PHYSIOLOGY - Ovarian Function and the Oestrous Cycle Flashcards
What are the three phases of the oestrous cycle?
Oestrus (and ovulation)
Luteal phase
Follicular phase
What are the four stages of the oestrous cycle?
Oestrus
Metoestrus
Dioestrus
Proestrus
What is recruitment?
The process of primordial follicle activation
Following recruitment, what are the two possible fates for the growing follicles?
Either growth until ovulation or follicular atresia
What is the difference between early antral and small antral follicles?
Early antral follicles: follicles directly after antrum formation
Small antral follicles: follicles containing a fully grown ovum within them
During follicular development, which stage of meiosis is the ovum arrested in?
Prophase 1
During follicular development, what keeps the ovum arrested in Prophase 1 of meiosis?
The cumulus cells surrounding the ovum have cytoplasmic endings with synapses which extend into the zona pellucida and release cAMP into the ovum, arresting the ovum in Prophase 1
What are the primary and pre-antral follicles dependent on for growth?
Growth factors
When do follicles develop FSH and LH receptors?
Primary follicles develop FSH receptors on their granulosa cell layer, LH receptors are developed on the thecal cells in pre-antral follicles
When do follicles become FSH responsive?
Primary follicles become FSH responsive when FSH receptors develop on their granulosa cell layer
When do follicles become LH responsive?
Pre-antral follicles become LH response when the LH receptors develop on their theca cell layer
Primary and pre-antral follicles are gonadotrophin responsive. What does this mean?
The binding of FSH and LH has a proliferative effect on the granulosa and theca cells however this effect is not required for the growth of the follicles - it just makes the process more efficient
When do follicles begin to synthesise steroid hormones and become gonadotrophin dependent?
Small antral follicles begin to synthesise steroid hormones and become gonadotrophin dependent
Describe the process of the ‘two cell theory’ in steroid hormone production
- Cholesterol is taken up by the theca cells where steroidogenic enzymes stimulated by LH binding cleave cholesterol into pregnenolone and progesterone which act as precursors for androgen synthesis
- 17α Hydroxylase converts pregnenolone or progesterone into androgens
- Androgens are taken up through the basement membrane into the granulosa cell layer and act as precursors for oestrogens
- FSH binding stimulates aromatase enzymes to convert androgens into oestrogen
What is the androgen precursor for oestrodiol?
Testosterone
What is cohort emergence?
Cohort emergence is the emergence of small antral follicles in preparation for selection
What drives cohort emergence of small antral follicles?
Cohort emergence is driven by the elevated levels of FSH in the circulation
Describe the process of dominant follicle selection
Cohort emergence is driven by the elevated levels of FSH in the circulation. This elevated FSH stimulates increased oestrodiol and inhibin synthesis from the cohort follicles. Increased oestrodiol and inhibin levels suppress FSH synthesis in the gonadotroph cells, leading to a decreased FSH concentrations in the circulation. This leads to follicular atresia of most of the subordinate cohort follicles as they are FSH-dependent. The remaining follicle is now known as the dominant follicle which is no longer FSH dependent (however is now LH dependent)
How does the dominant follicle remain dominant and how does this relate to LH dependency?
The dominant follicle develops LH receptors on its granulosa cell layer as well as its theca cell layer. This allows for increased oestrodiol and inhibin secretion independent of FSH, further inhibiting FSH synthesis and preventing the emergence of another cohort
Describe how the gonadotropin releasing hormone (GnRH) surge is stimulated in spontaneous ovulators
Frequent, low amplitude LH pulses ensure maximum oestrodiol secretion from the dominant follicle. This rapid oestrodiol production in the absence of progesterone induces the surge of gonadotropin releasing hormone (GnRH) which stimulates ovulation
Describe how the gonadotropin releasing hormone (GnRH) surge is stimulated in mating induced ovulators (queen, exotic animals)
A neuroendocrine reflex triggered by mating induces the gonadotropin releasing hormone (GnRH) surge which stimulates ovulation
How does the gonadotropin releasing hormone (GnRH) surge allow the ovum to proceed to the next stages of meiosis?
The gonadotropin releasing hormone (GnRH) surge causes the cytoplasmic endings of the cumulus cells to withdraw from the zona pellucida and separate outwards. This stops the secretion of cAMP into the ovum and allowing the ovum to undergo meiosis
At which stage of meiosis does the ovum arrest again to allow fertilisation to occur?
Metaphase 2
In canines, ova can only be fertilised two days after ovulation. Why is this?
Because in canines, ovulation occurs before the ovum has reached metaphase 2
What is the difference between a primary and secondary ovum/oocyte?
