**2. Definition of reticulocytes**

#### **2.1 Origin of the term and definition**

The "reticulocyte" term has derived from the reticulum of RNA and protein precipitated by the fixation and staining that seen microscopically after supravital staining [8, 9]. These supravital granules represent precipitated rough endoplasmic reticulum along with polyribosomes. Thus, the human reticulocytes are defined as mature red cells that form a reticulum network or granules on exposure to supravital stains, such as new methylene blue or brilliant cresyl blue [10].

Although this definition has been revised several times, in 1997 it was chosen to be the universal standard by the Clinical and Laboratory Standards Institute and the International Council for Standardization in Hematology (NCCLS-ICSH) [11].

## **2.2 Morphology**

Romanowsky type stain, such as May-Gru¨nwald Giemsa or Wright stain, causes the RNA to disappear during alcohol fixation and reticulocytes acquire slightly larger size than mature erythrocytes, with a uniform polychromatic blue-gray color. On staining with new methylene blue (NMB) or brilliant cresyl blue (BCB), the RNA precipitates and becomes visible with the characteristic scattered granules under the microscope.

Reticulocytes must have at least 2 blue staining granules, visible without fine microscope adjustment and present away from the cell margin, as standard morphological definition provided by NCCLS-ICSH, in 1997 [11]. These granules can mimic Heinz bodies.

#### **2.3 Properties of reticulocytes**

Reticulocytes in comparison with mature erythrocytes, have following characteristics [12]:

