gonadal function and hormone

1. Essential Reproduction, Sixth edition, Martin H. Johnson
Chapter 6
631 Zoo

2. Introduction
The hypothalamic–
pituitary axis controls
gonadal function

3. The pituitary
gland
• Location.
• Blood supply.

4. Structure
The pituitary
gland
Adenohypophysis
Neurohypophysis

5. • The anterior lobe of the
pituitary contains a
variety of cell types that
Synthesis and secretes
hormone.
• The posterior lobe of
the pituitary secretes
two nonapeptide
hormones: arginine
vasopressin (AVP or
anti- diuretic
hormone—ADH) and
oxytocin.
The pituitary
gland

6. • Location.
• The boundaries.
The
Hypothalamus

7. The
Hypothalamus
The third ventricle split the
hypothalamus symmetrically into left
and right halve. containing
cerebrospinal fluid and lying in the
midline so that the hypothalamus forms
its floor and lateral walls

8. Hypothalamus
Function
The control
of body
temperatur
e
The photic
input at retina
give it aware of
the cycles of
light and dark
that can, in turn,
affect
reproductive
function.
The integration
of the
cardiovascular
and hormonal
responses to
stress
01 02 03

9. ● Each function is
associated with
various
hypothalamic
areas or nuclei
The
hypothalamus

10. The hypothalamus connection with the pituitary
• Secretes oxytocin and
vasopressin from the posterior
pituitary
The hypothalamic
magnocellular
neurosecretory
system
• Controls anterior pituitary
hormone secretion
The hypothalamic
parvocellular
neurosecretory
system

11. • Magnocellular neurons
are located in the
supraoptic and
paraventricular nuclei.
• This site responsible of
synthesis of the two
major hormones of the
posterior pituitary.
The hypothalamic
magnocellular
neurosecretory system

12. The magnocellular neurosecretory system
Direct neural connections linking the hypothalamus and posterior pituitary

13. The hypothalamic
parvocellular
neurosecretory system
The hypothalamus
communicates with the
anterior lobe indirectly by a
vascular route .

14. The hypothalamic parvocellular neurosecretory system
Parvocellular
Clustered in
nuclei around
the third
ventricle
Diffusely located
in the medial
preoptic and
anterior
hypothalamic
area

15. The hypothalamic parvocellular neurosecretory system
The axons of these neurons
project to and terminate in
the pericapillary space of the
primary portal plexus of
vessels in the median
eminence.

16. The neurohormones are
released into the portal blood
and pass to the anterior pituitary
to act on the:
1. Gonadotrophs.
2. Thyrotrophs.
3. Corticotrophs.
4. Somatotrophs.
5. Lactotrophs.
In order to regulate the synthesis
and release of their various
hormones.
The hypothalamic
parvocellular neurosecretory
system

17. • The glycoprotein hormones LH and FSH are secreted by gonadotrophs in the
anterior pituitary.
• Immunocytochemical studies have revealed that each hormone is generally
elaborated in a different cell type, but occasionally both may be found in the same
cell.
• The synthesis and secretion of both FSH and LH depend on a hypothalamic
decapeptide, gonadotrophin-releasing hormone.
• Histochemical techniques have localized GnRH mRNA, GnRH itself and its
precursor peptide to two major subsets of the parvocellular neurons.
Hypothalamic pulsatile secretion of GnRH controls
gonadotrophin secretion

18. • GnRH is the most important final common mediator of all influences on
reproduction conveyed through the CNS.
 Abnormality in GnRH synthesis, storage, release or action will result in partial
or complete failure of gonadal function.
 Destruction of GnRH-producing neurons in the hypothalamus, generation of
genetically null GnRH mice, or immunization against the peptide, all prevent
gonadotrophin function and result in gonadal atrophy.
Gonadotrophin-releasing hormone (GNRH)

19. Experiment
ovariectomized ewes
Aim : Study the relationship between GnRH
and gonadotrophin secretion
Method:
1. Sampling of portal blood to measure GnRH
secretion.
2. Sampling of peripheral blood to measure
gonadorophins.

20. Result:
Both are secreted in a pulsatile manner,
approximately one pulse being measured
about every hour or so and therefore called
circhorial pulses.
Each peripheral LH peak coincides with a
GnRH pulse, but each GnRH pulse is not
necessarily followed by an LH pulse.
This GnRH pulse generator, or circhorial
clock, controlling GnRH secretion seems to
reside in the hypothalamus.

