Updated from 5:58 p.m. ESTMorgan Stanley's ( MS) well-known Co-President Zoe Cruz is retiring amid a raft of management changes at the investment firm. Cruz, who had risen within the firm to become one of the top executives at Morgan Stanley during her 25-year tenure, will retire effective Dec. 1, according to a release issued by the firm late Thursday. Celebrated as one of the country's top female executives, Cruz was considered in many eyes as a successor to CEO John Mack. The reasoning for Cruz's retirement was not immediately available, but it surprised many on Wall Street. A call to a Morgan Stanley spokeswoman was not immediately returned. "Zoe has always demonstrated a deep commitment to Morgan Stanley. She has helped to build some of our most important and successful businesses and worked tirelessly to strengthen and grow our global franchise," wrote Mack in the release. "We greatly appreciate the enormous contributions Zoe has made in a wide variety of roles, and we are confident that she will continue to do great things in the years ahead." As a part of the Morgan shifts, Walid A. Chammah in London and James P. Gorman have been appointed co-presidents. Institutional securities will report to Chammah and wealth management and asset management will report to Gorman. Both executives will report directly to Mack.