Larval development of tectal efferents and afferents in Xenopus laevis (Amphibia Anura).

The development of tectal connections in Xenopus laevis had been investigated using the degeneration technique to demonstrate the efferent pathways and the retrograde HRP transport to label the afferent pathways. Bilateral tectal efferents were present as soon as the beginning of metamorphosis. Ascending efferents originated from the anterior tectal part terminate in the secondary visual thalamic centres whereas the descending efferents coming from the posterior tectal part reached the tegmentum and the medulla oblongata. At this same time, the optic tectum already received secondary visual afferents originating in the ipsilateral pretectum and non-visual afferents from the ipsilateral semicircular torus and tegmentum. Some sparse bilateral isthmotectal connections were also present. Later, efferent pathways showed an increasing number of fibres whereas the sites of origin of afferents became more diversified: the dorsal thalamus, the suprachiasmatic area, the tegmental nuclei and in the medulla oblongata, the reticular and octavolateral areas sent bilateral projections to the optic tectum. At the end of metamorphosis, we noted ipsilateral olivotectal fibres and reciprocal connections between the tectum and the area of the Vth nerve. These last findings and the presence of the following direct projections, not previously reported in Anurans: the reciprocal connections between the tectum and the semicircular torus or the octavolateral area, underline the implication of the optic tectum in the multisensory (visual, acoustic, vibratory) integration elicited during the larval behavior. Also, the relations between the optic tectum and the lateral line system are particularly examined in the discussion.