DENDRITIC REORIENTATION AND CYTOLAMINATION DURING THE DEVELOPMENT OF THE ISTHMO-OPTIC NUCLEUS IN CHICK-EMBRYOS

In the mature isthmo-optic nucleus (ION, source of efferents to the co ntralateral retina), the neuronal perikarya are generally described as being arranged in a single convoluted lamina surrounding a U-shaped r egion of neuropil, into which their highly polarized (unidirectional) dendritic arbors project perpendicularly. We find, however, that the d etails are more complicated than this description suggests, and are va riable, as might be expected if the ION is self-organized through neur on-to-neuron interactions in development. The laminated conformation o f the ION first appears at embryonic day (E) 14. Our previous experime nts indicate that this involves the displacement of perikarya and is n ot due to sculpting by neuronal death. We here present a quantitative demonstration that the dendritic arbors reorient during the period of lamination. At E11, they are already highly polarized, but their direc tions are different from those in the adult, being mostly medio-rostro -ventral. Then, between E11 and E13, the arbors in the border region o f the ION undergo major changes in their direction of polarization, pr ojecting towards the center of the ION. The arbors within the core of the ION make more subtle changes. The dendritic reorganization seems t o be intrinsically linked to the process of cytolamination, since the two events occur synchronously and disruption of either affects the ot her. Mechanisms are discussed; interaction with afferents is not respo nsible for lamination. (C) 1996 Wiley-Liss, Inc.