Chemoaffinity hypothesis

Abstract: 1 Introduction.- 2 Material and Methods.- 2.1 Experimental Animal.- 2.2 Morphology.- 2.2.1 Light Microscopy.- 2.2.2 Electron Microscopy: Fixation and Embedding.- 2.2.3 Localization and Orientation.- 2.2.4 Measuring Procedures.- 2.2.5 Optimization of Parameter Values.- 2.3 Physiology.- 3 Topogenesis and Morphogenesis.- 3.1 Ganglion Cell Perikarya.- 3.2 Ganglion Cell Dendrites.- 3.3 Ganglion Cell Axons.- 3.4 Synaptogenesis in the Optic Tectum.- 4 Physiogenesis.- 4.1 Development of Fibre Activity.- 4.1.1 Evoked Potentials.- 4.1.2 Fibre Diameter and Conduction Velocity.- 4.2 Physiology of Synaptogenesis.- 4.3 Structural and Functional Development.- 5 The Formation of the Retinotopic Map.- 5.1 Retinotopy and Order.- 5.1.1 The Existence of the Retinotopic Map.- 5.1.3 Definition of Retinotopy and Order.- 5.2 The Chemoaffinity Hypothesis.- 5.3 The Organization of Fibres in the Visual Pathway.- 5.3.1 Minimal Conditions of Order.- 5.3.2 Retinotopy in the Optic Fibre Layer of the Retina.- 5.3.3 The Transformation of the Fibre Organization at the Entrance of the Optic Nerve.- 5.3.4 The Fibre Organization in the Middle of the Optic Nerve.- 5.3.5 The Course of Fibres in the Optic Nerve, Chiasm and Tract.- 5.4 The Origin of the Retinotopic Map.- 5.5 An Alternative View.- 6 The Final Maturation of Retinal Ganglion Cells.- 6.1 Axonal Branching and Formation of Dendrites.- 6.2 Increase of Fibre Diameter as a Function of Axonal Arborization.- 6.3 Competition and Degeneration.- 6.3.1 Degeneration of Ganglion Cell Perikarya.- 6.3.2 Degeneration of Ganglion Cell Axons.- 6.3.3 Interpretation With a Mathematical Model.- 6.3.4 Competition, Synaptogenesis and Selective Stabilization.- 7 Summary.- References.

Abstract: 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.

Abstract: Roger Sperry's chemoaffinity hypothesis, elegantly summarized in the pre­ ceding quotation (Sperry 1965), is one of the central ideas of contemporary neurobiology. When Sperry first proposed the chemoaffinity hypothesis, the experimental evidence for it consisted entirely of behavioral and anatomical studies. The proposed identification tags were necessary to explain the way axons grow during the regeneration of altered nervous pathways; the hy­ pot hesis gained widespread support because it was extremely difficult to imagine other reasonable explanations for the anatomical and behavioral results. Experimental support for the chemoaffinity hypothesis could theoreti­ cally be of two distinct kinds. In the first, the nervous system is altered and the assembly of nerve circuits is observed and compared to that in normal animals. If chemical identification tags offer a parsimonious explanation for the observed patterns of growth and connections, then the theory gains support. More direct support must come from experiments of a second kind, which study actual molecular content, in order to test the proposition