When Emperor Akihito delivered a speech this past August hinting that he would like the Japanese parliament to change the law to allow him to retire, the New York Times noted that changing the law might re-open the sensitive topic of allowing a woman to be in line for the Chrysanthemum Throne. I've been proof-reading the Kyoto chapter of "Gateway to Japan" and came across of bit of history regarding the circumstances which led to an empress ascending to the throne in the 17th century:
“Iemitsu's sister [the Shogun's sister] had become Go-Mizuno'o's empress, reviving the classical tradition of powerful families marrying off their daughters to emperors. When a son was born, the Tokugawa pressured Go-Mizuno'o to abdicate in favor of the child. He resisted, however, and then the child died. In 1627, the emperor had another fight with the Tokugawa, over the assignment of "purple robes" to the abbots of Daitokuji and Myōshin-ji. Humiliated, Go-Mizuno'o abdicated in protest in 1629 and, breaking tradition, left his throne to a daughter, who became the first reigning empress in more than 850 years...” (Excerpt From: June Kinoshita. “Gateway to Japan, Digital Edition.”)
Freed from the restrictions the Tokugawa placed on reigning emperors, Go-MIzuno'o made the most of his retirement, designing masterpieces of Japanese architecture and gardens.