Horse Reproductive Cycles: Oestrus, Follicles, Luteolysis

Animal Reproduction Science 124 (2011) 220–228
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Animal Reproduction Science
journal homepage: www.elsevier.com/locate/anireprosci
Reproductive cycles of horses夽
Christine Aurich a,b,∗
a
b
Centre for Artificial Insemination and Embryo Transfer, University of Veterinary Sciences, Vienna, Austria
Graf Lehndorff Institute for Equine Sciences, Neustadt (Dosse), Germany
a r t i c l e
i n f o
Article history:
Available online 18 February 2011
Keywords:
Horse
Reproductive cycle
Behaviour
Follicle
Luteolysis
a b s t r a c t
Horses are long-day breeders. During the breeding season, cycle length is about 22 days
with 5–7 days of oestrus. Gonadotroph cells are localized in the pars distalis as well the
pars tuberalis of the pituitary and heterogeneity in the pattern of LH and FSH storage
within the gonadotroph population is considered the basis for the differential regulation of
gonadotrophin secretion throughout the reproductive cycle. No short and distinct periovulatory LH peak exists in the mare. The equine ovary has an extreme large size and weight.
One to two major follicular waves develop per cycle. The preovulatory follicle reaches an
average size of 40 mm. Only granulosa cells develop into luteal cells. Progesterone increases
at the time of ovulation and reaches maximal concentrations on day 8. Functional luteolysis
occurs around day 15 and is initiated by endometrial secretion of PGF2␣ . In contrast to other
species, no significant luteal oxytocin synthesis exists in the mare. During the oestrous cycle,
uterus, vagina and endometrium undergo pronounced changes related to variations in the
endocrine milieu. Seasonal reproductive activity is stimulated by photoperiod together
with exogenous factors. The anovulatory season can be differentiated into an autumn transitional phase, a mid-anovulatory period and a spring transitional phase bringing the mare
back into cyclic activity. During the mid-anovulatory period, follicular development is minimal. The beginning of the spring transitional period is characterized by the development of
1–3 anovulatory follicular waves before ovulation occurs and the most important factor for
the re-initiation of ovulatory activity is the occurrence of repeated pronounced increases
in circulating LH.
© 2011 Elsevier B.V. All rights reserved.
1. Reproductive activity and cycle length
Horses are seasonal breeders with ovulatory activity
being related to long days. In less domesticated horse
breeds, ovulatory oestrous cycles occur between May
and October. Breeding selection has diminished seasonal
reproductive activity as conception early in the year is
advantageous to horse breeds performing early in life
(“official birth day” 1st January). In consequence, among
夽 This paper is part of the special issue entitled: Reproductive Cycles of
Animals, Guest Edited by Michael G. Diskin and Alexander Evans.
∗ Correspondence address: Centre for Artificial Insemination and
Embryo Transfer, University for Veterinary Sciences, Veterinärplatz 1,
1210 Vienna, Austria. Tel.: +43 1 250776400; fax: +43 1 250775491.
E-mail address: [email protected]
0378-4320/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.anireprosci.2011.02.005
riding and racing breeds, about 30% of mares show ovulatory cycles throughout the winter season. The horse is
predominately monovulatory. Among less domesticated
breeds, double ovulation rate is low, but varies between 7%
and 25% in domesticated horse breeds. During the breeding season in spring and summer, average oestrous cycle
length is about 22 days with 5–7 days of oestrus (Fig. 1).
Oestrous cycle length is also affected by reproductive stage
(e.g. 21.2 ± 1.8 days (±SEM) in lactating and 22.8 ± 1.4 days
in non-lactating mares; p < 0.01; Heidler et al., 2004) and
breed. It is approximately 2 days longer in ponies than in
horses. Between individual mares, significant differences
in oestrous length are seen.
Transition phases from winter anoestrus to the ovulatory season and back into the anovulatory season coincide
with irregular periods of oestrous behaviour that may last
C. Aurich / Animal Reproduction Science 124 (2011) 220–228
Fig. 1. (a) Concentrations of FSH and oestradiol in the peripheral circulation and size of the largest follicle and (b) concentrations of LH,
progesterone and prostaglandin F2alpha in the peripheral circulation
throughout the equine oestrous cycle.
from several days to weeks and thus vary considerably in
length. In fillies, puberty occurs approximately at an age
of 12–18 months. This age is again influenced by season.
