Week 2 - Formation of Bilaminar Layers Flashcards
Blastomere
“Any of the cells resulting from the cleavage of a fertilized ovum during early embryonic development”
Compaction
“The process during which blastomeres change their shape and align themselves tightly against each other to form the compact morula”
Morula
“The solid mass of blastomeres formed by cleavage of a zygote, containing 12-32 cells”
Inner cell mass (embryoblast)
“The mass at the embryonic pole of the blastocyst concerned with the formation of the body of the embryo”
Outer cell mass (trophoblast)
“Outer cell layer forming the embryonic placenta”
Blastocele
“the fluid-filled central cavity of the blastula of a developing embryo”
Blastocyst
“The modified blastula stage of mammalian embryos, consisting of the inner cell mass and athin trophoblast layer enclosing the blastocele. This is the developmental form which must leavethe fallopian tube, enter the uterus, and implant itself in the uterus to achieve actual pregnancy.”
Embryoblast
” inner cell mass; an aggregation ofcells at the embryonic pole of the blastocyst, destined to form the embryo proper.”
Trophoblast
“The peripheral cells of the blastocyst, which attach the blastocyst to the uterine wall and become the placenta and the membranesthat nourish and protect the developing organism”
Where does the blastocyte implant in the uterus in a normal pregnamcy?
By what day does this occur?
- the endometruim layer
- by day 7
The trophoblast seperates into which 2 cellular structures upon contacting the endometrium?
cytotrophoblast + syncytiotrophoblast
cytotrophoblast
“one layer of cells mitotically active that migrate to the syncytiotrophoblast, where they fuse and lose their cell membranes”
syncytiotrophoblast
” invades the endometrial connective tissue, the endometrial cells undergo apoptosis (programmed cell death), which facilitates the invasion”
It is looking for looking for maternal capillaries, nutrients, uterine glands
Embryonic Cavity
- appears as implantation of the blastocyst progresses
Morphologic changes in the embryoblast result in the formation of __________________?
embryonic disc
(bilaminar plate)
What are the layers/structures of the bilaminar (embryonic) plate?
•Epiblast
high columnar cells related to the amniotic cavity
•Hypoblast
small cuboidal cellsadjacent to the exocoelomic cavity.
Hypoblast
- forms the roof of the exocoelomic cavity
- is continuous with the exocoelomic membrane
- (This membrane + hypoblast, lines the primary umbilical vesicle)*
- ((primary umbilical vesicle = where RBC production starts))*
DAY 9: LACUNAR STAGE
- amnion, embryonic disc, + primary umbilical vesicle form
- isolated cavities (lacunae) appear in the syncytiotrophoblast
What fills the lacunae and where does it come from?
fill with maternal blood from ruptured endometrial capillaries and cellular debris from eroded uterine glands
Communication between the _eroded endometrial capillaries _and lacunae forms …?
primordial uteroplacental circulation
What is the vascular structure in the lacunae?
Oxygenated blood —> lacunae (spiral endometrial arteries)
lacunae —> Poorly oxygenated blood (endometrial veins)
@ Day 12-14 the extraembrionic mesoderm increases. What are its has 2 surfaces called?
Extraembryonic Splanchnic mesoderm; surrounds yolk sac (interior)
Extraembryonic Somatic mesoderm; lining cytotrophoblast (exterior)
During which days does the extraembryonic mesoderm expand and develop isolated spaces within it?
What are these spaces called?
What do they form when they fuse together?
During days 12-14
The spaces are called extraembryonic coelomic spaces.
They form the extraembryonic coelom when they fuse.
What fluid-filled cavity surrounds the amnion and umbilical vesicle?
What is the one part of these structures it DOESN’T surround?
- Extraembryonic coelom
- doesn’t surround where the connecting stalk attaches the amnion/unbilical vesicle to chorion
What occurs as the extraembryonic coelom forms?
the primary umbilical vesicle decreases in size and a smaller secondary umbilical vesicle forms.
Primary chorionic villi
- cellular extensions that grow into the syncytiotrophoblast
- produced by the proliferation of cytotrophoblastic cells
- thought to be induced by the underlying extraembryonic somatic mesoderm
- ((can observe chorionic villi to determin age of aborted embryo))*
Transvaginal ultrasonography
… used for measuring the chorionic (gestational) sac diameter
“uniquely thickened portion” of the endoderm that is in contact with ectoderm…?
prechordal plate
Ectopic Pregnancy
a pregnancy in which the fetus develops outside the uterus, typically in a Fallopian tube.
How is implantation of the blastocyst detected?
detected by ultrasonography and by highly sensitive radioimmunoassays of hCG
(as early as the end of the second week)
How often do ectopic pregnancies occur in the uterine tubes?
What are the 2 most comon places?
**- 95% - 98% ** occure in uterine tubes
- most often in the ampulla and isthmus
Sypmtoms of a tubal pregnancy include:
- normal pregnancy symptoms
PLUS:
- abdominal pain/tenderness
- abnormal bleeding/irritation of pelvic peritoneum
Acute Appendicitis is a common + incorrect diagnoses for _____________.
Tubal Pregnancy
What is used for early detection of ectopic tubal pregnancies ?
Transvaginal ultrasonography
(hCG assays can give false-negative results if performed too early)
What is “Lithopedion”
stonechild– dead, calcified embryo/fetus discovered in body
(i.e. from a missed/incomplete abortion)
What is a common cause of ectopic tumal pregnancies?
… associated with abnormalities in lumen of uterine tube
(if you have 1 you’re more likely to have another)
What can happen if a tubal pregnancy is not detected early?
