Fertilisation Flashcards
What are the requirements for fertilisation?
a sperm - maturation and capactitation
an egg - arrested at metaphase II
Synchronised receptive endometrium
How does the ovum enter the fallopian tube?
cumulus oocyte complex is picked up by ciliated fimbriae on the infundibulum at the end of the uterine tube
- action of fimbrae controlled by menstrual cycle
What happens to sperm when in the female tract?
decrease rapidly as they move through
- 20-250 million deposited in upper vagina
- seminal plasma, short term buffering, coagulates within minutes (seminogelin) PSA
- Cervical mucus least viscous during days 9-16 of cycle
- pro ovarian contractions of myometrium, sperm pulled towards ovum
- 1000 sperm enter tube via uterotubal junction
- chemotaxis in humans (progesterone)
- cilia line the tubes and move fluid surrounding them to assist sperm movement
- muscular actions of female genital tract and own motility
Where does fertilisation normally occur?
ampulla region of uterine tube
How long to sperm remain capable of fertilisation?
How long does an egg remain capable of fertilisation?
What does this mean?
- sperm - 5 days within female tract
- egg - 24 hours
- fertilisation can occur max of 5 days after intercourse
What are the 5 steps of sperm interaction with egg vestments?
- remote detected of occyte cumulus complex
- penetration of cumulus
- zona binding
- awesome reaction
- zona penetration
Describe penetration of the cumulus
- approx 3000 cells embedded in gelatinous matrix (huylaronic acid)
- closely apposed cells from tight organised layer
- others less organised
- sperm penetrate and can disperse cumulus (hyaluoronidase enzyme)
After penetrating cumulus, sperm bind to zona pellucida
How?
Acrosome reaction permits zona penetration
exposes new membrane for oocyte fusion
What is the zona pellucida?
extracellular protein matrix which surrounds all mammalian eggs
- 4 glycoproteins in humans (ZP1, ZP2, ZP3, ZP4)
- important for sperm-egg binding and induction of AR
- persists post fertilisation
Describe fusion
- Sperm penetrates the ZP and occupies to perivitelline space
- Equaltorial segment of sperm head fuses with oocyte plasma membrane
- oocyte engulfs the front of the sperm head
- sperm nucleus encased by a vesicle composed of internalised oocyte membrane
- large increase in Ca2+ sweep across egg from point of sperm fusion
What are the 2 proteins reponsible for fusion?
Izumo
- sperm membrane receptor for fusion
- detectable on sperm surface only after acrosome reaction
- KO completely abolishes fusion
Juno
- receptor for Izumo on the oocyte plasma membrane
- KO abolishes fusion
Describe oocyte activation
- within 1-3 minutes of fusion - large rise in Ca2+ which sweep across egg from point of sperm entry - lasts 2-3 minutes
- followed by Ca2+ oscillations every 3-15 minutes which may last for several hours
- triggered by PLC zeta - sperm specific PLC
What are the 2 key effects of Ca2+ increase and oocyte activation?
- Release of meiotic block
- maturation promoting factor (MPF) = cdk1 plus cyclin B
- MPF is established by cytostatic factor (CSF)
- raised calcium levels surpress CSF activity and destroy cyclin B
- triggers resumption of cell cycle in the oocyte and complete of meiosis II - block to polyspermy
- fast block = electrical, membrane depolarisation - minutes
- slow block = cortical reaction , triggered by increased Ca2+
What is the cortical reaction?
- cortical granules contain a mixture of enzymes, including several proteases, which diffuse into the ZP following exocytosis from the egg
- induces the zona reaction
What is the zona reaction?
- the alteration in the structure of the ZP catalysed by proteases from cortical granules
- cleavage of ZP2 by ovastacin protease
- sperm can no longer bind or penetrate
What is meant by the loss of Juno?
- Juno protein is shed from oocyte plasma membrane with the cortical granules
- no further sperm can fuse
What does the sperm contribute to the zygote?
