resulting in a number of changes in follicular growth. Among them, at least two Ž .
responses standout in their physiological importance: 1
development of a larger number of large follicles probably increases the rate of double ovulation and hence of
Ž . twin calving; and
2 early emergence of the preovulatory follicle lengthens the
dominance period, and this has been shown to be associated with lower fertility in Ž
. spontaneously cyclic dairy cows Bleach et al., 1998 or in heifers induced to ovulate
Ž .
persistent dominant follicles Mihm et al., 1994; Austin et al., 1999 .
3. Steroidogenic capacity
Earlier studies were inconsistent in their findings concerning plasma concentrations Ž
of estradiol under HS no change — Roman-Ponce et al., 1981; increase — Rosenberg .
et al., 1982; decrease — Gwazdauskas et al., 1981 . However, most recent studies indicated that plasma estradiol concentration was lowered during HS. Lactating cows
Ž and dairy heifers that were heat-stressed during the second half of the cycle Wilson et
. Ž
. al., 1998a,b or during the entire cycle Roth, 1998 had a reduced preovulatory surge in
plasma estradiol concentration. Likewise, plasma estradiol was lower during the first Ž
. follicular wave in HS cows than in non-HS ones Wolfenson et al., 1995 , and a
pronounced reduction in plasma estradiol was noted during the first follicular wave in Ž
. HS cows in September, compared with those heat-stressed in July Badinga et al., 1993 .
The difference between the two replicate months is the much longer period of exposure Ž
. to daily HS in late summer September , which may severely impair follicular function.
Ž .
In contrast, a seasonal study Badinga et al., 1994
detected a higher and faster Ž
. preovulatory rise of plasma estradiol concentration in August hottest month in Florida
than in April, June or November. The decreased steroidogenic capacity of follicles under HS involved a lower aro-
matase activity in the granulosa cells and a lower estradiol concentration in the follicular fluid of dominant follicles on day 8 of the estrous cycle, in late vs. early summer
Ž .
Badinga et al., 1993 . A seasonal study comparing first-wave dominant follicles on day Ž
. 7 of the cycle Wolfenson et al., 1997 showed lower follicular fluid estradiol in summer
than in winter. This decrease was due primarily to a drastic reduction of androstenedione Ž
5
. production by theca cells during the summer 4.1 vs. 1.1 ngr10 viable cells . Estradiol
production by granulosa cells decreased in summer to about 50 of that in winter, but the difference was barely significant. Furthermore, the percentage of viable granulosa
Ž cells in follicles collected in summer had fallen to 60 of its winter value Wolfenson
. and Meidan, unpublished data , contributing to the reduction in in vivo secretion of
estradiol to the circulation. Reduced androstenedione production by theca cells was also found in cells incubated at high temperature in vitro, and in follicles collected in winter
Ž from cows previously exposed to 3 days of acute HS in a hot chamber Wolfenson et al.,
. 1997 .
In an attempt to characterize the molecular events leading to the decrease in plasma estradiol, the expression of mRNA encoding cytochrome P
aromatase, cytochrome
450
P sidechain cleavage, 17a-hydroxylase, FSHr, and steroid acute regulatory protein
450
Ž .
were studied in the second-wave dominant follicles of HS heifers Wilson et al., 1997 . None of the above was affected except 17a-hydroxylase, which was markedly lowered
by HS. 17a-Hydroxylase activity is rate-limiting in the biosynthesis of androgens, serving as the substrates for estradiol synthesis by the granulosa cells. The reason why
theca cells are susceptible to HS is unclear; it may be related to the fact that theca cells differentiate earlier than granulosa cells.
4. The corpus luteum
