Morphology of mature erythrocytes
The red blood cell population is characterized by a colossal numerical predominance: in a blood smear, they outnumber leukocytes a thousandfold, and fields of view filled exclusively with them are frequently encountered.
The bulk of the population (about 85%) is represented by discocytes—biconcave disc-shaped cells with a mean diameter of about 7.5 µm. This geometry causes a characteristic appearance under light microscopy: a noticeable central pallor is observed in the center of the cell. With standard Romanowsky staining, erythrocytes turn pink 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 network of cytoskeletal elements.
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 making up 1–2% of the total number. Like mature cells, they no longer have a nucleus; however, remnants of organelles persist in their cytoplasm: mitochondria, ribosomes, and endoplasmic reticulum elements. They contain hemoglobin and membrane proteins, but in lesser amounts. With conventional Romanowsky staining, reticulocytes are visually indistinguishable from discocytes. To identify them, a special cresyl blue stain is used, which reveals a reticulofilamentous structure (reticulum)—aggregates of remaining organelles—inside the cell.
Aging forms and poikilocytosis As they age, erythrocytes undergo regular changes in shape. The crenation mechanism is triggered: the cell begins to rotate, assumes an ellipsoid shape, and transforms into an echinocyte (about 6% of the population)—a structure with spiky cytoplasmic projections. Subsequently, echinocytes can turn into spherocytes (about 1%)—spherical cells that have lost their spikes. Stomatocytes (about 2%) are also found, in which the biconcave shape changes to dome-shaped or cup-shaped.
Cytoskeleton and maintenance of shape
The erythrocyte membrane contains dozens of proteins. Their most crucial task is to preserve elasticity 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, providing erythrocyte elasticity. This spectrin network is attached to the cell membrane via a specialized complex: the protein ankyrin binds spectrin to an integral transmembrane protein known as band 3 protein.
Surface charge and transport function
On the outer side of the plasmalemma lies the integral protein glycophorin. It contains oligosaccharide chains terminating in sialic acid residues. The carboxyl groups of this acid create a strong negative charge on the erythrocyte surface. This charge is critically important as it causes cells to repel one another, preventing them from clumping together in the bloodstream.
The loss of sialic acid residues leads to a decrease in surface charge. This process serves as a marker of cell aging and acts as a physiological signal for the utilization of old erythrocytes.
The transport function of the membrane is ensured by membrane proteins that form specific ion channels. A characteristic feature of these channels is that they freely pass anions (e.g., chlorides $Cl^-$, bicarbonates $HCO_3^-$, hydroxyl ions $OH^-$), while remaining virtually completely impermeable to cations.
Briefly: answers to topical questions
What is the average lifespan of a mature erythrocyte in the bloodstream?
The average lifespan of a mature erythrocyte in the bloodstream is 120 days. Normally, the circulation time of these cells ranges from 100 to 120 days. After this time, their physiological destruction occurs, compensated by the influx of new erythrocytes from the red bone marrow to maintain a steady state.
In which organs and cells does phagocytosis and destruction of old erythrocytes take place?
The destruction and phagocytosis of old erythrocytes take place in the spleen, localized predominantly in the red pulp. The process itself is carried out by macrophages of the splenic cords. Macrophages recognize cells designated for elimination by the loss of surface charge (decrease in the number of sialic acid residues) and loss of elasticity, which causes rigid erythrocytes to become trapped in the reticular network of the cords.
Which types of hemoglobin are normally present in the erythrocytes of an adult human?
Sources indicate the following for an adult human:
- HbA — the main adult hemoglobin (97%), consisting of two $\alpha$- and two $\beta$-globin chains.
- HbA$_2$ — minor adult hemoglobin (2–3%), comprising two $\alpha$- and two $\delta$-chains.
HbF — fetal hemoglobin — is listed separately; by 4–6 months of life, its level is less than 1%.
What are the pathological inclusions in erythrocytes that represent nuclear remnants called?
Pathological inclusions in erythrocytes representing nuclear remnants are called Howell-Jolly bodies and Cabot rings.
- Howell-Jolly bodies (Howell-Jolly bodies) — small, round, purple inclusions that are remnants of nuclear chromatin and DNA.
- Cabot rings (Cabot rings) — thin, thread-like structures shaped like a ring or a figure eight.
It is easy to remember the role of the proteins through association: Spectrin provides the "Structure" (an elastic network), while Ankyrin acts as an "Anchor," firmly attaching this network to the membrane.
Class and exam questions
Why is a lighter central area visible in erythrocytes under light microscopy?
This is an optical effect caused by the biconcave disc shape (discocyte). The central part of the cell is significantly thinner, so it binds less dye and appears lighter.
How to distinguish a reticulocyte from a mature erythrocyte in a slide preparation?
With standard Romanowsky staining, they look identical. To identify a reticulocyte, a special cresyl blue stain is necessary, which stains the organelle remnants, forming a visible reticular structure.
What happens to an erythrocyte upon the loss of sialic acid?
Sialic acid provides a negative charge to the membrane, preventing cell aggregation. Its loss reduces the charge and serves as a specific marker of aging, signaling the need for cell disposal.
What else to study on this topic
- Mechanisms of discocyte transformation into echinocytes and spherocytes
- Organization of the spectrin-membrane attachment complex
- Properties and functions of band 3 protein
- Operation of erythrocyte plasmalemma ion channels
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the topic "Erythrocytes" and another 3,485 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the "Blood" section
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.