Prince Akishino, the younger son of Emperor Akihito, turned 50 on Monday, expressing concern over the health of his mother, Empress Michiko, who has battled heart problems.

"I believe her stress is partly due to the illness of the Emperor," the prince said at a Nov. 19 news conference, referring to the Emperor's 2003 surgery for prostate cancer and coronary-artery bypass surgery in 2012.

The Emperor's recovery from the bypass surgery "was not so fast," the prince said, indicating that anxiety about the health of her husband has put a strain on the Empress.