The development of retina and the optic tectum of petromyzon marinus, L. A light microscopic study.

Morphological evolution of the retina and optic tectum along the stage of ammocoete, transformation and young adult of sea lampreys (Petromyzon marinus L.) was studied using light microscopic techniques and quantitative morphometry. A retinal differentiated zone surrounding the optic nerve head with a kind of differentiated photoreceptors is present through all the stages studied until stage VI of transformation and its extension is almost unchanged since 60 mm. larve. From this length larval retina grows by extension of the lateral undifferentiated retina, that in large larvae subdivides in a lateral germinal zone and an intermediate differentiating zone more thickened were ganglion cells and the optic fibre layer differentiate early. In the largest larvae outer and inner neuroblastic layers were also recognized in these intermediate zone except in the most lateral retina. Mitotic activity was observed both in germinal and intermediate differentiating zones near the optic ventricle. The germinal zone disappears after the formation of an irideal retina in transforming stages and, with the exception of the photoreceptor layer, retinal layers were differentiated since stage III along the neural retina. The photoreceptor layer develops in the early stage VI along the retina. Adult pattern of retinal pigmentation is found in these stage. A periventricular and a lateral region were recognizable in the optic tectum of the larval period. Tectum of large larvae shows an outline of laminar organization. In the stage III of transformation the tectal lamination is the same of the young adults: the periventricular cell layer is subdivided by fibre bands and in the lateral region a stratum cellulare centralis and a stratum cellulare et fibrosum externum were distinguishable. A comparison between retinal and tectal growths was made. Most retinal and tectal growth and differentiation occurs before adult photoreceptors develop.