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hematopoiesis The process through which the
body generates new BLOODcells. In the adult, the
red BONE MARROWand the LY M P Htissues (primarily
the lymph nodes and the SPLEEN) manufacture the
blood cells the body needs, with extramedullary
resources for ERYTHROCYTEproduction available as
reserves from the LIVER, spleen (erythrocytes), and
yellow BONE marrow. Researchers do not fully
understand the mechanisms of hematopoiesis
though know complex interactions of hormones,
proteins, and chemicals regulate the processes by
which the body makes new blood cells. There are
two major divisions of hematopoiesis: erythro-
poiesis (production of erythrocytes) and
leukopoiesis (production of leukocytes).

Pluripotency, Differentiation, and Proliferation
As best researchers understand the mechanisms of
hematopoiesis, all blood cells arise from pluripo-
tent BLOOD STEM CELLSthat have the ability to

develop into any of the blood cell types. The first

level of hematopoiesis occurs when a blood stem

cell either proliferates, extending the volume of

pluripotent cells, or differentiates into one of two

committed lineages, myeloid or lymphoid. The

lymphoid lineage will produce lymphocytes and

monocytes, and the myeloid lineage will produce

erythrocytes, granulocytes (basophils, eosinophils,

and neutrophils), and platelets. Each lineage gen-

erates a number of differentiations or stages of

development. The length of time it takes for a

pluripotent cell to produce a mature blood cell

varies with the type of blood cell and other physi-

ologic factors, ranging from 6 days for an erythro-

cyte to 14 days for a neutrophil.

Erythropoiesis

Erythropoiesis begins with committed myeloid

cells that differentiate into myeloblasts or proery-

throcytes. Myeloblasts will become granulocytes,

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HEMATOPOIETIC STRUCTURES

Hematopoietic Structure Blood Cells the Structure Produces
red BONE MARROW erythrocytes, platelets, granulocytes, some monocytes

LIVER erythrocytes on demand (extramedullary resource)

LYMPHnodes lymphocytes, monocytes

SPLEEN lymphocytes, monocytes
erythrocytes on demand (extramedullary resource)

THYMUS lymphocytes

yellow bone marrow limited leukocytes
erythrocytes and platelets on demand (extramedullary resource)