Embryology Flashcards
Embryology
Study of embryonic development
Teratology
Study of abnormal embryonic development
Defines the lateral sides of a body
Sagittal plane
Defines the anterior and posterior sides of a body
Coronal plane
Defines the superior and inferior sides of a body
Transverse plane
Fertilization
Spermatozoan and oocyte unite to form the zygote (~24hrs)
Fertilization marks the start of …
the pre-embryonic period
Metabolic activation after fertilization initiates…
cleavage of the zygote (cell division)
Cleavage: Each cell is called a … and is surrounded by the ….
Each cell is called a blastomere and is surrounded by the zona pellucida.
Why does the zygote size remain the same during the initial stages of cleavage?
Because the blastomeres are surrounded by a membrane called the zona pellucida which constrains the growth of the zygote, leading to compaction
Describe what happens at the 8-cell stage of the zygote
Compaction: cells are tightly aligned, allowing for better cell interaction
Describe what happens at the 12-32-cell stage of the zygote
This stage is referred to as the morula stage because the blastomeres are so compacted. It is at this stage that the zygote enters the uterus.
Once the zygote enters the uterus (~day 5), it is referred to as the … because…
Once the zygote enters the uterus, it is called a “blastocyst” because the morula is hollowed out and the blastocystic cavity (fluid-filled) begins to form
The blastocystic cavity separates the blastocyst into 2 layers (inner and outer) called
trophoblast (outer layer - future placenta) and embryoblast (inner layer - future embryo)
The trophoblast secretes the …
Early Pregnancy Factor, which forms the basis of a pregnancy test during the first 10 days. EPF has immunosuppressant properties.
- Fertilization
- Cleavage
- ?
- Implantation
First step of implantation
Blastocyst binds the endometrial lining (epithelium) of the uterus
Upon implantation, the trophoblast differentiates into two layers…
- syncyntiotrophoblast (invades endometrial epithelium)
- cytotrophoblast (adjacent to the embryoblast)
Why does the syncyntiotrophoblast invade the endometrial epithelium?
To establish nutrient circulation (nourishment for the future embryo is derived from eroded maternal tissue).
Day 8: Blastocyst starts to become embedded in the endometrium.
Day 9: ?
Day 9: Lacunae (small cavities) start to develop in the syncytiotrophoblast
What is the role of lacunae?
They eventually become filled with maternal blood and establish the uteroplacental circulation (serve as a direct connection to maternal blood)
After the blastocyst sinks beneath the endometrial lining, the implantation site is filled by…
a closing plug (fibrin coagulum)
If closing plug does not form at the site of implantation, what can happen?
Abnormal bleeding
Fusion of the lacunae leads to the formation of…
the lacunar network
Once the lacunar network is established, the syncyntiotrophoblast begins to…
erode the endometrial capillaries in order to establish the uteroplacental circulation
After implantation , the embryoblast differentiates into the…
bilaminar disk (2 layers)
What are the two layers of the bilaminar disk?
Hypoblast (primary endoderm)
Epiblast (primary ectoderm)
Thick layer of columnar cells related to the amniotic cavity
Epiblast
Layer of cuboidal cells facing the blastocystic cavity
Hypoblast
Describe the formation of the amniotic cavity
A later of the epiblast becomes the amnioblast and separates from the epiblast to form the amnion enclosing the amniotic cavity.
Hypoblast migrates to populate the inner surface of the blastocystic cavity, forming the …
exocoelomic membrane surrounding the exocoelomic cavity
Hypoblast + exocoelomic membrane form the …
primary umbilical vesicle (yolk sac)
Epiblast forms the
amnion
Hypoblast forms the
yolk sac
The amniotic cavity and the exocoelomic cavity are separated by the…
embryonic disk
Layer of connective tissue formed between the exocoelomic membrane and cytotrophoblast
Extraembryonic mesoderm
Function of extraembryonic mesoderm
- Surrounds the amnion and primary umbilical vesicle
- Separates embryoblast from trophoblast
Around day 12-13, cavities form in the extra embryonic mesoderm. These cavities begin to merge to form the…
extraembryonic coelom (cavity)
The extraembryonic coelom separates the extraembryonic mesoderm into:
- Extraembryonic somatic mesoderm (lines the trophoblast - future body wall)
- Extraembryonic splanchic mesoderm (lines the yolk sac - future viscera)
The extraembryonic mesoderm surrounds the amnion and secondary umbilical vesicle everywhere except at the…
connecting stalk
The late 2nd week of pre-embryonic development marks the…
1st stage of the placenta
Placenta
Fetomaternal organ for metabolic exchange between the mother and embryo
The extraembryonic mesoderm induces the 2 layers of the trophoblast to grow into the endometrium, forming…
primary chorionic villi
Function of primary chorionic villi
Primary chorionic villi maximize the surface-area in contact with maternal blood for nutrient and gas exchange
Composed of the extra embryonic somatic mesoderm and 2 trophoblast layers
chorionic sac
The secondary umbilical vesicle (yolk sac), amnion and embryo are suspended in the … via the …
The secondary umbilical vesicle (yolk sac), amnion and embryo are suspended in the chorionic sac via the connecting stalk.
