of myelinogenesis proposed recently by Geren. The possible significance of these results with respect to cell surface membranes and cytoplasmic double membranes is discussed.