Final answer:
The bluish color of reticulocytes on Wright-Giemsa stain is due to the presence of residual ribosomal RNA, which comes from remnants of ribosomes that these immature red blood cells contain temporarily after their formation in the bone marrow.
Step-by-step explanation:
Reticulocytes, which are immature erythrocytes (red blood cells), have a bluish cytoplasm when stained with Wright-Giemsa stain. This coloration is due to the presence of residual ribosomal RNA from ribosomes, which is responsible for the color observed under the microscope. As the reticulocytes mature in the red bone marrow, they lose organelles such as the nucleus and other cellular structures to make room for hemoglobin, which is crucial for oxygen transport. However, reticulocytes retain some remnants of their organelles, primarily reticulum networks of ribosomes, for a short period while in circulation. It is this residual ribosomal RNA that reacts with the stain, giving the reticulocytes their distinctive blue tint. Notably, these cells lack mitochondria and endoplasmic reticula, and thus do not synthesize proteins, but they contain structural proteins like spectrin that help maintain their biconcave shape.