On a glittering ballroom set in LA, the “Grey’s Anatomy” cast huddles, trying to do the latest American dance craze. Sara Ramirez is flailing her arms and wildly shaking her rump. And Kevin McKidd, Jesse Williams and Sandra Oh are following her lead — but adding crazy derriere and tongue action.

“We were twerking,” says Ramirez, who plays orthopedic surgeon Dr. Callie Torres. “Or trying to figure out how to do it. So we were trying all kinds of variations. Almost everyone gave it a shot.”

Patrick Dempsey, who plays Dr. Derek “McDreamy” Shepherd, however, does not twerk. He’s talking to a circus performer on stilts, hired to perform at a fund-raiser for the hospital. And he’s on his cellphone. “I am racing in Texas this weekend,” he says. The racing aficionado has lost 15 pounds and looks terrific. “I haven’t been this thin since I was in my ’30s,” he says.

As the scene starts, the fun stops and doctors rush to help an acrobat who has plunged to the ground.

Not everyone comes to the rescue. Sarah Drew, who plays Dr. April Kepner, and Jessica Capshaw, the show’s seductress, Dr. Arizona Robbins, are in a closet, doing who knows what?

“We were sipping champagne,” says Drew. “And celebrating.”

So why the all the fizz and fun on the set?

It is the 200th episode of “Grey’s Anatomy,” and the actors are clearly celebrating the milestone.

“My kids are getting to college age,” says Justin Chambers, who plays the perpetually single Dr. Alex Karev, but in real life is a father of five. “Thank God I can pay for it.”

“It’s so much fun that most of the cast is here. We are feeling rowdy,” says McKidd, who has played Dr. Owen Hunt since Season 5.

Of course, they have every reason to be rowdy. “Grey’s” is now up there in TV history with shows like “Friends,” “Murphy Brown,” “CSI: Miami” and “Two and a HalfMen.” They all went at least 10 seasons.

That kind of run was the furthest thing from the mind of creator and executive producer Shonda Rhimes when she met with ABC executives to pitch a show she had written sitting in bed in her pajamas.

“I was terrible at pitching,” Rhimes says. “I pitched the show by holding the document in front of my eyes and reading it in a monotone as fast as I could. I was just desperate to get it over with. It was wonderfully kind of them to buy the show since I was having an anxiety attack. I am pretty sure they felt sorry for me.”

Her mythical kingdom, called “Shondaland,” is the primary producer of hits for ABC, including her latest show, “Scandal.”

From day one, Dempsey and Ellen Pompeo, as Dr. Meredith Grey, have led the cast.“We were at a VA hospital in the San Fernando Valley and I wondered, will anyone believe me as a doctor? I thought it was quite funny,” says Dempsey.

The Shonda Rhimes drama was groundbreaking, touching every aspect of modern life, including abortion, gay marriage, bigotry and the pressures of being transgender, but the very first episode was pure soap opera, and much more clever. The pilot on March 27, 2005, opened with a bar hookup, a night of wild sex and an awkward goodbye as Meredith and Derek rushed off to work, only to learn that they were both doctors at Seattle Grace. And he was her boss.

“You know they wanted Isaiah Washington to be my boyfriend,” Pompeo says, calling from her home a few days later. “Shonda really wanted to put a black man in the mix. I didn’t think they were really going to put an interracial couple on the show and I didn’t want him. It was too close to home.”

Pompeo is married to Chris Ivery, an African-American music producer. “I said I wanted that Dempsey kid. I think that once Isaiah did not get the role it backfired,” she says. (Washington used a derogatory slur against homosexuals on the set and then dropped the same f-bomb backstage at the 2007 Golden Globe Awards. His contract was not renewed for season 4.)

He was the first original cast member to go. Soon T.R. Knight, who played Dr. George O’Malley, and Katherine Heigl, who played Dr. Izzie Stevens, followed.

Heigl left the show much the same as she spent her years on the show, amid controversy. In 2008, the actress wanted her name taken out of Emmy consideration because the writing for her character didn’t merit a nomination.

Pompeo witnessed all of the shenanigans and for the first time is willing to talk about it.

