Reticulocytes and polychromasia
New red cells contain RNA, which they lose after 1-2 days in the circulation. Enumerating reticulocytes therefore gives a measure of marrow red cell production. The RNA can only be seen in a specially-stained blood film, as a 'reticulum' (hence the name). On the regular blood film, reticulocytes have a greyish tinge, which is known as 'polychromasia'.