Fertilization - Gastrulation Flashcards
Where does fertilization typically occur?
Usually occurs in ampulla of uterine tube
List the stages of the first week of embryonic development:
- Fertilization
- Cleavage formation
- Blastocyst formation
- Implantation of blastocyst
What clinical correlations are specific to the first week of fertilization?
- Ectopic pregnancy
- Hydatidiform moles
When does the morula form?
During cleavage formation. The morula forms after fusion of pronuclei the blastomeres, division of the blastomeres forms a morula (mulberry)
Each cell in the morula is totipotent
When does compaction occur?
During cleavage formation. Compaction of blastomeres by formation of tight junctions allows fluid to be pumped into the center of the morula.
The peripheral cells of the morula form adhesions (via tight junctions) to seal off the outside of the embryo - outside cells become placenta/trophoblast. Inside cells become embryo.
What is blastocyst formation?
Result from compaction of blastomeres, fluid pumped into the center of the morula forms a blastocyst that has a fluid filled cavity and an inner cell mass=embryo and an outer cell mass = trophoblast (fetal contribution to the placenta).
During the first week of pregnancy, what secretes HCG to maintain the corpus luteum?
If pregnancy occurs, then syncytiotrophoblast secretes hCG that maintains the corpus luteum that makes progesterone to maintain pregnancy.
What happens if fertilization does not occur?
- No fertilization = no hCG = corpus luteum regresses = no more progesterone = menstruation leaving basal layer to regenerate endometrial glands for next cycle
List the stages of embryonic development (days 8-10)
- Trophoblast differentiation into two layers
- Embryonic Disc formation into two layers = inner cell mass lies at one pole of blastocyst and forms the germ disc that has two layers
- Two cavities form
§ Amniotic =dorsal to epiblast
§ Yolk sac= ventral to hypoblast (day 9/10)
When the trophoblast differentiates, what layers are formed?
- Cytotrophoblast (makes cells)
- Syncytiotrophoblast forms syncytium = responsible for invasion of the blastocyst into the uterine endometrium and production of hormones
What are the two layers of the embryonic disc?
- Hypoblast
- Epiblast
List the stages of embryonic development (days 11-13)
o Uteroplacental circulation established and
o Chorionic cavity formed
o Extraembryonic Mesoderm, two layers of extraembryonic mesoderm continuous at the connecting stalk (umbilical region)
o Bleeding may occur at day 13, confused with menustration. About 12-14 days, the embryo penetrates uterine vessels = some bleeding = may misdiagnose as menstrual bleeding = misdiagnose pregnancy and age of embryo
o Completely formed two cavities
Where are the common implantation sites of an ectopic pregnancy?
Most common is in the ampullary region of the uterine tube (tubal pregnancy); Most common site in the abdominal cavity is the Rectouterine (Douglas’) pouch.
What two cavities are completely formed on days 11-13 of embryonic development?
The amniotic cavity forms before the chorionic cavity
What are tertiary villi?
Where are they found and what stage of development?
Tertiary villi = blood vessels in mesoderm core with most of cyto-degenerated covered by syncytio: = vessels in tertiary villi are formed by extraembryonic mesoderm; these vessels will connect extraembryonic mesoderm formed around the outside of the amnion and yolk sac splits into two layers = somatic and splanchnic. Cavity between = chorionic cavity
Formed on days 11-13 of development and found in chorionic cavity
Why is week two of embryonic development referred to as “the week of 2’s?”
- 2 layers to trophoblast: synctio- and cytotrophoblast
- 2 layers to embryoblast: epiblast and hypoblast
- 2 cavities: amniotic and yolk sac (actually 3 since the chorionic cavity forms later in the week)
- 2 layers of extraembryonic mesoderm: somatic and splanchnic
What occurs during the third week of embryonic development?
- Gastrulation
- Formation of notochord
- Establishment of the body axes
- Trophoblast development
What are the three germ layers and their corresponding tissue designation?
- Ectoderm (skin, CNS, PNS, eyes, internal ear, neural crest cells,
bones/connective tissue of face and part of the skull) - Mesoderm (blood, bones, connective tissue, urogenital system, and heart)
- Endoderm (gut, gut derivatives -liver, pancreas, lungs -, parenchyma of glands)
epiblast forms all 3 germ layers= all of the embryo, the hypoblast disap
When does the formation of the germ layers occur?
During the beginning of week three of embryonic development (gastrulation is the process of making three germ layers)
What is the first proccess that occurs during gastrulation?
- Formation of primitive streak, forms at the caudal end of the embryo at the beginning of the 3rd week:
- Formation of primitive node, node = organizer =cranial end of streak
“Gastrulation makes the icing for the cookie”
In gastrulation, what is the functional purpose of the primitive streak?
Epiblast cells migrate toward and through the streak and node to form mesoderm and endoderm
When does creation of the oropharyngeal membrane occur?
During week three of embryonic development (gastrulation)
What primitive cells give rise to all three germ layers?
Epiblasts
After germ layer formation, hypoblasts dissapear
What two embryonic structures induce the CNS?
The prechordal plate and the notochord
After full development, what does the notochord form?
The nucleus pulposus in intervertebral discs
When are the risk of birth defects being induced the highest?
Most birth defects occur between 0-8 weeks (before the mother knows she is pregnant)
What creates the amniotic cavity?
The epiblast (7.5 days after fertilization)
What creates the yolk sac?
The hypoblast (9 days after fertilization)
What secretes the sonic hedgehog (SHH) gene?
The the prechordal mesoderm cells of the prechordal plate
