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Chapter 27
Control of the Ovarian Cycle of the Sheep
© 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition
FIGURE 27.1 Hormonal patterns during the ovine estrous cycle. Changes in mean (± standard error of the mean) secretion rate of
GnRH into hypophysial portal blood, peripheral concentrations of gonadotropins and major ovarian hormones, and uterine vein
concentrations of PGF2α throughout the ovine estrous cycle. Data for all hormones (except PGF2α) during the periovulatory period (days
−1 to +2 and 13–17) are normalized to the peak of the LH surge (day 0 and day 16) and represent values monitored every 4 h. During
the remainder of the cycle, data collected every 12 or 24 h are presented. By convention, the day of estrous behavior is designated Day
0; this is also the day of ovulation and thus the start of the luteal phase. The follicular phase begins with the first sustained fall in
progesterone concentration (on Day 14 in this figure), but the transition between luteal and follicular phases occurs over the next 24 h
as progestrone concentrations fall to a minimum. Source: GnRH values were calculated from data in Moenter et al.,9 and LH and
ovarian steroid concentrations are from Goodman et al.10 (Copyright 1981, The Endocrine Society.) FSH and inhibin A from Knight et
al.,11 and PGF2α values are derived from data in Inskeep and Murdoch.12
© 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition
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FIGURE 27.2 Episodic GnRH and LH secretion varies with stage of estrous cycle. Pulsatile patterns of
hypophysial portal GnRH secretion and jugular concentrations of LH during the late luteal phase (days 12–13)
and late-follicular phase (39–51 h after progesterone withdrawal). Note the difference in the scale of the y-axis
between the luteal and follicular phases. Source: Drawn from data provided by F. J. Karsch from work described
in Moenter et al.9
© 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition
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FIGURE 27.3 Follicular waves and hormonal patterns early in the luteal phase. Relationships among the
diameter of the largest follicle (top panel, solid lines), peripheral FSH concentrations (top panel, dashed lines),
and concentrations of estradiol (middle panel) and inhibin A (lower panel) in ovarian venous plasma during the
first and second follicular waves of the estrous cycle. Values (mean ± standard error of the mean) are normalized
to the day of first emergence (diameter >2.5 mm) of the largest follicle, for waves 1 (from days 1 to 4 of the
cycle) and 2 (from days 5 to 9 of the cycle). Source: Data replotted from Souza et al.20; reproduced by
permission of the Society for Endocrinology.
© 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition
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FIGURE 27.4 Sequence and sites of the process of follicular rupture and ovulation in the ewe in
response to the preovulatory surge of luteinizing hormone. Hormones and cytokines are in light gray print in
stippled boxes, tissues are in dark gray boxes, enzymes are in light gray boxes, and results are in dark gray
ovals or structures. Hormonal and enzymatic changes in granulosal (G) and thecal (T) cells are indicated on
arrows. Source: Adapted from Murdoch et al.36 and information provided by E. K. Inskeep from work described in
Senger PL Pathways to Pregnancy and Parturition, 3rd Ed., Current Conceptions Inc., Redmon, OR; 2012: 173.
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FIGURE 27.5 Diencephalic areas important for reproductive function in the ewe. Top panel: Schematic drawing of a parasagittal
section through the ovine preoptic area and hypothalamus illustrating the major areas involved in the regulation of estrous behavior and
GnRH secretion. ac: anterior commissure; AHA: anterior hypothalamic area; ARC: arcuate nucleus; dBB: diagonal band of Broca; MB:
mammillary body; ME: median eminence; OCh: optic chiasm; POA: preoptic area; Pit: pituitary; VMN: ventromedial nucleus. Bottom
panel: Distribution of GnRH perikarya (striped bars) and GnRH neurons activated (percentage containing Fos) during the preovulatory
GnRH surge (gray bars) and during pulsatile GnRH secretion (black bars). Data are mean ± standard error of the mean; note that the
scale for percentage of GnRH neurons expressing Fos is on the right y-axis. Source: Values for GnRH perikarya and Fos expression
during pulsatile secretion (Copyright 1999: The Endocrine Society) are replotted from Boukhliq et al. 46; those for Fos expression during
the surge plotted are from data presented in Moenter et al.47
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FIGURE 27.6 Model for role of KNDy neurons in driving episodic GnRH secretion. Representative GnRH pulse pattern is
presented in the bottom panel, which depicts the GnRH secretion rate into hypophysial portal blood samples collected every 30 s for 2.5
h in OVX ewes. The model proposes that each pulse is triggered by an initial release of NKB (gray terminals) within the KNDy circuit,
which increases the activity of these neurons. This initiates a positive- feedback loop that further increases NKB release onto KNDy
neurons and kisspeptin (striped terminals) release that stimulates GnRH neurons, which have kisspeptin receptors (R Ks) Within 1–2
min, the inhibitory effects of dynorphin (stippled terminals) within the KNDy circuit dominate, possibly due to a decrease in NK3R (RNKB)
or an increase in κ-receptor (RDyn) expression. Dynorphin inhibits KNDy neural activity, breaking the positive-feedback loop and
suppressing kisspeptin and, thus, GnRH release to terminate the pulse; see the text for further details. Note that the pattern in each
terminal indicates the biologically active transmitter (e.g., due to selective expression of postsynaptic receptors), not selective transport
of that peptide to the terminal. Source: Revised from Lehman et al.73
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FIGURE 27.7 Effects of ovarian steroids on episodic LH secretion and Kiss1 mRNA expression in the caudal ARC of ewes.
