As we reported earlier today, Katherine Heigl is taking a break from Grey's Anatomy to film a movie. That news alone was enough to cause a stir among fans this afternoon.

But what's shocking, and what shapes the entire sixth season of the series, is why recently married cancer survivor Dr. Izzie Stevens is vanishing for multiple episodes.

The reason for the leave of absence?

According to inside sources, this season of Grey's Anatomy is overwhelmingly defined by the merger of Seattle Grace and its oft-mentioned crosstown rival, Mercy West.

Reportedly, Seattle Grace is about to double in size, bringing in a slate of new doctors and paving the way for shake-ups in the season(s) of Grey's Anatomy to come.

What does this merger mean for our favorites?

Well, Dr. Izzie Stevens is getting fired. Yes, fired, as in dismissed or downsized. Izzie gets laid off in the big merger and vanishes for a handful of episodes.

The shake-up also provides cover for Ellen Pompeo's maternity leave.

She'll fade into the background while new doctors, administrators and nurses (played by Robert Baker, Jesse Williams and Nora Zehetner) fill in gaps regulars leave behind.

Producers are, however, working to keep Meredith Grey in as many episodes as possible. Ellen is banking footage for her leave by shooting scenes for two episodes at once.

The Seattle Grace-Mercy West merger storyline is part of a long-term plan hatched by producers and ABC execs to make Grey's Anatomy a viable series for years to come.

They are thinking ER style here - 15 years or more.

No matter which stars are pregnant, moonlighting, looking to get out or otherwise unavailable, the writers want a large, expandable palette of characters and situations.

So, going forward, if Patrick Dempsey needs to shoot a movie (he will, later this season), a new and improved Seattle Grace offer wiggle room in schedules and stories.

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.