Neuroscience Letters, 106 (1989) 1~5 I Elsevier Scientific Publishers Ireland Ltd. NSL 06417 Ganglion cells and retinopetal fibers of the larval lamprey retina: an HRP ultrastructural study Encarnacirn De Miguel, Maria Celina Rodicio and Ram6n Anad6n University of Santiago de Compostela, Faculty of Biology, Santiago de Compostela (Spain) (Received 22 March 1989; Revised version received 26 June 1989; Accepted 10 July 1989) Key words: Ganglion cell; Larval retina; Centrifugal fiber; Lamprey; Ultrastructure The ultrastructure of ganglion cells and centrifugal fibers of the larval lamprey retinas were studied using horseradish peroxidase (HRP) as a marker. Larval ganglion cells were found both in the inner nucle- ar layer and the inner plexiform layer of the differentiated retina, and also were present in the undifferen- tiated retina. Direct photoreceptor ganglion cell contacts and the presence of centrifugal fibers are de- scribed for the first time in the lamprey. The centrifugal fibers contact directly with ganglion cells in this species. Lampreys are living species of the most primitive vertebrates, the Agnatha. As in teleosts [21], differentiation of the first retinal photoreceptors in the lamprey begins by the time of hatching [25]. Unlike the gnathostomous vertebrates, further differen- tiation soon stops in larvae (ammocoetes), although addition of new cells around the early differentiated retina produces a significant retinal growth in pre-metamorphic larvae. As a result, two retinal zones become distinguishable in the late larval period, one differentiated and the other undifferentiated [4, 6, 13, 25]. Two of us have recently suggested that in the undifferentiated retina the first gang- lion cells appear during the larval period and not at metamorphosis as was previously thought [4]. The precise location of ganglion cells in the retina of larval or adult lam- preys is still uncertain; different authors have located these cells either in the inner nuclear layer [10, 14] or in the inner synaptic layer [8, 22]. In this study we report the first light and electron microscopic experimental evidence concerning the pres- ence and location of ganglion cells in both zones of the larval retina, and describe direct connections between these cells and photoreceptors in the differentiated retina. In addition, the presence of centrifugal fibers and contacts of these fibers with gang- lion cells have been demonstrated for the first time in the retina of lampreys. Experiments were carried out in larvae (ammocoetes) of the sea lamprey Petromy- zon marinus L. ranging from 65 to 120 mm in length. The larvae were anaesthetized Correspondence: R. Anadrn, Departamento de Biologia Fundamental, Facultad de Biologia, Universidad de Santiago de Compostela, Santiago de Compostela, 15706 Spain. 0304-3940/89/$ 03.50 © 1989 Elsevier Scientific Publishers Ireland Ltd.