In fillies born very early in the year puberty will not occur
before the beginning of the breeding season of the following year. Senescence in old mares is rarely seen and most
mares continue to cycle independent of age. However, in
old mares, the interovulatory interval may be longer than
in young and middle-aged mares due to a slower growth
rate of the dominant follicle (Ginther et al., 2008).
2. Reproductive behaviour
Oestrous behaviour of the mare is characterized by
increased interest in stallions and proceptive behaviour in
response to the sexual attractivity of a stallion (CrowellDavis, 2007). The oestrous mare will turn her hindquarters
to the stallion and show a characteristic posture with lowered pelvis and straddled hind limbs. This is accompanied
by deviation of the tail and exposition of the perineal region
together with “clitoral winking” (rhythmic eversion of the
clitoris) and frequently voidance of small quantities of
urine. Frequent urination ensures contact to the stallion
who will show an olfactory response including frequent
flehmen. The behaviour of the mare thus may function
in chemosensory priming of the stallion for reproduction
(Crowell-Davis, 2007). In oestrous mares, a unique facial
expression occurs that is characterized by relaxed facial
muscles, ears turned to side and a lowered head. In contrast
to oestrous mares, luteal phase mares will be less interested in a stallion. If a stallion approaches, they will start to
squeal, strike and kick at the stallion. Their face will express
a kind of aggressiveness characterized by tension of the
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facial muscles, ears turned back and often threatening to
bite at the stallion (Crowell-Davis, 2007).
Under feral conditions, horses live in stable family
groups that typically consist of adult mares, their young
offspring and one adult harem stallion. Therefore, strong
social bonds between mares and the stallion exist. Under
today’s domesticated breeding conditions, the situation is
completely different. Frequently, a mare will be exposed
to a stallion to test her reproductive behaviour (“teasing”).
However, in most cases, this stallion is not familiar to
the mare and no social bond is formed. In addition, many
mares show individual preferences for particular stallions
and may avoid other stallions completely. These factors
contribute to the situation that an oestrous mare does
not always exhibit characteristic oestrous behaviour when
exposed to a stallion.
Equine oestrous behaviour is inhibited by progesterone
and facilitated by oestradiol. However, even in anovulatory mares with small and inactive ovaries as well as in
ovariectomized mares, oestrous behaviour is seen at irregular intervals. The behaviour ranges from mild interest in a
stallion to the complete repertoire of oestrous behaviour
including the acceptance of mounting and copulation
(Hedberg et al., 2007). Oestrous behaviour in nonovulatory
mares is most likely caused by small amounts of oestradiol synthesized by the adrenal cortex together with a high
sensitivity of the mare to steroid hormones (Crowell-Davis,
2007). In contrast to other species, in horses even the first
oestrus in life as well as the first oestrus after the anovulatory winter season or after foaling is accompanied by
characteristic oestrous behaviour. In the post partum mare,
this facilitates early conception in foal heat and allows the
production of a foal every year in spring which is the most
appropriate time of the year under feral conditions.
3. Endocrine regulation of the oestrous cycle
3.1. Hypothalamic function
In the horse, pituitary venous blood can be experimentally collected by a direct catherization technique. This
allows determination of GnRH and gonadotrophin release
directly from hypothalamic and pituitary outflow (Irvine
and Alexander, 1987). In the mare, the gonadotrophins
LH and FSH are considered to be under the control of
GnRH alone. So far there is no evidence that a specific
FSH-releasing factor exists in the horse. In the mare, most
hypothalamic GnRH pulses are followed by LH pulses
from the pituitary and more than 80% of LH pulses are
accompanied by a pulse of FSH (Alexander and Irvine,
1987). The GnRH pulse frequency changes with the stage
of the oestrous cycle and is low during the luteal phase
with a GnRH pulse interval of approximately 120 min.
On the day of ovulation, the GnRH pulse interval lasts
approximately 30 min. This is similar to the GnRH pulse
interval observed during the LH surge induced in ovariectomized ewes by oestradiol treatment (Alexander and
Irvine, 1987). Hypothalamic GnRH release is modulated by
steroid feedback mechanisms. However, the existence of
steroid receptors on GnRH neurons has not been proven in
the horse so far. Therefore, feedback mechanisms have to
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C. Aurich / Animal Reproduction Science 124 (2011) 220–228
Table 1
Follicular development during the oestrous cycle of the mare: size of the largest follicle, concentrations of hormones in the peripheral circulation and
occurrence within the largest follicle.
