Acrosome Reaction
Upon contact with the corona radiata, the overlying plasma membrane of the acrosome becomes unstable as a glycoprotein coat and seminal plasma proteins are removed from the plasma membrane
Cortical Reaction
- Prevents polyspermy
- Triggered by acrosome reaction
- Oocyte membrane becomes impenetrable to other sperm
Zona reaction
- prevents polyspermy
- triggered by acrosome reaction
- Zona Pellucida alters its structure o prevent other sperm binding and penetration
List the steps to fertilization
- Acrosome reaction
- Fusion of plasma membranes of sperm and oocyte
- Formation of male pronucleus
- Completion of second meiotic division of oocyte
- Fusion of pronuclei to form diploid zygote
Syngamy
Fusion of male and female pronuclei to form a diploid zygote
Results of zygote cleavage
- mitotic divisions produce cells called blastomeres
- cells become smaller with each division (still contained within ZP)
- cells arranged in loose clump until 8 cell stage when compaction begins to take place
Compaction
- at 8 cell stage, blastomeres begin to maximize their contact with each other, forming a ball of cells held together by tight junctions
Morula
- “Mulberry”
- dense 16 to 32 cell ball comprised of inner cell mass and outer cell mass
- forms at ~3 days
What does the morula form?
- After ~4 days, the morula enters the uterus
- The fluid from the uterus begins to penetrate the ZP into the intercellular spaes of the inner cell mass
- now called a blastocyst
Trophoblast
- derived from outer cell mass of morula
- differentiates into two layers: cytotrophoblast and synctiotrophoblast
cytotrophoblast
- inner layer of mononucleated cells with mitotic capacity
- derived from trophoblast at ~8 days
synctiotrophoblast
- Outer multinucleated zone of the trophoblast
- over the embryoblast
- invades the uterine endometrium to implant the blastocyst
- derived from trophoblast at ~day 8
embryoblast
-derived from inner cell mass of blastocyst at 4-5 days
What are the 2 layers of the embryoblast?
- epiblast and hypoblast
- differentiates at ~ day 8
epiblast
- dorsal layer of the embryonic disc
- site of formation of amniotic cavity
- cells adjacent to the cavity form the amnion and are individually called amnioblasts
hypoblast
- ventral layer of embryonic disc
- at day 9, thin layer of cells from the hypoblast (Exocoelomic membrane) migrate along inner surface of cytotrophoblast to form the primitive yolk sac (exocoelomic cavity)
Why is hatching of the blastocyst important?
- Allows for growth of the blastocyst
- allows for implantation into the uterine endometrium
What produces hCG during week 2 of pregnancy?
- synctiotrophoblast takes over at week 2
- hCG enters meternal bloodstream via trophoblastic lacunae
- stimulates progesterone production by corpus luteum
- without hCG at this stage, menstruation occurs
What takes over production of hCG at week 8?
The placenta
Lacunae
- “lakes”
- form within synctiotrophoblast at ~9 days
- contain hCG and anastomose with maternal sinusoids, initiating uteroplacental circulation
Where and how do the amniotic cavity develop?
- first develops from clefts in the epiblast after 8 days
- cells surrounding the amniotic cavity become amniblasts and together form the amnion
Where and how do the Chorion and Chorionic cavity develop?
- Chorion has origins in extraembryonic mesoderm, which prolifereates between the exocoelomic membrane and the cytotrophoblast at day 10-11
- cavities form within the extraembryonic mesoderm, forming the chorionic cavity (aka extraembryonic coelom)
Chorionic plate
- 3 layers: (from deep to superficial)
1. extraembryonic mesoderm
2. cytotrophoblast
3. synctiotrophoblast
Primary chorionic villi
- projections that proliferate from the cytotrophoblast into the surrounding synctiotrophoblast
- grow out into the lacunae, bringing a surrounding layer of synctiotrophoblast
- happens in conjunction with anastomosis of maternal sinusoids with trophoblastic lacunae
