Reticulocytes-Mother of Erythrocytes

*Ashish Kumar Gupta and Shashi Bhushan Kumar*

### **Abstract**

Reticulocytes are immature red blood cells (RBCs) that is seen in the bone marrow after through nuclear extrusion from the orthochromatic normoblasts. They are released into the peripheral blood as mature RBCs, after completion of maturation in the bone marrow. The reticulocyte count reflects the erythropoietic activity of the bone marrow, the rate of reticulocyte delivery from the bone marrow into the peripheral blood, and the rate of reticulocyte maturation. Reticulocyte enumeration is also of value in monitoring bone marrow regenerative activity after chemotherapy or bone marrow transplantation. Manual counting of reticulocytes by light microscopy with supravital dyes for RNA remains the standard method of reticulocyte enumeration. However, automated methods of reticulocyte enumeration developed during the past decade are much more accurate, precise, and cost-effective than manual counting, and are increasingly being performed in the clinical laboratory. The differentiation of the reticulocyte is based on the presence of RNA. The newer techniques provide a variety of reticulocyte related parameters, such as the reticulocyte maturation index and immature reticulocyte fraction, which are not available with light microscopy. These new parameters are under evaluation in the clinical diagnosis and monitoring of hematological disorders.

**Keywords:** reticulocyte count, new methylene blue, immature reticulocyte fraction, reticulocyte maturation index, reticulocyte production index, high-fluorescence ratio, medium-fluorescence ratio, low-fluorescence ratio, reticulocyte mean hemoglobin content

## **1. Introduction**

Reticulocytes are immature red blood cells (RBCs) produced in the bone marrow and released into the peripheral blood where the terminal maturation into RBCs occurs for next couple of days. Any alteration in reticulocyte count is an indicator of active or failed erythropoiesis, in response to anemias or other causes of bone marrow dysfunction [1].

The first description of reticulocytes was made in 1865 by Wilhelm Heinrich Erb, a German neurologist when he discovered the population of granulated erythrocytes while observing the effect of acetic and picric acid on the development of erythrocytes. He erroneously regarded these cells as transitional forms between leucocytes and erythrocytes. Ehrlich described the stained material as fine, dense, and elegant networks as a feature of senescent erythrocytes rather than of young ones [2]. The reticular substance was first regarded as a degenerative material, a "coagulation

#### **Figure 1.**

*Maturation stages of reticulocytes according to Heilmeyer classification [4].*

necrosis" or a substance produced by the action of certain deleterious agents on corpuscles.

Theobald Smith brilliantly asserted that reticulocytes represented young red cells. Further investigators designated these cells as erythrocytes with "substantia granulo-filamentosa," the Americans used such terms as "reticulated red cell" or "vital-staining erythrocyte."

In 1922, Edward Bell Krumbhaar, first coined the term "reticulocyte" when he stated that: "Erythrocytes revealing granular filamentous substance by the methods of vital staining may be conveniently designated 'reticulocytes" [3].

Ludwig Heilmeyer, a German internist, proposed the still well-known classification of reticulocytes maturity in 1932 (**Figure 1**). In 1944, Pierre Dustin, a Belgian pathologist, showed that granular "reticulated" substance is RNA [5]. In 1947, Giovanni Astaldi, an Italian hematologist, classified reticulocytes into 3 stages of maturity, as in the current maturation classification used in flow cytometry. In 1918, it was demon-

strated that reticulocytes are able to synthesize hemoglobin and absorb iron [6]. Over the years acridine orange, a fluorescent dye, replaced the use supravital stain, improving the sensitivity of manual microscopic count of reticulocytes [7].
