Tokyo stocks opened sharply lower Friday, tracking overnight falls on Wall Street, while a firmer yen against the U.S. dollar dented exporter issues. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 625.87 points, or 1.57 percent, from Thursday to 39,147.27. The broader Topix index was down 30.74 points, or 1.13 percent, at 2,701.26. On the top-tier Prime Market, precision instrument, electric appliance and transportation equipment issues were among the main decliners. At 9 a.m., the dollar fetched 151.28-31 yen compared with 151.30-40 yen in New York and 151.68-70 y…