Fertilisation and pregnancy - reproduction 3 Flashcards
Fertilisation
Implantation
Placentation
Learning Objectives
* Fertilization and blastocyst development
* Process of Implantation and placental formation
* Activity of Key hormones
* Gestation and Maternal adaptations during
pregnancy
Fates of ejaculated sperm? 5
– Leak out of the vagina immediately after
deposition
– Destroyed by the acidic vaginal environment
– Fail to make it through the cervix
(mucus, anti sperm antibodies)
– Dispersed in the uterine cavity or destroyed
by phagocytic leukocytes
– Reach the uterine tubes
During fertilisation the sperm travels through what 3 parts?
Cervical canal, Uterus and Upper 1/3 of oviduct (fallopian tube)
How many sperm reach the fallopian tubes?
0.001% - Only ~100 reach the ampullary portion
of the FT where fertilization normally occurs
The female reproductive tract aids in migration via what 4 mechanisms?
- Thin cervical mucous (oestrogen from mature
follicle) aids sperm travel
*Contractions of myometrium – Sperm to
oviduct - drive sperm - Upward contractions of oviduct smooth
muscle drive sperm to egg - Allurin – Sperm chemoattractant released by
mature eggs to attract sperm towards it
When must fertilisation must occur?
Must occur within 24 hours after ovulation (viability of ovum)
How long can sperm last?
Sperm usually survive about 48 hours; can survive up to 5 days in female tract
What must the sperm undergo before they can penetrate the oocyte?
Capcitation
Capacitation is a cAMP mediated process what does it involve?
-> Ca^2+ influx which leads to this erratic whiplash motion developed by sperm which causes enhanced penetration of the ovum - not travelling in a linear way
-> Remove of surface proteins and cholesterol: weakening of sperm membrane allows readiness for penetration of outer layers for this acrosomal reaction (ability to fertilise an ovum)
How long must ejaculated sperm be “capacitated” in the female for fertilisation leading to activation of spermatozoa?
4-6 hours
Activation of spermatozoa - capacitation is mediated by what?
cAMP mediated process
Are ejaculated sperm capable of fertilisation?
Ejaculated sperm are incapable of fertilisation. They must be “capacitated” in the female genial tract
What is the corona radiata?
Corona radiata: layer of cells surrounding eggs
Receptors on head that recognise zona pellucida?
Z3 particularly but also ZP1,3+4 relevant in detection of egg whites - these are glycoproteins
Steps of sperm tunneling through the barriers surrounding an ovum?
5 Steps
- Fertilising sperm penetrates the corona radiata via membrane-bound enzymes in the plasma membrane of its head and bins to ZP3 receptors on the zona pellucida. (Zona Pellucida glycoproteins ZP1,3,4)
- Binding of sperm to these receptors triggers the acrosome reaction, in which hydrolytic enzymes in the acrosome are released into the zona pellucida.
- Acrosomal enzymes digest the zona pellucida creating a pathway to the plasma membrane of the ovum. When the sperm reaches the ovum, the plasma membranes of the 2 cells fuse.
- The sperm head with its DNA enters the ovum cytoplasm.
- The sperm stimulates release of enzymes stored in cortical granules in the ovum, which in turn, inactivates ZP3 receptors and harden the zona pellucida, leading to the block to polyspermy.
The 1st sperm to reach the ovum fuses with what and what does this cause?
Fuses with plasma membrane of ovum and the head of the fused sperm gradually pulled into ovum’s cytoplasm and this fusion triggers meiotic division of oocyte
Number of mechanisms of polyspermy - different between species paricualrly between sea urchin dvelopmental biology model but theya re differnet
Can more than 1 sperm fertilise an egg?
Diandritic triploidy resulting from dispermic fertilisation represents how many triploids?
Many cases where more then 1 sperm can fertilise an egg however if more than 1 is done normally it is unsuccessful and the embryos die
- Diandric triploidy resulting from dispermic fertilization represents 60−80% of human triploid
conceptuses (most die in embryogenesis)
2 Mechanisms to ensure monospermy?
Zona pellucida block and Membrane block
Mechanisms to block polyspermy are dependent on? This occurs in?
Ca2+ release from ER of oocyte in response to release of sperm head-associated phospholipase C zeta (PLCζ) into the egg’s cytoplasm
Occurs in waves in cytoplasm
Explain the zoda pellucida block mechanism to monospermy?
Ca2+ signal induces exocytosis of cortical granules
* Fusion of cortical granules with oocyte plasma membrane releases contents into the
perivitelline space (between the plasma membrane and the ZP)
* “Hardening” of zona pellucida - Increased resistance to proteolytic digestion and changes
in mechanical behavior
* Cleavage and modification of ZP glycoproteins
Explain the membrane block mechanism to ensure monospermy?
Not as well understood - Ca^+2 is involved however not just by itself - not independent there is more that they do not know
- Conversion of the membrane to a state that is unreceptive to sperm
- Lesser understood (Ca2+ elevation required but not sufficient alone to trigger membrane
block)
When fertilised ovum divides mitotically this causes formation of?
Is there cleavage of cells?
Totipotent yes or no and until when?
– Forms morula
– Cell cleavage (no increase in overall size)
– Totipotent up to 32 cell stage