Fetilizaton, Implantation And Placental Development Flashcards
Fertilization
Needs to occur within 24 hrs after ovulation in order for the ovum to not degenerate
- sperm can last 6-7 days but remain arrested in the isthmus of the fundus until Zona pellucidum of the ovvum releases chemical factors to promote union (needs to be close enough though)
- in the uterus, factors are also produced to induce capacitated factors (help keep sperm acrosome intact)
Union of the two gametes form a zygote
- this occurs via capacitated sperm penetrating the corona radiata to bind to the ZP3 ligand of the zona pellucida and enter inside to fuse. (ONLY capacitated sperm can do this)
- male pronucleus is ejected into the ovum and the oocyte membrane depolarizes and excess growth of the zona pellucida occurs
Once fertilization has completed = meiosis 1 and meiosis 2 completes and forms the female pronucleus with hypertrophy of the zona pellucida
How does the oocyte prevent polyspermy?
Once the spermatozoon enters the oocyte, the cortical membrane of the oocyte depolarizes the calcium channel membranes and results in both a release of cortical granules and down regulates membrane receptors of ZP3
- both function to prevent multiple spermatozoa from binding
Zygote formation
End of fertilization 24hrs after ovulation**
Pronucleus membranes fuse and breakdown
Chromosomes line up and cleavage spinde/furrow forms to initiate metaphase of meiosis
this restores the diploid condition and also determines the chromosomal sex of the zygote
Polyspermy
Multiple stream fuse with oocyte membrane and contribute multiple male pronucleus
- results in polyploidy and usually fetal demise since this is not comparable with life
Mosaicism
Somatic cells of an individual have a different genetic chromosomal configuration
Occurs via chromosomal abnormalities via non-disjunction and also normal lyonization reactions in females
- also gene mutations after zygote stage can cause this
- the degree of mosaicism is related to the timing of the anomaly
Heteroploidy
Occurs when fertilization involves gametes with missing or extra chromosomes
- always due to non-disjunction during game to genesis
- can casue non-viable blastocysts but can cause trisomy or Turner syndrome
Morula formation
Forms from the zygote at roughly day 3
- at the point before this, there are 4 blastomeres present in the oocyte (4-cell stage)
The morula is a 16 cell stage in which the cells divide into a inner cell mass and outer cell mass
Begins blastualtion when fluid is drawn in tilt he morula (usually around day 4-5)
**looks like a raspberry with its bumps
Blastulation and formation of the blastocyst
Generates from the morula when fluid is drawn into he center of morula
- this starts at day 4-5 usually
Generates two layers
- trophoblasts = outer cell mass
- embryoblast = inner cell mass
also the Zona pellucida degenerates at this point and a blastocoel is formed surroundings he trophoblast cells
Implantation
Blastocyst enters the uterus and ruptures from the Zona pellucida (allows for protection against implantation in the fallopian/uterine tubes) and implants on the uterus lining
- poccurs at usually day 7-8
Trophoblasts adhere to the uterine epithelium and proliferate/migrate into the he endometrium lining
Blastocysts migrate into the lamina propria of the endometrium functional layer and begin to generate the placenta
Conceptus formation
Usually occurs at day 8-14
- “week of twos”
Trophoblast cells differentiate even more into two layers
- cytotrophoblasts = inner layer that provides a protective barrier to developing fetus
- syncytiotrophoblasts = outer layer that invades uterine stroma and gathers nutrients from degeneration of the uterus for the developing fetus
- *syncytiotrophblasts secrete b-HCG to keep the corpus luteum alive and prevent menstrual phase**
Embryoblast differentiates into a bilaminar embryonic disc (2 layers)
- epiblast = columnar cells on floor of amniotic cavity
- hypoblast = cuboidal cells on the ventral surface of the epiblast layer
Two sacs form above/dorsal surface (amniotic sac) and below/ventral surface(yolk sac) the embryonic disc
Two layers of extraembryonic mesoderm form
- somatic layer underlays trophoblasts
- splanchnic layers overlays the amnotic sac and umbilical vesicles
At day 13 the connecting stalk forms which is the site of umbilical vessels from placenta
Molar pregnancy or GTDs
Abnormal form of pregnancy due to implantation of a non-viable blastocyst
- becomes a mass of swollen chorionic vili (hydatid cysts) that secretes mass amounts of B-HCG and can metastasis the chorionic villi
Complete = cysts present throughout the placenta and embryonic tissues are absent
- usually due to lack of any chromosomes in a ovum that is fertilized
Partial = portion of placenta develops normally and embryonic tissue may be present
- usually results from polyplodiy zygote
Development of the placenta
Starts at usually day 14-15
The chorionic villi increases growth as extensions of cytotrophoblasts and extraembryonic somatic mesoderm form between the lacunae of the syncytiotrophoblast
- stem villi = extend from chronic plate to decidual plate
- free villi = branches that project out into intervillous spaces
Outer cytotrophoblast layer adheres to the surrounding endometrial tissue (decidua)
Chorionic plate is also formed
- thickened portion of the extraembryonic somatic mesoderm (inner layer of chorion) that forms blood vessels that pass through connecting stalk and into he surrounding mesoderm
Decidual plate forms in the stroma of the endometrium (completely surrounds the conceptus). Forms in response to signals from outer cytotrophoblast cell
- uterine vessels penetrate the decidual plate and outer layer of the cytotrophoblast shell to give oxygen and nutrients tot he fetus ( this becomes the placenta only near the embryonic pole)
Utero placental circulation
Spiral arteries bring oxygen into the placenta which then is carried via umbilical vein into the fetus
Umbilical arteries send oxygen poor blood out of the fetus and into the endometrial veins and uterine veins back to the mother circulation
In vitro fertilization and embryo transfer
Implants sperm in oocytes and then transfers diving zygotes or the blastocyst into the uterus
High risk for multiple pregnancies and spontaneous though via this process
using subset of intracytoplasmic sperm inject is very helpful in cases of ashermann syndrome or olgiospermia
Spontaneous abortion rate
Actually very high around 50% but most of them occur in the blastocyst stage so most are not clinically recognized
Usually occur due to inadequate production of progesterone and estrogen by the corpus luteum or due to chromosomal abnormalities of the blastocyst
if the last menstrual period was delayed or had an unusually profuse menstrual cycle and who is sexually active and not peripmenopausal = often times this is an early spontaneous abortion
