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
How does the notochord form?
Cells migrate through the cranial region of the node, forming the prechordal plate followed by the formation of the notochord.
What establishes the cranial-caudal axis?
The Anterior Visceral Endoderm (AVE): Secretes genes essential for head formation.
How is the head mesoderm organized? (Molecular signals)
The node secretes goosecoid and other genes that antagonize BMP-4.
The node is the organizer
What is the role of BMP-4?
Embryonic development
BMP-4 secreted throughout the embryonic disc = ventralizes mesoderm = forms intermediate and lateral plate mesoderm:
What controls formation of dorsal mesoderm in regions caudal to the head?
Brachyury (T gene) - Expressed by the node and notochord: If decreased T gene, then results in caudal dysgenesis.
What is responsible for the formation of laterality during the third week of embryonic development?
Gene regulation: PITX2 = master gene for laterality = establishes left sidedness: Upregulated by serotonin (5HT), nodal and FGFs
What abnormalities are associated with laterality?
- Situs inversus = transposition of the viscera = usually no other defects
- Laterality sequences = incomplete situs inversus such that organ
reversal only involves a few organs = often have other defects.
Antidepressants (Selective Serotonin Re-uptake Inhibitors [SSRIs])
cause laterality problems Trophoblast = forms villi for placenta:
Primary villi = core of cytotrophoblast covered by
syncytiotrophoblast; Secondary villi = core of extraembryonic
mesoderm, covered by cytotrophoblast, covered by
syncytiotrophoblast: Tertiary villus = Capillaries and venules in the
extra-embryonic mesoderm, cytotrophoblast breaking down, still
covered by syncytiotrophoblast.
What is caudal dysgenesis and when does it occur?
Caudal dysgenesis (Sirenomelia; Mermaid syndrome) = insufficient mesoderm formed by gastrulation = missing kidneys, fused lower limbs.
What occurs during trophoblast development?
- Villus differentiation
- Cytotrophoblast, connecting stalk
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)
After fertilization, how long until the embryo reaches the uterine cavity?
3-4 days
What cells give rise to the placenta?
Trophoblast cells
Trophoblast cells are formed during the first week of embryonic development
When do embryonic cells go from totipotent to pluripotent?
When the blastocyst is formed, embryonic cells are now pluripotent.
What is syncytium?
“They do not form boundaries. They are like a bag full of nuclei.”
Syncytiotrophoblast cells invade the uterine wall during implantation of the blastocyst (during day 6 of embryonic development).
Where do syncytiotrophoblast cells arise?
Cytotrohpoblast cells feed into the syncytiotrophoblast cells. They express binding proteins (selectins) which allow them to bind to the uterine epithelium.
Syncytiotrophoblast make hormones - secrete
Choriocarcinomas can arise from invasive embryonic cells
During embryonic development, how does the mother’s immune system shift to protect the embryo?
The mother’s immune system shifts from cell-mediated to humoral immunity
Mother’s risk of infection and death from infection rise
Cell-mediated autoimmune disorders improve (i.e. multiple sclerosis and rheumatoid arthritis)
Antibody-mediated autoimmune disorders become worse (i.e. systemic lupus erythematosis)
During the implantation of the blastocyst into the uterine epithelium, what genomes are expressed by embryonic cells and trophoblasts?
Trophoblasts - paternal genome
Embryonic cells - maternal genome
Problems with ART vs. unassisted pregnancy
ART - 41% vs. 3.5% multiple birth deliveries
29% low birth weight vs. 8%
34% preterm vs. 11%
Birth defects are 28% higher with in vitro techniques (intracytoplasmic injection is 32% higher than in vitro)
Increase in stillbirths
When are 50% of embryos lost?
Implantation of the blastocyst into the uterine epithelium.
What creates the amniotic cavity?
The epiblast (7.5 days after fertilization)
What creates the yolk sac?
The hypoblast (9 days after fertilization)
What two layers does the extraembryonic mesoderm form after 13 days?
The visceral layer - evelopes embro
The parietal layer - adheres to trophoblast layers
These two layers are continuous through the connecting stalk (where umbilical cord forms)
Which layer is important for cell signaling and which is important for the embryo?
Hypoblast - cell signaling
Epiblast - composes the embryo
Overexpression of goosecoid genes during week three most likely leads to what?
Conjoined twins
What is the “master gene” for left sided development (occurs during week 3)
PITX2 (activated by nodal)
What secretes the sonic hedgehog (SHH) gene?
The the prechordal mesoderm cells of the prechordal plate
What is one cause of the development of two heads during embryogenesis?
Abnormal signaling from the anterior visceral endoderm.
What is the most “laterality sensitive” organ in the body?
The heart
What signal is disrupted in response to SSRIs?
During embryonic development
Nodal, first present during gastrulation. Nodal acts to ultimately produce PITX2 (master gene for laterality), which induces laterality by marking the left side of the embryo.
What is a synonym for the parietal layer of the extraembryonic mesoderm?
The somatic extraembryonic mesoderm
When does cranial/caudal anterior/posterior signaling begin?
After implantation of the blastocyst, as the epiblast and hypoblasts differentiate
What begins to erode the mother’s cappilary vessels?
The syncytiotrophoblast
Where does the umbilical cord arise?
The connecting stalk of the extraembryonic membrane
What is gastrulation?
The bilaminar disc is turned into a three layer disc by migration of epiblast cells through the primitive streak, below the ectoderm, forming the mesoderm
Why is the hypoblast important during early embryogenesis?
Early signaling to the epiblast (AVE)
What is heterotaxy?
Defects in laterality (Situs Ambiguus)
Asplenia (left)
Polysplenia (right)
Where is the node located?
At the terminal cranial end of the primitive streak
What embryonic tissue layer is the notocord composed from?
The mesoderm
