Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

III. Developing Erythroid Cells
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growth hormone and IGF-1 are reported to decrease
Epo synthesis in rat kidneys ( Sohmiya and Kato, 2005 ).
Glucocorticoids promote the differentiation of embryonic
stem cells to hematopoietic cells and prolong the proliferation
of erythroid progenitor cells but reduce the rate of
spontaneous differentiation and terminal maturation of erythroid
cells ( Leberbauer et al. , 2005 ; Srivastava et al. , 2006 ).
Glucocorticoids appear to be important in stress erythropoiesis
(e.g., following hemorrhage or increased RBC destruction)
when a substantial increase in erythropoiesis is required
(Bauer et al. , 1999 ). The thyroid hormone 3,5,3 -triiodothyronine
(T3) promotes the differentiation and maturation of
erythroid cells toward enucleated RBCs ( Leberbauer et al. ,
2005 ). Thyroid hormones may also promote the synthesis of
Epo in the kidney ( Ma et al. , 2004 ).
Epo production in adult mammals occurs primarily
within peritubular interstitial cells that are located within
the inner cortex and outer medulla of the kidney. The liver
is an extrarenal source of Epo in adults and the major site of
Epo production in the mammalian fetus ( Jelkmann, 2007 ).
Bone marrow macrophages and erythroid progenitor cells
themselves have also been shown to produce Epo, suggesting
the possibility of short-range regulation of erythropoiesis
( Stopka et al. , 1998 ; Vogt et al. , 1989 ).
The ability to deliver oxygen to the tissues depends on
cardiovascular integrity, oxygen content in arterial blood,
and Hb oxygen affinity. Low oxygen content in the blood
can result from low partial pressure of oxygen (pO 2 ) in
arterial blood, as occurs with high altitudes or with congenital
heart defects in which some of the blood flow
bypasses the pulmonary circulation. Low oxygen content
in blood can also be present when arterial pO 2 is normal, as
occurs with anemia and methemoglobinemia. An increased
oxygen affinity of Hb within RBCs results in a decreased
tendency to release oxygen to the tissues ( McCully et al. ,
1999 ).
Epo production is stimulated by tissue hypoxia, which
is mediated by hypoxia-inducible factors (HIFs) that are
heterodimers consisting of α and β subunits. An α subunit
denoted 2 α is most important in Epo production, at least for
definitive erythropoiesis. Both α and β subunits are continuously
translated, but α subunits are labile and regulated
by tissue oxygen levels. At normal tissue oxygen levels in
tissue (pO 2 36 mmHg), α subunits are hydroxylated by
prolyl hydroxylases, polyubiquitinated, and removed by
proteasomal degradation. When tissue oxygen levels are low
(pO 2 36 mmHg), α subunits are no longer hydroxylated
and degraded, allowing them to translocate into the nucleus
and combine with β subunits to form heterodimeric transcription
factors. These HIF heterodimers activate the transcription
of the Epo gene, and many other target genes, by
binding to the hypoxia responsive elements (HREs) in their
promoter/enhancer regions. Binding to the Epo gene results
in increased Epo synthesis when tissue hypoxia is present
(Gruber et al. , 2007 ; Jelkmann, 2007 ).
Other tissues also exhibit Epo receptors, and Epo also
stimulates nonhematopoietic actions including promoting
proliferation and migration of endothelial cells, enhancing
neovascularization, stimulating the production of modulators
of vascular tone, and exerting cardioprotective and neuroprotective
effects ( Jelkmann, 2007 ).
III . DEVELOPING ERYTHROID CELLS
A . Morphological and Metabolic Changes
Rubriblasts are large cells (approximately 900 fl in
humans) that are continuously generated from progenitor
cells in the extravascular space of the bone marrow. The
division of a rubriblast initiates a series of approximately
5 divisions over a period of 3 to 5 days to produce about
32 metarubricytes that are no longer capable of division
( Prchal, 2006 ). These divisions are called maturational
divisions because there is a progressive maturation of the
nucleus and cytoplasm concomitant with the divisions.
Each division yields a smaller cell with greater nuclear
condensation and increased Hb synthesis. An immature
RBC, termed a reticulocyte , is formed following extrusion
of the nucleus ( Harvey, 2001 ).
Early precursors have intensely blue cytoplasm, when
stained with Romanowsky-type bloodstains, owing to the
presence of many basophilic ribosomes and polyribosomes
that are actively synthesizing globin chains and smaller
amounts of other proteins. As the cells are nonsecretory,
rough endoplasmic reticulum is scant and limited to early
erythroid precursors ( Bessis, 1973 ). Hb progressively accumulates
in these cells, imparting a red coloration to the cytoplasm.
Cells with both red and blue coloration are described
as having polychromatophilic cytoplasm ( Harvey, 2001 ).
Kinetics of erythroid cells and changes in biochemical and
metabolic pathways are depicted in Figure 7-1 ; time intervals
were determined for cattle ( Rudolph and Kaneko, 1971 ).
B . Iron Metabolism
Erythroid precursors have iron requirements that far exceed
the iron requirements of any other cell type because of the
need for Hb synthesis. Developing erythroid cells generally
extract about 75% of the iron circulating in plasma ( Smith,
1997 ).
1 . Transferrin and Transferrin Receptors
Plasma iron is bound to apotransferrin, a beta globulin that
can maximally bind two atoms of ferric iron per molecule.
The proportion of apo-, mono-, and diferric forms of transferrin
present in serum depends on the percentage saturation
of transferrin with iron. Diferric transferrin is more
efficient than monoferric transferrin in delivering iron