“Hurt feelings, combined with instant success and huge paychecks started things spinning out of control,” she says. “The crazier things got, as I watched all the tumult with Isaiah and then the Katie thing, I started to focus on the work. Maybe it is my Boston, blue-collar upbringing. I just tried to not pay attention to all the noise around me.”

“When Katie left, it was tough,” she says. “You could understand why she wanted to go — when you’re offered $12 million a movie and you’re only 26. But Katie’s problem is that she should not have renewed her contract. She re-upped, took a big raise and then tried to get off the show. And then her movie career did not take off.”

The show’s leads did not leave, though. Dempsey, who had made movies before joining “Grey’s,” says, “Of course I have thought about leaving. It’s always at the end of the season, when you are tired of having no control over your schedule,when you can’t see the forest for the trees, to know how fortunate you are.”

Pompeo has always known that she would go the distance. “Actors always think the grass is greener somewhere else. I didn’t want to do that,” she says. “I have an amazing life. I have a house five minutes from the studio, I have a house in the Hamptons, a house in Malibu, a beautiful daughter whom I see every night, and it’s all because I didn’t leave ‘Grey’s Anatomy.’ ”

The latest member of the original cast to announce her departure is Sandra Oh, who plays Dr. Cristina Yang.

For McKidd, who plays Oh’s on-screen love, it is both a curse and a blessing. “I am really sad but it could be interesting and exciting to discover things about Owen apart from Christina,” he says.

Despite the comings-and-goings, the “Grey’s” engine keeps going. Rhimes has even taken some of the actors who were guest stars, such as Scott Foley, who played Henry Burton, and Jeff Perry,who played Meredith Grey’s dad, and promoted them to series regulars on “Scandal.”

If that show’s ratings are any indication, they will have a long run like the “Grey’s” actors still standing — Chambers, Dempsey, Oh, James Pickens Jr, Pompeo and Chandra Wilson, all of whom have been there from the pilot.

“We knew it was a crapshoot,” says Pickens. “So we were always, in the back of our minds, looking for the next job.”

One day it may be time to go, but right now it’s time to celebrate a job well done.

THE WAY THEY WERE

So many surgeries and season finales, but these “Grey’s” vets have their favorite moments

Chandra Wilson (Dr. Miranda Bailey)

“It was the middle of the second season. Miranda’s husband was in an accident and in surgery,shew as pregnant and determined to hold the baby until he could be there. But they said ‘you are going to have to push,’ and I thought I would have a breakdown without my husband, but O’Malley was behind her, and he assured her the baby was great.And suddenly she says, ‘O’Malley, stop looking at my vajayjay.’ It was the first time we hear the word ‘vajayjay’ said out loud, and we had to stop shooting because we couldn’t stop laughing.”

James Pickens Jr. (Dr. Richard Webber)

“The gunmen episode [at the end of Season 6]. The shooter had wounded a couple of the interns and Patrick was wounded, and he found the gunmen. He had a gun on Webber, and he thought to himself,‘I have lived and I battled back from my own demons, I am ready. If that is what you have to do, my wife is not coming back, I am going to do it’. The gunman had created heinous acts, and wanted revenge. I said, ‘Either kill me or kill yourself.’ I walk out of the room and he killed himself.”

Jessica Capshaw (Dr. Arizona Robbins)

“For me, it is all my lovely moments with Sara [Ramirez] and Justin [Chambers] and particularly the beginning when Callie and Arizona are getting into a relationship. There were no words, it was so joyful.This was going to be the first regular prime-time lesbian character on TV, and I made myself not think about the totality of the character in the sociopolitical space of life, but how human and invigorating it was,and so we danced like wild little girls together.”

Justin Chambers (Dr.Alex Karev)

“I had a lot of great stuff with Izzie [Katherine Heigl] — the proposal scene, picking her up off the bed when Denny [Jeffrey Dean Morgan] died, and being there for her when she was dying, fighting the weaknesses and insecurities. I remember on that bed, trying to revive her, it felt good as an actor can feel. I love Katie. She is a doll, and she was a pro, always on her mark; a great talent.”