21. Summary
Hypothalamic
GnRH neuron
• Regulate the synthesis and secretion of FSH and LH.
• The GnRH is released as a series of pulses into the
portal vessels.
Anterior
pituitary
• GnRH reaches and binds to receptors on the
gonadotrophs, and drives gonadotrophin secretion in
a similar, pulsatile manner.
LH and FSH
output
• By increasing or decreasing either the amplitude or
the frequency of these pulses of GnRH
• modulating the response of the gonadotrophs to the
pulses

22. There is a dynamic relationship between pituitary and ovary.
Ovarian hormones regulate gonadotrophin secretion in females
A depressant effect on
gonadotrophin output
induced by elevation of
the plasma
concentrations of
oestrogens,
progestagens and
inhibins.
Negative
feedback
An increase, or
surge, of LH and
FSH secretion
induced principally
by oestradiol
Positive
feedback

23. Ovarian hormones
regulate
gonadotrophin
secretion in females
oestradiol
FSH and LH
Progesterone
FSH and LH
Inhibins
Only FSH

24. Oestradiol regulates FSH and LH
secretion
The menopause or ovariectomy
FSH and LH increase at
plasma.
Due to removal of oestradiol.
Infusion of this hormone.
Rapid Decline in FSH and LH.
Circulating FSH (green) and LH (red) levels in:
(a) a woman at day 7 in the (early) follicular
phase; and (b) a postmenopausal woman. The
difference between the two represents the
negative feedback effect of oestradiol.

25. Oestradiol regulates FSH and LH secretion
Only low
circulating
levels of
oestradiol are
required to
effect
The effect
very rapid in
onset,
detectable
within 1 h
and
maximal by
4–6 h.
Oestradiol is
acting to
suppress
gonadotrophin
levels, the
process is
termed
negative
feedback.

26. Oestradiol regulates FSH and LH secretion
plasma
concentratio
ns of
oestradiol
increase
greatly, and
remain at
this high
level for 48 h
LH and FSH
secretion is
enhanced,
not
suppressed,
a surge of
LH and
FSH
The high levels of
oestradiol increase
the secretion of
gonadotrophins,
Postive feedback

27. Progesterone
regulates FSH and LH
secretion
Negative feed back
High plasma concentration of
Progesterone enhance the
Negative feed back of oestradiol
FSH and LH down to low level
Positive feed back
Block the effect of
oestradiol

28. The inhibins regulate FSH secretion

29. Steroid hormones and the inhibins feedback regulates the
menstrual cycle
re-examine the blood levels of steroids,
inhibins and gonadotrophins during a normal
human menstrual cycle
Follicular phase Luteal phase

30. Negative feedback is gradually reexerted and so FSH levels fall and LH levels
plateau
The selection of a dominant follicle leads to a further rise in oestrogen (together with their
biosynthetically associated androgens), culminating in an oestrogen (and androgen) surge.
Negative feedback inhibition is relaxed, and both FSH and LH levels rise
These rises permit antral growth to proceed, resulting in first the rising output of inhibin B
followed by androgens and oestrogens
Menstruation is initiated of Luteolysis
falling levels of luteal oestrogen, progesterone and inhibin A
The follicular phase of the cycle

31. The length of the follicular phase appears to be determined by the rate at
which the principal preantral follicle matures, since it is the main source of
oestrogen and thereby a major arbiter of cycle length. It is therefore
sometimes described as an ovarian or pelvic clock.
During the preovulatory phase, the surge in oestrogens triggers a rapid rise
in LH and FSH levels via its positive feedback effect, and ovulation follows.
As result of follicular collapse, androgen and thus oestrogen outputs fall, and
progesterone levels rise. The LH and FSH levels now fall equally precipitously
because, at least in part, they lack a continuing positive feedback stimulus.
Switch from inhibin B to inhibin A output under the combined stimulation of LH
and FSH.
This output of oestrogen and inhibin A reflects the development of only
the most advanced follicle(s), and this measured near ovulation

