Figure 4. Tissue plasminogen activator (tPA) activity of the visual cortex during postnatal development. Histograms show tPA activity measured as a function of the conversion of plasminogen to plasmin by tPA. Each bar represents the mean±SD (n=9). tPA activity is highest at PW3, and then declines at later developmental stages with the lowest value at PW14. Values at PW14 were significantly lower compared to all other ages (p<0.01). Activity at PW3 was not significantly different from that at PW5 but was higher than that at PW7 (p<0.01). Values were compared using a least significant difference (LSD) multiple comparison t-test.