<P> Red blood cells in mammals anucleate when mature, meaning that they lack a cell nucleus . In comparison, the red blood cells of other vertebrates have nuclei; the only known exceptions are salamanders of the Batrachoseps genus and fish of the Maurolicus genus with closely related species . </P> <P> The elimination of the nucleus in vertebrate red blood cells has been offered as an explanation for the subsequent accumulation of non-coding DNA in the genome . The argument runs as follows: Efficient gas transport requires red blood cells to pass through very narrow capillaries, and this constrains their size . In the absence of nuclear elimination, the accumulation of repeat sequences is constrained by the volume occupied by the nucleus, which increases with genome size . </P> <P> Nucleated red blood cells in mammals consist of two forms: normoblasts, which are normal erythropoietic precursors to mature red blood cells, and megaloblasts, which are abnormally large precursors that occur in megaloblastic anemias . </P> <P> Red blood cells are deformable, flexible, are able to adhere to other cells, and are able to interface with immune cells . Their membrane plays many roles in this . These functions are highly dependent on the membrane composition . The red blood cell membrane is composed of 3 layers: the glycocalyx on the exterior, which is rich in carbohydrates; the lipid bilayer which contains many transmembrane proteins, besides its lipidic main constituents; and the membrane skeleton, a structural network of proteins located on the inner surface of the lipid bilayer . Half of the membrane mass in human and most mammalian red blood cells are proteins . The other half are lipids, namely phospholipids and cholesterol . </P>

Did red blood cells ever have a nucleus