Day of cycle
Stage of the cycle
Hormone concentration in the
peripheral circulation
Occurrence within the (later)
dominant follicle
7
Development of
the follicular wave
• FSH ↑
• Emergence
• Growth (all follicles of the wave)
13
17
Follicle deviation
Beginning of the
preovulatory phase
• FSH ↓
• Continued growth (3 mm per day)
• Oestradiol-17␤ ↑
• Inhibin ↑
• Activation of deviation mechanisms:
• Sensitivity to FSH ↑
• Free IGF-I ↑
• Inhibin ↑
• Oestradiol ↑
• LH ↑
• Continued growth (3 mm per day)
• Oestradiol-17␤ ↑
19
21
Preovulatory phase
Ovulation
• LH →
• Stagnation of growth
• Oestradiol-17␤ ↑
• Maturation (characteristic pattern in
the expression of different factors, e.g.
PGE ↑, PGF2␣ ↑, prostaglandin
dehydrogenase ↑)
• LH ↑
• Granulosa cells → luteinization
• Progesterone ↑
• Progesterone ↑
• Vascularization ↑
be mediated by higher brain areas. In the horse, endogenous opioidergic systems are activated by progesterone
together with oestradiol. During the luteal phase, opioidergic systems inhibit hypothalamic GnRH and subsequent
pituitary LH release while during the follicular phase, they
are inactive and allow an increase in LH secretion (Aurich
et al., 1995).
3.2. Pituitary function and gonadotrophin release
Gonadotroph cells are localized in the pars distalis
as well the pars tuberalis of the equine pituitary. Subsets of gonadotrophs that store and produce either
LH or FSH (monohormonal gonadotrophs) and bihor-
monal gonadotrophs that store both gonadotrophins exist
together. Whereas in the pars distalis all three types of
gonadotrophs have been identified, few if any FSH monohormonal cells exist in the pars tuberalis of the equine
pituitary. This heterogeneity in the pattern of LH and FSH
storage within the gonadotroph population is considered
the morphological basis for the differential regulation of
LH and FSH secretion throughout the equine reproductive
cycle (Tortonese et al., 2001).
The divergent pattern of LH and FSH release is more
pronounced in the mare than in many other domesticated
animal species (Fig. 1a and b). An early periovulatory rise
in peripheral concentrations of LH is accompanied by a
modest increase in FSH subsequently declining to its nadir
C. Aurich / Animal Reproduction Science 124 (2011) 220–228
concentration (Bergfelt et al., 1991) while LH is reaching
its maximum. In the mid-luteal phase, a second and robust
FSH rise occurs with no concomitant increase in LH. This
second FSH surge occurs on different days of the cycle
among individual mares (Ginther et al., 2005). In contrast
to other domestic animal species which exhibit a short and
pronounced preovulatory LH surge, no distinct periovulatory LH peak exists in the mare. However, during oestrus
the period of elevated concentrations of LH lasts for several
days. The gradual increase in LH is transiently disrupted
after ovulation due to absorption of oestradiol from follicular fluid that is suggested to be discharged into the
abdomen at ovulation (Ginther et al., 2010). Peak concentration of immunological LH activity is reached not earlier
than one day after ovulation, but is preceded by biological
LH activity. Maximal LH bioactivity occurs shortly before
ovulation (Alexander and Irvine, 1982). In pituitary blood,
gonadotrophin pulse frequency rises from 0.5 pulses per
hour early in the LH surge to 1.9 pulses per hour at the
time of ovulation. In jugular blood, gonadotrophin pulses
are relatively small and therefore difficult to detect. This
is most likely the result of the long plasma half-life of
both equine gonadotrophins (approximately 5 h). During
the periovulatory period, the mean frequency of detectable
gonadotrophin pulses in peripheral circulation increases
to approximately 1 pulse per hour (Alexander and Irvine,
1987), while during the luteal phase the LH pulse frequency
is as low as 0.1 pulses per hour (Alexander and Irvine, 1986).
3.3. Follicular development, follicular hormone
production and ovulation
Compared to other domestic animal species, the ovary
of the mare has a unique structure characterized by an
extremely large size and weight (35–120 cm3 in volume;
40–80 g in weight; Kimura et al., 2005), the presence of
an ovulation fossa and an inverted location of its cortex
and medulla. One to two distinct follicular waves develop
during an oestrous cycle. A first major follicular wave can
occur early in the luteal phase. The dominant follicle of
this early wave may be anovulatory, but despite increased
concentrations of progesterone ovulation can occur. The
development of an ovulatory follicular wave during the
luteal phase is a phenomenon considered unique to the
equine species. However, it does not occur in all breeds
of horses to the same extent: ponies usually develop one
major follicle wave, while two follicular waves is typical of
thoroughbreds and closely related sport horse breeds (e.g.
warmbloods; Ginther, 2000).
