Red Bone Marrow

Hemopoietic Islands and Differentiation Lineages

Hemopoietic cells in red bone marrow are not scattered randomly; instead, they form specialized colonies called islands. The bulk of these islands consists of maturing cells (Class V). Nearly all hemopoietic lineages are represented here.

Erythroid Lineage Erythrocyte maturation is accompanied by a decrease in cell size and a color change of the cytoplasm from blue to pink:

• Proerythroblasts (Class IV) — the largest cells with a basophilic nucleus.
• Basophilic erythroblasts (Class V) — blue nucleus and cytoplasm.
• Polychromatophilic erythroblasts — cytoplasm becomes grayish-pink due to hemoglobin accumulation and residual ribosomes.
• Orthochromatic erythroblasts (normoblasts) — small cells with pink cytoplasm and a small pyknotic nucleus.
• Reticulocytes and erythrocytes (Class VI) — post-cellular structures lacking a nucleus.

Granulocytic Lineage Divided into neutrophilic, eosinophilic, and basophilic lines. The process begins with myeloblasts (rarely seen). Primary (azurophilic) granules appear at the promyelocyte stage. The key stage is the myelocyte, where secondary (specific) granules form in a cell with a round nucleus. Next, the nucleus becomes indented (metamyelocytes or juvenile), horseshoe-shaped (band cells), and finally segmented (segmented granulocytes).

Thrombocytic Lineage Characterized by the presence of giant polyploid cells — megakaryocytes. They possess an extensive oxyphilic cytoplasm and are in close contact with blood capillaries. Cytoplasmic processes of megakaryocytes extend directly into the vessel lumen, where platelets are pinched off via clasmatosis.

Stromal Component and Microenvironment

The stroma creates a three-dimensional framework and the necessary conditions for hemopoiesis. It is represented by reticular tissue and specialized cells:

1. Reticular cells: branched elements producing reticular fibers (type III collagen) and amorphous ground substance. They also secrete hemopoiesis-regulating factors.
2. Osteogenic cells: localized within the endosteum (stem cells of bone and cartilage tissue). They secrete factors that stimulate the proliferation and differentiation of hemopoietic cells.
3. Adventitial cells: poorly differentiated fibroblast-like cells covering sinusoids from the outside. Under the influence of erythropoietin, they can contract, facilitating the exit of blood cells into the bloodstream.
4. Adipocytes (fat cells): contain a large lipid droplet that pushes the nucleus to the periphery. They provide local energy supply and produce hemopoietins. Notably, the fat reserves of these cells are not depleted during generalized starvation.

Macrophage System

Red bone marrow macrophages develop from blood monocytes and are divided into three functional types:

• Typical macrophages: rich in lysosomes and phagosomes. They clear foreign agents and hemopoietic "debris" (such as nuclei extruded from orthochromatic erythroblasts), and secrete growth factors.
• Nurse cells: located in the center of erythropoietic islands. Their main task is to capture iron ions ($Fe^{2+}$) from the blood and transfer them to developing erythroblasts for hemoglobin synthesis. They also secrete erythropoietin.
• Osteoclasts: multinucleated cells formed by the fusion of monocytes. They are localized in the endosteum and are responsible for bone tissue lysis during bone remodeling.

Vascular System

Blood supply ensures trophism and the exit of mature formed elements into the blood. Lymphatic vessels are absent in red bone marrow.

The vascular bed includes narrow capillaries (responsible for nutrition only) and sinusoidal capillaries (venous sinuses). Sinuses are wide, thin-walled vessels whose endothelium and basement membrane feature slit-like pores.

Functions of the sinuses:

• Selective migration: only mature cells enter the bloodstream, recognized by the endothelium via specific surface glycoproteins.
• Maturation: erythrocyte maturation concludes within the lumen of the sinuses.
• Regulation: endothelial cells secrete factors that stimulate hemopoiesis.

Blood outflow proceeds from the capillaries into the central sinus, then into collecting veins piercing the bone, and further into periosteal vessels.

Histological Preparations of Bone Marrow

The study of red bone marrow is performed using two types of preparations, each with its own features:

• Bone marrow section (H&E stain): Typically prepared from a mouse embryo head. The tissue is located between developing bone trabeculae. It appears as a dense accumulation of cells with dark purple nuclei. Individual hemopoietic stages are difficult to distinguish in sections, but giant megakaryocytes, wide sinusoidal capillaries with flat endothelium, and pale pink reticular cells are clearly visible. It is important not to confuse bone marrow with surrounding embryonic tissues (muscles, glands, organ rudiments).
• Bone marrow smear (Azure II and eosin stain): Unlike sections, cells lie loosely. Surrounding tissues are absent, and only hemopoietic elements are present. This technique allows for detailed examination of cell morphology and accurate diagnosis of maturation stages (e.g., distinguishing a basophilic erythroblast from an oxyphilic one).