What is the fine-grained topic of the following text: In terms of migration, the three major urban centres (Tokyo, Osaka and Nagoya) saw net gains of more than 400,000 people between them each year during the 1950s and 1960s, leading to a great housing shortage. Only return the topic.
Migration trends and housing shortage in Tokyo, Osaka and Nagoya during the 1950s and 1960s.