Folliculogenesis, Oogenesis and Ovulation Flashcards
Define folliculogenesis
development of follicles to produce mature ova
Define oogenesis
development of primary egg cells into mature ovum competent for fertilisation
Define ovulation
release of the ova from the ovary
What tissues are present in the ovary cortex?
endocrine
gametogenic
What tissue does the ovary medulla consist of?
supportive tissue - blood and nerves
What are the differences between species in the ovary cortex and medulla?
- cow/sow/sheep - medulla inner surface and cortex outer surface
- mare - inversed
Where does ovulation occur in different species?
- cow/sow/sheep - over surface of ovary
- mare - ovulation fossa
What are the 2 stages of oogenesis?
prenatal
after puberty during oestrus cycle
Describe prenatal oogenesis
- occurs during foetal development on outer surface of ovary where cortex will form
- primordial germ cells diploid
- 2N oogonia formed via mitosis - become diploid again
- primary oocyte division via meiosis with aim of forming haploid cell
What does the process of prenatal oogenesis maximise?
no. gametes in order to be as prolific as possible
Describe primary mieosis oocyte division in prenatal oogenesis
- interphase and prophase I of meiosis I
- mieotic inhibitors halt at prophase I and cause nuclear arrest
- female born with no. gametes for lifetime
Describe post puberty oogenesis
- Not all primary oocytes come out of nuclear arrest all at once
○ Recruitment and selection phase - protects primary oocyte as nucleus remains dormant
○ Still see activity - oocytes have cytoplasmic and formation of Zona Pelucida
§ Translucent ring around cytoplasm of the oocyte- Of primary oocytes selected:
○ During oestrus cycle a no. follicles containing primary oocytes start to develop
§ Some become atretic and die
○ Meiosis I has potential to resume in a primary oocyte that has the potential to ovulate and has a mature cytoplasm
○ Meiotic inhibitors removed by LH following surge of LH close to ovulation
○ Meiosis I resumes and completes
§ Secondary oocyte (1N) will ovulate
§ Most oocytes won’t get to this stage
○ In bitch - meiosis I won’t resume until after ovulation
Meiosis II can only occur after fertilisation - produces mature ovum
- Of primary oocytes selected:
What are the 4 stages of oocyte maturation and what is essential to support these?
- prenatal mitotic division
- nuclear arrest
- cytoplasmic growth
- resumption of meiosis
folliculogenesis essential
Describe the overall process of Folliculogenesis
- oogonia - no follicle
- meiotic division of oogonia to form primary oocyte
- primordial follicle develops into primary follicle
- develops to secondary then tertiary follicle
- preovulatory follicle developed
Describe the meiotic division of oogonia in folliculogenesis
- primary oocyte held in nuclear arrest until post-pubtery & oestrus
- primodrial (first) follicle appears - very immature, flattened granulosa cells in close proximity to primary oocyte and outer basement membrane
Describe how primordial follicle develops into primary follicle in folliculogenesis
- primary oocyte still present
- granulosa cells more cuboidal
- females born with lifetime supply of primordial and primary follicles but not all will be ovulated
Describe how the secondary follicle is developed in folliculogenesis
- 2 or more layers of granulosa cells
- small number of theca cells that surround granulosa cells - have receptors for hormones
- zona pelucida starts to form around primary oocyte
Describe how the tertiary follicle is developed
- primary oocute surrounded by granulosa cells
- basement membrane and then 2 extra layers of theca cells
- as follicle develops - antrum (cavity) forms
Describe the 2 layers of theca cells in the tertiary follicle
- theca externa: connective tissue, fairly loose structure that supports developing follicle
- theca interna: role in hormone production and sensitivity
Describe the antrum in the tertiary follicle
- filled with follicular fluid from oocyte and granulosa cells
- fluid contains hormones and proteins to support developing oocyte
Describe the preovulatory follicle and how it is formed
- follicle that ovulates
- called graafian follicle or mature follicle
- FORMED:
- antrum increases in size due to fluid
- granulosa cells group together to form cumulus oophorus in which secondary oocyte sits on
- theca interna and externa still present
When manipulating the cycle, which phase is targeted and why?
antral phase because it is dependent on gonadotropin hormones therefore occurs within oestrus cycle
Describe the preantral phase
oogonia
primordial follicle
primary follicle
secondary follicle
Describe the antral phase
tertiary hormone and preovulatory hormone
gonadotropic hormone dependent
Describe antral follicle dynamics
- recruitment from ovarian follicular pool
- selection
- dominance
- antresia
Describe selection in antral follicle dynamics
- small follicles recruited and start to grow & secrete oestradiol in small quantities
- some selected to become medium follicles: grow and secrete more
- some selected again to become dominant follicles
Describe dominance in antral follicle dynamics
dominant follicles will go onto produce more oestradiol and become the pre-ovulatory follicles
Describe atresia in antral follicle dynamics
over 90% of follicles die due to insufficient blood supply, hormone supply and response
What is survival and development of follicles driven by?
FSH and LH
Describe how FSH and LH recruit follicles
- follicles respond to FSH, less to LH
- FSH drives follicular growth
- become atretic without FSH
What role does LH play in selection and dominance of follicles?
increases concentration and is more effective than FSH
How do theca cells function?
- contain LH receptors
- cholesterol converted to androstenedione which is transferred to granulosa cells
How do granulosa cells function?
- FSH receptors
- androstenedione converted to oestrodiol
What are the different sizes of follicle in each species?
- mare >3cm
- cow 1-1.5cm
- ewe 0.5-1cm
- sow 0.5cm
- bitch <0.25cm
- queen <0.25cm
How many oocytes are prepared for ovulation in each species?
- mare, cow 1-2
- ewe 1-3
- sow 4-20
- bitch, queen 8-10
What determines number of oocytes ovulated?
whether the species is monotocous
What triggers ovulation?
preovulatory LH surge
How does the LH surge trigger ovulation?
- increased blood flow to follicle and ovary
- ovarian smooth muscle contractions
- connective tissue destruction
Describe connective tissue destruction in ovulation
- tunica albuginea - sits over surface of ovary/ovulation fossa
- weakens wall coupled with increased pressure from follicle - leads to rupture
- follicular fluid secreted along with secondary oocyte which washes into infundibulum
- ovum waits in ampulla and if fertilized - enters uterus via utero-tubular function in isthmus
Describe what happens to the empty follilce post-ovulation
- follicle wall collapses
- theca, granulosa and connective cells mix
- blood vessels rupture -> clot -> corpus haemorrhagicum
- small blood clot in cavity
Describe the formation of the corpus luteum
-
- increases in size 3-5 days post ovulation
- luteinisation of theca interna and granulosa cells
- more structure to luteal tissue than early CH
- active CL is red/orange/yellow
- produces progesterone, precursor cholesterol
Describe the breakdown of the corpus luteum
- luteolysis breaks down CL to form corpus albicans
- CL regresses and becomes smaller - no glandular tissue
- scar tissue, light yellow-white
Which waves occur whilst the CL is present?
- recruitment
- selection
- dominance
- atretic
- NO ovulation
Which hormone breaks down CL in order for the cycle to resume?
PGF2 alpha from the uterus
