Oogenesis Flashcards
structure of ovaries
- 2-3cm long
- attached to broad ligament
helpful: circles at ends of fallopian tubes
fallopian tubes and egg
- egg released from one ovary into abdominal cavity during ovulation
- fimbriae (finger-like projections) at end of fallopian tube
- fimbriae sensitive to hormonal cues - when egg released the fimbriae closest to ovary become mobile and actively sweep across surface of ovary to guide egg into entrance of fallopian tube
egg travel after release from ovary
ovary -> FT: fimbriae (part of IF) into infundibulum -> FT: ampulla -> FT: isthmus -> intramural portion -> uterus
describe ampulla structure
- widest part of fallopian tube
- longest part of fallopian tube
describe isthmus structure
- narrower portion of fallopian tube
- portion closer to uterus
what is the infundibulum in female reproductive tract
funnel-shaped entrance into fallopian tubes
where does fertilisation occur?
in ampulla or isthmus
where does implantation of embryo (fertilised egg) occur
in uterine epithelium (uterine wall)
what is intramural (interstitial) portion [not always needed]
- fallopian tube that extends into top of uterus
- opens into uterine cavity
what is the centre of the ovary called?
ovarian medulla
where do we see follicles in ovary?
in ovarian medulla scattered at varying points of progression
list the progression of follicles
primordial follicle -> primary follicle (preantral) -> secondary follicle -> tertiary follicle (pre-ovulatory follicle - may ovulate)
fate of more than 99% of follicles
become atretic
only 1% of follicles….
ovulate
after ovulation occurs what occurs to follicle
change in structure
to form corpus luteum
function of corpus luteum
produce progesterone
to signal that implantation may occur in body
what happens to oogonia before birth
enter meiosis and become primary oocytes
when do primary oocytes become suspended
during prophase 1 during diplotene phase
around time of birth
how many primary oocytes around time of birth
300,000
how many primary oocytes will end up ovulated
3-400
what happens to the rest of the primary oocytes that are not ovulated and when
lost to atresia until time of menopause (~early 50s)
before puberty what happens to primordial follicles and primary oocytes
every day a few primordial follicles differentiate to primary follicles
primary oocytes remain at diplotene
is differentiation into primary follicles gonadotrophin dependent, if not what is it?
no (hence happens before puberty)
controlled by intraovarian factors
what happens to all developing follicles prior to puberty
become atretic (degenerate)
structure of a primordial follicle
small oocyte
surrounded by squamous granulosa cells
surrounded by basement membrane - separates follicle from stroma
structure of primary follicle
oocyte grows & secretes zona pellucida (+ more organelles incl mitochondria being produced)
increase amount granulosa cells and become cuboidal
formation of thecal layer from stroma cells
basement membrane separating from thecal later
what is zona pellucida
surrounds oocyte
layer primarily composed of glycoproteins
formation at primary follicle stage
what hormones do we see the production of after puberty
- steroid hormones - progesterone, oestrogen
- increases in FSH and LH
WHAT is regulated by hormonal changes across the menstrual cycle
ovulation
what is further development of primary follicles dependent on
hormonal changes across menstrual cycle after puberty
what hormone and feedback prevents ovulation
negative feedback by progesterone (high levels of progesterone)
what happens after some time to progesterone if implantation hasn’t occurred (progesterone is high following ovulation)
progesterone declines from influence of prostaglandins produced by uterus, then menstrual cycle begins again
(implantation occurs after ovulation)
when do we see an increase in production of secondary follicles
from puberty
structure of secondary (antral) follicles
- increase in thecal and granulosa cells
- theca organised into vascularised theca interna & outer fibrous theca externa
- accumulation of fluid between granulosa cells
- slight increase oocyte size
- development of follicular antrum that surrounds oocyte(follicle fluid-filled cavity)
- oocyte still attached to granulosa cells through specialised subpopulation of granulosa cells called cumulous oophorus which suspended from mural granulosa of follicle wall
cumulous cells function
attached to oocyte at time of ovulation
important for subsequent maturation of oocyte
what does production of oestradiol by secondary follicle rely on
uptake of cholesterol from thecal layer
what do secondary follciles produce
- ovarian steroids - oestradiol, progesterone
explain how thecal layer takes up cholesterol and makes androgens
- theca interna highly vascular
- cells of theca interna take up cholesterol from blood and synthesise androgens (testosterone, DHT)
- androgens cross basement membrane to granulosa cells that convert androgens to oestradiol
what dictates more oestradiol production
size of secondary follicle, in particular the follicle destined to ovulate
is ovulation initiated by positive or negative feedback
positive feedback
outline positive feedback in ovulation (3 steps)
- decline in progesterone & increase in oestradiol late in luteal phase - triggers ovulation
- creates positive feedback on hypothalamus causing increased LH and FSH secretion by anterior pituitary
- LH surge causes preovulatory follicle to rupture -> resulting egg gets ovulated
explain how LH causes ovulation
- LH surge acts via LH receptors on granulosa cells
- follicle rupture induced by proteases (inside follicle)
- follicle collapses
-> granulosa cells lutenised (change function)
-> corpus luteum formed (& produces progesterone)
what happens at the same time as LH surge in oocyte
resumption of meiosis
what happens in resumption of meiosis for oocyte
meiosis I completed -> forming secondary oocyte (suspended) & extrusion of first polar body
what does the first polar body contain?
chromosomes that are redundant during meiosis
when is meiosis II completed? / resumption of meiosis II on suspended secondary oocyte
at time of fertilisation - when sperm penetrates membranes surrounding oocyte
when is cAMP relevant and at what levels
decreased cAMP during resumption and completion of meiosis I (prior to ovulation)
at menopause are there many follicles left
no, nearly all gone
what is steroid hormone production by ovary like during menopause
little / no steroid hormone production => negative feedback is removed causing increased FSH levels which are a menopause indicator
at what age do we see a decline in female fertility
~35 years old
what causes decline in female fertility
decrease in oocyte quality (developmental competence)
what characterises decreased oocyte quality
increased incidence of aneuploidy (incorrect chromosome number) = increased rate of miscarriage in women aged 35 and over even though pregnancy can occur
at what age does male fertility decline
~40years old but not as dramatically
what testing allows embryos to be screened for aneuploidy => improve success rates of IVF
preimplantation genetic testing (PGT-A)
compare success rates of women aged 35 vs under 35
> 35 yrs - less than 10% - still relatively low
<35 yrs - over 30%
describe PGT-A
take usually 5 cells from blastocyst embryo stage
taken to lab to determine chromosome number, particularly whether aneuploid embyro
blastocysts frozen then transferred at a subsequent cycle once results known
why is there an increase in freezing of oocytes below age 35
to preserve fertility
~10% women use these oocytes, get pregnant nonetheless without accessing these
