In today's crowded TV space, where shows are lucky to secure a second season, few have endured the way that Grey's Anatomy has. On Thursday, the Shondaland series surpassed ER to become the longest-running medical drama in prime time television history, which was no easy feat considering it took 15 seasons and 322 episodes to get there. And while the series may have broken ER's impressive record, showrunner Krista Vernoff admitted that Grey's owes a great deal to the proceeding drama.

"The truth is, Grey's Anatomy doesn't exist without ER," Vernoff told TV Guide. "John [Wells] broke the mold with ER and laid the path for shows like Grey's Anatomy to come along."

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Considered one of TV's most influential shows, ER set the standard for emergency room dramatics and turned confusing medical jargon into thrilling, tense storytelling. As avid fans of the seminal series, Vernoff and Grey's creator Shonda Rhimes recognized the show's significance, thinking deeply about how to build on its success without turning Grey's into a copycat.

"We would talk about how can we make sure that we are not ER? We used to talk all the time about, do we wanna crash a medevac helicopter?" Vernoff said. "That comes up in pitches often and we've never done it because ER did it so memorably more than once. So there are things we've always tried to stay away from."

Kim Raver and James Pickens Jr., <em>Grey's Anatomy</em>Kim Raver and James Pickens Jr., Grey's Anatomy

But the comparisons to ER were inevitable — they're both medical dramas, after all — and that heavily influenced Grey's first few seasons with Rhimes pushing her show in a different tonal direction. "We leaned into the romantic comedy for that reason and away, initially, from cases coming in through the emergency room. More often, cases were up on the surgical floor," Vernoff added.

It's a gamble that paid off; Grey's is now in the middle of its fifteenth season. As ABC's most-watched show among the 18-to-49 demographic and second only to The Good Doctor in total viewers, the beloved series is showing no signs of slowing down just yet.

"What's pretty incredible to me in hitting this milestone is that there's not really an end in sight. In the writers' room, we're not building toward an end," Vernoff said.

Grey's Anatomy has yet to be renewed for Season 16, but audiences shouldn't panic. Given that star Ellen Pompeo is contractually obligated to stay on through then and the series continues to dominate in ratings, it's a safe bet that the show will be back next year.

Grey's Anatomy airs Thursdays at 8/7c on ABC.

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