Retinal and tectal connections of embryonic nucleus superficialis magnocellularis and its mature derivatives in the chick.

In a companion paper (Puelles et al, this issue), the cytoarchitectonic development of the thalamic primordium called nucleus superficialis magnocellularis (SM) and its adult configuration in the chick were studied, correcting the misinterpretations that have impeded proper study of this neuronal group. Given its superficial position in the diencephalon, in contact with the optic tract and neighbouring retinorecipient grisea (SS, GV), as well as with the tecto-recipient n. rotundus, SM was suspected to have connections with centers of the visual pathway. In this paper we report the existence of a non-topographic retinal projection over the superficial adult derivate of SM (n. interstitialis tractus opticus, ITO) and a non-topographic, diffuse projection of the whole SM-derived population (area perirotundica, ApR, and ITO) onto the optic tectum. The latter was demonstrated throughout the late embryonic period in which SM loses its embryonic unitary character and becomes dispersed into its ill-defined, definitive adult portions (ITO, ApR). Golgi-like HRP- or DiI-labeling of SM cells showed a protracted immature appearance of their dendrites, expressed coincidently with a capacity to translocate superficially into the optic tract.