fertilisation and implantation

Question 1

meiosis

Answer 1

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.

Question 2

coagulum

Answer 2

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)

Question 3

pH of vagina

Answer 3

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

Question 4

path of sperm

Answer 4

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

Question 5

cervical mucus changes

Answer 5

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

Question 6

characteristic of cervical mucus

Answer 6

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

Question 7

sperm transport survival times

Answer 7

vagina = 2.5hrs

cervix = 48hrs

uterus = 24hrs

oviduct = 48hrs

clinically to patients indicate ~5 days of survival time

Question 8

what percentage of spermatozoa enter the cervix

Answer 8

1%

99% of spermatozoa (in an ejaculate) do not enter the cervix

Question 9

transport time in fallopian tubes and effects of speed

Answer 9

2-7hrs

speed is related to fertility
speed decreases with time
speed decreases with freezing and thawing

Question 10

number of sperm in an ejaculate and in cervix

Answer 10

anywhere between 20-200million sperm in human ejaculate however only 1 million enter cervix

Question 11

where does fertilisation occur

Answer 11

usually near the isthmus/ampulla junction

Question 12

sperms immune response

Answer 12

sperm produce an innate immune response

sperm arriving in the uterus induce an infiltration of leucocytes

invading leucocytes phagocytose dead or dying sperm

Question 13

capacitation

Answer 13

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

Question 14

acrosome reaction

Answer 14

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

Question 15

unfertilized oocyte

Answer 15

can only survive for 24hrs or less

Question 16

first stage of fertilisation

Answer 16

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

Question 17

after the sperm has penetrated the cumulus cells

Answer 17

encounters the corona radiata, a layer of tightly packed follicle cells and digests its way past these to encounter the zona pellucida

Question 18

zona pellucida

Answer 18

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

Question 19

perivitelline space and process

Answer 19

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

Question 20

sperm beyond the perivitelline space

Answer 20

one sperm will attach to the oolemmal membrane (at microvilli) and the two membranes fuse together allowing the sperm nucleus ot enter the ovum

Question 21

cortical reaction

Answer 21

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)

Question 22

polyspermy

Answer 22

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

Question 23

resumption of meiosis

Answer 23

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)

Question 24

zygote

Answer 24

is a fertilised egg

Question 25

zygote transport

Answer 25

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)

Question 26

timeframe in the division of blastomeres

Answer 26

2 cells = 30hrs

16 cells = 72hrs (Morula)

~60 cells = 96 hrs (bastocyst)

~100 cells = 4.5 days

Question 27

blastocyst structure and locations

Answer 27

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

Question 28

nidation

Answer 28

name given to the contact between the trophectoderm (blastocyst) and uterine epithelium (decidua)
-adhesion via receptors