The Fine Structure of a Special Type of Nerve Fiber Found in the Ganglia of Armadillidium vulgare (Crustacea-Isopoda)

In the past few years a large body of experimental data has been amassed on the physiology of the arthropod nervous system (see the review by Roeder (8)) but, unfortunately, the full significance of these data is difficult to assess for lack of information on the fine structure of the arthropod neuropile. Some papers have been published that treat the electron microscopic morphology of the arthropod nervous system (7, 4, 3, 1, 2, 9, 5). The present work is a brief report of observations on the fine structure of a distinctive type of nerve fiber found in the abdominal ganglia of Armadillidium vulgare (Crustacea-Isopoda). The ganglion chains of adult specimens were fixed for 30 minutes in a 1 per cent OsO4 solution (veronal buffer pH 7.9) and were embedded, following the usual procedure, in n-butyl methaerylate. Observed under the electron microscope, ArmadiUidium ganglia have the well known appearance of arthropod ganglia in general, that is: neurons and glial cells are arranged at the periphery of the ganglion while the central region is a dense assembly of interwoven fibers called the neuropile. An attentive study of the neuropile reveals that it is formed of a great number of nerve fibers together with thin glial processes. The majority of the nerve fibers have the structural pattern already described in other arthropods. In a few fibers, however, the structural components appear in an unusual arrangement thus constituting a special type of nerve fiber. Two main features characterize this special type of nerve fiber: (a) the existence of regularly spaced and transversely oriented cisternae of the endoplasmic reticulum (ER) and (b) the existence of a great number of mitochondria in close contact with the limiting membrane. The transverse cisternae lend to this type of fiber a typical cross-striated appearance. These cisternae are not parts of a continuous septa, but rather, fenestrated structures that allow the flow of material within the neuroplasm. Their spatial arrangement can be inferred from images such as those shown in Figs. 1 and 2 in which the transverse cisternae are seen as discontinuous