Immature red blood cells (RBCs). They're normally found mostly in the bone marrow and make up less than 2% of RBCs in the blood stream.
The concentration of reticulocytes is determined by a blood test called a reticulocyte count, which helps determine if RBCs are being created in the bone marrow at a normal rate. The test is often given to see if a patient's anemia treatment is working, as well as to help determine the cause of anemia. A low concentration can indicate anemia due to lack of RBC production by the bone marrow. This can be due to bone marrow failure, liver cirrhosis, folate-, iron- or vitamin B12 deficiency, radiation therapy, or kidney disease with decreased erythropoietin production. A high concentration indicates increased production of RBCs, for example following chemotherapy or an anemia caused by bleeding or increased RBC destruction; the body increases RBC production and sends them into circulation before they're fully mature. Other causes include erythroblastosis fetalis, a splenectomy, and kidney disease with increased erythropoietin production.
Reticulocytes typically have a network of filaments and granules on the surface, unlike mature RBCs. They also have RNA, which mature RBCs do not. The reticulocyte count is done by dyeing RNA and counting how many reticulocytes are in a blood sample.
Reticulocytes were first described as transitional forms of RBCs by Wilhelm H. Erb, a German neurologist, in 1865.