Orthochromic normoblasts are the last nucleated stage of erythroid maturation. In this stage, the nuclei of the cells completely shrink to a pyknotic remnant. The cytoplasm color approaches the color of a peripheral RBC as it becomes fully hemoglobinized. This is the stage that is most commonly seen when NRBCs are found in the peripheral blood.
In the top image on the right there are many orthochromic normoblasts scattered across this section of bone marrow. Note the pyknotic-appearing nuclei which make them easy to spot, even at lower magnification. It is also evident that the cytoplasm is well hemoglobinized and the color is just slightly more blue than the non-nucleated red bloods cells present.
In the higher magnification (second image), notice the orthochromic normoblast (blue arrow) to the right of the basophilic normoblasts. The color of the cytoplasm of the orthochromic normoblast is almost identical to the background RBCs. Notice how condensed the nucleus has become as well. You can actually observe the nucleus in the early stages of extrusion/elimination from the cell. Once the nucleus has been extruded, the slight blue color, also known as polychromasia, will begin to fade and the now non-nucleated RBC will be indistinguishable from any other circulating RBC.