Ewes (n = 4–5/group) were ovariectomized (OVX) and given either no implants (clear symbols) or implants that produced luteal-phase
levels of estradiol (stippled symbols), progesterone (striped symbols), or both (shaded symbols). Eight days later, LH pulse patterns
were monitored for 4 h, and tissue was immediately collected and frozen. Samples from ovary-intact (INT) luteal-phase ewes (black
symbols) were collected at the same time. The caudal ARC was then microdissected, mRNA extracted, and Kiss1 mRNA determined
by qRT–PCR. Bars on the right depict mean ± standard error of the mean values for LH pulse amplitude (top panel), LH pulse
frequency (middle panel), and Kiss1 mRNA (bottom panel). *P < 0.05 versus OVX. Right panels present correlations of the mean LH
(top), LH pulse amplitude (middle), and LH pulse frequency (bottom) with Kiss1 mRNA expression in individual ewes. Source: From
Goodman RL, Rao A, Smith JT, Clarke IJ. Negative feedback control of Kiss-1 gene expression by estradiol and progesterone.
Program: 1st World Congress on Kisspeptin Signaling in the Brain, Cordoba, Spain, 2008.
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FIGURE 27.8 Effects of estradiol (E) and an EOP antagonist (naloxone) on the shape of GnRH pulses in
OVX ewes. Left panel: Mean GnRH secretion rate, normalized to the peak of each pulse, in samples collected
every 2 min from acutely OVX ewes treated with either no steroid or estradiol implants to produce luteal-phase
(low E) or peak follicular-phase (high E) concentrations of E. Right panel: Mean GnRH secretion rate, normalized
to the peak of each pulse, in samples collected every 10 min from ewes OVX 2 weeks earlier, before (OVX) and
during (OVX + naloxone) intravenous infusion of naloxone. Note the difference in axes between two panels.
Source: Reprinted from Goodman et al.107
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FIGURE 27.9 Time course of changes in the pattern of GnRH secretion leading up to and during the
estradiol-induced preovulatory surges of GnRH and LH. GnRH secretion rates (solid circles) monitored
every minute and peripheral LH concentrations (open circles) monitored every 10 min in an individual ewe are
presented. Note the differences in the y-axis between the top and bottom panels. The shaded area depicts the
time of overlap of values from the top and bottom graphs. Source: Reproduced from Evans et al.18 (Copyright
1995, The Endocrine Society).
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FIGURE 27.10 Neural systems postulated to participate in the estrogen-induced LH surge in the ewe. Note that solid
connections are supported by direct evidence, and dashed ones by indirect evidence. This model proposes that these mechanisms are
activated by estradiol acting on ARC KNDy and VMH somatostatin (SS), and, possibly, A1 noradrenergic (NE) neurons (top panel).
During the transmission phase (middle panel), stimulatory NE and possibly kisspeptin inputs to GnRH neurons are counterbalanced by
the inhibitory effects of β-endorphin at GnRH cell bodies and terminals. Sometime after β-endorphin release falls, GnRH secretion is
initiated by kisspeptin from KNDy and POA kisspeptin neurons and maintained by a positive-feedback loop between KNDy and NK3Rpositive neurons in the RCh (bottom panel). NE and somatostatin may also contribute to the stimulation of GnRH neurons at this time.
Note that for simplicity, two aspects of this system have not been depicted here: (1) GnRH neurons in the AHA and MBH that participate
in the surge, and (2) stimulatory inputs to GnRH terminals in the median eminence.
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FIGURE 27.11 Effects of FSH, with and without LH, on the development of, and hormone secretion from,
follicles in ewes with an ovary autotransplanted in the neck. Gonadotropin secretion was suppressed by
long-term treatment with a GnRH antagonist. FSH was infused intravenously (IV) for 72 h, with (closed symbols)
and without (open symbols) episodic LH injections (every 4 h). Top panel depicts FSH (circles) and LH (squares)
concentrations; bottom panels present follicular diameter and secretion rates of estradiol and inhibin. Estrogenic
capacity of the follicles (peak estradiol secretion rate in response to a single IV injection of LH) was determined
daily. Source: Data redrawn from Campbell et al.201; reproduced by permission of the Society for Endocrinology.
