Fertilisation Flashcards
what is coagulated?
ejaculated sperm
why is ejaculated sperm coagulated?
to prevent loss, later liquifies
what happens to the coagulated sperm?
it’s jelly-like, takes about 15 mins to liquify and the sperm swim out
importance of sperm moving through cervical mucus?
removes seminal fluid, abnormally morphological sperm and cellular debris
effect of progesterone on cervical mucus?
cervical mucus is less viscous in the absence of progesterone allowing sperm to pass
what do cervical crypts do?
-ridges on the side of the cervix
that trap sperm which then swim round and round in the ridges and come out at different times
-increases the window of time that the sperm has to meet the egg – increases the chance of them being around when the egg is released
when does fertilisation occur?
typically within 24-48 hours
give a suggestion as to how the sperm finds and travels to the oocyte cumulus complex?
-chemo-attractants (pH and other factors) released from the oocyte cumulus complex may attract the sperm
what are CatSper channels?
VGCC’s on the surface of the sperm which open in response to pH and other factors released from the egg
- sperm become hyperactivated
- forceful tail beats with increased frequency and amplitude mediated by Ca2+ influx via CatSper channels.
what does hyperactivity mean?
means that means as the sperm gets near the egg it swims much faster
where do the sperm and egg meet?
the ampulla (wide portion of the uterine tube, close to the fimbrial end)
where does the embryo spend the first 5-6 days of its life?
uterine tube
what is capacitation and how is it achieved?
- biochemical rearrangement of the surface glycoprotein and changes in membrane composition
- partly achieved by removing sperm from seminal fluid
- uterine/tubal fluid may contain factors which promote capacitation
why is capacitation needed?
prepares the sperm to be able to undergo the acrosome reaction
how long does capacitation take?
4-18 hours
does every sperm undergo capacitation for the same length of time - what is the importance of this?
each sperm has a slightly different time to capacitate
-widening the window of time that capacitated sperm might meet the egg
when does the sperm undergo the acrosome reaction?
when sperm comes into contact with zona cumulus complex, binds to ZP3 receptor
what is the acrosome?
invagination of the sperm membrane to make a fluid-filled compartment that contains enzymes to digest though cells surrounding the egg
what happens in the acrosome reaction
the acrosomal membrane on the sperm head fuses releasing enzymes that cut through the complex
what breaks in ovulation?
basement membrane, blood pours into the middle
what mix in ovulation?
theca and granulosa become mixed
another name for the follicular phase (of the ovary)?
proliferative phase (of the uterus)
another name for the luteal phase (of the ovary)?
secretory phase (of the uterus)
what happens in the follicular/proliferative phase?
- dominated by oestrogen, levels go up causing endometrium proliferation
- LH surge, meiosis 1 completion, ovulation
what happens in the luteal/secretory phase?
dominated by progesterone, which:
- makes the endometrium secretory and receptive to implantation
- supresses cilia in uterine tubes once oocyte has already passed.
- makes cervical mucus viscous again to prevent further sperm penetration
also some oestrogen secreted from CL, which helps maintain endometrium in luteal phase
role of oestrogen in follicular vs luteal phase
follicular is proliferation of endometrium, luteal is maintenance of endometrium
why would the CL need beta-HCG to survive?
because otherwise its essentially killing itself
- it is producing progesterone, which will negatively feedback on the HPG axis, decreasing LH levels
- less LH is able to bind to CL, so CL would die
- hence beta-HCG from embryo binds to LH receptors on the CL to maintain its survival
what happens as a consequence of the CL dying?
endometrium sheds, menstruation
-because CL stops making progesterone, which is needed to maintain the endometrium
what is the zona pellucida?
secreted by the egg, impenetrable after the cortical reaction (inner layer)
what is the Cumulus oophorus?
derived from granulosa cells, protects the egg (very outside layer)
what is the corona radiata?
the innermost layer of cumula cells, in contact with the ZP
-formed by granulosa cells adhering to the oocyte before it leaves the ovarian follicle (middle layer)
explain sperm binding and penetration?
- sperm comes into contact with the zona-cumulus complex, penetrates cumulus and comes into contact with ZP via ZP3 receptor
- enzymes digest through ZP, sperm fuses with plasma membrane and taken in by phagocytosis
- phospholipase Zeta activated by basal Ca2+ inside egg
- PIP2 → DAG + IP3
- release of intracellular Ca2+, large Ca2+ spike
- cortical reaction as wave of Ca2+ sweeps around egg
- release of proteases, peroxides and hyaline prevents polyspermy
ca2+ release causes what?
completion of meiosis II, expelling of the second polar body and the cortical reaction
explain what syngamy is and what happens
it is the fusion of 2 nuclei
- sperm nuclear membrane breaks down, the chromatin decondenses and chromosomes separate
- 4-7 hours after sperm penetration the two sets of haploid chromosomes (male and female) become surrounded by distinct membranes forming two pronuclei
- pronuclei synthesise DNA in preparation for the first mitotic division
- pronuclei fuse, spindle forms and chromosomes line up
- completion of mitosis
- one cell zygote becomes a two cell embryo
