The main function of erythrocytes is transporting oxygen to all cells, as well as removing carbon dioxide as a product of cell oxidation. The oxygen concentration in tissues is in indirect proportion to the level of erythrocytopoiesis.
Erythrocytopoieses proceeds normally in the bone marrow and involves a lot of biochemical and morphological alterations of different shapes - from a directed stem cell, through proerythroblast, basophilic erythroblast, polychromatophilic erythroblast, acidophilic erythroblast, and reticulocyte to the mature erythrocyte.
The synthesis of DNA and RNA in the proerythroblast phase is very active.
Large amount of RNA gives distinct basophilic appearance to the cytoplasm. Decrease in the RNA synthesis results in the change of the cytoplasm color from polychromatophilic erythroblast to acidophilic erythroblast.
Acidophilic erythroblast expels the nucleus and becomes the reticulocyte. Loosing its nucleus, acidophilic erythroblast becomes the reticulocyte and stays in the bone marrow for a few more days. When it loses ribosomes, mitochondria, Golgi's complex (components of the cytoplasm), and transferrin receptors, the reticulocyte decreases in size and becomes the mature erythrocyte.

The mature erythrocyte looks like a flexible biconcave cell of a discoid shape, 7-8.5 µm in diameter, and 7.5 mm thick. Erythrocyte does not have nucleus and mitochondria. It contains a significant quantity of enzymes. The only source of energy is glucose that is subjected to the process of glycolysis after entering the cell.
In the bone marrow, the developing stages of erythrocytes last 4 to 5 days. The bone marrow is able to create about 900 billions of erythrocytes per hour, thus replacing the same number of erythrocytes that are simultaneously degraded by mononuclear macrophages of the histomonocytic system.
Life-span of a normal erythrocyte is 100 - 120 days.
Erythrocyte degradation is a consequence of "aging", i.e., the loss of enzymes involved in glycolysis.

Decomposition of old erythrocytes follows this scenario:

 fragmentation,
 osmosis,
 erythrophagocytosis,
 cytolysis by complement, and
 hemoglobin denaturation.

In normal conditions, aged erythrocytes are mostly ingested and degraded in the cells of histiomonocytic system in the bone marrow, spleen, and liver.
In the fetus, or after birth in the case of vitamin B₁₂ or folic acid deficiency, the cells of the erythrocytic line are formed by maturation along the megaloblastic pathway. The difference between the cells of this lineage and the cells of the erythroblastic lineage is in the disturbance of the cell maturation.
All cells of the megaloblastic lineage may be found in the patient's bone marrow. In the peripheral blood, megalocytes and rare megaloblasts can be found.

 DISTURBANCES OF ERYTHROCYTE MORPHOLOGY
The maturation of the cells of the erythrocytic line normally happens in the bone marrow. Only mature erythrocytes pass into the peripheral blood, those which have specific size, shape (biconcave appearance), and the ability to be dyed.
Morphological disturbances of erythrocytes in the peripheral blood may be of different types:

Disturbed erythrocyte coloration (anisochromia, hypochromia, hyperchromia, polychromia),
Disturbed erythrocytee size (anisocytosis, mycrocytosis, macrocytosis),
Disturbed erythrocyte shape (spherocyosis, ovalocytosis, leptocytosis, acanthocytosis, megalocytosis, poikilocytosis, drepanocytosis, rouleaux formation)

 

 ERYTROCYTES WITH NUCLEAR REMNANTS
Erythrocyte does not have a nucleus. In certain conditions, mature erythrocytes retain nuclear remnants. Nuclear remnants can be in the form of:

Howell-Jolly bodies,
Cabot's rings, and
Nuclear dust.

 INCLUSION BODIES IN ERYTHROCYTES
The appearance of erythrocyte can be altered if different abnormal components appear. Pathological inclusions noticed in erythrocytes are:

Basophilic stipplings,
Heinz-Ehrlich bodies,
Siderocytes, and
Malarial plasmodium.