32. The luteal phase of the cycle
At the end of the luteal phase, if conception has not
occurred, oestrogens, progesterone and inhibin A decline at
luteolysis, the negative feedback effect of these hormones
is relaxed and LH and FSH levels start to rise
Which result in another cycle
Growth of antral follicles is suppressed and so androgens
are also at a low level.
The luteal phase of the cycle is characterized by rising concentrations of
plasma progesterone and 17α- hydroxyprogesterone, which peak around 8
days after the LH surge
In higher primates, the luteinized cells of the corpus luteum
also make large amounts of oestrogen and inhibin A

33. Positive and negative feedback are
mediated at the level of both
hypothalamus
and pituitary
• The anterior pituitary
the hormones might regulate FSH and LH
secretion by a direct action on the gonadotrophs
1. Decreasing (negative feedback)
2. Increasing (positive feed back)
their sensitivity to hypothalamic GnRH pulses.
• This might be achieved through regulation of
the GnRH receptors.
• There are abundant receptors for oestrogens,
progestagens and inhibins in the anterior
pituitary, emphasizing the potential importance
of this site.

34. • The hypothalamus
the ovarian hormones might change the GnRH output
signal:
1. Directly by affecting the GnRH neurons in the
hypothalamus.
2. Indirectly by changing the activity of other neural
systems that exert a modulatory influence on GnRH
release.
3. Ovarian hormones alter both the GnRH output signal
and the response of the anterior pituitary to it.
• Large regional concentrations of receptors for
oestradiol and progesterone exist in deffrnt area in the
hypothalmus.
• No hypothalamic site of action of inhibin.
Positive and negative feedback are
mediated at the level of both hypothalamus
and pituitary

35. Method:
1.Large lesions of the mediobasal hypothalamus, which
destroy the arcuate and ventromedial nuclei.
2. A large part of the median eminence.
Result:
1. abolition of GnRH output.
2. A decrease in serum FSH and LH to undetectable
levels.
The anterior pituitary can mediate feedback effects of
steroids and inhibins
Experiments
ovariectomized
rhesus monkeys
1

36. Method:
3. Hourly pulses of exogenous GnRH delivered by a
programmable intravenous infusion pump.
4. The subsequent injection of oestradiol, so as to reach
surge levels.
Result:
3. Restore pulsatile LH and FSH secretion.
4. First in a fall, and then in a dramatic rise (surge) in
serum FSH and LH levels.
The anterior pituitary can mediate feedback effects of
steroids and inhibins
Experiments
ovariectomized
rhesus monkeys
2

37. Method:
1. Measuring the change in plasma levels of LH and FSH
induced by pulses of exogenous GnRH administered on
different days of the menstrual cycle.
Result:
1. Increase the Secretion of FSH and LH at the follicular
phase.
2. Increased sensitivety, will rising level of ostradiol.
3. However, notice that the responsiveness of the
pituitary to GnRH remains very high in the luteal phase
of the cycle. What about normal??
The anterior pituitary can mediate feedback effects of
steroids and inhibins
Experiments
Studies on the
pituitary sensitivity
of women undergo
-ing normal cycles

38. Result

39. The demonstration that inhibin is able to reduce the FSH
secretory response to GnRH strongly indicates that its
effects are mediated by actions on the anterior pituitary.
The anterior pituitary can mediate feedback effects of
steroids and inhibins
Experiments
ovariectomized
ewes
Complete
absence of an
FSH response
to GnRH in
the inhibin-
treated ewes

40. The hypothalamus mediates steroid hormone feedback
only
• LH is secreted in a series of:
• High-frequency,
• Low-amplitude
• Pulses occurring approximately
once every hours.
The
follicular
phase
• LH is secreted as:
• A pattern of high amplitude,
• low-frequency,
• Irregular LH pulses, often with long
intervals between them of up to 6 h.
The luteal
phase
The pattern of LH and FSH pulses varies during the menstrual cycle

41. Fig. 6.14 LH pulsatility at different points during a human
ovulatory cycle (ovulation occurred on day 16) in 10-min serum
samples: (a) early follicular phase; (b) mid-follicular phase;
(c) late follicular phase; (d) early luteal phase.
Note the unique high-amplitude, low-frequency pulses
characteristic of the luteal phase when progesterone plasma
concentrations are high. *, LH peaks; OV, ± number of days
after/before ovulation; E2, 17β- oestradiol; P, progesterone; FD,
follicular diameter (mm).
The hypothalamus mediates steroid hormone feedback
only

