Folliculogenesis Flashcards
What has to be achieved to reproduce?
much more than just sexual intercourse
- Differentiation into male or female
- Sexual maturation
- Production, storage and release of sufficient supply of eggs and sperm
- correct number of chromosomes in eggs & sperm
- Eggs and sperm have to meet ie. gamete transport
- Creation of new individual with genes from both parents
- To nurture individual until capable of “independent life”
cell that become eggs or sperm originate from where?
primordial germ cells
primordial germ cells are first identifiable where?
in the yolk sac of the developing foetus at 3 weeks
what happens to primordial germ cells
they undergo many rounds of mitosis, migrate to genital ridge which because the gonad
-PGC’s differentiate into male or female games depending on the gonadal development
when germ cells are in the ovary what do they become?
oogonia
what are oogonia?
egg-precursors
summary of oogonia - primary oocytes - primordial follicles
- oogonia are diploid and multiply by mitosis
- once mitosis stops they enter into the first stage of meiosis
- no more division occurs and they are primary oocytes
- surrounded by layer of GC – now known as primordial follicle
- establishment of the true “ovarian reserve”
before birth what happens?
massive loss of oocytes
-when you’re born you have your entire stock of primordial follicles
how long do primary oocytes remain in the fist phase of meiosis?
until they are ovulated or die
when do chromosomes replicate?
s phase of cell cycle
what happens during chromosome replication?
- remain attached at the centromere
- each copy known as a chromatid
- 2 copies are identical to each other and are called “sister” chromatids
- exact copy of original chromosomes
phases of mitosis?
prophase
metaphase
anaphase
telophase
how does meiosis differ to mitosis?
- in meiosis the homologous chromosomes line up adjacent to 1 another
- allows recombination and variation to occur
- line up at the spindle and arrest at this stage for decades
where are primary oocytes located?
they are packed into the outer layer of the ovary
-the cortex
formation of primordial follicles
- primary oocytes will remain in this phase for many years. therefore, each one gets surrounded by many protective layers
- in the foetal ovary, surrounding cells condense around the oocyte and differentiate into granulosa cells
- GC’s secrete an acellular layer called the basal lamina
- whole structure is now a primordial follicle
why do we have a massive loss of oocytes when we are born?
because those which haven’t formed a follicle properly will die off, those which haven’t undergone mitosis properly or the first stages of meiosis will die off.
what is folliculogenesis?
the growth and development of follicles from the resting stages through to ovulation
after puberty what happens?
a cohort of follicles grow every day
primordial follicle growth
- granulosa cell multiplication
- secretion of another protective acellular layer, zona pellucida - stays attached after ovulation and protects the egg
- in the adolescent ovary, second layer of cells differentiates around the basal lamina - theca cells
what are steroid producing?
the theca and granulosa cells
-sustain the follicle and feed into the menstrual cycle
what does FSH drive?
most of folliculogenesis, but early growth is independent of FSH
what happens in FSH deficient patients?
follicles will still continue early growth but then die
formation of an antral/secondary follicle
- follicle grows and rapidly increases in diameter
- granulosa cells divisions increase, gaps form in granulosa cell layers
- gaps consist of follicular fluid-filled spaces which form an antrum
- as the antrum and follicular fluid volume expands, oocyte is displaced to one side
(follicles with an antrum are known as antral/secondary follicles)
what is an antral follicle
-characterised by an “antrum” (cavity)
-contains fluid formed as exudate of plasma containing secretory products of oocyte & GC
Known as “follicular fluid”
preovulatory or Graafian follicle
- as the fluid filled spaces expand, the granulosa cells are pushed towards the edge
- eventually the egg is sitting within the follicular fluid surrounded by granulosa cells, now known as cumula cells
- this is a cumulus oocyte complex, and it is then ovulated
follicle initiation
cohort of early follicles leaving the resting pool and grow continuously
follicle recruitment
cohort of early follicles recruited will NOT continue to grow unless they reach the correct size to respond to FSH changes in the menstrual cycle
(only one will be selected for ovulation)
gonadotrophin independent growth?
primordial and preantral follicle
gonadotrophin dependent growth?
antral and ovulatory - need FSH to grow
role of the theca?
- very detailed blood supply
- bringing growth factors and hormones to sustain follicle growth
two-cell–two-gonadotropin theory
- LH drives androgen production from cholesterol in theca cells
- androgens go into circulation and across into granulosa cells
- FSH binding to FSH receptors on granulosa cells drives oestrogen production in granulosa cells, formed from androgens (aromatase)
what does aromatase catalyse?
the conversion of androgens to oestrogens
-only exists in granulosa cells
what happens to the majority of primordial follicles?
die
when do the follicles finish meiosis 1?
just before ovulation
