Fig. 1. Induction of neurite outgrowth by retinoic acid. Comparison of the effect of retinoic acid in neurite outgrowth in cultured embryonic day 13.5 mouse spinal cord (A-C) and 10-month-old adult mouse spinal cord (D-F). Pieces of spinal cord were cultured in cellagen in the presence of 10% delipidated serum with or without RA for a period of 5 days. The medium was changed every 2 days. In A and D there is no RA. The embryonic cord extends neurites in the absence of RA, whereas the adult cord does not. In B and E, 1x10^-6M RA was added. Many more neurites are extended from the embryonic cord after RA addition, whereas the adult cord is still unresponsive. C and F show RT-PCR analysis of cultured cords to analyze the expression of RARß2 after after 5 days. The presence of GAPDH was used to indicate equal amounts of cDNA in the samples. (C) Embryonic day 13.5 cord. Lane 1, no RA; lane 2, 1x10^-8 M RA; lane 3, 1x10^-7 M RA, lane 4, 1x10^-6 M RA. RARß2 is upregulated by RA. (F) Adult cord. Lane 1, no RA; lane 2, 1x10^-8 M RA; lane 3, 1x10^-7 M RA, lane 4, 1x10^-6 M RA. RARß2 fails to be upregulated at any concentration of RA. Arrows on the right of F show the position of GAPDH and RARß2. (G) A western blot of proteins from embryonic mouse spinal cord (lane 1) and adult mouse spinal cord (lane 2) incubated with a RARß2 antibody. This part of the blot, at a molecular weight of approximately 45 kDa, confirms the presence of RARß2 protein in embryonic but not adult cord.