The emergence of each follicular wave is temporally
associated with an FSH surge (Fig. 1a; Table 1). FSH reaches
a plateau when the largest follicle reaches a size about
13 mm in diameter (Gastal et al., 1997). Subsequently,
FSH declines to a concentration that does not support
pronounced further growth of subordinate follicles but
is sufficient for continuing growth of the dominant follicle. This size dissociation of the members of a follicular
wave is known as follicle deviation or selection. Emergence
of the future dominant follicle occurs at a size of 6 mm
in diameter, approximately 6 days before follicle diameter deviation. At the time of deviation the largest and
223
second-largest follicle has an average size of 22 and 19 mm,
respectively (Ginther, 2000). This phase of the dominant
follicle is considered the decisive developmental stage. It
coincides with rapid activation of deviation mechanisms
that block further development of the second largest follicle to a similar decisive size.
The inhibition of growth of the smaller follicles does not
depend on follicle-to-follicle inhibitory mechanisms, but
follicle deviation involves important changes in the largest
follicle. These are characterized by an increased sensitivity
to circulating concentrations of FSH. Dramatic changes in
the insulin-like growth factor (IGF) system (IGF-I and -II,
IGF binding protein, IGF binding protein proteases) in the
largest follicle before the beginning of size deviation play
a crucial role (Beg and Ginther, 2006). Simultaneously, the
dominant follicle suppresses circulating concentrations of
FSH, most probably due to follicular synthesis and release of
oestrogens and inhibin. This hypothesis is supported by the
finding of an increased oestradiol production in the future
dominant follicle one day before follicle deviation (Gastal
et al., 1999) together with an inverse relationship between
circulating concentrations of FSH and inhibin in the cyclic
mare (Bergfelt et al., 1991). It is feasible that systemic LH
is not involved in follicle deviation but is required for further growth of the dominant follicle after the beginning of
deviation (Ginther, 2000).
With regard to preovulatory follicular development and
ovulation horses differ from other farm animal species.
The preovulatory follicle is much bigger and ruptures at
a specific region of the ovary – the ovulation fossa. From
deviation onwards, the preovulatory follicle grows at an
average rate of 3 mm per day to a diameter of approximately 35 mm four days before ovulation. Continued
growth occurs up to 2 days before ovulation when follicular size reaches a plateau of approximately 40 mm (Fig. 3a;
Ginther et al., 2008). However, equine follicles may grow
up to a preovulatory size of 55 mm and more, with mares
ovulating consistently from similar preovulatory diameters in consecutive cycles (Cuervo-Arango and Newcombe,
2008). Histological maturation of the equine preovulatory
follicle is characterized by an extensive expansion of its
entire mural granulosa cell layer. Furthermore, accumulation of extracellular matrix is abundant. The ovulatory
process of the equine follicle involves a specific and unique
pattern of gene regulation in theca and mural granulosa
cells. This includes differences in the expression of a variety
of factors among them prostaglandins and prostaglandin
metabolizing enzymes (Sayasith et al., 2007). During ovulation, oocyte and corona radiata enter the oviduct, while
most of the follicular fluid passes into the peritoneal cavity. Hormones from this fluid are rapidly absorbed into the
circulation leading to a pronounced increase in concentrations of inhibin on the day of ovulation (Bergfelt et al.,
1991).
Double ovulations may occur in horses. The double ovulation rate is affected by various factors such as breed,
reproductive status and age and pharmacological manipulation of the oestrous cycle. The incidence of spontaneous
double ovulation varies between approximately 2% in
ponies and 25% in thoroughbreds, respectively. When two
dominant follicles (two follicles >28 mm) develop in the
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C. Aurich / Animal Reproduction Science 124 (2011) 220–228
same follicular wave, double ovulations occur in about
40% of mares (Ginther et al., 2008). These may occur synchronously (within 12 h), but intervals up to two days
and more have been reported between ovulations and
can lead to the establishment of a twin pregnancy. During the 2.5 immediately days before ovulation, the rate
of dominant follicle growth in double ovulating mares is
less pronounced than in single ovulating mares resulting
in a lower preovulatory follicle diameter in twin ovulating mares. The reduced follicular growth is related to lower
FSH concentrations, most probably due to higher oestradiol
concentrations from the two preovulatory follicles (Ginther
et al., 2008).