Primary ovum/oocyte: Ovum that has not resumed meiosis until after ovulation
Secondary ovum/oocyte: Ovum that has resumed meiosis before ovulation
During the follicular phase, how does oestrodiol prepare the uterine horns for fertilisation?
Increased myometrial motility
Vascularisation
During the follicular phase, how does oestrodiol prepare the oviducts for fertilisation?
Enhanced myosalpinx motility
Enhanced ciliary motility
Enhanced serous secretions
During the follicular phase, how does oestrodiol prepare the cervix for fertilisation?
Serous gland development
During the follicular phase, how does oestrodiol prepare the vagina for fertilisation?
Enhanced keratinisation and proliferation of the vaginal epithelium
Acidic serous secretions
How does oestrodiol enhance the myometrial motility of the reproductive tract in preparation for fertilisation?
Oestrodiol increases the oxytocin receptors present on the myometrial cells thus increasing smooth muscle excitability and motility
What is luteinisation?
Luteinisation is the process by which the granulosa and theca cells convert to luteal cells in response to reduced oestrodiol synthesis and begin elevated progesterone production
Why does luteinisation occur before ovulation as well as in the luteal phase?
Luteinisation induces elevated progesterone production which is essential in the remodelling and lysis of the follicular wall, a process required for ovulation to occur
Other than progesterone production, what else is required for follicular wall remodelling?
Angiogenesis
Inflammatory response
Describe the process of follicular wall remodelling and lysis essential for ovulation
Progesterone activates proteases which lyse the follicular cell wall, allowing the antral follicular fluid to transport the ovum into the oviducts
Describe the formation of the corpus haemorrhagicum after ovulation has occurred
Slight haemorrhage can occur after the rupture of the ovulatory follicle and this haemorrhage can accumulate in the antral cavity, forming the corpus haemorhagicum
Describe the formation of the corpus luteum after ovulation?
After the formation of the corpus haemorragicum, the granulosa and theca cells continue to luteinise into large and small luteal cells which form the corpus luteum
What is the main function of the corpus luteum?
The production of progesterone to prepare the tubular tract for a possible pregnancy
What regulates progesterone production in the corpus luteum?
Luteinising hormone (LH)
Prolactin (only in mono-oestrus animals)
Describe the regulation of progesterone production during the formation of the corpus luteum
During the formation of the corpus luteum, progesterone production is LH dependent due to the LH receptors present on the small luteal cells
When the corpus luteum is fully formed, why does progesterone production shift from being LH dependent to LH independent?
LH stimulates progesterone production however progesterone actually inhibits the production of gonadotropin releasing hormone (GnRH) which is required for LH production. Because of this negative feedback loop, the corpus luteum becomes dependent on progesterone production by the large luteal cells which are LH independent
During the luteal phase, how does progesterone prepare the uterine horns for pregnancy?
Increased myometrial thickness
Leukocyte recruitment
Decreased myometrial motility
During the luteal phase, how does progesterone prepare the
oviducts for pregnancy?
Decreased myosalpinx motility
Decreased ciliary activity
Reduced serous secretions
During the luteal phase, how does progesterone prepare the
cervix for pregnancy?
Increased mucus secretions
What is luteolysis?
The regression of the corpus luteum
Describe the process of active luteolysis in seasonally polyoestrous animals (ruminants, horses and pigs)
- The endometrium secretes prostaglandin F2α (PGF2α) into the uterine vein
- PGF2α is transported from the uterine vein into the ovarian artery via the counter-current transport system (however this doesn’t occur in horses) into the ovary
- PGF2α binds to PGF2α receptors present on the small luteal cells and induces regression of the corpus luteum
Describe the process of passive luteolysis in mono-oestrus animals (small animals)
- LH and prolactin concentrations in the bloodstream are reduced due to the inhibition of GnRH secretion due to the negative feedback loop exerted by elevated progesterone production by the corpus luteum
- The corpus luteum is no longer supported in progesterone production and begins to regress
How does the regression of the corpus luteum allow ovulation to occur?
Regression of the corpus luteum reduces the progesterone in the bloodstream which allows the selected dominant follicle to be stimulated by frequent, low amplitude LH pulses and stimulation of the gonadotropin releasing hormone (GnRH) surge leading to ovulation
What is oestrus?
Oestrus is a recurring period of sexual receptivity and fertility in many female mammals, also known as ‘heat’
What are some of the signs of oestrus in the cow?
Vulval swelling
‘Bulling string’
What are some of the signs of metoestrus in the cow?
Dry mucus on the tail
Metoestrus bleeding
What are some of the signs of oestrus in the mare?
