Page 221 - Atlas of Histology with Functional Correlations

SECTION 2 Bone Marrow

Although bones provide important structural support for the body, they also

serve as important sites for blood cell formation. Bone marrow is a highly
cellular tissue that is located in the medullary cavities of the bone. Red bone
marrow is the principal site of blood cell formation or hematopoiesis located
between the bony trabeculae of the bone. Red bone marrow consists of densely
packed cords and islands of blood-forming (hematopoietic) stem cells. They are

surrounded by numerous macrophages and abundant branching sinusoidal
capillaries that open into the thin venous sinuses. These sinuses provide the main
exit route through the openings in their endothelial lining for the newly

differentiated blood cells to enter the systemic circulation. A connective tissue
stroma of reticular cells and reticular fibers form a delicate meshwork that
surrounds the islands of hematopoietic cells and provides support for the bone
marrow.

The active red bone marrow in selected bones provides a steady rate of blood

cell renewal to replace those that are worn out or lost. Also, the red bone marrow
is the site where tissue macrophages engulf and phagocytose worn-out
erythrocytes and store the iron recovered from the hemoglobin breakdown for
the next generation of blood cells.

Supplemental micrographic images are available at

www.thePoint.com/Eroschenko13e under Blood Cells.

Figure 6.12 | Development of Different Blood Cells

in Red Bone Marrow (Decalcified Section)

In a section of the red bone marrow, all types of developing blood cells are
difficult to distinguish. The cells are densely packed, and different cell types are
intermixed. During the maturation process, hematopoietic cells become smaller

and their nuclear chromatin more condensed. As the blood cells pass through a
series of developmental stages, they exhibit morphologic changes and become
microscopically identifiable.

This section of bone marrow is stained with hematoxylin and eosin stain. At

this magnification, little differentiation of cytoplasm is visible. In the

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