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FIGURE 27.12 Time course of intraluteal events in response to a bolus injection of PGF 2α. Data on
messenger RNA are presented above the timeline, and data on functional and structural changes are presented
below the timeline. aContinues through 48 h. bContinues through 6 h. cContinues through 8 h. dContinues through
24 h. eMore often detected by 2–4 h because of sampling at same times as for blood flow. fContinues through 12
h. gLow-density lipoprotein, PGF2α, LH, Flt (receptor for VEGF). and Tie-2 (receptor for ANGPT2). hStAR,
3βHSD, and P450scc. jContinues through 20 h. kContinues through 16 h. mOccurs in luteal macrophages.
pThrough 36 h and beyond. Source: Data were adapted from references cited in the text244,246–252 and
References 342,378,384–386,389,402 from the previous edition’s version of this chapter.1 Effects of pulsed
PGF2α may vary as discussed in the text based on data in the cow.242,243)
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FIGURE 27.13 Model for the two major feedback systems controlling the ovine estrous cycle. The top half
of the figure presents a schematic depiction of circulating concentrations of FSH and inhibin, the feedback cycle
thought to be responsible for waves of follicular development during the luteal phase, and the role of tonic LH
secretion in the final stages of follicular growth in the follicular phase. The bottom half of the figure depicts a
schematic pattern of progesterone (Prog), LH, estradiol (E2), and PGF2α secretion, and the sequence of causal
relationships that determines the timing of major events during the estrous cycle. In the latter, solid arrows
represent direct effects, and dashed arrows represent permissive actions. See the text for further details.
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FIGURE 27.14 Seasonal changes in response to estradiol negative feedback. Top graph: Mean ± standard
error of the mean (shaded area) LH concentration throughout the year in ovariectomized (OVX) or estradioltreated ovariectomized (OVX + E) ewes. Bottom graph: Estradiol was administered with silastic capsules that
produced relatively constant serum estradiol concentrations throughout the year. Histogram in the middle graph
indicates the breeding and anestrous seasons in a separate group of 14 ovary-intact ewes. Source: Reproduced
from Legan et al.131 (Copyright 1995, The Endocrine Society).
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FIGURE 27.15 Model for the control of seasonal breeding in the ewe. Left to right: endocrine events: (a)
during the follicular phase in the breeding season; (b) at luteolysis during the transition to anestrus; (c)
throughout anestrus; and (d) during the first follicular phase at the transition to the breeding season. See the text
for further details. Source: Reproduced from Karsch et al.298
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FIGURE 27.16 Model for the seasonal changes in response to estradiol (E2) negative feedback in the ewe that postulates that
an E2-responsive inhibitory neural system is active only during anestrus. During the breeding season (left panel), this system is
inactive so that E2 cannot inhibit GnRH pulse frequency and progesterone is the primary regulator of tonic GnRH secretion. During
anestrus (right panel), this system is active, allowing E2 to act on ERα-containing neurons (stippled) in the vmPOA and retrochiasmatic
area (that may contain glutamate), which in turn stimulate activity of A15 DA neurons (gray). These DA neurons act in the ARC to inhibit
kisspeptin (striped) release from KNDy neurons and thus decrease GnRH and LH pulse frequency. Possible seasonal changes in this
circuitry include an increase in responsiveness of the ERα-containing neurons and/ or in their ability to stimulate A15 DA neurons.
These changes are induced by a long-day (LD) melatonin pattern that increases synthesis and release of TSH in the pars tuberalis
(PT); TSH then acts locally to increase conversion of T 4 to its active form, T3, which initiates changes that activate the anestrous neural
circuit. The short-day (SD) melatonin pattern, in turn, acts in the premammillary region (PMR) to inactivate this system during the
transition to the breeding season.
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FIGURE 27.17 Effects of introduction of a novel or familiar ram on pulsatile LH secretion in ovary-intact
anestrous ewes. Reintroduction of a familiar ram (top panels) had no effect on episodic LH secretion, whereas
a novel ram (middle panels) produces a dramatic increase in LH pulse frequency. Ewes can remember a familiar
ram for at least 2 weeks, but by 1 month pheromonal and social signals are able to induce a modest increase in
episodic LH secretion (bottom panels). Rams introduced at time 0 (arrows). Source: Adapted from Jorre de St
Jorre et al.320
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FIGURE 27.18 Control of ovulation in the ewe. The frequency of pulsatile LH secretion, by controlling the preovulatory estradiol (E2)
rise, determines whether or not ovulation occurs. Left panels: During the breeding season, progesterone (Prog) is the primary regulator
of LH pulse frequency. Bottom left: During the luteal phase, Prog inhibits the hypothalamic pulse generator, producing anovulation. Top
left: Following luteolysis, LH pulse frequency increases and initiates the follicular-phase events leading to ovulation. Right panels:
During anestrus, the long-day melatonin pattern activates an E2-sensitive inhibitory neural system. Top right: In anestrous ewes isolated
from rams, E2 stimulates the inhibitory neural system, which inhibits GnRH and LH pulse frequency and ensures anovulation. Bottom
right: Pheromones and social signals from a novel ram activate a neural pathway that directly stimulates the GnRH pulse generator,
increasing LH pulse frequency and producing ovulation during anestrus.
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