Bertrand Serlet, Apple's senior VP of Mac software engineering, developed the Mac OS X operating system. Photo courtesy Apple.

Apple announced Wednesday that Bertrand Serlet, lead developer of Mac OS X, is leaving the company, just a day before the operating system turns 10 years old.

Though less publicized than Apple executives Steve Jobs and Tim Cook, Serlet is a legend at the company. He was basically the Jony Ive of Apple’s software design. Mac OS X, which originally released March 24, 2001, has played a crucial role in driving the success of Apple’s Macs and mobile products.

“I’ve worked with Steve for 22 years and have had an incredible time developing products at both NeXT and Apple, but at this point, I want to focus less on products and more on science,” said Serlet, Apple’s senior VP of software engineering, in a press statement. “Craig has done a great job managing the Mac OS team for the past two years, Lion is a great release, and the transition should be seamless.”

Serlet has a lot to brag about. His baby, Mac OS X, not only powers Apple’s Mac computers, but also the iPhone, iPad and iPod Touch — all blockbuster products.

After Apple fired Jobs in a power struggle in 1985, the exiled CEO founded NeXT to build a Mac-like computer for education that would put Apple out of business. Serlet, a former Xerox PARC employee, was on the NeXT team.

Then when Apple nearly went bankrupt in 1996, the company acquired NeXT to build a new Mac OS. That brought both Jobs and Serlet on board at Apple, and the two have been working together for 22 years.

When Mac OS X debuted in 2001, it had a somewhat rocky start: Many features were missing, and there were some compatibility problems with external hardware. Over the past decade, Apple pruned out OS X’s issues and polished the OS. Apple released iOS in 2007. It’s a specialized version of Mac OS X for the iPhone, and later the iPod Touch and iPad.

The next version of Mac OS X, dubbed Mac OS X Lion, is due for release sometime 2011.