Development of the visual system of the chick--a review.

This article reviews recent and earlier findings that yield the present knowledge about the embryonic development of retina, tectum, and the retinotectal projection in the chick. Data and concepts dealing with cell proliferation, migration, and differentiation, the processes underlying the generation of cytoarchitecture in the nervous system are discussed for the avian visual system. Emphasis is also laid on the presentation of hypotheses and experiments about directed axonal growth along the visual pathway and concerning the mechanisms responsible for the establishment of specific connections between retinal ganglion cells and their central targets. Among the results, the following topics deserve special attention: 1) Investigations of morphogenetic factors in vitro, and the application of recombinant retroviruses in vivo to study cell lineages rendered new insights into the processes of cell determination and differentiation. The evolving picture in this progressing field is discussed. At present, however, the research of retinal and tectal histogenesis is still largely in the state of morphological description. 2) Both systems, retina and optic tectum, develop independently from each other but in corresponding spatio-temporal patterns, which provide that ingrowing retinal axons encounter receptive target tissue at appropriate locations at the time when connections are due to be formed. 3) Possible mechanisms of directed fibre growth are being elucidated by increasing efforts in research devoted to cell surface molecules, neurotrophic, and inhibitory substances, and their receptors. The axons of the primary visual pathway seem to be guided by local cues on glial endfeet and perhaps in the extracellular matrix, but so far, instructive molecules to which functional significance can be assigned have eluded discovery. 4) The question, how the retinotopic projection upon the tectum is created during development, remains still unsolved, although most results point to modified forms of the chemoaffinity hypothesis for its explanation. Sequential maturation and growth, selective fasciculation of orderly entering axons, recognition of positional tectal markers, and functionally controlled refinement may together contribute to the correct retinotectal projection.