Biomedical Engineering Reference
In-Depth Information
sion and biochemical adaptive responses (Fujita
1999
; Lindquist
1986
) . Biologically, the ability
to survive and adapt to thermal stress is a funda-
mental requirement of cellular life, as cell stress
responses are ubiquitous among both eukaryotes
and prokaryotes, and key heat-shock proteins
(HSPs) involved in these responses are highly
conserved across all species (Lindquist
1986
;
Parsell and Lindquist
1993
) . In euthermic species,
in which core temperature is very precisely regu-
lated, considerable variations in core temperature
can occur during severe environmental stress,
exercise and fever. The ability to survive and adapt
to severe systemic physiological stress is critically
dependent on the ability of cells to mount an
appropriate compensatory stress response.
The effects of heat stress on cellular function
include:
1. Inhibition of DNA synthesis, transcription,
RNA processing and translation
2. Inhibition
Brahman, Senepol and Angus in the amount of
heat-shock protein 70 (HSP70) in heat-shocked
lymphocytes (Kamwanja et al.
1994
) , and the
nonsignificant lower HSP70 amounts in Brahman
and Senepol may indicate that protein denatura-
tion in response to elevated temperature (one of
the signals for HSP70 synthesis; Ananthan et al.
1986
) is reduced in Brahman and Senepol.
Therefore, magnitude of transcription in response
to stressor or high temperature seems to be impor-
tant for expression of genetic differences in breeds
as there are no differences between Brahman and
Holstein embryos in resistance to elevated tem-
perature at the two-cell stage (Krininger et al.
2003
), a time when the embryonic genome is
largely inactive (Memili and First
2000
) .
11
Genetic Improvement
for Adaptation
of
progression
through
the
cell
cycle
3. Denaturation and misaggregation of proteins
4. Increased degradation of proteins through
both proteasomal and lysosomal pathways
5. Disruption of cytoskeletal components
6. Alterations in metabolism that lead to a net
reduction in cellular ATP
7. Changes in membrane permeability that lead
to an increase in intracellular Na
+
, H
+
and Ca
2+
In mammalian cells, nonlethal heat shock pro-
duces changes in gene expression and in the
activity of expressed proteins, resulting in cell
stress response (Jaattela
1999
; Lindquist
1986
) .
This response increases thermotolerance (i.e. the
ability to survive subsequent, more severe heat
stresses) that is associated with increased expres-
sion of HSPs. A cell stress response leading to
HSPs production can be induced by other stres-
sors like toxins, chemicals, pyrogens and heavy
metals, and the response initiated by one stressor
often leads to cross-tolerance to others (Parsell
and Lindquist
1993
) . At increasingly severe heat
exposures, heat shock leads to activation of the
apoptotic process and to cellular necrosis (Creagh
et al.
2000
). The apoptosis of cells exposed to
different stressors appears to depend critically on
the sequence of exposure (DeMeester et al.
2001
) .
There were no significant differences between
Acclimation and adaptation are two different
processes in response to a stressor. Animals are
considered acclimated to a given stressor when
body temperature returns to prestress levels
(Nienaber et al.
1999
) . Systemic, tissue and
cellular responses associated with acclimation
are coordinated, require several days or weeks
to occur and are therefore, not homeostatic in
nature (Bligh
1976
). Furthermore, when stress is
removed, these changes decay. Adaptation, on the
other hand, requires modifications of the genetic
makeup and is a process involving populations
and exposure for very long periods.
Genetic improvement is an evolutionary
action; evolution should be defined as a conti-
nuous process of adaptation of the populations
of organisms to the ever-changing geological,
biological and climatic conditions (Dobzhansky
1970
). Because of the almost infinite number of
combinations of environmental factors, organi-
sms must have a great variety of genetic types
capable to deal with a range of climatic, nutri-
tional or other conditions. In a word, any popula-
tion must be genetically heterogeneous - that is,
with a great genetic diversity - in order to be able
to survive under the challenge of the changing
environment. Therefore, any population in a
specific ambient is composed by a majority of
