In vitro growth properties of Xenopus retinal neurons undergo developmental modulation

To determine whether Xenopus retinal neurons undergo intrinsic developmental changes in growth properties, retinal explants from embryos and tadpoles of different stages were grown on laminin, fibronectin, and collagen I in serum-free media. Growth was assayed in terms of a neurite growth index (NGI) and the appearance of clockwise bundles, or a clockwise growth index (CGI). The first neurites from stage 25 optic vesicles are pioneers and display a unique growth phenotype; they emerge rapidly, survive for a short time, show little substrate preferences for growth (they grow almost as well on BSA as they do on laminin and fibronectin), and form no clockwise bundles under any conditions. Neurites from progressively older retinas (stages 32-37) share with stage 25 neurites the rapid outgrowth pattern, but begin to show substrate preferences and clockwise growth. From stage 40 to 50, the mature growth pattern is expressed; a lag in initial outgrowth, long-term survival, distinct substrate preferences (they grow 10 times better on laminin and fibronectin than on BSA) and display robust clockwise growth patterns on laminin and fibronectin. The acquisition of clockwise growth is independent of optic fiber contact with the tectum or exposure to diffusible factors from mature brain tissues. The results suggest that retinal neurons undergo developmental modulation of surface adhesive properties and/or cytoskeletal organization.