Fertilisation, Implantation and Gastrulation Flashcards
For sperm to be successful when should it arrive in the female reproductive tract
+/- 1 to 2 days form ovulation ( day 14 of the 28 day cycle )
What happens at ovulation
1- Cervical mucus increases in amount, less acidic and becomes less thick to allow easier sperm transport.
2- Fallopian tube come in close contact with ovary and fimbriva sweep it into the tube
3- peristaltic waves of Fallopian tube bring ovum into ampulla of Fallopian tube where it can be fertilized
How long can sperm survive in the female reproductive tract
5 days
What is Capacitation
the readying of the sperm :
1- Final step of spermatozoan maturation is in the female reproductive tract
2- Binding site on plasma membrane on spermatozoan is exposed
3- spermatozoan is now available and has to binding proteins to fertilize oocyte
Cant spermatozoa fertilize oocytes when newly ejaculated
No because membrane molecules are masked by plasma proteins. Glycoprotein coat has to be remodelled before sperm is ready
Where is capacitation done
In the uterus and oviduct
Explain the stages of fertilization ( binding of sperm to zona pellucid )
1- sperm passes through cumulus layer of oocyte
2- zona pellucida ( glycoprotein shell ) facilitates and maintains sperm binding also induces acrosome reaction
3- capacitated sperm will penetrate cumulus and bingo adhere to ZP , this triggers acrosome reaction
4- ZP opens Ca channels in spermatozoan cell membrane resulting in exocytosis of acrosomal contents
How can sperm pass through the cumulus layer
1- Hyaluronidase released from sperm acrosome
2- tubule mucosal enzymes
3- flageller action
What is the acrosome
modified lysosomes around the head of the spermatozoan that release acorosome enzymes to facilitate binding and penetrating of ZP
What is the acrosome action
Prerequisite action for sperm to fuse with oocyte
What happens once the sperm binds membrane of oocyte ( to stop too much sperm form entering )
1- cortical reaction : exocytosis of cortical enzymes into ZP
2- destroys sperm receptors on ZP & hardens the ZP
3- blocks polyspermy
What happens when more than one sperm enters the ovum ( dispermy , triploidy )
Fetus nearly always aborts
Explain stage 3 & 4 of fertilization
1- Fusion of plasma membranes of oocyte and sperm
2- triggers completion of meiosis II ( making a haploid )
3- head and tail of spermatozoan enters cytoplasm of oocyte
Explain stage 5 of fertilization
Formation of male and female pronuclei ( hasn’t fused yet, 2 nuclei )
1- Cytoplasm of oocyte gives rise to organelles of zygote and nutrition that will be needed
2- membranes of pronuclei break down and chromosomes are arranged for mitotic cell division ( Cleavage division )
3- egg is completely fertilized and now called a Zygote
Explain the cleavage process
A series of mitotic divisions that result in cells called blastomeres. After 3 divisions blastomeres undergo compaction. Compact blastomeres will divide and form 16 cell morula ( Cells start to act like a tissue and form tight junctions)
How is a blastocyst formed
Morula will develop two distinct cell types and the central cavity will be filled with fluid ( blastocoel). Trophoblast cells will be pushed to outside that gives rise to placenta. Embryoblast will be the inner cell mass that gives rise to the embryo. These cells hatch from the ZP. Now cell is called blastocyst
What is a blastocyst
Morula form Blastocyst which is an embryo 5-7 days after fertilization.
What happens to the ZP after the blastocyst hatches from it
It disintegrates
List the steps of In Vitro
1- Hormonal stimulation of egg maturation ( overstimulating ovary to get ovulation )
2- Egg is removed by laparoscopy through vagina
3- Sperm sample is collected and most active sperm is concentrated
4- in vitro fertilization
5- early cleavage in vitro
6- extra embryos frozen
7- preimplantation of up to 3 embryos
Explain the implantation process
Trophoblast outer cell mass divides and differentiates into:
1- Syncytiotrophoblast outer
2- cytotrophoblasts inner
Inner cell mass embryoblalst divides :
1- epiblast
2- hypoblast
What stage is the uterus in during implantation
Uterus is at the secretory phase where the blastocyst implants in the endometrium along the anterior or posterior wall
What happens to the endometrium if fertilization doesn’t occur
Endometrium is shed as menstruation but basal layer stays to regenerate other layers during the next cycle
Once the embryoblalst divides which type of cell is in the upper layer and which is the inner layer
Upper : Epiblast
Lower: Hypoblast
What is the role of the synctiotrophoblasts
Will secrete hormone of pregnancy , HCG ( human chorionic gonadotrophin. This will signal to corpus luteum to keep producing estrogen and progesterone which will tell endometrial lining of uterus to stay in secretory phase and not go into mensis phase
