b. During the first two months of embryonic development, the mesoblastic phase
occurs. The blood cells are formed in the blood islands of the yolk sac. Immature blood
cells develop, having large nuclei containing a vesicular chromatin meshwork. The
nuclei are surrounded by a thin rim of cytoplasm. The cells gradually develop
hemoglobin to become a nucleated red blood cell. Mitosis occurs and daughter cells
contain more hemoglobin. Finally the nucleus is lost and the nature erythrocyte is
c. At two months, the hepatic phase begins. Blood cell development shifts to
the body of the fetus as the organs of the reticuloendothelial system (liver, spleen,
thymus, etc.) are formed. During this phase, red cells begin their normal development.
Granulocytes and thrombocytes are formed in the liver while lymphocytes and
monocytes are formed in the spleen and thymus.
d. The myeloid phase begins during the fifth month of gestation. Blood cells are
formed in the bone marrow and lymphatic system that at the time of birth constitute the
total sources of hematopoiesis. The bone marrow is the principle source of production
of erythrocytes, granulocytes, and thrombocytes. The lymph nodes are the primary
sites of production of lymphocytes, monocytes, and plasmacytes.
a. Blood formation at birth is confined primarily to the bone marrow (central
medullary structure of the bone). Blood cells multiply by mitosis and then mature to a
specific cell type. The mature cells lose the ability to reproduce and develop a definite
life span. Regeneration of blood cells after birth involves multiplication of precursor
cells, evolution of the definitive characteristics of each type, and release of mature cells.
b. Myelopoiesis is the production of blood cells and bone marrow by the bone
marrow (medullary site of production). The red bone marrow is the principle source of
production of red cells and white cells of the granulocytic series. At birth, the central
medullary structure of bones is red bone marrow and it is actively engaged in
hematopoiesis. At about 5 years of age non-hematopoietic type of marrow (fatty yellow
bone marrow) that is a reserve potential tends to replace most of the red bone marrow.
This partial replacement of red marrow is complete when the individual reaches maturity
(about 18 years of age) at which time, active hematopoietic centers in bone tissue are
limited almost exclusively to the sternum, pelvic area, vertebrae, skull, ribs, clavicle,
scapuli, and the epiphyses of the long bones.
c. Extramedullary hematopoiesis is blood production that occurs in sites other
than the bone marrow. Active sites are the spleen, thymus, lymph nodes, and other
lymphoid tissues. Cell production is largely limited to lymphopoiesis. The lymph nodes
are the primary source of lymphocytes and plasmacytes (see figure 1-3).