Dendrite development and target innervation of displaced retinal ganglion cells of the chick (Gallus gallus)

Abstract

The avian accessory optic system (AOS) processes visual signals of translational and rotational flowfields resulting from self-motion. It has been investigated extensively with physiological methods and, because of its anatomical distinction from other retinofugal projections, is well suited for the investigation of dendritic differentiation and axonal pathfinding. Displaced retinal ganglion cells (dRGC) constitute the retinal origin of the AOS. Since little is known about the time course of the development of this projection, we studied the dendritic differentiation of dRGC, their innervation of the nucleus of the basal optic root (nBOR) and the histological development of this target area. dRGC, visualized by retrograde 1,1′-dioctadecyl-3,3,3′,3′-tetramethyl-indocarbocyanine perchlorate labeling, migrated into the inner nuclear layer of the retina and subsequently developed their characteristic dendritic morphology between E9 and E14. At this stage, dendrites were unistratified in the inner plexiform layer and displayed characteristic branches with 45–90° angles. The frequency of dendritic branches increased from an average of 44 branches per cell at E9 to an average of 155 at E15. This phase was followed by a period of dendritic pruning, E15–E17, where a large number of small branches were eliminated. At the time of hatching, dRGC were morphologically mature with mean dendritic field sizes of 0.28 mm2 and an average of 108 dendritic branches per cell. Retinal innervation of the nBOR occurred between E8 and E11, and tracer injections at later stages revealed no further changes. In addition to the predominant contralateral projection, we have also described a connection to the ipsilateral nBOR. This ipsilateral pathway persisted at least to juvenile stages (P14). The histological development of the nBOR proceeded such that calretinin-immunoreactive neurons were observed from E10 onwards and morphologically described cell types evolved after E12.