The 144,000

To accomplish the gathering of Israel, the Lord chooses 144,000 priesthood holders, "to  administer the everlasting gospel . . . out of every nation, kindred, tongue, and people, by the angels to whom is given power over the nations of the earth, to bring as many as will come to the church of the Firstborn” (D&C 77:11). But why 144,000? What is special about that number?