Erythrocytes in cytologic preparations

Site of blood cell formation

All formed elements of the blood, including erythrocytes, develop in the red bone marrow. This is the central organ of hematopoiesis, which in an adult is located in the spongy substance of the pelvic bones, sternum, ribs, vertebral bodies, cranial bones, and the proximal parts of long tubular bones.

Histologically, this tissue consists of three interrelated components:

• Stromal component: formed by reticular connective tissue. The branching reticular cells interconnect, are in close contact with reticular fibers, and form a three-dimensional network. This structure provides the necessary microenvironment for developing blood cells.
• Vascular component: represented by a dense network of sinusoidal capillaries. They are characterized by a wide lumen and the presence of a discontinuous basement membrane. This structure is critical for mature cells to leave the tissue unhindered and enter the systemic circulation.
• Hemal component: includes the hematopoietic cells themselves at various stages of maturation.

Cell development and cytological picture

The process of hematopoiesis begins with hematopoietic stem cells. Their division gives rise to multipotent progenitor cells responsible for myelopoiesis and lymphocytopoiesis. Subsequently, they go through multiple stages of differentiation before becoming mature cells.

In cytologic preparations, early precursors (belonging to classes I–III) visually resemble small lymphocytes. It is morphologically impossible to distinguish them from each other using routine microscopy—exact identification requires immunophenotyping, which evaluates surface antigens. These cells are characterized by the phenomenon of repopulation: they are capable of entering peripheral blood (where they are present in extremely low concentrations), circulating through the body, and re-seeding hematopoietic tissues.

In addition to erythrocyte precursors, giant multinucleated cells—megakaryocytes—can be found in bone marrow preparations. Small, anucleated fragments 2–3 µm in size (several times smaller than an erythrocyte) are constantly pinched off from their cytoplasm. These fragments are thrombocytes, which immediately enter the bloodstream.

Cell research methods

For the in vivo study of the cellular composition and histological architecture of hematopoietic tissue, two main methods are used in clinical practice:

1. Cytological examination (myelogram): performed on a smear obtained by sternal puncture. The procedure is considered safe even against the background of severe thrombocytopenia. When analyzing the smear, the physician assesses the overall cellularity of the preparation, the morphological features of the elements, and counts the number of megakaryocytes and sideroblasts (erythrokaryocytes containing iron granules).
2. Histological examination (trephine biopsy): involves the collection of a solid cylinder of bone together with bone marrow. The material is usually taken from the iliac bone. Bone marrow extraction from cranial bones is not performed. In pediatric practice until the age of 18, this procedure requires the mandatory use of general anesthesia. In the laboratory, the obtained biopsy specimen is often stained using immunohistochemical (IHC) methods.

Changes in the erythroid lineage and bone marrow in pathologies

Various diseases specifically alter the picture in cytological and histological preparations:

• Iron deficiency anemia: overall bone marrow cellularity remains within normal limits, but moderate hyperplasia of the erythroid lineage is observed. At the same time, the maturation of erythroblasts is delayed. The most important diagnostic criterion is a sharp decrease in the number of sideroblasts. The Prussian blue reaction in the punctate yields a negative result, indicating the absence of hemosiderin.
• Aplastic anemia: the cytologic preparation (punctate) appears hypocellular. Megakaryocytes are completely absent, against which background relative lymphocytosis is detected. Histological examination of the biopsy reveals aplasia—active red bone marrow is replaced by adipose tissue.
• Myelodysplastic syndromes (MDS): preparations show impairments in cell differentiation, although the capacity for maturation is partially retained. The main sign is the presence of morphologically defective progenitor cells accompanied by cytopenia in the peripheral blood.
• Chronic myeloid leukemia (CML): granulocytes and their precursors (myelocytes, promyelocytes) totally predominate in the bone marrow. Cells with "bizarre" (dystrophic) changes are detected.
• Fibrous osteomyelitis: characterized by subacute inflammation without sequestration (destruction). The bone marrow tissue is gradually replaced by connective tissue, which subsequently undergoes sclerosis and reverse metaplasia into bone.

Compensatory mechanisms

When resources are depleted or the demand for blood cells (including erythrocytes) sharply increases, the body triggers compensatory processes:

1. Fatty marrow reversion: yellow bone marrow, consisting mainly of adipose tissue, can transform back into active red marrow. This occurs when there is a high demand for blood cells or due to tumor proliferation (e.g., in CML, the tissue acquires a pyoid, greenish, or juicy gray-pink hue).
2. Extramedullary hematopoiesis: when the regenerative capacities of the bone marrow are exhausted, foci of hematopoiesis begin to form outside of it. Organs are involved in this process in a strict sequence: first the liver, then the spleen, and lastly the lymph nodes.

Class and exam questions

What else to study on this topic

• Structural features of sinusoidal capillaries and the basement membrane
• The mechanism of platelet shedding from megakaryocytes
• Immunophenotyping: markers of early progenitor cells
• Pathogenesis of extramedullary hematopoiesis and the sequence of organ involvement

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.