Biomedical Engineering Reference
In-Depth Information
the scrotum is insulated, the testes are internalised
(i.e. cryptorchidism induced) or body temperature
is raised because of fever or thermal environment
(Setchell
1998
) . However, low body temperature
is not an absolute requirement for spermatogenesis.
Birds, which have body temperatures higher than
mammals (Prosser and Heath
1991
) , have internal
testes. The cells that are most susceptible to dam-
age by high temperature are the spermatocyte and
spermatid (Setchell
1998
) , although B sper-
matogonia are also damaged. Oxidative stress is
a major cause for thermal damage to spermato-
genic cells which leads to apoptosis and DNA
strand breaks (Pe´rez-Crespo et al.
2008
; Paul
et al.
2008,
2009
). Effects of cryptorchidism on
spermatogenesis enhance in superoxide dismus-
tase-1 knockout mice (Ishii et al.
2005
) .
Heat stress significantly impairs bull fertility
during summer. Semen quality decreases when
bulls are continually exposed to ambient tempera-
tures of 86 °F for 5 weeks or 100 °F for 2 weeks
despite no apparent effect on libido (De la Sota
et al.
1998
). Heat stress decreases sperm concen-
tration, lowers sperm motility and increases
percentage of morphologically abnormal sperm
in an ejaculate. When exposed to long period of
heat stress, semen quality does not return to nor-
mal for approximately 2 months because of the
length of the spermatic cycle, adding to the carry-
over effect of heat stress on male reproduction.
High temperatures cause degeneration of mei-
otic germ cells in the seminiferous tubules (Lue
et al.
1999
) , in fl uence the structure of spermatozoa
DNA (Love and Kenney
1999
) and sperm counts
and sperm motility are lowered (Setchell
1998
) .
The progressive sperm motility is more sensitive to
temperature variation as compared to the number of
spermatozoa produced at each ejaculation (Moreira
et al.
2001
). If heat damaged sperm cells are used to
fertilise normal oocytes, premature embryonic death
may occur (Burfening and Ulberg
1968
) .
Paul et al. (
2008
) observed that in vitro fertilisa-
tion with sperm collected from male mice, in which
the scrotum was heated to 42°C, resulted in embryos
with reduced ability to complete development. In
addition, females mated to males exposed to scrotal
heating were observed to have conceptuses with
smaller fetal and placental weights compared with
controls (Jannes et al.
1998
; Paul et al.
2008
) .
5
Male Reproduction
The mammalian species have testes located out-
side in groin region, and the testes are suspended
in scrotum outside the body cavity to keep intra-
testicular temperature slightly lower than core
body temperature. This intricate thermoregula-
tory system in the testis, involving countercurrent
heat exchange from warm blood entering the
testis and cool blood draining from the testis
through an arteriovenous plexus, that is, pampini-
form plexus, helps in maintaining optimum
temperature for sperm development. The degree
of descend or ascend for cooling is further con-
trolled by two muscles, the tunica dartos in the
scrotum that regulates scrotal surface area and
the cremaster muscle that controls the position of
the scrotum relative to the body. Evolution of the
scrotum occurred because of the need for low
temperatures either for spermatogenesis, sperm
storage or to minimise mutations in gamete DNA
(Werdelin and Nilsonne
1999
; Bedford
2004
) .
Different mammals have different location of tes-
tes in the body and system for optimising sperm
development. Heat is lost from the testis and
scrotum to the environment through the scrotal
skin, which is well endowed with sweat glands
(Setchell and Breed
2006
) .
B. indicus
bulls are
less sensitive to the effects of high temperatures
than
B. taurus
or crossbred bulls, but as they are
actually more sensitive to the effects of scrotal
insulation (Brito et al.
2003
) , this would appear
to be due to the greater ability of
indicus
animals
to keep their testes cool (Brito et al.
2002
) .
B. indicus
bulls have greater testicular artery
length to testicular volume ratios and smaller tes-
ticular artery wall thickness and arterial to venous
distances, which may be responsible for greater
cooling of the arterial blood in the spermatic cord
(Brito et al.
2004
) .
Regardless of the evolutionary reason for the
location of the testis and epididymis outside the
body, a rise in testicular temperature in mammals
with external testes reduces sperm output, dec-
reased sperm motility and an increased propor-
tion of morphologically abnormal spermatozoa
in the ejaculate. Such effects can be observed
when a local heat source is applied to the testis,