42. The experimental manipulation of the steroid
environment confirms that:
1. Progesterone acts primarily to reduce pulse
frequency.
2. Oestrogen acts to reduce pulse amplitude.
The
hypothalamus
mediates steroid
hormone
feedback only

43. Method:
1. Modulation of GnRH secretory activity by steroids
2. measurement of the peptide in portal blood.
Result:
1. GnRH secretory activity is subject to modulation by
steroid feedback during the menstrual and oestrous
cycles.
2. in rats, GnRH secretion is increased on the afternoon
immediately before the LH surge. Similarly, the LH surge
induced by exogenous oestradiol administration in
rhesus monkeys and ewes is associated with elevated
concentrations of GnRH in portal blood.
The hypothalamus mediates steroid hormone feedback
only
Experiments
rats, sheep and
rhesus monkeys

44. Fig. 6.15 GnRH concentration
in portal blood and peripheral
plasma LH levels in an
ovariectomized ewe given an
injection of
50 mg oestradiol
monobenzoate to induce an
LH surge.
Note the increased frequency
of GnRH and LH pulses during
the LH surge.
Result

45. 2
The cellular mechanisms of feedback control
Anterior pituitary
Hypothalamu
s
1

46. • The effect of oestradiol positive
feed back:
1. By inducing and maintaining GnRH
receptors.
2. Sensitizing the self-priming process
GnRH induces its own receptors.
3. Small-ampliude GnRH pulses
cause an LH pulse.
The cellular mechanisms of
anterior pituitary feedback
control

47. • May prime a full LH response to the
next adequate GnRH pulse.
1. the presence of oestradiol enhances
this interaction between GnRH and its
receptor, perhaps thereby contributing
to the magnitude of the oestradiol-
induced LH surge.
The cellular mechanisms of
anterior pituitary feedback
control

48. The cellular mechanisms of the hypothalamus feedback
control
Fig. 6.18 Schematic diagram
to show some of the postulated
neurochemical interactions that
may control GnRH secretion.
GnRH neurons (blue) lie in the
medial preoptic area and
arcuate nucleus in primate
species. They project to the
portal vessels in the median
eminence, especially to the
lateral palisade zone.

49. The cellular mechanisms of the hypothalamus feedback
control
Dopamine neurons (purple) in
the arcuate nucleus modulate
prolactin release (and may
affect GnRH output but this is
controversial)

50. The cellular mechanisms of the hypothalamus feedback
control
Neurons within the
hypothalamus that contain β-
endorphin (green) also
modulate anterior pituitary
secretion, perhaps by
modulating GnRH neuron
activity in the medial preoptic
area.

51. The cellular mechanisms of the hypothalamus feedback
control
Noradrenergic and
adrenergic neurons in the
medulla oblongata project to
the medial anterior
hypothalamus and preoptic
area and may also participate
in the regulation of GnRH
secretion

52. • The neuroendocrine mechanisms
that govern testicular function are
fundamentally similar to those that
regulate ovarian activity.
• The major difference between the
sexes is the absence of positive
feedback in the male, arising from
the non-cyclic nature of male
reproduction,
Testicular hormones regulate
gonadotrophin secretion in
males

53. • Relation between LH and Leydig cell.
• Negative feed back effect.
• Testosterone and inhibin effect on
FSH.
Testosterone regulates the pituitary–Leydig cell axis

54. • Inhibin levels in testicular lymph,
rete testis fluid and semen are
some 100-fold lower than the level
in follicular fluid.
• The inhibin B in humans (inhibin A
in rams), and the blood levels
reflect the number of functional
Sertoli cells.
• FSH stimulates Sertoli cells
directly, a negative feedback loop
is postulated, especially during
puberty, as a way of regulating
Sertoli cell function.
Inhibin regulates the
pituitary–seminiferous
tubule axis

55. Important of inhibin
An increased output of
testicular inhibin related
to the successful
completion of
spermiogenesis
The depressed inhibin
levels and elevated
serum FSH levels will
cause failure to
complete
spermatogenesis in man.