3.4. Luteal function
Due to the specific anatomy of the ovary, the corpus
luteum (CL) of the mare enlarges within the internal part
of the ovary and does not protrude to the outer ovarian
surface as in other species. The equine CL has a pearlike shape and comprises many small compartments with
rough surface textures (Kimura et al., 2005). Structures of
the equine CL are formed by luteal and non-luteal cells.
As in other species luteal cells can be distinguished into
large and small luteal cells. However, in contrast to many
other species, luteal cells of the mare are not of thecal origin, but derive exclusively from the granulosa cells of the
preovulatory follicle (Van Niekerk et al., 1975). At ovulation, thecal cells are at various stages of degeneration
and are subsequently replaced by hypertrophied fibroblasts (Van Niekerk et al., 1975). Non-luteal cells of the
corpus luteum are mainly fibroblasts, smooth muscle cells,
macrophages and endothelial cells that originate from vascular endothelium of the postovulatory follicle. In the early
luteal phase, progesterone producing luteal cells show
pronounced mitotic activity (Aguilar et al., 2006). Luteal
development is further characterized by intense angiogenesis regulated by angiogenic factors such as vascular
endothelial growth factor (VEGF) produced by luteal cells
(Al-zi’abi et al., 2003).
In the mare, circulating concentrations of progesterone
immediately increase at the time of ovulation (Roberto
da Costa et al., 2005). Maximal concentrations of progesterone are reached on day 8 after ovulation and then
slowly decrease until the onset of luteolysis that begins
at approximately day 14 (Fig. 1b). Transrectal ultrasound
has elucidated a parallel and progressive decrease in the
mean cross-sectional area of the CL from day 4 to day 19
of the cycle (Ginther et al., 2007). However, the volume
of hormone producing luteal cells significantly increases
from early to mid-luteal phase when a maximal volume is
reached (Van Niekerk et al., 1975; Aguilar et al., 2006). As
in other species, in the mare, CL function is under control of
LH and progesterone. However, expression of progesterone
receptors can only be detected in large luteal cells but not in
small luteal cells. This finding together with an inconsistent
or failure to detect steroidogenic enzymes in small luteal
cells has led to the suggestions that, in the mare, synthesis
of progesterone is restricted to large luteal cells (Roberto
da Costa et al., 2005; Ferreira-Dias et al., 2007). This is in
contrast to many other species where both luteal cell types
are involved in progesterone production. The hypothesis is
supported that equine small luteal cells are just precursors
of large luteal cells that advance in size and function during the development of the CL (Van Niekerk et al., 1975;
Roberto da Costa et al., 2005).
3.5. Luteolysis
Functional luteolysis in the mare is characterized by a
pronounced decrease in blood concentrations of progesterone at around days 15–17 of the cycle. Morphologic
regression of the CL develops later and more slowly
(Ginther et al., 2005, 2007). Functional luteolysis coincides with a pronounced decrease in expression of P450scc
(Watson et al., 2005). Simultaneously, the majority of
hormone-producing luteal cells undergo cell death characterized by an increase in apoptotic markers. However,
during that time a subset of luteal cells still enters the early
stages of cell division. Expression of vascular endothelial
growth factor (VEGF) declines at early regression of the
CL but does not precede apoptosis in luteal cells. Therefore, in the mare functional luteolysis is not initiated by
endothelial cell death within the CL (Aguilar et al., 2006)
and no acute changes in blood flow contribute to luteolysis
(Ginther et al., 2007). A regulatory role of the IGF system
not only in follicular but also in luteal function in the mare
has been suggested. Most probably, an increase in IGFBP2 at the time of luteolysis decreases the bioavailability of
IGFs in the CL. Thus, functional luteolysis is facilitated by
inhibiting the protective effect of IGFs on apoptosis and
their stimulatory effect on steroidogenesis (Watson et al.,
2005).
In the mare, the initial signal for luteolysis is endometrial secretion of PGF2␣ during the late luteal phase. In
contrast to ruminants, PGF2␣ is secreted into the peripheral circulation and no local counter current system (i.e.
between uterine veins and ovarian arteries) exists in the
horse. However, the equine corpus luteum is believed to
have a much higher binding affinity for PGF2␣ resulting
in a greater sensitivity to this hormone. At day 15 of the
cycle, COX-2 expression in uterine epithelial cells of nonpregnant mares is markedly increased. No parallel changes
in expression of other enzymes involved in prostaglandin
synthesis were found, but COX-2 expression is inhibited in
the endometrium of pregnant animals. The regulation of
endometrial expression of COX-2 is therefore considered
a key event in either induction of luteolysis or maternal
recognition of pregnancy in the mare (Boerboom et al.,
2004).
