Reticulocyte is immature RBCs. 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 rRNA that becomes visible under a microscope with certain stains such as methylene blue.