Morphology of Mature Erythrocytes
The red blood cell population is characterized by a colossal numerical dominance: in a blood smear, they outnumber leukocytes a thousandfold, and fields of view filled exclusively with them are common.
The bulk of the population (about 85%) consists of discocytes — cells shaped like biconcave discs with an average diameter of about 7.5 µm. This geometry creates a characteristic appearance under light microscopy: a noticeable central pallor. With standard Romanowsky staining, erythrocytes acquire a pink color due to oxyphilia (high affinity for eosin).
Electron microscopy demonstrates a key feature of mature forms: the complete absence of a nucleus and all intracellular organelles, including mitochondria. The internal space of the cell is filled with hemoglobin, and its shape is maintained exclusively by a specialized cytoskeletal network.
Life Cycle and Population Heterogeneity
The bloodstream contains not only mature discocytes but also cells at various stages of their life cycle.
Precursors (Reticulocytes) These are young forms representing 1–2% of the total count. Like mature cells, they lack a nucleus; however, their cytoplasm retains remnants of organelles: mitochondria, ribosomes, and elements of the endoplasmic reticulum. They contain hemoglobin and membrane proteins, though in smaller amounts. With standard Romanowsky staining, reticulocytes are visually indistinguishable from discocytes. Special cresyl blue staining is used to identify them, revealing a reticular (mesh-like) structure inside the cell consisting of aggregated organelle remnants.
Aging Forms and Poikilocytosis As they age, erythrocytes undergo predictable shape changes. The crenation mechanism is triggered: the cell begins to rotate, assumes an ellipsoidal shape, and transforms into an echinocyte (about 6% of the population) — a structure with spike-like cytoplasmic projections. Subsequently, echinocytes can turn into spherocytes (about 1%), which are spheroidal cells that have lost their spikes. Stomatocytes (about 2%) are also encountered, in which the biconcave shape changes to a bowl-like or dome-shaped profile.
Cytoskeleton and Shape Maintenance
The erythrocyte membrane contains dozens of proteins. Their crucial task is to preserve flexibility and the cell's ability to deform when passing through narrow capillaries.
The primary cytoskeletal protein is spectrin. It has the shape of flexible rods and forms a dense network on the inner surface of the plasmalemma, ensuring erythrocyte elasticity. This spectrin network is anchored to the cell membrane via a specialized complex: the protein ankyrin links spectrin to an integral transmembrane protein known as band 3 protein.
Surface Charge and Transport Function
The integral protein glycophorin is located on the external side of the plasmalemma. It contains oligosaccharide chains terminating in sialic acid residues. The carboxyl groups of this acid create a powerful negative charge on the erythrocyte surface. This charge is critical because it causes cells to repel one another, preventing them from clumping together in the bloodstream.
Loss of sialic acid residues leads to a decrease in surface charge. This process serves as a marker of cellular aging and acts as a physiological signal for the clearance of old erythrocytes.
The transport function of the membrane is ensured by membrane proteins forming specific ion channels. A unique feature of these channels is that they readily allow anions (e.g., chlorides $Cl^-$, bicarbonates $HCO_3^-$, hydroxyl ions $OH^-$) to pass freely while remaining practically impermeable to cations.