As in other species, the release of equine endometrial
PGF2␣ is stimulated by oxytocin. During late dioestrus, initial oxytocin secretion is most probably originating from
the hypophysis. In contrast to other species, no significant luteal oxytocin synthesis exists in the mare, but the
horse is the only domestic species were oxytocin has been
localized in the endometrium. Specific secretory endometrial cells containing oxytocin have been described (Bae
and Watson, 2003). Therefore, a paracrine–autocrine system involving endometrial oxytocin and PGF2␣ release is
accelerating luteolysis in the non-pregnant mare (Fig. 2).
C. Aurich / Animal Reproduction Science 124 (2011) 220–228
225
Fig. 2. Suggested endocrine regulation of luteolysis in the mare.
4. Oestrous cycle-induced changes of uterus, vagina
and endometrium
During the oestrous cycle, the uterus, vagina and
endometrium of the mare undergo pronounced changes
related to variations in the endocrine milieu. They can
easily be differentiated by clinical examination. During
oestrus, high concentrations of oestrogen and low concentrations of progesterone contribute to increasing uterine
wall oedema together with opening of the cervix and flattening of uterus and vagina. Histologically, the endometrial
oedema is most apparent in the stratum compactum. It
is often associated with accumulation of varying small
amounts of fluid within the uterine lumen (Tunón et al.,
1995). Using transrectal ultrasonography the oestrous
oedema of the endometrium is easily detectable as it
results in the appearance of individual endometrial folds
(Fig. 3b). These show areas of different echotexture: nonechogenic areas are corresponding with the oedematous
portions of the folds while echogenic areas are corresponding with their tissue-dense central portions. Altogether
the endometrial oedema results in a characteristic ultrasonographic picture of the uterus of the oestrous mare.
In contrast, during the luteal phase the uterine wall has a
mild contractility and the cervix is firmly closed. The uterine echotexture is homogenous and no oedema should be
present.
In the mare, patterns of secretory and ciliary activity
of the endometrium are similar to other species. During
oestrus, cells of the polygonal microvillous type dominate the cell population. Secretory cells located around
the openings of endometrial glands are of the apocrine
(Samuel et al., 1979) or merocrine type (Tunón et al., 1995).
During oestrus, secretory activity of the endometrium
together with increased myometrial activity contributes
to uterine clearance mechanisms that void uterine infection. Equine endometrial secretory cells produce, store
and secrete oxytocin and are thus most likely involved
in stimulating and maintaining uterine contractility (Bae
and Watson, 2003). Uterine clearance after breeding is
further supported by invasive cells, mainly granulocytes
and intraepithelial macrophages located particularly in the
luminal epithelium of the uterine body (Tunón et al., 1995).
After oestrus, the number of secretory endometrial cells
rapidly declines and only few are present during the luteal
phase. In parallel, the number of ciliated cells increases and
reaches a maximum at mid dioestrus but again declines
towards the end of the luteal phase (Samuel et al., 1979).
Fig. 3. Transrectal ultrasonography of (a) a preovulatory follicle and (b)
uterine oedema in an oestrous mare.
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C. Aurich / Animal Reproduction Science 124 (2011) 220–228
Endometrial changes related to the oestrous cycle are regulated by the ovarian steroid hormones progesterone and
oestrogens interacting with their endometrial receptors.
Progesterone and oestrogen receptors exist in the luminal
epithelium, glandular epithelium and stroma of the equine
endometrium. Their expression is markedly influenced by
the stage of the oestrous cycle. It is maximal during oestrus
and minimal during late dioestrus suggesting that steroid
hormone receptor expression in the equine endometrium
is stimulated by oestrogens and down-regulated by progesterone. This is in agreement with the situation in ruminants
(Hartt et al., 2005).
5. Regulation of seasonal reproductive activity
In the horse, seasonal reproductive activity is stimulated by long days and short nights (Palmer and Guillaume,
1992). In addition to photoperiod, exogenous factors such
as age, reproductive state, nutrition, body condition and
environmental temperature affect seasonal reproductive
activity of the mare tremendously. Therefore, in most
horse populations a proportion of mares continue to cycle
throughout the year (see Section 1).
