Reticulocytes are immature red blood cells, typically composing about 1% of the red cells in the human body. Reticulocytes develop and mature in the red bone marrow and then circulate for about a day in the blood stream before developing into mature red blood cells. Like mature red blood cells, reticulocytes do not have a cell nucleus. They are called reticulocytes because of a reticular (mesh-like) network of ribosomal RNA that becomes visible under a microscope with certain stains such as new methylene blue.

The reticulocyte count increases during rapid blood loss or in the course of certain diseases in which red blood cells are destroyed prematurely. Also, spending an extended period of time at high elevations may cause reticulocyte counts to increase to help you adapt to the lower oxygen levels found at high altitudes.

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