Biomedical Engineering Reference
In-Depth Information
in temperature makes cells more resistant to a
subsequent severe temperature elevation or makes
them refractory. This thermo-adaptive response
may not develop at an early stage or until day 4 in
cattle (Paula-Lopes and Hansen
2002a
) and the
eight-cell stage in mice (Ar´echiga et al.
1995
)
and varies in other animals like buffalo and goat.
Acquisition of the capacity for induced ther-
motolerance involves synthesis of heat-shock
protein 70 (HSP70) which helps stabilisation of
intracellular proteins and organelles, and apopto-
sis is inhibited (Brodsky and Chiosis
2006
). High
temperature can induce HSPs as early as the two-
cell stage in cattle (Edwards and Hansen
1996
)
and mice (Christians et al.
1997
) , that is, before
dividing fertilised cell acquire thermotolerance.
Therefore, other molecular mechanisms are most
likely involved in thermotolerance or adaptive
response of a fertilised cell in earlier stage of
development. Glutathione has been reported to
be required for induced thermotolerance in mice
(Ar´echiga et al.
1995
), and changes in redox
status may be an important determinant of deve-
lopment of induced thermotolerance.
Inhibition of apoptosis in bovine embryos
with a caspase inhibitor has been observed to
increase the magnitude of the reduction in deve-
lopment caused by high temperature (Paula-Lopes
and Hansen
2002b
). Thus, apoptosis, if limited to
the most damaged cells of the embryo, may allow
the embryo to continue to develop after thermal
challenge. In cattle, induction of apoptosis by
high temperature does not occur until the 8-16-
cell stage at day 4 after insemination (Paula-
Lopes and Hansen
2002a
). Most of the effects of
thermal challenges that occur at the early stage of
development after fertilisation of ovum may not
occur due to direct impacts of heat, and some of
these effects of elevated temperature on embry-
onic survival in utero could be indirect results
due to the changes in maternal physiology rather
than a direct effect on the embryo.
placental weights. The concentrations of placental
hormones in the blood are influenced, and effects
on growth are greater during mid-gestation than
that occurring during later gestation (Wallace
et al.
2005
). Some effects of heat stress on pla-
cental function represent redistribution of blood
to the periphery and reduced perfusion of the
placental vascular bed (Alexander et al.
1987
) .
However, reduced perfusion to the placenta is not
the only reason of reduced fetal weights because
placental blood flow per gram of fetus was
similar between heat-stressed and control ewes
(Wallace et al.
2005
). Perhaps more important is
an increase in vascular resistance in the placenta
(Galan et al.
2005
) caused by alterations in angio-
genesis as reflected by aberrant patterns of
expression of genes such as vascular endothelial
growth factor, its receptors and placental growth
factor (Regnault et al.
2002
). Heat stress has more
effects during mid-gestation than during late
gestation because angiogenesis is more extensive
during mid-gestation. Glucose transport capacity
across the placenta is also reduced by maternal
heat stress (Thureen et al.
1992
) , and this effect
involves reduced expression of GLUT8 genes in
cotyledonary placenta (Limesand et al.
2004
) .
Similar effects of maternal heat stress on pla-
cental functions and fetal development have also
been observed in the cows (Collier et al.
1982
) .
Reduced secretion of placental hormones as a
result of heat stress may reduce milk yield in cows
(Collier et al.
1982
; Wolfenson et al.
1988
) .
4.6
Uterine Environment
Heat stress reduces blood flow to the uterus, and
there is increase in uterine temperature which
may affect implantation and embryonic morta-
lity. These effects are likely to be associated with
the production of heat-shock proteins by the
endometrium during heat stress and reduced
production of interferon-tau by the conceptus.
Heat stress may affect endometrial prostaglandin
secretion leading to premature luteolysis and
embryonic loss (Malayer and Hansen
1990
) .
There are distinct breed differences between
Brahman and Holstein cows in endometrial
responses to culture at high temperature.
4.5
Fetal Development
In large animals, heat stress affects fetal growth
during gestation. Exposure of pregnant ewes to
heat stress has been observed to reduce fetal and
