Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Organs of Hematopoiesis and the Immune System. The Lymphatic System
Central Organs of the Immune System
Bone Marrow

Bone marrow is a hematopoietic organ and a central organ of The Immune System. A distinction is made between Cytology/practical/86.html">Red bone marrow, which in adults is located in the spongy spaces of flat and short bones and in the epiphyses of long (tubular) bones, and yellow bone marrow, which fills the medullary cavities of the diaphyses of long (tubular) bones. The total mass of bone marrow in an adult is approximately 2.5–3 kg (4.5–4.7% of body weight). About half of it is red bone marrow, and the rest is yellow.

Red bone marrow has a dark red color and a semi-fluid consistency. It consists of a network of Connective Tissue (reticular) fibers, within the meshes of which lie Blood and immune system Cells at various stages of maturity—erythrocytes, various leukocytes, and B lymphocytes.

Red bone marrow is located around arterioles in the form of cylindrical cords and cellular islets. These cords are separated from one another by wide blood capillaries known as sinusoids. Mature Blood Cells (erythrocytes, leukocytes) and B lymphocytes, generated from stem cells within the bone marrow, penetrate into the lumens of the sinusoids (into the bloodstream) between endothelial cells and through slit-like openings—pores that form in the Cytoplasm of endothelial cells only at the moment of Cell passage. Immature cells enter the bloodstream only in certain diseases (of the bone marrow or blood).

Yellow bone marrow consists mainly of adipose tissue that has replaced the reticular stroma. The yellowish color of fat inclusions within degenerated reticular cells gave this part of the marrow its name. Hematopoietic elements are absent in yellow bone marrow. However, in cases of severe blood loss, red bone marrow can develop in place of the yellow bone marrow.

Age-related Features of the bone marrow. In newborns, red bone marrow occupies all medullary cavities. Individual fat cells in the red bone marrow first appear after birth (1–6 months). After 4–5 years, the red bone marrow in the diaphyses of tubular bones gradually begins to be replaced by yellow bone marrow. By the age of 20–25, yellow bone marrow completely fills the medullary cavities of the diaphyses of tubular bones. As for the medullary cavities of flat bones, fat cells account for up to 50% of the bone marrow volume there. In old age, yellow bone marrow may acquire a mucus-like consistency (gelatinous bone marrow).