- haploid male genomee- sex determination
- centriole - the oocyte has none, forms spindle for first cell division
What does the egg contribute to the zygote?
haploid female genome
cytoplasm
all organelles
mitochondria
Describe the zygotic/pronucleate (2PN) stage
- decondensation of sperm DNA - protamine/histamine exchange
- male and female pronuclei replicate their DNA
- pronuclei migrate towards each other
- guided by sperm aster - microtubules radiating from centrosome
What is syngamy?
- 18-24 hours
- pronuclear membrane break down
- chromatin intermixes
- nuclear envelope reforms around zygote nucleus
- cleavage begins - end of fertilisation/beginning of embryogenesis
Describe the transport of the embryo to the uterus
- increased progesterone: oestrogen ratio relaxes musculature of female reproductive tract - isthmic sphincter
- mostly transported under action of cilia
Describe the cleavage stages
- zygote cleaves to form two blastomeres
- 4-8 cells stage - PGD/embryo transfer
- morula (16-32 cells) - near end of uterine tube
- no cytoplasmic synthesis so blastomere size decreases wth each division
- ZP still present
What controls development of the embryo
> 2 cell stage = dependent on oocyte cytoplasm
4-6 cell stage = major burst of transcription
Many maternally derived proteins persist until blastocyst stage - poor oocyte maturation has big effect
Embryo metabolise, and growth is stimulated by a number of growth factors - both autocrine and paracrine -
What is compaction?
- 8-cell stage onwards - inside out polarity starts to develop
- outer cells - trophobalast
- inner cells - inner cell mass
- late morula - fluid absorntio - formation of intercellular junctions between trophoblast cells - Na+K+ATPase
Describe blastocyst formation
Day 5
Blastocoel
Discitnon ICM and single layer epithelial trophoblast layer
embryonic and anembryonic pole
What is hatching?
Late day 6 onwards
Blastocyst expands out of hole in ZP - anembryonic pole
Implantation
What are the risks with monochorionic twins?
What are the risk with mono amniotic twins?
monochronionic = twin to twin transfusion monoamniotic = umbilical cord compression
Describe the structure of the endometrium
- Underlying muscular myometrial layer
- Upper functional layer = undergoes proliferation then shedding
- Lower basal layer = attached to myometrium; remains intact during menstruation
- Functional layer is resconstriuted out of the underling basal layer
- stromal matrix covered with luminal epithelium
- glandular epithelial extensions penetrate into stroma
- stroma contains a rich supply of blood vessels - the spiral arteries
What happens to the endometrium during the proliferative phase?
- after menstruation, endothelium is very thin and consist of only a few lawyers of basal cells
- in the first 14 days of menstrual cycle, endometrial cells proliferate - oestrogen
- thickening due to stromal cell proliferation and stromal oedema
- surface epithelium increases in surface area and metalbolic activity
- increase in number and size of glandular invagination of the stroma
- expression of intracellular progesterone receptor
What happens to the endometrium during the luteal phase?
- after ovulation, ovaries begin to produce progesterone
- progesterone stimulates synthesis of secretory material by the glands - rich in glycogen, glycoproteins and AAs - provides nurtrion for blastocyst
- stromal cells becoming larger and plumper
- spiral arteries become fully developed
- cellular secretions are released into the glandular lumen
- must be oestrogen-primed = receptive endometrium
What is the window of implantation?
last four days - 20-24
- characterised by the appearance of small elevations at the apical pole of the epithelial endometrium cells - pinopodes
- pinopodes are involved in the absorption of the uterine fluid
- brings the blastocyst nearer to the endometrium
- also immobilises it
Describe menstruation
- spiral arteries of the functional layer are hormone sensitive - constrict when the progesterone conc falls
- collapse and necrosis of the functional layer
- blood and necrotic tissue is lost (menses)
What is the clinical relevance of all this?
Ectopic Pregnancy
Chromosomal Problems
