Transformations of the retinal topography along the visual pathway of the chicken

It is still unclear how the retinotectal map of the chick is formed during development. In particular, it is not yet known whether or not the organization of fibres plays a role in the formation of this map. In order to contribute to the solution of this problem, we analysed the representation of the retinal topography at closely spaced intervals along the fibre pathway. We injected HRP into various sites of the tectal surface and traced the labelled fibre bundles back to the retina. The retinal topography was reconstructed at ten different levels, i.e. in the retina, the optic nerve head, the middle of the optic nerve, the chiasm (three levels), the optic tract (three levels), and the optic tectum. We obtained the following results: (1) The labelled fibre bundles as well as the fields of labelled retinal ganglion cells were always well delimited and coherent. (2) The reconstructions show that transformations of the retinal topography occur in the fibre pathway. The first and most important transformation is found in the optic nerve head where the retinal image is mirrored across an axis extending from dorsotemporal to ventronasal retina. In addition, the retinal representation is split in its temporal periphery. Thus, central and centrotemporal fibres are no longer in the centre of the image but close to the dorsal border of the nerve. Peripheral fibres are found along the medial, ventral and lateral circumference of the nerve. In the optic tract a second transformation occurs. The retinal topography is rotated clockwise by about 90 degrees and flattened to a band. The flattening is accompanied by a segregation of fibre bundles so that eventually central and centrotemporal retinal fibres are located centrally, ventral fibres dorsally and dorsal retinal fibres ventrally in the tract. By these two transformations an organization of fibres is produced in the optic tract which can be projected onto the tectal surface without major changes given that dorsal and ventral fibres remain in their relative positions, and that deep lying fibres project to the rostral and central tectum, superficial fibres to the caudal tectum. The transformations which we have observed follow specific rules and thus maintain order in the pathway although retinotopy is lost. In conjunction with our earlier studies on the development of the retinotectal system we conclude that fibres are laid down in a chronotopic order. The transformations take place under particular structural constraints.(ABSTRACT TRUNCATED AT 400 WORDS)