56. The hypothalamic–pituitary–gonadal axis may be sexually
dimorphic
Female
Postive
feedback
Sex steroid
Inhibins
Negative
feedback
oestradiol

57. The hypothalamic–pituitary–gonadal axis may be sexually
dimorphic
The testis at birth has
prevented subsequent
support of ovarian
cyclicity, Testicular
androgens are
responsible for this
effect
Experiment
ovaries
transplanted into
recipient male rats
castrated at birth
No cycle
change
If adult male rats
are castrated and
receive ovarian
transplants
Undergo
cyclic
ovulation

58. The hypothalamic–pituitary–gonadal axis may be sexually
dimorphic
Experiment
Female rats injected
with testosterone
during the first few
days after birth
•No oestrous cycles in adulthood.
•Their ovaries contain follicles that
secrete oestrogens but as ovulation does
not occur there are no corpora lutea.
Neonatal androgen
causes acyclicity by
suppression or
modification of the
oestradiol positive
feedback
mechanism

59. The hypothalamic–pituitary–gonadal axis may be sexually
dimorphic
Experiment
Male or female rats are
castrated in adult hood
and are subsequently
injected with
oestradiol,
Only the females
show a surge of
gonadotrophins
If female rats given
testosterone during the
first few days of life
No surge is
observed
.
1

60. This is taken to be
evidence of sexual
differentiation of the
hypothalamus, neonatal
androgens in the male
preventing the ability to
respond to an oestrogen
surge with an LH surge
in the adult.
2 The positive feedback effect of oestradiol in
castrated male and female adult rats. The LH
response is shown to be present only in the
female (red) and not in the male (green).

61. • The ovaries or the pituitaries of androgenized females are
capable of secreting surge levels of oestrogen or LH if
transplanted into normal females, so their functional capacity
does not seem to be grossly impaired.
• The ‘masculinizing’ effect of neonatal testosterone is exerted
on the hypothalamus.
Does neonatal testosterone act on the ovary, pituitary or
hypothalamus to suppress the positive feedback response?

62. • This ‘masculinization’ of the brain occurs in most species (rodents,
sheep and some carnivores).
• The critical period of sensitivity to the effects of androgens varies
considerably.
• For example, guinea-pigs have a gestation period of 68 days,
compared to 21 days in the rat, and are born in a state of relative
maturity.
• The critical period during which androgens exert their effects on the
brain is pre- and not postnatal.
• Similar considerations apply to the large domestic animals and
carnivores.
Masculinization
1

63. • Different than other species.
• That the capacity for positive
feedback exists in normal male
monkeys and men. Thus, in
castrated male monkeys (as well as
in hypogonadal and castrated men)
an administered oestrogen surge
reliably induces a gonadotrophin
surge.
Masculinization of the brain (
primates)
2
Fig. 6.21 Positive feedback effects of
oestradiol in a castrate male talapoin monkey

64. Masculinization of the brain (
primates)
3
The positive feedback action of oestradiol cannot be elicited in intact male
monkeys for reasons that are not clear.
Undergo apparently normal monthly ovulatory cycles,
a marked contrast to the results of similar experiments in rats.
Ovaries transplanted into castrated male monkeys

65. Masculinization of the brain (
primates)
4
In some of these monkeys the external genitalia are so ‘masculinized’ that
menstruation occurs through a penis- like phallus.
Undergo menstrual cycles as adults, although puberty occurs slightly
later than usual.
Female rhesus monkeys exposed to high levels of testosterone during
fetal life

66. Masculinization of the brain (
primates)
5
Undergo menstrual cycles as adults, often after a delayed
puberty.
Human females exposed to high levels of androgens during gestation, for
example as the result of the adrenogenital syndrome

67. Conclusion
The less overt effects that neonatal androgens
appeared to have on sexual behaviour in adult
primates, as compared to their more dramatic
effects on sexual behaviour in rats and other
non-primate species.
Masculinization

68. • Prolactin is made in the pituitary
lactotrophs, which are distributed
evenly throughout the anterior
pituitary.
• It is stored in secretory granules and
released in a pulsatile manner, which
probably reflects the pulsatile release
of controlling hypothalamic hormones.
Prolactin has reproductive functions

69. Prolactin is secreted spontaneously in large
amounts when the vascular links between
the pituitary and hypothalamus are
disconnected.
This means that regulation of secretion is
mainly by inhibition
The hypothalamus controls
prolactin secretion

