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

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Chapter | 7 The Erythrocyte: Physiology, Metabolism, and Biochemical Disorders
proteins (e.g., the anion transporter and glycophorin A) are
retained and concentrated ( Geminard et al. , 2002 ; Ponka
et al. , 1998 ). The heat shock protein 70 (Hsp70) is concentrated
in exosomes and may promote the formation of exosomes
during reticulocyte maturation ( Jeong et al. , 2005 ).
Some membrane proteins such as the nucleoside transporter,
glucose transporter, Na,K-ATPase, insulin receptor, and
adrenergic receptors decrease to variable degrees depending
on the species involved ( Chasis et al. , 1989 ; Geminard
et al. , 2002 ). Examples of reticulocytes from a species that
exhibit a complete or nearly complete loss of a protein that
is retained in mature RBCs from other species include the
adenosine transporter in sheep ( Jarvis and Young, 1982 ),
the glucose transporter in pigs ( Zeidler and Kim, 1982 ), and
Na,K-ATPase in dogs ( Maede and Inaba, 1985 ).
Although loss of membrane components accounts for
much of the change in membrane protein composition during
reticulocyte maturation, certain proteins such as protein
4.1 and glycophorin C increase because they are still being
synthesized in reticulocytes ( Chasis et al. , 1989 ). These
membrane alterations result in increased mechanical stability
of blood reticulocyte and RBC membranes compared
to marrow reticulocyte and nucleated erythroid cell membranes
( Waugh et al. , 2001 ).
Mitochondria undergo degenerative changes in a
programmed death phenomenon (mitoptosis) owing to
15-lipoxygenase attack and subsequent ATP-dependent proteolysis
( Geminard et al. , 2002 ). Degenerating mitochondria
are either digested or extruded following entrapment in structures
resembling autophagic vacuoles ( Simpson and Kling,
1968 ). The polysomes separate into monosomes and decrease
in number and disappear as reticulocytes mature into
RBCs. The degradation of ribosomes appears to be energy
dependent; it presumably involves proteases and RNAases
(Rapoport, 1986 ).
4 . Species Differences in Marrow Release
Reticulocyte maturation begins in the bone marrow and is
completed in the peripheral blood and spleen in dogs, cats,
and pigs. As reticulocytes mature, they lose the surface receptors
needed to adhere to fibronectin and thrombospondin
components of the extracellular matrix, presumably facilitating
their release from the bone marrow ( Telen, 2000 ).
Residual adhesion molecule receptors on newly released
reticulocytes may explain their tendency to concentrate in
the reticular meshwork of the spleen ( Patel et al. , 1985 ).
Reticulocytes become progressively more deformable as
they mature, a characteristic that also facilitates their release
from the marrow ( Waugh et al. , 2001 ). To exit the extravascular
space of the marrow, reticulocytes press against
the abluminal surfaces of endothelial cells that make up the
sinus wall. Cytoplasm thins and small pores (0.5 to 2 μm)
develop in endothelial cells, which allow reticulocytes to
be pushed through by a small pressure gradient across the
sinus wall ( Lichtman and Santillo, 1986 ; Waugh, 1991 ).
Pores apparently close after cell passage.
Relatively immature aggregate-type reticulocytes are
released from canine bone marrow; consequently, most of
these cells appear polychromatophilic when viewed following
routine blood film staining procedures ( Laber et al. ,
1974 ). Absolute reticulocyte counts oscillate with a periodicity
of approximately 14 days in some dogs, suggesting
that canine erythropoiesis may have a homeostatically
controlled physiological rhythm ( Morley and Stohlman,
1969 ). Reticulocytes are normally not released from feline
bone marrow until they mature to punctate-type reticulocytes;
consequently, few or no aggregate reticulocytes
( 0.4%), but up to 10% punctate reticulocytes, are found
in blood from normal adult cats ( Cramer and Lewis, 1972 ).
The high percentage of punctate reticulocytes results from
a long maturation time with delayed degradation of ribosomes
( Fan et al. , 1978 ). Reticulocytes are generally absent
in peripheral blood of healthy adult cattle and goats, but a
small number of punctate types (0.5%) may occur in adult
sheep ( Jain, 1986 ). Based on microscopic examination of
blood films stained with new methylene blue, equine reticulocytes
are absent from blood normally and rarely released
in response to anemia. However, low numbers of reticulocytes
have been reported in the blood of normal and anemic
horses using an Advia 120 (Siemens Medical Solutions
Diagnostics, Tarrytown, New York) automated analyzer
(Cooper et al. , 2005 ). Either the instrument is more sensitive
than microscopic evaluation, or values reported in
normal horses represent “ noise ” in the instrument.
5 . “Stress ” Reticulocytes
Except for horses, increased numbers of reticulocytes are
released in response to anemia, with better responses to
hemolytic anemia than to hemorrhage. When the degree of
anemia is severe, basophilic macroreticulocytes, or so-called
stress reticulocytes, may be released into blood. It is proposed
that a generation in the maturation sequence is skipped
and immature reticulocytes, about twice the normal size, are
released ( Rapoport, 1986 ). Increased Epo results in a diminution
in the adventitial cell and endothelial cell barrier separating
marrow hematopoietic cells from the sinus, thereby
potentiating the premature release of stress reticulocytes from
the marrow ( Chamberlain et al. , 1975 ). Although a portion
of these macroreticulocytes apparently is rapidly removed
from the circulation ( Noble et al. , 1990 ), it is clear from
studies in cats that some can mature into macrocytic RBCs
with relatively normal life spans ( Weiser and Kociba, 1982 ).
E . Abnormalities in Erythroid Development
1 . Ineffective Erythropoiesis
Ineffective erythropoiesis is a term used to describe the
destruction of developing erythroid cells in marrow.