apanese Emperor Akihito is undergoing heart surgery today in Tokyo, casting light on rules that limit the line of succession to the world's oldest hereditary monarchy.

Akihito, 78, is having coronary bypass surgery at the University of Tokyo Hospital to treat a narrowed artery, the Imperial Household Agency said last week. The emperor, who had prostate surgery in 2003 and was hospitalized with pneumonia in November, was on medication after experiencing arterial problems last year.

Concerns over his health have prompted the government to consider altering the 1947 law for the world's oldest hereditary monarchy that mandates only men succeed to the throne and requires princesses to give up their titles if they marry commoners. Akihito's grandson Hisahito in 2006 became the first male born into the family in more than four decades, increasing the number of potential heirs to three.

"By the time he assumes the throne, he will be the imperial family," Colin Jones, a law professor at Doshisha University in Kyoto, said of the 5-year-old prince. "You're looking at a future where the imperial family consists of a single nuclear family. That's problematic in that, if he doesn't have a son, then what do you do?"