THE JOURNAL OF COMPARATIVE NEUROLOGY zyx 307:335-349 (1991) The Optic Tectum of the Dogfish zy Scyliorhinus canicula L.: A Golgi Study M. JESUS MANSO AND RAMON ANADON Colegio Universitario, Universidad de la Corufia, 15071-Spain (M.J.M.) and Departamento de Biologia Fundamental, Facultad de Biologia, Universidad de Santiago de Compostela, 15706-Spain (R.A.) ABSTRACT The optic tectum of the dogfish zyxwvuts Scyliorhinus canicula L. was studied by using the methods of Nissl, reduced silver nitrate, Golgi-aldehyde, and Golgi-Cox. Six layers and eight types of neurons were recognized. These cell types are not restricted to one layer; in fact some are found in all six tectal layers. The types of cells found are A) monopolar, B) triangular, C) radial bipolar, D) horizontal fusiform, E) large tectal, F) small tectal, G) pyriform, and H) stellate cells. In at least six of the cell types a series of dendritic specializations can be observed, such as spines in the form of "drumsticks" and thin varicose appendages, similar to those reported previously in the optic tecta of amphibians and teleosts. The optic tectum of the dogfish shows a degree of complexity comparable to that of amphibians and teleosts. Key words: neuron types, Golgi methods, elasmobranchs The optic tectum of the dogfish is not conspicuously laminated as are the tecta of most other nonmammalian vertebrates. Up to 14 layers have been described in teleosts, amphibians, reptiles, and birds (Ramon y Cajal, '11; Ariens Kappers et al., '36; Szekely and Lazar, '76; Schroeder et al., zyxwvu 'SO), whereas only six layers have been identified in elasmo- branchs (Smeets et al., '83). The studies of the optic tectum in various species of elasmobranchs indicate the absence of clearly defined cellular strata (Schroeder and Ebbesson, '75; Witkovski et al., '80). Smeets et al. ('83) considered that the tecta of the elasmobranchs species also differ in the degree of internal lamination as well as in the size of the ventricular cavity. The afferent fibers of the tectum in the elasmobranchs are similar to those of other vertebrates (see Ebbesson, '84). The most characteristic afferents of the tectum of the elasmobranchs are the retinal projections but, in addition, the optic tectum receives afferents from other regions of the brain and spinal cord. The efferent tracts of the tectum originate principally in the stratum medullare internum and are strikingly similar to those of other vertebrates except that they are more diffuse and, in the thalamus, overlap with retinal and spinal inputs (see Ebbesson, zyxwvut '84). Although the cytoarchitecture of the optic tectum has been the object of numerous studies in all classes of vertebrate, that of the elasmobranchs has received little attention. The first description, Houser's zyxwvut ('01) study of Mustelus, mentioned only small, medium, and large tectal cells. Later studies of several species by Golgi methods demonstrated a variety of cell types and suggested that substantial differences exist between the general organiza- tion of the elasmobranch tectum and that of other verte- brates (Leghissa, '62; Schroeder and Ebbesson, '75; Witk- ovsky et al., '801, while Ebbesson andMeyer ('80) considered that the cytoarchitectural variations among the different species of elasmobranch are very limited. The present study reports the types and distribution of neurons in the optic tectum of the dogfish. New cell types have been found, as well as new locations of types that were already known. In addition, six types of neurons were found to have specialized dendritic appendages similar to those described in the frog optic tectum. The significance of these results is discussed in relation to the widely accepted idea that the tectum of the elasmobranchs is poorly differenti- ated and has nonspecialized neuron types. MATERIALS AND METHODS Atotal of 38 adult dogfish, Scyliorhinus canicula L., were used in this study. The animals were anaesthetized in 0.01% MS-222 (Sigma Chemical Co., St. Louis, MO) in sea water. We used "in-block'' silver impregnation methods, Nissl stains, and Golgi-aldehyde and Golgi-Cox techniques. Silver impregnation methods After the fish had been anaesthetized, the brains were carefully removed and placed in the appropriate fixative for subsequent staining. Accepted January 9,1991 zyxwv o 1991 WILEY-LISS, INC.