During the winter anovulatory season which lasts
approximately from October to March, prolonged phases
of melatonin secretion contribute to reduce hypothalamic
GnRH content and release. However, annual reproductive rhythms also occur in pinealectomized horses, but
are no longer strictly synchronous with the geophysical
year (Grubaugh et al., 1982). This indicates that seasonal
changes in reproductive activity in the horse are driven by
an endogenous rhythm that is synchronized to the geophysical year by photoperiod.
While mean plasma concentrations of FSH and pituitary content of FSH are relatively constant throughout the
year, mean plasma concentrations of LH reach a maximum
in summer and are lowest in anovulatory mares. In the
peripheral circulation, LH pulses are almost undetectable
during winter anoestrus (Alexander and Irvine, 1986). Pronounced seasonal effects on the incidence of gonadotrophs
in the pars tuberalis of the equine pituitary exist. In cyclic
animals during the breeding season as well as in sexually
active mares during the non-breeding season, the density
of gonadotrophs in the pars tuberalis is four to five times
higher than in seasonally anovulatory mares. In contrast,
no effects of season or reproductive activity on the density of the gonadotrophs in the pars distalis of the pituitary
occur. The seasonal pattern in the density of gonadotrophs
of the pars distalis is most probably the main mechanism
underlying the differential release of LH and FSH throughout the annual reproductive cycle of the mare (Tortonese
et al., 2001).
In the mare, the anovulatory season can be differentiated into an autumn transitional phase from cyclic activity
to deep anoestrus, a mid-anovulatory period and a second transitional phase to cyclic activity in spring. During
the autumn transitional phase, inadequate gonadotrophin
stimulation in early dioestrus seems to be a critical event
leading to suboptimal follicular development and subsequent acyclicity. In the last ovulatory cycle of the season,
the periovulatory nadir in concentrations of FSH is pro-
longed and the early dioestrus FSH surge is reduced when
compared to ovulatory cycles during the breeding season.
Simultaneously, LH secretion is impaired with a shortened
LH surge. Subsequently, the LH surge fails to occur at the
expected time of the cycle and results in anovulation (Irvine
et al., 2000). Failure of ovulation is followed by a phase of
variable levels of follicular activity before follicular growth
is reduced to deep anoestrous levels. During this period,
follicular growth is minimal, only a few follicles have a
diameter of more than 15 mm and the maximal diameter
of the largest follicle does not exceed 16 mm. A dominant
follicle does not develop during that time. Low circulating
concentrations of LH contribute to this reduction in follicular growth. However, FSH surges and follicular waves
can be distinguished throughout the anovulatory season
(Donadeu and Watson, 2007).
The spring transitional period has a variable length that
ranges from 30 to 90 days. Its beginning is characterized
by the re-initiation of follicular deviation, i.e. the development of a dominant follicle reaching a size between 20 mm
and 30 mm in diameter. In addition, an increasing number
of follicles with a diameter greater than 15 mm occur. Subsequently, 1–3 anovulatory follicular waves develop before
ovulation occurs (Donadeu and Watson, 2007). The switch
in follicular development between the deep anovulatory
phase and spring transition is not attributable to differences in pituitary FSH release. As in sheep, a low expression
of follicular FSH receptors during deep anoestrus and a
much higher expression in ovulatory mares explain a different sensitivity of follicles to FSH. Most likely, changes
at the ovarian level contribute to these differences, among
them changes in the expression of dopamine D2 receptors
in the ovarian cortex (King et al., 2008) or in the expression of members of the IGF system within ovarian follicles.
Seasonal changes in the concentrations of circulating hormones such as prolactin or growth hormone are most likely
involved (Donadeu and Watson, 2007). The most important factor for the re-initiation of ovulatory activity at the
end of the transitional phase is the occurrence of surges
in circulating LH. As during the ovulatory season (Ginther,
2000), systemic LH is required for further growth of the
largest follicle after the beginning of deviation. In contrast to ovulatory follicles, the dominant follicles of the
anovulatory follicular waves during spring transition in
the presence of lower circulating LH concentrations still
show a poorly developed theca layer, scant vascularisation
and a deficiency in steroidogenesis early in the steroidogenic pathway (Donadeu and Watson, 2007). Resumption
of steroidogenesis increases gradually in successive anovulatory follicles during spring transition. A further increase
in LH release seems to be the key event that finally leads to
the first ovulation.
