Warner Bros.
Original "ER" cast members Anthony Edwards, left, George Clooney, Sherry Stringfield, Noah Wyle and Eriq LaSalle in 1994.

Tonight, "ER" airs its 300th episode. And it is not the show it was when it premiered in 1994.

That's partly because the entire cast of "ER" (9:01 p.m., Ch. 5) has changed. We've actually been through several rounds of turnover. But it's mostly because the format and focus of the show are completely different.

When it began, "ER" was the medical-drama equivalent of "Law & Order." It was about what the doctors and nurses in the emergency room did, not who they were.

Here's a bit of my original review of "ER," which ran on Sept. 18, 1994: "Whether it's a relatively minor complaint or a life-threatening injury, it's the patients who drive 'ER' along. There's some hint of the personal lives of the doctors — particularly of Dr. Greene's life in tonight's pilot — but this is a show about medicine, not a soap opera about the doctors."

Today, "ER" is a soap opera about the doctors and nurses. The patients are there to drive the drama surrounding the regulars.

That's been true for, oh, about 275 episodes or so.

"At the beginning, we were going through eight or 12 (medical cases) and, on some of the big episodes, we did 20, 22 stories," executive producer John Wells said about 155 episodes ago. "And what we discovered is that, truthfully, there aren't that many stories in the ER medically. (And) those stories aren't as interesting as the characters and how our (regular) characters, in particular, become involved in those stories."

So viewers became involved in the stories of characters played by George Clooney, Anthony Edwards, Eriq LaSalle, Julianna Margulies, Sherry Stringfield and Noah Wyle. And all of those actors are gone.

So are a lot of actors who came aboard later, such as Laura Innes, Alex Kingston, Ming-Na, Paul McCrane, Erik Palladino, Gloria Reuben, Kellie Martin, Sharif Atkins, Maria Bello and Shane West.

Maura Tierney and Goran Visnjic weren't on the show until Season 6 and are now in their eighth year. That's longer than most series last.

Given what "ER" became, the fact that there has been so much turnover has actually worked to the show's advantage. After you've taken Dr. Mark Greene (Edwards) through divorce and remarriage and getting beaten up and the birth of a new daughter and troubles with the teenage daughter and a brain tumor, what else could you do but kill him off?

You need new characters to bring new stories to a show that now counts its episodes in the hundreds.

Other characters have exited via death — medical student Lucy Knight (Kellie Martin) was stabbed, and Dr. Robert Romano (Paul McCrane) had a helicopter dropped on him — but most who left survived. They've just gone on to other things.

It's all part of keeping a show on the air for 14 seasons — something that doesn't happen all that often.

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