The relationship between retinal and tectal growth in larval Xenopus: implications for the development of the retino-tectal projection.

The modes of retinal and tectal histogenesis, as well as the patterns of terminal distribution of optic axons in larval Xenopus were studied, using anatomical techniques. We show that the retina grows by adding strips of cells at its ciliary margin. The pattern of retinal growth is asymmetrical along the dorso-ventral axis of the retina. On the other hand, the tectum grows by adding newly formed cells caudo-medially. The most rapid tectal growth takes place between stages 50 and 53, and thereafter only a small proportion of cells are added near the midline of the caudal tectum. Despite such incongruent modes of retinal and tectal growth, retinotopic order is maintained throughout larval life. We present here further evidence supporting the idea that connexions between the arrays of retinal and tectal cells shift progressively caudo-medially on the tectum during the period of growth. When the temporal pole of the retina is destroyed at various developmental stages, the density of degenerating synapses is always highest in the rostral pole of the tectum. Moreover, optic terminals stemming from the central retina spread caudally, invading newly maturing regions of the tectum. Quantitative analysis of the terminal distributions of optic axons suggest that orderly shifts in synaptic contacts between optic axons and tectal dendrites take place in the course of development. Ultrastructural observations on the maturing tectal neuropil are consistent with this view.