**3. Reticulocyte maturation**

When the orthochromatic normoblast (late stage of erythropoiesis) loses its nucleus, it becomes a reticulocyte that persist in the bone marrow for next few days and is subsequently released into the circulation for terminal maturation [13, 14].

Maturation of reticulocyte is a continuous process with various morphologic, biochemical, and functional changes that lead to remodeling of membrane, changes in volume, and elimination of membrane-bound organelles and ribosomes [15]. Immature or early reticulocytes are biochemically more active than mature ones with intact cellular functions of Hb production and absorption of iron [16]. Circulating reticulocytes are unable to synthesize Hb and cannot further increase their Hb content.

Maturation of reticulocytes is a complex sequential mechanism of enucleation, caused by condensation of chromatin, vesicular trafficking, and selective autophagy [17]. The ultimate maturation occurs when the basophilic reticular filamentous substance in the reticulocyte disappears [18]. Intracytoplasmic organelles such as the mitochondria, ribosomes, and endosomal vesicles are eliminated by a mitochondrial death program which includes physiologic events of macroautophagy and mitoptosis [19, 20].

Early reticulocyte maturation is characterized by the selective elimination of unwanted plasma membrane proteins (CD71, CD98, and β1 integrin) through the endosome exosome pathway. In contrast, late maturation is characterized by the generation of large glycophorin A coated vesicles of autophagic origin [21, 22].

Recent studies have suggested that the small amount of RNA that remains in reticulocytes might still be essential for reticulocyte maturation to form normal biconcave erythrocytes [23].

During the maturational remodeling of the membrane cytoskeleton, by vesiculation and endocytosis, reticulocytes lose about 24% of their volume and surface area, and increase their stability and deformability [24].

As already mentioned, a series of progressive physiological and biochemical changes occur during the differentiation of reticulocytes into mature RBCs [25]. The most important of these changes include:

