* Ex-dividend effect shaves off about 90 points from Nikkei - traders * Aeon rises on tender offer to become top shareholder of Daiei * Japan market is in "structural bull market" - JPMorgan By Ayai Tomisawa TOKYO, March 27 (Reuters) - Japan's Nikkei average was flat on Wednesday morning in directionless trade after many shares went ex-dividend, offsetting the boost to sentiment from Wall Street's data-led gains. Traders said the effect of shares going ex-dividend - Tuesday was the deadline for investors to buy shares and be eligible to receive dividends for the current financial year - took about 90 points off the Nikkei and about 9 points off the Topix. The Nikkei was flat at 12,476.65 by the midday break after venturing into both positive and negative territory. The index is 1.4 percent below a 4-1/2 year high of 12,650.26 struck last Thursday. Analysts said that investor sentiment remained positive on expectations of expansionary policy measures emerging from the Bank Of Japan's policy meeting on April-3-4 under its new leadership, but that trading may stay choppy on Wednesday. "If you take ex-dividend price declines into account, overall gains will likely be limited at the end of the day," said Toshihiko Matsuno, a senior strategist at SMBC Friend Securities. He added that the market remained nervous about the euro zone's debt problems because of the negative implications for future bailouts in the current rescue plan for Cyprus. "In terms of global market news, there is a mix of both positives and negatives, so most investors may stay on the sidelines for now," Matsuno said. The Dow Jones industrial average on Tuesday climbed to another record close and the S&P 500 came within striking distance of its all-time closing high, as strong data on home prices and manufacturing fed optimism about the economy. The broader Topix was also flat at 1,044.31. On Wednesday, Aeon Co rose 4.7 percent after sources told Reuters that the company would raise its stake in Daiei Inc to more than 40 percent through a tender offer to become the supermarket operator's top shareholder after acquiring 24 percent from Marubeni Corp. Daiei dropped 11 percent. Real estate companies gained on hopes for the government's reflationary policy, with Mitsui Fudosan rising 3.4 percent and Mitsubishi Estate adding 2.4 percent. Exporters were mixed, with Toyota Motor Corp adding 0.2 percent, Sony Corp rising 0.7 percent while Nikon Corp dropping 0.9 percent. "If the dollar trades above 95 yen, we may even see the index climb back to near a recent high quickly on exporters' gains," said Naoki Fujiwara, a fund manager at Shinkin Asset Management. The dollar last traded at 94.74 yen. COMPANIES' FORECASTS IN SIGHT The benchmark Nikkei has rallied about 44 percent since mid-November, when Prime Minister Shinzo Abe started calling for bold monetary easing and expansionary fiscal policy to revive the economy. Analysts said that gains seem to have paused for the time being before the end of the fiscal year, but long-term sentiment on the Japanese market remains bullish. "We think Japanese equities are poised to stay in a structural bull market. While a 'pause for breath' is long overdue, the fundamental case remains strong, with rising visibility of earnings set to pull the market higher over the coming nine to 12 months, in our view," Jesper Koll, head of Japanese Equity Research at JPMorgan wrote in a report. "Micro factors rather than macro ones look set to become the key catalyst for the next up-leg in the market."