Alterations Of Hematology
Alterations Of Hematology
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Fetal and Neonatal Hematopoiesis
As the developing embryo becomes too large for oxygenation of tissues by simple diffusion, the production of erythrocytes begins within the vessels of the yolk sac. Shortly after 2 weeks of gestation, circulating erythrocytes play a major role in delivering oxygen to the tissues. At approximately the eighth week of gestation, the site of erythrocyte production shifts from the vessels to the liver sinusoids, and the production of leukocytes and platelets begins in the liver and spleen. Erythropoiesis in the liver and, to a lesser extent, in the spleen and lymph nodes, reaches a peak at approximately 4 months. Hepatic blood formation declines steadily thereafter but does not disappear entirely during the remainder of gestation. By the fifth month of gestation, hematopoiesis begins to occur in the bone marrow and increases rapidly until hematopoietic (red) marrow fills the entire bone marrow space. By the time of delivery, the marrow is the only significant site of hematopoiesis.
In neonates and young infants, hematopoietic marrow progressively fills the bony cavities of the entire axial skeleton (skull, vertebrae, ribs, sternum), the long bones of the limbs, and many intramembranous bones. (These structures are described in Chapter 45.) Fatty (yellow) marrow gradually replaces hematopoietic marrow in some bones. During childhood, hematopoietic tissue retreats centrally to the vertebrae, ribs, sternum, pelvis, scapulae, skull, and proximal ends of the femur and humerus.
In diseases characterized by hemolysis, erythrocyte production can increase as much as eight times the normal because erythropoietin causes hematopoietic marrow to increase in volume. Initially, hematopoietic marrow expands from the ends of the long bones toward the middle of the shafts, replacing fatty marrow. Next, blood cell production begins to occur outside the marrow cavities, especially in the liver and spleen. Extramedullary hematopoiesis is more likely to occur in children than in adults because the bony cavities of children already are filled with red marrow (Figure 30-1). This is why hemolytic disease causes especially pronounced enlargement of the spleen and liver in children.
