Editing Red blood cell (section)

===Nucleus===
Red blood cells in mammals are ''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 [[salamander]]s of the family ''[[Plethodontidae]]'', where five different clades has evolved various degrees of enucleated red blood cells (most evolved in some species of the genus ''[[Batrachoseps]]''), and fish of the genus ''[[Maurolicus]]''.<ref>{{cite journal |doi=10.1007/BF01283036 |title=The cytomorphic system of anucleate non-mammalian erythrocytes |year=1982 | vauthors = Cohen WD |journal=Protoplasma |volume=113 |pages=23–32|s2cid=41287948 }}</ref><ref>{{cite journal | vauthors = Wingstrand KG | title = Non-nucleated erythrocytes in a teleostean fish Maurolicus mülleri (Gmelin) | journal = Zeitschrift für Zellforschung und Mikroskopische Anatomie | volume = 45 | issue = 2 | pages = 195–200 | year = 1956 | pmid = 13402080 | doi = 10.1007/BF00338830 | s2cid = 12916049 }}</ref><ref>{{cite journal | pmc=2435017 | date=2008 | last1=Mueller | first1=R. L. | last2=Gregory | first2=T. R. | last3=Gregory | first3=S. M. | last4=Hsieh | first4=A. | last5=Boore | first5=J. L. | title=Genome size, cell size, and the evolution of enucleated erythrocytes in attenuate salamanders | journal=Zoology | volume=111 | issue=3 | pages=218–230 | doi=10.1016/j.zool.2007.07.010 | pmid=18328681 | bibcode=2008Zool..111..218M }}</ref>

The elimination of the nucleus in vertebrate red blood cells has been offered as an explanation for the subsequent [[C-value enigma|accumulation of non-coding DNA in the genome]].<ref name="The bigger the C-value, the larger"/> 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.

[[Nucleated red blood cell]]s 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 anemia]]s.