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Zusammenfassung:
The mechanisms underlying axonal navigation during embryogenesis are still not understood. Environmental and intrinsic properties of the neurons as well as mutual interactions between axons are thought to play a role in the formation of the neural network found in the adult animal. I studied the growth behavior of axons and the formation of the optic fiber pattern in early retinal morphogenesis. Whole mounts of embryonic chick, quail and pigeon retinae were examined with silver staining, Golgi impregnation, scanning and transmission electron microscopy. Area measurements show that the retina as well as the optic fiber layer increase in size exponentially, but with different rates. Consistently, in none of the species the retinal fiber layer expands symmetrically. It enlarges with polarities along the dorso-ventral and the naso-temporal axes. The very first axon-bearing ganglion cells appear at stage 16 in the dorsal and central retinal portion and grow ventrally to merge at the optic disk. As development proceeds, the retina and the optic fiber layer expand peripherally and ventrally so that the optic nerve head, starting from an initially ventral position, comes to lie close to the geometrical center of the retina. From stage 23 on, the optic fiber layer expands faster in the temporal than in the nasal side. Quantitative measurements on the initial polarisation of young axonal processes show that the axonal growth is highly directed towards the optic fissure and the optic nerve head. The growth polarisation is found at the very onset of growth cone formation and in axons far distant from the next ganglion cells and axons. This excludes fiber-fiber interaction to be essential for the initial fiber orientation.
