Quantitative and morphological study of cell proliferation during morphogenesis in the trout visual system

Cell proliferation in the retina and tectum was investigated in a teleost (the trout Salmo gairdneri Rich.) at embryonic and larval stages in order to determine the pattern of growth in retinotectal organization. The tritiated thymidine was injected into the coelom: 1 microCi at stage 27 and 30, 2 or 5 microCi at stage 34. The morphological and quantitative analysis of the growth in the retina and tectum indicate the existence of two modes of development. In the retina, the thymidine labelled cells are located in a peripheral area with an important dorso-ventral asymmetry near the optic disc. This asymmetry decreases at hatching and disappears at stage 34. In the tectum, the proliferative zone is periventricular and lies along the medial, lateral and caudal rim. During ontogenesis, the lateral and caudal proliferative zones are moving back. The quantitative analysis of the retina, tectum and of their proliferative zones show that the volumes are highly correlated for all the specimens over the three stages. The volume variation is subordinate to the growth, low between stages 27 and 30, high between stages 30 and 34. Nevertheless the tectal proliferative zone exhibits a typical change: it decreases at hatching then increases at stage 34. At this stage the volume of the proliferative zones of the retina and tectum are discordant. The peculiar quantitative development of tectal proliferative zone predict the shifting of the retino-tectal projections and may be correlated with the timing of tectal developmental events.