Development of the visual pathway in Xenopus laevis (Daudin)

Abstract

Aspects of the development and distribution of the primary retinal afferent projection to the contralateral optic tectum were studied by light microscopy, electron microscopy, and microelectrode recording from adult and tadpole optic nerves and optic tecta of the toad Xenopus laevis (Daudin). The adult tectum was found to be innervated by four populations of optic nerve afferents with different conduction velocities and retinal receptive field properties. Current source density analysis of the postsynaptic potentials recorded extra cellularly from tectal cell populations revealed the spatial distribution of optic nerve afferent synapses. The dendrites of cells that constitute tectal cell layer 8 were found to receive the majority of retinal afferent fibre input. The earliest time at which ganglion cell terminal activity was recorded in the tectum was tadpole Stage 46, at which time the optic nerve was entirely composed of unmyelinated fibres. The excitatory receptive fields of these units were large, and these units were easily habituated. Stage 46 was also the earliest time at which postsynaptic field potentials ('u' waves) could be recorded in the tectum and synapses observed morphologically. Evoked potentials ('m' waves), characteristic of a myelinated fibre innervation of the tectum, were first seen at tadpole Stage 59, as was the appearance and development of optic nerve myelination. No obvious correlations, other than the division of optic nerve axons into my elimated and unmyelinated fibre populations, could be made between optic nerve fibre conduction velocity groups and fibre diameter histograms. However, an obvious correlation between optic nerve conduction velocity groups and postsynaptic field potentials recorded in the tectum was observed. In addition, - a clear correlation was discovered between the distributions of the current sinks of postsynaptic activity and the location of recording sites of ganglion cell axon terminals having known receptive field properties, A gradual emergence of the adult pattern of tectal innervation by optic nerve afferents, during the time at which the tadpole undergoes metamorphosis, was observed.