Ten MLB umpires, which includes seven crew chiefs, are set to retire at the end of the month, making it the largest turnover at that job since 1999, sources familiar with the situation tell ESPN.

Some the of retirements are due to nagging injuries while others are coincidental as a group of them entered the league at around the same time -- after a labor dispute saw 22 former umps resign at the end of the last century.

Well respected crew chiefs Ted Barrett, Greg Gibson, Tom Hallion, Sam Holbrook, Jerry Meals, Jim Reynolds and Bill Welke are among the group to hang up their chest protectors while Marty Foster, Paul Nauert and Tim Timmons will join them in retirement as well.

"I'm so grateful to have the career that I did and to be a part of baseball history," Barrett said in a phone interview. "I'm incredibly proud of the crews that I worked with and everything baseball provided for me. For all of us."

The retirements have nothing to do with the upcoming on-the-field rule changes for 2023, nor the possibility of the ABS (automatic balls and strikes) system being instituted at the MLB level in the near future.

The league will promote/hire 10 new umpires next month with a commitment to making it a diverse group -- though a first ever female MLB umpire won't be among them. Jen Pawol could eventually break that barrier as she worked in Double-A last season.

The retirements come a year after Joe West, who umpired the most games in history, called it quits which means MLB is losing decades of experience over the past two offseasons.

"Such a great group of men," Chicago Cubs manager and former catcher David Ross said. "They're such a big part of our game. Teddy Barrett can diffuse any situation. Tom Hallion's got one of the most aggressive punch outs in the game. You always see him reaching for the sky...one of those signature moves you see all the time. I'll miss that."

Among the crew chiefs alone, they've called 16 World Series with Barrett leading the way having worked five Fall Classics. He was behind the plate for David Cohen's perfect game in 1999 and Greg Maddux's 300th win in 2004.

Meals sat behind the catcher for Kerry Wood's 20 strikeout game in 1998 and a Justin Verlander no-hitter in 2011.

Gibson was the first umpire to have a call overturned based on a manager's challenge in 2014 then later that season was behind the plate for a Clayton Kershaw no-hitter. In all, the retiring umpires have worked over 200 combined seasons in major league baseball.

The last time the league added as many as eight new umpires was in 2014, to account for instant replay. Current crews are part of a rotation who work the instant replay room in New York.

Barrett acknowledged there are good days and bad in the job. One of his toughest came early in his career when he was warned of former pitcher David Cohen's backdoor slider.

"I had seen plenty of sliders in the minors so I thought I was prepared," Barrett said. "I called his a ball way too early, then it broke over the plate and into (catcher) Mike Macfarlane's glove. Cohen was just shaking his head. Macfarlane said 'first time having him kid?'

"I knew I was in the big leagues."