fertilisation and implantation Flashcards
meiosis
Following LH surge, meiosis is resumed and the first division is completed.
The process stops again, however, at second metaphase.
Thus at ovulation, the egg consists of the oocyte and a small polar body surrounded by the zona pellucida.
Meiosis is resumed, for the second time, on fertilization.
coagulum
formed when the prostatic and seminal vesicle fluids mix (in the vagina)
holds the ejaculate together and adheres the ejaculate to the female reproductive tract
after 2-15 minutes coagulum is broken down to release the sperm which swims from vagina to fallopian tubes
(breakdown of coagulum = liquefaction)
pH of vagina
median = 4.7
ranges from 3.5-7.4
this pH is inhibitory to sperm motility and survival
seminal plasma buffers the vaginal pH to around 7.2 so sperm can become motile
path of sperm
sperm must migrate from vagina through the uterus into the fallopian tubes (where fertilisation occurs)
first barrier to sperm ttransport is cervix and cervical mucus
cervical mucus changes
under influence of high estrogen the cervical mucus changes
it consistency changes throughout the menstrual cycle
mid cycle (high E2) mucus becomes spinnbarkeit, stretchy and permissive to sperm passing through it
characteristic of cervical mucus
filters poor sperm
tails of normal sperm cause the fibres of the cervical mucus to vibrate in rhythm (helps sperm move through cervix)
failure of abnormal sperm to induce this vibration in the cervical mucus may retard the progress of abnormal sperm
sperm transport survival times
vagina = 2.5hrs
cervix = 48hrs
uterus = 24hrs
oviduct = 48hrs
clinically to patients indicate ~5 days of survival time
what percentage of spermatozoa enter the cervix
1%
99% of spermatozoa (in an ejaculate) do not enter the cervix
transport time in fallopian tubes and effects of speed
2-7hrs
speed is related to fertility
speed decreases with time
speed decreases with freezing and thawing
number of sperm in an ejaculate and in cervix
anywhere between 20-200million sperm in human ejaculate however only 1 million enter cervix
where does fertilisation occur
usually near the isthmus/ampulla junction
sperms immune response
sperm produce an innate immune response
sperm arriving in the uterus induce an infiltration of leucocytes
invading leucocytes phagocytose dead or dying sperm
capacitation
occurs in the female reproductive tract
freshly ejaculated sperm are not capable of fertilising an ovum until capacitated
involves the removal of capacitation inhibitory substances from the sperm and a loss of cholesterol from the sperm membrane
sperm then becomes hyperactivated and facilitate movement of sperm through tortuous fold of fallopian tube
only capacitated sperm can undergo the acrosome reaction
acrosome reaction
the acrosome and plasma membranes of the sperm head fuse, allowing release of the acrosomal contents to the environment around the sperm (e.g. receptors can bind to oocyte)
reaction is triggered by an influx of calcium into the cell
-Ca2+ triggered by progesterone, ZP3 etc
if it happens to early sperm will not be able to fertilise egg as it cant get to the zona pellucida
must take place in close proximity to an oocyte
unfertilized oocyte
can only survive for 24hrs or less
first stage of fertilisation
requires the sperm to penetrate the layer of cumulus cells that surround the egg
assisted by hyaluronidase secreted from the acrosome which digests the BM allowing sperm to squeeze between cumulus cells
after the sperm has penetrated the cumulus cells
encounters the corona radiata, a layer of tightly packed follicle cells and digests its way past these to encounter the zona pellucida
zona pellucida
protein coat surrounding the ovum
3 major proteins ZP1, ZP2, ZP3
ZP-3 is the primary sperm receptor and contact between ZP-3 and its sperm ligand induces acrosomal reaction completely
perivitelline space and process
space between zona pellucida and ovum
enzymes of the acrosome aid sperm to pernetrate through the ZP and into this space
- takes about 10 mins
- race between sperm
sperm beyond the perivitelline space
one sperm will attach to the oolemmal membrane (at microvilli) and the two membranes fuse together allowing the sperm nucleus ot enter the ovum
cortical reaction
entry of sperm into the oocyte causes a release of intracellular calcium
followed by regular spikes of Ca2+ in the oocyte
- inducing the resumption of meiosis (2nd polar body)
cortical granules also released into pervitelline space
- release enzymes to harden ZP and prevent other sperm binding (blocks polyspermy)
polyspermy
a result of sperm arriving at the oocyte at exactly the same time or from the ZP not hardening
end results would be >2 nuclei therefore baby isnt fertilised
resumption of meiosis
dependent of Ca2+ spikes
destabilizes a protein complex that held the chromosomes in metaphase
loss of complex allows chromosomes to migrate to the poles and for meiosis to resume
2nd polar body is shed and 1st may also divide (not usually)
zygote
is a fertilised egg
zygote transport
zygote begins cleavage in the tube and takes approx 5 days to traverse the tube and enter uterus
blastomeres (cleavages) divide symmetrically
-produce identical daughter cells
when you get to 16 cells the structure is referred to as a morula
-totipotent (8-16)
timeframe in the division of blastomeres
2 cells = 30hrs
16 cells = 72hrs (Morula)
~60 cells = 96 hrs (bastocyst)
~100 cells = 4.5 days
blastocyst structure and locations
2 differentiated cell populations trophectoderm
=forms placental trophoblast
inner cell mass at one end = forms embryo proper
remains free-floating in the uterine cavity for 2-3 days before hatching
then attaches to the decidual/endometrial epithelium
nidation
name given to the contact between the trophectoderm (blastocyst) and uterine epithelium (decidua)
-adhesion via receptors