6. Manipulation of reproductive activity in the mare
Reasons for the manipulation of reproductive activity
in the mare are numerous. They range from induction of
ovulatory cycles in seasonal anovulatory mares to produce
early offspring over timing of ovulation in mares bred with
semen of limited availability to suppression of oestrous
behaviour in performance mares. However, the availabil-
C. Aurich / Animal Reproduction Science 124 (2011) 220–228
ity of the drugs used may change and often differs between
countries. Moreover, it has to be considered that in brood
mares, pharmacological manipulation of the oestrous cycle
increases the incidence of twin pregnancies (e.g. 17% in
treated vs. 7% in untreated Thoroughbred mares; Veronesi
et al., 2003).
In luteal phase mares, oestrus is induced by injection of
PGF2␣ and its analogues that will cause immediate regression of the corpus luteum. In comparison to ruminants,
small doses of PGF2␣ are efficient (Douglas and Ginther,
1975). The treatment is effective from day 5 after ovulation.
Onset of oestrus and ovulation occur approximately 3–4
and 8–12 days, respectively, after injection. The interval
to ovulation depends on size and stage of ovarian follicles
present at drug administration. When a preovulatory follicle at the end of its growing phase is present, ovulation
may occur within 48 h after PGF2␣ treatment. In contrast,
in the presence of a small preovulatory follicle or an already
atretic follicle, the interval to ovulation is prolonged.
For induction of ovulation, oestrus should be confirmed by the presence of a preovulatory follicle (diameter
≥35 mm) and uterine oedema. In horses, a single injection
of GnRH or its analogues, e.g. buserelin does not predictably
result in ovulation making these drugs impractical in the
mare (Camillo et al., 2004). In contrast, either a shortterm implant containing the GnRH analogue Deslorelin or
human chorionic gonadotrophin (hCG) effectively induce
ovulation between 36 and 48 h after treatment (Squires,
2008). Deslorelin may result in receptor down-regulation
at the pituitary. If the mare fails to conceive, the following inter-ovulatory interval can thus be prolonged (Johnson
et al., 2002). Therefore, removal of the implant after confirmed ovulation is recommended (McCue et al., 2002).
Disadvantages of hCG include a diminished response after
repeated use due to formation of neutralizing antibodies
and reduced sensitivity to hCG in older mares (Squires,
2008). Recently, recombinant equine LH has been proven
a safe and reliable drug for induction of ovulation (Yoon
et al., 2007).
Oestrous can be suppressed either by daily injection
of progesterone or oral administration of the synthetic
progestin altrenogest. However, ovulation is not always
inhibited. Even after long-term treatment with altrenogest,
negative side effects neither on fertility nor on general
health have been reported (Shideler et al., 1983). Treatment
with altrenogest for one or two days was successfully used
to postpone ovulation in oestrous mares without impairing
fertility (Squires, 2008).
Exposure of mares to artificial photoperiod is a routine programme for advancing the onset of the breeding
season (Squires, 2008). A 15-h fixed programme starting
early in December (in the Northern hemisphere) has been
proven to be most effective and advances the onset of ovulatory cycles by 75 days in comparison to untreated mares
(Palmer et al., 1982). As treated mares will still experience a
transition period, administration of progestins for a period
of 12–15 days as soon as ovarian follicles ≥25 mm in diameter develop is common and supports the onset and timing
of ovulation (Squires, 2008). Alternatively, in transitional
mares with considerable ovarian activity, ovulation was
successfully hastened by twice daily injections of GnRH,
227
GnRH analogues or eFSH. Experimentally, treatment with
the dopamine antagonists sulpiride or domperidone has
been used to induce ovulation in transitional phase mares
(reviewed by Squires, 2008).
7. Conclusion
Despite a general similarity in the endocrine regulation
of the oestrous cycle between the horse and other domestic animal species, the mare is unique in some features.
These include a long oestrous together with a prolonged
and gradual increase in preovulatory LH concentration, a
pronounced divergency in the pattern of LH and FSH release
and the development of only one to two follicular waves.
Effects related to seasonal reproductive activity of the horse
further complicate the situation. Therefore, the schematic
application of drugs for manipulation of the mare’s oestrus
cycle will be less successful than in ruminants. On the other
hand, the reproductive stage of an individual mare can be
easily diagnosed due to typical changes of the genital tract.
Therefore, only profound knowledge of the mare’s reproductive physiology together with in-depth examination of
the individual animal will provide a basis for successful
reproductive management of the female horse.
Conflict of interest statement
None.
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