Biomedical Engineering Reference
In-Depth Information
steroid production and follicular characteristics
in both medium-sized and preovulatory follicles
has been related to the low fertility of cattle in
the autumn.
achievement of a continuous rise in milk pro-
duction is associated with a gradual decline in
plasma progesterone concentration, consistent
with the negative relationship between milk pro-
duction and progesterone concentration (Lucy
and Crooker
1999
) . This relationship can proba-
bly be attributed to accelerated progesterone
metabolism in the liver (Parr et al.
1993
) and/or
to decreased synthesis of the steroid. Decreased
progesterone production by the corpus luteum
under heat stress conditions could result from:
(a) A suboptimal luteinisation process during
hyperthermia
(b) Depressed progesterone synthesis at high
temperatures
(c) Heat-induced impairment of the ovulatory
follicle, of which the corpus luteum (CL) is
subsequently formed
Under chronic summer heat stress conditions,
progesterone production is markedly reduced in
luteinised theca cells and less so in luteinised
granulose cells. These seasonal effects of heat
stress are carried over from an impaired follicle
to an impaired CL. The decreased progesterone
production found in vitro corresponds well with
the mild but significant decrease in plasma pro-
gesterone during the summer. Alleviation of
thermal stress and cooling cows and buffaloes in
summer might help in restoring their normal CL
function and could contribute to the improvement
of fertility. Therefore, the need is to elevate
progesterone concentrations as the hormonal
strategy to increase the summer conception in
dairy cows (Wolfenson et al.
2000
) .
Information on increase in FSH preceding the
emergence of the follicular wave and the role it
plays in follicular wave turnover has been well
documented (Adams et al.
1992
; Ginther et al.
1996
). The increase in the number of medium-
sized follicles in heat-stressed cows most likely
occur as a result of the higher plasma FSH
increase that preceded the second follicular wave
(Gibbons et al.
1997
). A lower plasma FSH surge
in heat-stressed cows has been observed (Gilad
et al.
1993
). Inhibin is an important factor in the
regulation of FSH secretion (Findlay
1993
;
Kaneko et al.
1993
) . An inverse relationship
between plasma FSH and immunoreactive inhibin
4.2
Steroid Production
The reduction in reproductive performance under
heat stress has been attributed to steroidogenic
capacity and its effects on oocyte function
(Roth et al.
2001b
; Al-Katanani et al.
2002b
;
Roth and Hansen
2004b
). Under heat stress, low
oestradiol concentration in the follicular fluid of
dominant follicles involves reduced aromatase
activity in the granulosa cells (Badinga et al.
1993
) and reduced androstenedione production
by theca cells (Wolfenson et al.
1997
) .
In terms of steroid production, the thecal cells
are more susceptible than granulosa cells to heat
stress and expressed a delayed effect of heat
stress in both classes of follicles. The consistent
decrease in androstenedione production in both
medium-sized and preovulatory follicles in pre-
viously heat-stressed cows has been correlated
with the decreased concentrations of androstene-
dione in their follicular fluid. The delayed heat
stress response on androstenedione production
by thecal cells indicates that the decrease in
androstenedione production by dominant folli-
cles is observed in the comfortable season like
that in autumn (Wolfenson et al.
1997
) . Decreased
oestradiol concentration in the follicular fluid
more likely occurred after exposure to long-term,
chronic (summer) heat stress than due to acute
heat stress. This response would be consistent
with the finding that after chronic summer heat
stress, an eight times decrease in androgen pro-
duction by thecal cells in the autumn was accom-
panied by a significant decrease in oestradiol
concentration in the follicular fluid (Wolfenson
et al.
1997
) .
Low progesterone has been reported to alter
ovarian follicular dynamics and persistence of
dominant follicles (Sirois and Fortune
1990
) and
to induce changes in uterine morphology and
PGF
2a
secretion (Shaham-Albalancy et al.
2001
) .
The situation has been aggravated because the
