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 contralateral retina), the neuronal perikarya are generally described as being arranged in a single convoluted lamina surrounding a U-shaped region of neuropil, into which their highly polarized (unidirectional) dendritic arbors project perpendicularly. We find, however, that the details are more complicated than this description suggests, and are variable, as might be expected if the ION is self-organized through neuron-to-neuron interactions in development. The laminated conformation of the ION first appears at embryonic day (E) 14. Our previous experiments indicate that this involves the displacement of perikarya and is not 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 directions are different from those in the adult, being mostly medio-rostro-ventral. Then, between E11 and E13, the arbors in the border region of the ION undergo major changes in their direction of polarization, projecting towards the center of the ION. The arbors within the core of the ION make more subtle changes. The dendritic reorganization seems to be intrinsically linked to the process of cytolamination, since the two events occur synchronously and disruption of either affects the other. Mechanisms are discussed; interaction with afferents is not responsible for lamination.