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Because birth represents one temporal event in the development
of the infant, the interpretation of the blood picture in the newborn
requires an understanding of the maturational processes that precede
it.
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The anatomic sites of hematopoiesis change during the development of
the embryo and fetus (Fig. 429-1). Red blood
cell (RBC) formation first occurs within the yolk sac at 2 weeks’ gestation.
In this primitive phase of hematopoiesis, clusters of cells called
blood islands form in the yolk sac. The peripheral cells of these
blood islands differentiate to become the walls of blood vessels,
whereas the central cells become primitive blood cells or hemocytoblasts.
Similar blood islands are seen throughout the body stalk at 4 weeks’ gestation.
This primitive, intravascular phase of hematopoiesis declines by
6 weeks’ gestation and is no longer observed after the
third month.
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Blood formation begins to shift to the sinusoids of the liver
at the fifth week of gestation. The liver is the first site of definitive
hematopoiesis where leukocyte precursors and megakaryocytes (platelet
precursors) are also produced. Erythroid elements are the predominant
cell type, however. The liver is the main site of hematopoiesis
from the third to the fifth month of fetal life (Fig.
429-1) and declines thereafter, although continuing to produce
blood cells until the first week of postnatal life. A smaller amount
of definitive hematopoiesis also occurs in the spleen during this
phase.
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Hematopoiesis in the bone marrow begins in the third and fourth months
of gestation, and it becomes the predominant site of hematopoiesis
by the sixth month and remains so thereafter—throughout
the rest of gestation and extrauterine life (Fig.
429-1). The bone marrow is also the site where significant
myeloid hematopoiesis (production of white blood cells [WBCs] and
platelets) begins. During the last trimester, most blood cells are
produced in the marrow. The cellularity of the bone marrow becomes
maximal at 30 weeks’ gestation. In the newborn and during
early infancy, red or hematopoietic marrow fills ...