- rupture of the uterine tube
- hemorrhage into the peritoneal cavity (first eight weeks)
- death of the embryo
- mother’s life is threatened
- affected tube and conceptus usually surgically removed
What happens if the blastocyst implants in the isthmus of the uterine tube?
tube tends to rupture early (narrow part, relatively unexpandable)
(intramural pregnancy may develop beyond 8 weeks before expulsion occurs)
Blastocysts that implant in the ampulla or on fimbriae of the uterine tube…
…may be expelled into the peritoneal cavity where they implant in the rectouterine pouch
How long can abdominal pregnancies usually continue?
to full term
- (delivered by abdominal incision)*
Cervical implantations…
may result in **hysterectomy **
Which type of abnormal implantation may result in a hysterectomy?
Why?
Cervical implantation
due to firm attachment of the placenta to the muscular tissues of the cervix
What, in large doses, can inhibit implanataion?
Progestins and/or estrogens
(does NOT prevent fertilization, just implantation)
What does diethylstilbestrol do?
(given daily for 5 to 6 days) may also accelerate passage of the cleaving zygote along the uterine tube; helps prevent implantation
(Ex. The “Morning After” pill)
In what percentage of cases is the “Morning After” pill ineffective, allowing the blostocyst to implant?
What effects may be seen?
- less than 2%
- hormones in pill will have negative impact on embryo development = birth defects
- (that’s why pills with fewer estrogens = drug of choice)*
Good to Know
Also helpful
What important structures form during week 2?
bilaminar disc
amniotic cavity
amnion
umbilical vesicle (yolk sac)
connecting stalk
chorionic sac
Day 1 of Last Menstrual Period
Pre-ovulation Follicular Development
Week 1 Development
Week 2 Development
Week 3 Development
Week 4 Development
Week 5 Development
Week 6 Development
Which side of the blastocyst usually attaches to the endometrium layer?
usually near the embryonic pole (embryoblast side)
It is suggested the the hypoblast arises from what?
the delamination of blastomeres from the embryoblast
What produces enzymes to erode the maternal tissue?
the syncytiotrophoblast
Preimplantation Genetic Diagnosis
- can be carried out 3 to 5 days after in vitro fertilization of the oocyte
- One or two blastomeres are removed from the embryo known to be at risk of a specific genetic disorder
- analyzed before transfer into the uterus
- sex of the embryo can be determined from one blastomere (from a 6-8-cell dividing zygote)
A patient states that her last menstrual period was delayed by several days and that her last menstrual flow was unusually profuse.
What could this mean?
Very likely she had an early spontaneous abortion
Early spontaneous abortion rate is thought to be _____________?
approximately 45%
Why does early spontaneous abortion occur?
a variety of reasons
More than half of all known spontaneous abortions occur because of chromosomal abnormalities
What is the restricted time period for blastocyst implantation?
6 to 10 days after ovulation
What factors/structures play a role in making the endometrium receptive to implantation?
microvilli of endometrial cells (pinopodes)
cell adhesion molecules
cytokines
prostaglandins
homeobox genes
growth factors
matrix metalloproteins
decidual cells
- connective tissue cells around the implantation site
- accumulate glycogen and lipids
- degenerate adjacent to the penetrating syncytiotrophoblast
- providing a rich source of embryonic nutrition
What hormone is produced by the syncytiotrophoblast?
human chorionic gonadotrophin (hCG)
What does hCG do during implantation?
- enters the maternal blood via lacunae
- maintains the hormonal activity of the corpus luteum in the ovary during pregnancy
corpus luteum
… is an endocrine glandular structure that secretes estrogen and progesterone to maintain the pregnancy
After 2 weeks, pregnangy tests can test for increased levels of which hormone?
human chorionic gonadotrophin (hCG)
What small space appears in the embryoblast during implantation?
amniotic cavity
What forms the amnion?
What does it do?
amniogenic cells - *amnioblasts *- separate from the epiblast and form the amnion
- it encloses the amniotic cavity
The 10-day human conceptus…
(embryo and extraembryonic membranes)
is completely embedded in the endometrium
closing plug
- fibrinous coagulum of blood
- fills a defect in the endometrial epithelium (approx. 2 days)* *
- (By day 12, an almost completely regenerated uterine epithelium covers the closing plug) *
decidual reaction
when endometrium cells begin to swell with lipids & glucogen
- provide nutrition for the early embryo
- provide an immunologically privileged site for the conceptus
Endometrial capillaries around the implanted embryo become ____________?
What does this form?
- congested and dilated
- form sinusoids
- (thin-walled terminal vessels that are larger than ordinary capillaries)*
How does maternal blood enter the lacunar network?
syncytiotrophoblast erodes the sinusoids
Where does the trophopblast get nutrients?
absorbs nutritive fluid from the lacunar networks (transferred to the embryo)
secondary umbilical vesicle
- formed by extraembryonic endodermal cells that migrate from the hypoblast inside the primary umbilical vesicle
The umbilical vesicle
- has a primary, then secondary vesicle
- contains no yolk
- important functions (e.g., it is the site of origin of primordial germ cells)
- may have a role in the selective transfer of nutrients to the embryo
End of the second week is characterized by ….?
…appearance of _primary chorionic villi _
- Proliferation of cytotrophoblastic cells produces cellular extensions that grow into the syncytiotrophoblast
- first stage in the development of the chorionic villi of the placenta
chorion
forms the wall of the chorionic sac
(contains embryo and its amniotic sac and umbilical vesicle)
extraembryonic coelom —> chorionic cavity
prechordal plate
- future site of the mouth
- important organizer of the head region
Good to Know
Placenta Previa
- implantation of a blastocyst in the inferior segment of the uterus (near internal os)
- may cause bleeding because of premature separation of the placenta during pregnancy or at delivery of the fetus