70. Dopamine secreted into the portal blood from the
terminals of this tuberoinfundibular dopamine (TIDA)
system and carried to the lactotrophs.
Action:
Suppress prolactin secretion
How to suppress?
1. Dopamine carried to lactotrophs, which express the
D2-like subtype of dopamine receptors.
2. Binding of prolactin to D2-like receptors activates
coupled G-proteins, resulting in several functional
consequences.
3. Acute prolactin release is inhibited, transcription of its
RNA is reduced, and lactotroph mitosis is suppressed.
The overall outcome is reduced prolactin release.
Dopamine is the prolactin inhibitory
factor

71. Short loop feedback control of hypothalamic TIDA neuron activity
The increase in dopamine turnover is related to an increase in tyrosine
hydroxylase activity, the enzyme that is rate-limiting in the intraneuronal
synthesis of dopamine
An increase in dopamine turnover within TIDA neuron terminals of the median
eminence
A reduction therefore in prolactin secretion.
Increases in circulating prolactin levels

72. • Oestrogens induce hyperprolactinaemia by binding to ERα in the lactotrophs and
stimulating prolactin synthesis.
• Chronic oestrogen exposure results in increased lactotroph numbers, which are
therefore more numerous and larger in females, who also have higher ambient
prolactin levels than males.
• An oestrous rhythm of prolactin secretion is observed in some animals, such as
the rat, with a mid-cycle prolactin surge coincident with LH.
• This appears to result from the preovulatory surge oestrogen, and is associated
with a decline in dopamine receptor level and thereby a dimin- ished inhibitory
influence of DA. Blocking the oestrogen surge, blocks the prolactin surge
Oestrogen stimulates prolactin release

73. • In women, no clear menstrual rhythm in serum prolactin levels,
and prolactin secretion does not alter significantly after the
menopause.
• During pregnancy, when oestrogen levels rise, prolactin output
also rises
Oestrogen stimulates prolactin release

74. Function of prolactin (women)
Modulate the number of ovarian receptors for LH
Assist in preventing premature progesterone secretion in the early stages of
follicular growth enhancing it in the luteal phase
Facilitate steroidogenesis in ovarian follicles
Lactation

75. Function of prolactin (male)
Prolactin also potentiates the effects of testosterone on the seminal
vesicles.
prolactin increases the uptake of androgen and increases 5α-reductase
activity in the prostate, acting synergistically with testosterone
prolactin may increase the number of LH receptors and potentiate the
steroidogenic effect of LH on Leydig cells

76. Function of prolactin (Male +
Female)
Its non-reproductive functions in mammals are numerous, such
as regulation of kidney and adrenocorticotrophic activity
prolactin as an ancillary hormone, promoting the activities of
other hormones.

77. Hyperprolactinaemi
a suppresses
fertility
Men
Impaired
fertility
Decreased
testosterone
Impotence and
loss of libido
Women
Amenorrhoea
infertility
loss of libido.
with or without
galactorrhoea
(abnormal milk secretion,

78. 3.Pathology.
Pituitary tumours
2.Iatrogenic
use of dopamine receptor-
blocking neuroleptic drugs
1.Physiologic
al
In pregnancy and during the
first few months of breast-
feeding
5.High-
amplitude
prolactin
pulses
4.Absence of
the fall in
prolactin
secretion
The causes of
hyperprolactinaemia

79. Dopamine D2 receptor
agonists, are used to lower
serum prolactin
concentrations immediately,
and daily treatment results in
the return of ovulation and
cyclicity in the vast majority of
women within 2 months.
Treatment of hyperprolactinaemia
Resumption of prolactinoma
activity and surgical removal
In the case of prolactin-
secreting tumours,
dopamine receptor agonists
have an antimitotic action,
reduce tumour size.
However, in the majority of
cases, cessation of
treatment is followed by
bromocriptin
e
01 02

80. • The CNS mediates the effects of environmental factors, such as coital
stimuli, olfactory stimuli and light, on the regulation of reproductive
activity in many species.
• In addition, factors arising from social interactions, including anxiety or
other forms of emotional distress, can have profound effects on
cyclicity and fertility in men and women, and are also mediated by the
CNS.
The environment influences reproduction

