Glial Cells of the Central Nervous System in the Crab Ucides cordatus

Abstract

Glial cells and their processes were characterized in the fasciculated zone and in the protocerebral tract of the crab Ucides cordatus by light and electron microscopy. Thiery and PAS procedures indicate the presence of carbohydrates, particularly glycogen in cells. Immunohistochemistry was used to observe tubulin distribution in the glial cells. Our results demonstrate at least two types of glial cells in the fasciculated zone and in the protocerebral tract, separable by their location and electron density. Judging by their position, electron-lucent cells may correspond to periaxonal cells and electron-dense ones may correspond to perineurial cells. The electron-dense processes have previously been interpreted as extracellular matrix, but since they feature an enveloping membrane and contain glycogen and mitochondria (intact and with varying degrees of disruption) we consider them to be part of one type of glial cells. Additional key words: Crustacea, Malacostraca, Brachyura, glia Invertebrate glial cells have traditionally been classified according to certain relatively general morphological or functional criteria and also by their anatomical position (Haimori & Horridge 1966; Radojcic & Pentreath 1979). Because invertebrates do not have true compact myelin, no glial class equivalent to vertebrate oligodendrocytes can be identified. In addition, immunocytochemical markers for vertebrate astrocytes in most cases fail to label invertebrate glia. (One of the exceptions is glutamine synthetase, a specific marker for astrocytes in vertebrates, which has also successfully labeled non-neuronal cells in lobster olfactory regions: Linser et al. 1997.) The glial cell types of arthropods share many morphological characteristics. First, in many species, the chromatin of glial cell nuclei is clumped in the periphery. This feature is rarely seen in neuronal nuclei. Second, mitochondria, endoplasmic reticulum, and Golgi structures are generally common. Third, between neighboring glial cells and also between glial cells and neurons, diverse membrane specializations are found (see Pentreath 1987). In the nervous system of some species of higher crustaceans, i.e., Decapoda, there is good anatomical and physiological evidence for an insulating sheath with nodes and rapid saltatory conduction (Heuser & Doggenweiler 1966). Where present, the sheath coma Author for correspondence. E-mail: sallodi@chagas.biof.ufrj.br prises numerous laminae, each containing cytoplasm and forming a seam. The nodes are loosely wrapped by a characteristic type of glial cell called the nodal cell. The axons can be wrapped either simply by a single glial process or in groups, by complex, tightly packed multiple layers, resembling vertebrate myelin (Bullock et al. 1977). Important differences from vertebrate myelin are: (a) the crustacean laminae do not connect to form a spiral; (b) the nuclei of the sheath cells lie on the inside of the sheath; (c) desmosomes join adjacent laminae; (d) sometimes an adjacent extracellular matrix may occur between neighboring glial cell processes (Heuser & Doggenweiler 1966). The participation of neuroglia in the blood-brain barrier is an important function of glial cells in both vertebrates and invertebrates (Abbott et al. 1986; Abbott 1995). Unlike the situation in higher vertebrates, in which the blood-brain barrier is formed by tight junctions of endothelial cells lining capillaries of the central nervous system (see Peters et al. 1991), the barrier in crustaceans is formed by glial perineurium surrounding the ganglia. The sealing function of the barrier is thought to be due to the junctions between the perineurial cells and the extracellular matrix, which may be expanded and contain collagen-like fibrils (Abbott 1972; see Pentreath 1987). The most widely accepted function of glial cells is that they interact metabolically with neurons, providing nutrients, exchanging metabolites, and removing catabolites. Such a role is supported by glial stores of glycogen This content downloaded from 157.55.39.231 on Wed, 05 Oct 2016 04:16:00 UTC All use subject to http://about.jstor.org/terms Allodi, Silva, & Taffarel and large numbers of vesicular and granular inclusions in the cytoplasm (Pentreath 1987; Tsacopoulos & Magistretti 1996). Our goal was to characterize the organization of glial cells and their processes by light microscopy (LM) and transmission electron microscopy (TEM) and to correlate those findings with the distribution of extracellular matrix in the fasciculated zone (FZ) and in the protocerebral tract (PT) of the crab Ucides cordatus. The FZ comprises retinular cell axons that run from the retina to the lamina ganglionaris (Bell & Lightner 1988) and the PT is the tract that links the terminal medulla and the hemiellipsoid body to the anterior medial protocerebral neuropil and other areas of the brain (Sandeman et al. 1992). These neuropils were chosen because they are primarily composed of axons and glial cells.