Biomedical Engineering Reference
In-Depth Information
5.1
Mechanism of Effects of Heat
observed to occur about 2 weeks after thermal
stress and return to normal takes about 8 weeks
after the heat stress. Culture of bull spermato-
zoa at 40°C was observed to have no effect on
their fertilising capability and the competence
of the resultant embryos to develop to the
blastocyst stage (Hendricks et al.
2009
) .
Ejaculated bull and stallion spermatozoa
were observed to induce no apoptotic changes
when cultured at temperatures characteristic of
physiological hyperthermia (Hendricks and
Hansen
2009
). However, there may be some
epigenetic changes in embryonic development
associated with damage to the sperm in the
reproductive tract. Insemination of rabbit does
with sperm exposed to elevated temperature
in vitro (Burfening and Ulberg
1968
) or in the
female reproductive tract (Howarth et al.
1965
)
was observed to result in reduced preimplanta-
tion and post-implantation survival (Burfening
and Ulberg
1968
) . The X and Y spermatozoa
are likely to be affected differentially by elevated
temperature. The sex ratio of embryos has been
observed to skew towards female when female
mice were bred to males experiencing scrotal
heat treatment on the day of mating (Pe´rez-
Crespo et al.
2008
). Incubation of sperm at
40°C for 4 h compared to 38.5°C has been
observed to reduce the proportion of embryos
(Hendricks et al.
2009
) .
The effect of heat stress on the testes and testi-
cular functions varies widely in animals. Different
cell types are affected in different ways, and
response of one cell type is likely to be different
from other cells. The Leydig cells are either not
directly affected by heat or are only minimally
affected; however, the primary site of action of
heat stress may be the Sertoli cell (Setchell
2006
) .
Because of its position in the seminiferous epi-
thelium, it may be influenced. The germ cells
depend almost entirely on the Sertoli cells for
nutrients, and their development is influenced by
the environment components of Sertoli cells. The
Sertoli cells could have an influence on cells such
as the spermatogonia and preleptotene spermato-
cytes. Secretion of fluid appears to be reduced
under some circumstances, but not in the first
24 h after the testes were made cryptorchid
(Setchell
2006
). The changes may occur in com-
position of the secreted fluid without any influence
on total volume secreted, and the composition of
the secretion is likely to be affected at different
temperatures. An effect on the Sertoli cells
could also influence chromosome behaviour dur-
ing the meiotic prophase, and investigations on
the effects of heat on the synaptonemal complex
may be required to further elucidate responses
(Setchell
2006
) .
Increased metabolism in the testis after heat
stress may not be met by a suffi cient increase in
blood fl ow and may become hypoxic (Setchell
1998
). The response or damage due to hypoxic
condition on exposure to heat may not be so much
directly, as compared to that caused by the gen-
eration of reactive oxygen species and the effect
of scavengers for ROS during heating or immedi-
ately afterwards. Therefore, changes in enzymes,
heat-shock factors and heat-shock proteins need
to be understood for their protective or harmful
responses on the testis.
Semen characteristics are not likely to be
immediately affected by changes in testicular
temperature as damaged spermatogenic cells
do not enter ejaculates immediately after heat
stress. In the bull, where spermatogenesis takes
about 61 days, alterations in semen have been
5.2
Hormone Secretion
Limited studies have been carried out in livestock
species on hormonal mechanisms involved in
depression of male gonadal functions during
summer. There are few experiments on the effects
of high environmental temperature on the secre-
tion of hormones controlling reproductive func-
tions. Data from both bulls and boars indicate
that heat stress causes an initial decline in cir-
culating concentrations of testosterone lasting
2 weeks, but concentrations are restored even in
the face of continued heat stress (Rhynes and
Ewing
1973
; Wettemann and Desjardins
1979
) .
Thyroxine levels show a positive correlation with
seminal volume and initial motility while T
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