Kate Walsh shared a heartfelt tribute to her late Grey’s Anatomy costar Eric Dane, who died at 53 on Thursday, Feb. 19.

“I’m at a loss for words to try to express the sadness around Eric’s passing,” Walsh, 58, began in an Instagram post shared on Friday, Feb. 20. “First and foremost I am thinking of his girls and Rebecca and holding them in my prayers and my heart.” The post included several behind-the-scenes and promotional photos of Walsh alongside her former Grey’s costar.

Walsh, who played Dr. Addison Montgomery on Grey’s Anatomy, went on to recount the first scene she shared with Dane, who played Dr. Mark “McSteamy” Sloan. Their fan-favorite characters notoriously had a torrid affair at the start of the show.

“I literally remember my very first scene with Eric…an elevator scene at Seattle Greys  So, so long ago. I believe it was his first scene on the show too and he was nervous,” Walsh continued. “He was so handsome and I thought ‘but can this guy act?’  and of course he could, and did, and the rest is history.”

Walsh noted that she “loved” Dane’s “sensitivity and vulnerability, (and intelligence, of course),” and reminisced about how the two actors supported each other in the early days of making the longtime ABC medical drama.

“As with most network tv shows, we spent more time with each other than with anyone else and so we became a family of sorts and Eric was such a source of support and love,” Walsh continued, adding that ALS is “an awful disease” and that “too many people have lost loved ones to it.”

“I know it was Eric’s mission to help find a cure. I would encourage you to look into @targetals in his memory. ” the actress wrote, directing fans to check out the nonprofit Target ALS Foundation.

Dane died 10 months after he went public with his amyotrophic lateral sclerosis (ALS) ALS diagnosis.

“With heavy hearts, we share that Eric Dane passed on Thursday afternoon following a courageous battle with ALS,” the actor’s family said in a statement to PEOPLE. “He spent his final days surrounded by dear friends, his devoted wife, and his two beautiful daughters, Billie and Georgia, who were the center of his world.”

The family’s statement continued, “Throughout his journey with ALS, Eric became a passionate advocate for awareness and research, determined to make a difference for others facing the same fight. He will be deeply missed, and lovingly remembered always. Eric adored his fans and is forever grateful for the outpouring of love and support he’s received. The family has asked for privacy as they navigate this impossible time.”

Dane is survived by his daughters, Billie and Georgia, whom he shared with his ex, actress Rebecca Gayheart.