Straight from the horses mouth. Ontario budget 2005 Page 159

https://www.poltext.org/sites/poltext.org/files/discours/ON/ON_2005_B_38_01.pdf

"PROFESSIONAL CORPORATIONS
In 2001, the right to incorporate was extended to all regulated professionals. Under existing provisions, non-members of a profession cannot own shares in a professional corporation. Recent negotiations with the Ontario Medical Association have resulted in the government’s commitment to extend the share structure of physician professional corporations to include non-voting shares for family members. The government is also proposing to implement this change for dentists who operate their practices through a professional corporation."

Thanks to the poster who alerted me to this.