In Shoho 3 (1646) soon after Koshin Sosa was employed by the Kii Tokugawa family, Sotan retired and made Koshin his successor at Fushin'an. This was the start of the present Omotesenke and the name Sosa was inherited by all of the successive Iemotos from that time.
After he had retired, Sotan built a retreat on the land behind the Omotesenke residence and lived there with his 4th son, Senso Soshitsu. Later the residence was passed down to Soshitsu who started Urasenke on that land. Of Koshin's two elder brothers, the eldest, Kano Sosetsu left home early on. The second eldest, Ichio Soshu, trained first as a lacquer craftsman, but in his later years took the name Sen again and started Mushakoujisenke. So three of Sotan's sons laid the foundations of the three Sen families (sansenke) that have continued until the present day.
Koshin also made a record of teachings since the time of Rikyu addressed to his son adopted from the Hisada family, the 5th generation Zuiryusai (1650-91). In this way, thanks to the efforts of Koshin, Rikyu's style of chanoyu has been transmitted to us through the successive Iemotos.