Raised tail that is moved to the side
Winking (eversion of the clitoris)
Spraying urine
Oedema of the uterine horns (can be seen on ultrasound)
Which animals are non-seasonally polyoestrous?
Cow and Sow
How long is the average cow oestrous cycle?
(include the length of the luteal and follicular phase)
21 days (17 day luteal phase, 4 day follicular phase)
How long is a cow in oestrus?
2-16 hours of oestrus
When does ovulation occur in the cow?
In the cow, ovulation occurs at the end of oestrus
What does it mean when a cow has a multiple follicle wave cycle?
The cow has several dominant follicles selected during her oestrous cycle but only the final one develops into an ovulatory follicle due to the time of selection, the other selected follicles become atretic
Which type of cows are more likely to have a two follicle wave cycle and which type of cows are more likely to have a three follicle wave cycle?
Dairy cows: two wave cycle
Beef cows: three wave cycle
Which animals are seasonally polyoestrous?
Mare, Ewe and Queen
Which hormone regulates the onset of the oestrous cycle in seasonally polyoestrous animals?
Melatonin produced by the pineal glands which has a feedback effect on GnRH pulsatility
Mares are described as long day breeders. What does this mean?
Mares breed within the summer months (‘long days’) and are anoestrus during the winter months
How long is the average mare oestrous cycle?
(include the length of the luteal and follicular phase)
21 days (14 day luteal phase, 7 day follicular phase)
How long is the mare in oestrus?
5-7 days of oestrus
When does ovulation occur in the mare?
In the mare, ovulation occurs 1-2 days before the end of oestrus
What causes ovulation to occur before the end of oestrus in the mare?
The mid-cycle FSH peak occurring within the luteal phase which stimulates cohort emergence and dominant follicle selection during the FSH decline
Ewes are described as short day breeders. What does this mean?
Ewes breed within the winter months (‘short days’) and are anoestrus during the summer months
How long is the average ewe oestrous cycle?
(include the length of the luteal and follicular phase)
17 days (14 day luteal phase, 3 day follicular phase)
How long is a ewe in oestrus?
24-36 hours of oestrus
When does ovulation occur in the ewe?
In the ewe, ovulation occurs at the end of oestrus
How many follicle waves do most ewes have within their oestrous cycle?
Most ewes have a 2-4 follicle wave cycle
Give an example of an animal that is a mating induced ovulator?
Queen (feline)
Are cats long-day or short-day seasonal breeders?
The queen is a long-day seasonal breeder
During the cat breeding season, how long is the queen on and off of oestrus?
The queen experiences 7 days of oestrus followed by 9 days of inter-oestrous
When ovulation is induced due to mating of the queen, what are the two possible resulting consequences?
Pregnancy
Pseudopregnancy
How long can pseudopregnancy last in the queen?
The lifespan of the corpus luteum, and thus the pseudopregnancy, can last between 5-6 weeks
Other than unsuccessful mating, what else can induce ovulation and a pseudopregnancy in the queen?
Presence of a tom cat near by
The owner simply stroking the queen when she is in oestrus
How long is the oestrous cycle in laboratory rodents?
4 days
What is pseudopregnancy?
Pseudopregnancy is the prolonged lifespan of the corpus luteum
Which species is monoestrous?
Bitch (canine)
What causes pseudopregnancy in laboratory rodents?
Pseudopregnancy in laboratory rodents is due to the luteotrophic action of prolactin pulses following vaginal stimulation
What are two of the signs of bitch oestrus?
Vulval swelling
Diapedesis bleeding
How long do bitches typically exhibit heat behaviour?
Three weeks
How long is the inter-oestrous period for the bitch?
The inter-oestrous period for the bitch is approximately seven months
What is the only breed of dog that is a long-day seasonal breeder?
Basenji’s are long-day seasonal breeders
What leads to pseudopregnancy in the bitch?
Pseudopregnancy in the bitch is caused by the prolonged lifespan of the corpus luteum and prolonged progesterone production. When progesterone levels begin to decline, prolactin levels rise also contributing to symptoms of pseudopregnancy
What is puberty?
Puberty is the process of acquiring reproductive competence
What are the two main hormonal influences on puberty onset?
Kisspeptin
Melatonin
How do kissneurones influence the onset of puberty?
Kissneurones produce kisspeptin which binds to GnRH neurones to increase the pulsality of GnRH and thus the gonadotrophin hormones (LH and FSH)
What are the two locations of kissneurones in the female brain?
Preoptic area kissneurones
Arcuate kissneurones
What are the main two endogenous influences on puberty onset?
Metabolic influences
Body weight
What are the main two exogenous influences on puberty onset?
Day-length
Social cues
