A Golgi-electron microscopic study of goldfish optic tectum. I. Description of afferents, cell types, and synapses.

A study of goldfish optic tectum was performed with conventional electron microscopy and with the Golgi-EM technique described by Fairén et al. ('77). Five types of tectal afferents, three types of interneurons and three types of efferent neurons were investigated. Afferents from the torus longitudinalis, which terminate in the marginal layer, contain round synaptic vesicles with a mean diameter of 43 nm. Optic afferents, which terminate in the superficial gray and plexiform layer, are characterized by pale mitochondria with dilated cristae and round vesicles with a mean diameter of 49 nm. Afferents of unknown origin, terminating in several tectal layers, can be subdivided in three types; one containing round vesicles and two containing pleomorphic vesicles with different degrees of ellipticity. The three types of interneurons studied (type I, III and XIV, of Meek and Schellart, '78) were selected on basis of their high frequency of occurrence. The apical dendrites of type I neurons make many synaptic contacts with the marginal axons. All three types have dendrites in the superficial gray and plexiform layer making contacts with optic nerve terminals. In addition, their dendrites and cell bodies make synaptic contacts with several types of unidentified presynaptic elements. The axon terminals of type I and of type XIV contain round vesicles with a mean diameter of 45 and 46 nm respectively. Three of the four types of efferent neurons present in the goldfish tectum were studied (type VI, XII and XIII). Two of them make contact with optic terminals (type VI and XII) and two make contact with tectal afferents of unknown origin in the central white layer or in the lower part of the central gray layer (type XII and XIII). The axons of all three types become myelinated at some distance from their origin. Their initial unmyelinated parts are covered with a so-called "outer surface coating", have no collaterals and are occasionally (type VI and XII) or frequently (type XIII) postsynaptic to other elements. The archiform axons of type XIII and to a lesser extent also the sherpherds-crook shaped axons of type XII, have a close apposition to looping and narrowing dendrites in the inner plexiform layer. The present results concerning neuronal circuitry of the goldfish optic tectum are summarized in a tentative scheme.