Why do Jehovah’s Witnesses see 144,000 as the total saved, and how is this reconciled with “multitudes from every nation” in Revelation?

In Revelation 7:4–8, John mentions 144,000 people sealed from the twelve tribes of Israel. Jehovah’s Witnesses interpret this number literally as the total number of anointed Christians who will go to heaven and rule with Christ. Immediately afterward, Revelation 7:9–10 describes “a great multitude that no one could count, from every nation, tribe, people and language, standing before the throne and before the Lamb.” How do Jehovah’s Witnesses reconcile the idea of a fixed number of 144,000 heavenly Christians with the depiction of innumerable “multitudes” standing before God’s throne? Do official Watch Tower publications clarify the relationship between the 144,000 and the great crowd?