Page 305 - Anatomy and Physiology of Farm Animals, 8th Edition

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were isolated from bone marrow and
maintained in cell culture. These studies
VetBooks.ir demonstrated that in the right conditions,

certain individual cells would form in vitro
colonies that contained immature and
mature cells of a specific cell line. The most
undifferentiated cells that could form the
cells of a specific lineage were termed col­
ony‐forming units (CFUs). More current
literature uses the stem cell terminology;
for example, a myeloid stem cell is equiva-
lent to a colony‐forming unit–granulo- Figure 15-2. Red blood cells as seen by scanning
cytic, erythrocytic, monocytic, and electron microscopy. Source: Cohen and Wood,
megakaryocytic (CFU‐GEMM). 2000. Reproduced with permission of Lippincott
Obviously, the proliferation and differ- Williams & Wilkins.
entiation of bone marrow stem cells must
be highly regulated. For example, the gen- exchange across the cell membrane.
eration of erythrocytes should increase in Erythrocytes have no nuclei and few
response to blood loss after hemorrhage, organelles (see Fig. 1‐9). Total erythrocyte
while the generation of leukocytes should counts are expressed as number of cells per
increase in response to an infection. A microliter of whole blood, and most
large variety of circulating chemical mes- domestic animals have about 7 million per
sengers have been found to regulate the microliter (Table 15‐2). (Total leukocyte
proliferation and differentiation of bone and platelet counts are also expressed per
marrow stem cells. The general term for microliter of blood.)
such agents is hematopoietin. An individ- The protein hemoglobin is the major
ual hematopoietin may stimulate commit- intracellular constituent of erythrocytes.
ted cells within development pathways to Hemoglobin is a complex molecule con-
give rise to specific blood cells or may have taining four amino acid chains (globin
a more general effect by stimulating less portion) held together by noncovalent
committed stem cells. For example, eryth­ interactions. Each amino acid chain con-
ropoietin is the hematopoietin that stimu- tains a heme group (red porphyrin pigment),
lates a specific increase in erythrocyte and each heme group contains an atom of
production, and interleukin‐2 stimulates iron. Hemoglobin concentration is meas-
increases in the production of several leu- ured in grams per 100 mL of blood, and
kocytes. More information on individual typical values for normal hemoglobin
hematopoietins is provided in later sec- concentrations range from about 11 to
tions of this chapter and in Chapter 16. 13 g/100 mL in domestic mammals.
Based on the CFU terminology, hemat- Hemoglobin functions in the transport
opoietins have also been described as col­ of both oxygen and carbon dioxide.
ony‐stimulating factors. Oxygen binds to the ferrous iron in the
heme group to form oxyhemoglobin
(HbO ); this process is termed oxygenation
2
Erythrocytes (not oxidation). The amount of oxygen that
can be bound is proportional to the amount
Erythrocytes (from the Greek erythro‐, of iron present, with one molecule of
red; ‐cyte, cell) range from about 5 to 7 μm oxygen combining with each atom of iron.
in diameter. They are biconcave disks with Because of the binding to hemoglobin,
a thick circular margin and a thin center blood can contain about 60 times as much
(Fig. 15‐2). The biconcave shape provides oxygen as would be dissolved in a similar
a relatively large surface area for gas quantity of water in the same conditions.