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In order to test the preference of growing axons for membrane-associated positional specificity a new in vitro assay was developed. In this assay, membrane fragments of two different sources are arranged as a carpet of very narrow alternating strips. Axons growing on such striped carpets are simultaneously confronted with the two substrates at the stripe borders. If there is a preference of axons for one or the other substrate they become oriented by the stripes and grow within the lanes of the preferred substrate. Such preferential growth could, in principle, be due to affinity to attractive factors on the preferred stripes or avoidance of repulsive factors on the alternate stripes. This assay system was used to investigate growth of chick retinal axons on tectal membranes. Tissue strips cut from various areas of the retina were explanted and the extending axons were confronted with stripes of cell membranes from various areas within the optic tectum. Tectal cell membranes prove to be an excellent substrate for the growth of retinal axons. Nasal and temporal axons can grow well on membranes of both posterior and anterior tectal cells. If, however, temporal axons are given a choice and encounter the border between anterior and posterior membranes they show a marked preference for growth on membranes of the anterior tectum, their natural target area. Nasal axons do not show a preference in this assay system. The transition from nasal to temporal properties within the retina is abrupt. In contrast, the transition from anterior to posterior properties of the tectal cell membranes occurs as a smooth gradient. Significantly, the positional differences of tectal membrane properties are only seen during the period of development of the retinotectal projection and are independent of tectal innervation by retinal axons. These anterior-posterior differences disappear by embryonic day 14.
Walter et al. (Tue,) studied this question.