81. A. Circadian rhythms control reproductive function in some species
The temporal control of reproductive activity in females is
complicated because the production of a viable oocyte is itself a
cyclical event that must be matched to other cyclical events
occurring within the life of an animal.
In a nocturnally active rodent, for example, potential encounters with
mates will be restricted to the hours of darkness. This selection
pressure has led to the development of an oestrous cycle that is
tightly locked to the best indicator of external time, the daily light–
dark cycle.
Daylight affects fertility

82. Fig. Serum hormone levels during the oestrous
cycle in the rat.
The orange bars represent dark periods (18.00–
06.00 h) centred around midnight,
and the arrow denotes the time of ovulation.
Also shown are the levels of GnRH measured in
portal vein blood, which show a clear surge just
before the FSH and LH surges. Note that the surge
in prolactin is coincident with the gonadotrophin
surges.
Circadian rhythms control
reproductive function in some species
(oestrous cycle)

83. Circadian
It is important to realize that the circadian system
controls a wide range of other behavioural and
endocrine rhythms, both reproductive and non-
reproductive, which are held in a very strict, temporal
relationship to each other.

84. • In seasonal environments, where adverse climate and the availability
of food are major determinants of offspring survival and therefore of
the reproductive success of the parents.
• It is adaptive to ensure that young are born in the equable, productive
conditions of spring or early summer
• This tight control over birth season, apparent in many domestic and
wild species, is achieved by a precise regulation of the month(s) of
fertility and hence the timing of conception.
B. Circannual rhythms control reproductive function in
seasonal breeders

85. • In species with short gestation times, such as hamsters and birds, winter
is a time of infertility with gonadal development suspended until spring.
• In species with longer gestation times, such as sheep and deer, the
anticipation of spring must begin much earlier and seasonal changes in
autumn act as a stimulus to reproductive function.
• In a third group, which includes marsupials, mustelids and seals, the total
length of the gestation period can be varied because of delayed
implantation and embryonic diapause. These processes are sensitive to
environmental influences and provide a second level of control over the
timing of the birth season
B. Circannual rhythms control reproductive function in
seasonal breeders

86. • In some species, such as deer and ground squirrels, there is good evidence that
seasonal cycles are under the control of an endogenous circannual oscillator, a
biological clock with a period of approximately 1 year.
• In other species, there is no endogenous rhythmicity and the seasonal rhythms
observed in the field are triggered by cyclical stimuli within the environment.
• Of these, photoperiod is by far the most important; this is exemplified in the
laboratory, where artificial manipulation of day lengths can be used to drive all of
the components of the annual reproductive cycle.
B. Circannual rhythms control reproductive function in
seasonal breeders

87. The seasonal sexual cycle of a Soay ram.
Changes in the plasma concentration of FSH
(red), LH (green) and testosterone (shading)
are shown at three times of the year.
Testis size (blue) is shown inset at each time:
(a) in the non-breeding season, when the
testes are fully regressed, and LH, FSH and
testosterone levels are all low;
(b) towards the onset of the breeding season,
when the testes are redeveloping and,
associated with this process, FSH
concentrations are very high;
(c) during the mating season, when
testosterone levels are very high and this
reflects the marked increase in the frequency
of pulsatile LH discharge
B. Circannual rhythms control reproductive function in
seasonal breeders

88. • In humans and other primates, sheep, rats and many other mammals, ovulation
is said to occur ‘spontaneously’.
• It depends on an endogenous event timed by the ovary, the oestradiol surge,
which results in an ovulatory discharge of LH.
• The neurally mediated variable controlling ovulation, concerns the induced or refl
exovulators, such as cats, rabbits and ferrets.
• These animals remain in behavioural oestrus for long periods of time without
ovulating until they copulate with a male. Stimulation of the cervix and vagina.
• Even in these species it seems that the hypothalamic–pituitary axis must be
primed with high levels of oestrogen for the neural input effective.
Coitus affects fertility in some species

89. • Studies of primates living in social groups have revealed that the social
context in which individuals interact can change their endocrine and
fertility status.
• For example, if plasma testosterone levels are measured in male
talapoin monkeys in the absence of females, all males, whether single
or together as a group, have very similar testosterone levels. However,
when oestrogen-treated females are introduced to an all-male group,
one male becomes dominant, displays sexual activity with the females
and is aggressive.
Social interactions and stress can affect fertility

90. Thank you
Any Question