Special Histology and Embryology: Practical Course - V. K. Napkhanyuk 2001

Organs of Hematopoiesis and Immune Defense
Bone Marrow

The Bone Marrow (medulla ossium) is the central hematopoietic organ, containing a self-maintaining population of hematopoietic stem Cells belonging to both myeloid and lymphoid lineages.

In adult humans, a distinction is made between Cytology/practical/86.html">Red bone marrow and yellow bone marrow.

Red bone marrow (medulla ossium rubra) is the hematopoietic portion of the bone marrow. It fills the spongy substance of flat bones and the epiphyses of long bones. In the adult Organism, it accounts for approximately 4–5% of total body weight. The red bone marrow consists of a coarse stroma (Skeleton); a delicate stroma; and parenchyma.

The coarse stroma is formed by the endosteum of the trabeculae of spongy bones and provides a structural support for the delicate stroma.

The delicate stroma is formed by reticular Connective Tissue, which serves as a supportive framework for Blood Cells at various stages of their development.

The parenchyma comprises blood cells at various Selection/3.html">Stages of development.

A vast network of microcirculatory Blood Vessels runs through the reticular (delicate) stroma, interspersed with the following hematopoietic cells: stem cells; hemopoietic stem Cell progenitors (semi-stem cells); blasts; proerythroblasts; promyelocytes; metamyelocytes; prolymphocytes; promonocytes; megakaryoblasts; megakaryocytes; and mature blood formed elements.

Hematopoiesis is most intense near the endosteum, where the concentration of hematopoietic stem cells is roughly 3 times higher than in the center of the medullary cavity.

Hematopoietic cells are arranged in islets within the bone marrow stroma. As the cells mature, their motility and ability to enter the vascular bed increase; this bed is represented by blood capillaries, postcapillary sinusoids, or sinuses. The walls of the sinuses consist of flattened endothelial cells pierced by slit-like openings or pores through which formed elements of the blood and plasma migrate. Fixed macrophages are interspersed among the endothelial cells.

Granulocytopoietic cells also form segregated islets. During maturation, granulocytes are stored in the red bone marrow, where they are three times as numerous as erythrocytes and 20 times as numerous as granulocytes in the peripheral blood.

Megakaryoblasts and megakaryocytes maintain close contact with the sinuses, allowing the peripheral region of their Cytoplasm to project through the pores into the vessel lumen. Cytoplasmic fragments pinch off and enter the bloodstream directly as platelets.

Under normal physiological conditions, only mature blood-formed elements cross the bone marrow sinusoidal wall. Immature cell forms (myelocytes and erythroblasts) enter the blood exclusively under pathological conditions.

Among the myeloid cell islets, small clusters of bone marrow lymphocytes and monocytes surrounding a blood vessel can occasionally be found.

Slides for Study

Slide 1. Red bone marrow (puncture biopsy smear) (Fig. 28).

High magnification. Examine the red bone marrow slide.

In the bone marrow smear, differentiating Cells of the erythrocytic and granulocytic series can be identified. The myeloblast is a large cell with a round or oval Nucleus containing dispersed Chromatin and pale blue cytoplasm. The promyelocyte resembles the myeloblast in appearance, but its cytoplasm contains non-specific azurophilic and specific granules. The number of specific granules is small; therefore, without immersion Microscopy, it is generally impossible to distinguish basophilic, eosinophilic, and neutrophilic promyelocytes.

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Fig. 28. Red bone marrow (puncture biopsy smear). Pappenheim staining. X 300:

1 — myeloblast; 2 — proerythroblast; 3 — erythroblast; 4 — neutrophilic myelocyte; 5 — normoblasts; 6 — neutrophilic metamyelocyte; 7 — erythrocytes; 8 — megakaryocyte; 9 — polychromatophilic erythroblast; 10 — acidophilic erythroblast

The myelocyte features a round or oval nucleus with more condensed chromatin than that of the promyelocyte, alongside a significant amount of specific granulation, which makes it easy to differentiate basophilic, eosinophilic, and neutrophilic myelocytes.

Eosinophils contain numerous densely packed pink granules, while basophils exhibit coarse dark-purple granulation.

In metamyelocytes, the shape of The Nucleus changes: it becomes Kidney-shaped (bean-shaped), followed by nuclear segmentation leading to The formation of a mature (segmented) granulocyte.

Erythrocytic Lineage cells differ from granulocytes primarily by their smaller size and homogeneous cytoplasmic staining. The nuclei of erythroblasts—ranging from basophilic to oxyphilic (acidophilic)—decrease in size and undergo pyknosis. The basophilic erythroblast has a denser nucleus compared to the blast and dark-blue cytoplasm. The polychromatophilic erythroblast is distinguished by a lighter, grayish cytoplasmic tone, occasionally with a pinkish or bluish tinge. The nucleus is small, with compact chromatin, and is located eccentrically. The oxyphilic erythroblast stains pink and possesses a very small, dark nucleus situated at the periphery of The Cell.

Megakaryocytes are large cells characterized by a broad expanse of cytoplasm and large, multilobed nuclei. The cytoplasm exhibits weakly basophilic staining.

On the figure, indicate: 1) myeloblast; 2) proerythroblast; 3) erythroblast; 4) neutrophilic myelocyte; 5) normoblast; 6) neutrophilic metamyelocyte; 7) erythrocytes; 8) megakaryocyte; 9) polychromatophilic erythroblast; 10) acidophilic erythroblast; 11) mitosis of a proerythroblast.