Why do mammalian red blood cells lack a nucleus?

Question

How did the red blood cell in humans get to lose its nucleus (and other organelles)? Does the bone marrow just not put the nucleus in, or is it stripped out at some stage in the construction of the cell?

Answer 1

Red blood cells have a nucleus through most of their development. When immature red blood cell precursor mature to fully functioning red blood cells, they enucleate (spit out the nucleus). This is an evolutionary adaptation that enhances the ability of the red blood cell to carry oxygen.