Normally, implantation of the blastocyst occurs on the …
uterine endometrium (superior part of the uterus)
Define ectopic pregnancies
An ectopic pregnancy occurs when the blastocyst implants and grows outside the main cavity of the uterus.
Ectopic pregnancy = extrauterine implantation
Two types of ectopic pregnancy
- Tubal pregnancy (most common)
- Abdominal pregnancy (rare)
Tubal pregnancy characteristics (2)
- Implantation occurs in the fallopian tube (ampulla or isthmus)
- Most common type of ectopic pregnancy (95-98%)
Tubal pregnancy symptoms (3)
- Abdominal pain & tenderness (due to tube distension)
- Abnormal bleeding
- Peritonitis (inflammation of the abdomen)
Where does implantation occur in an abdominal pregnancy?
Implantation in the rectouterine pouch of the peritoneal cavity (i.e. outside of the uterus, in the abdomen).
What type of ectopic pregnancy may continue to full term? How will the baby be delivered?
Abdominal pregnancy (baby will be delivered by laparotomy)
The third week after fertilization marks the …
beginning of the embryonic period
Formation of the embryonic germ cell layers involves 4 steps (simultaneous and in parallel)
- Gastrulation
- Notochord
- Somites
- Neurulation
Gastrulation
Bilaminar disk is converted to 3 germ cell layers i.e. trilaminar embryonic disk called gastrula
2 key events of gastrulation
- Beginning of morphogenesis (i.e. development of the body)
- Creation of bodily axes observed in mature adults
Why is the embryo at its most vulnerable during gastrulation?
Because errors in the differentiation of gastrula layers will lead to developmental defects
Primitive streak is formed by..
migration of epiblastic cells towards midline (caudal to cranial)
What defines bilateral symmetry observed in the mature adult?
Primitive streak
Primitive node is formed by…
proliferation of epiblastic cells at the cranial end
Primitive pit lies within… and is formed by…
Primitive pit lies within primitive node and is formed by invagination of epiblastic cells from the primitive streak and node
Primitive groove is formed by…
migration of epiblastic cells as primitive pit progresses cranially
Mesenchymal cells
multipotent stem cells (formed by migrating epiblastic cells)
Epiblastic cells migrate towards the primitive pit, where they detach and penetrate through the epiblast layer to form 3 new germ cell layers:
- Embryonic ectoderm
- Intraembryonic mesoderm
- Embryonic endoderm
2 zones where ectoderm and endoderm remain fused (no mesoderm)
- Prechordal plate
- Cloacal membrane
Prechordal plate (2)
- Will give rise to oropharyngeal membrane (future site of oral cavity)
- Important signalling centre for cranial development
Cloacal membrane
Future site of urogenital meatus and anus
Embryonic ectoderm
1. Initially called…
2. Becomes…(6)
- Initially called epiblast
- Becomes epidermis, CNS & PNS, eyes, internal ears, neural crest cell, connective tissue of the head
Intraembryonic mesoderm
1. Made from…
2. Becomes (5)
- Made from migrating epiblast cells
- Becomes skeletal muscles, connective tissue, blood cells, lining of blood vessels, viscera’s smooth muscle
Embryonic endoderm
1. Made from…
2. Becomes
- Made from hypoblast replaced by migrating epiblast cells
- Becomes epithelial lining of respiratory, GI tract and associated glands
Cardiogenic area
At the cranial end, where intraembryonic mesoderm and extraembryonic mesoderm meet (i.e. heart begins to develop near the head of the embryo)
Fate of primitive streak (3)
- Forms mesoderm by invagination until ~4weeks
- Diminishes in size to an insignificant structure in the sacrococcygeal region
- Should disappear by the end of week 4
What can happen if the primitive streak does not disappear?
Sacrococcygeal teratoma (tumor)
- most common tumour in neonates (85% female)
- contains tissue from all 3 germ cell layers at different stages of differentiation
1st step of notochord formation:
Prechordal plate (at the cranial end) signals to the …. cells to migrate towards it, leading to the formation of the…
Prechordal pate (at the cranial end) signals to the mesenchymal cells to migrate towards it, leading to the formation of the notochordal process.
2nd step of notochord formation:
Notochordal process undergoes changes to become the…
notochordal plate
3rd step of notochord formation:
At the cranial end, the … proliferates and undergoes infolding to form the …
At the cranial end, the notochordal plate proliferates and undergoes infolding to form the notochord.
Hollow cord formed on the medial (middle) axis of the embryo
Notochordal process
Flattened grooved plate on the medial (middle) axis of the embryo
Notochordal plate
Rod-like structure spanning from the primitive node to the prechordal plate, detached from the endoderm & umbilical vesicle
Notochord
Name the 3 roles of the notochord
- Provides signals for the development of the axial musculoskeletal system
- Induces overlying ectoderm to thicken and form the neural plate (CNS)
- Defines primordial longitudinal axis of the embryo + provides some rigidity
Around day 17-18, the mesoderm proliferates into columns on each side of the midline (notochord). These 2 columns are called…
paraxial mesoderm
Each paraxial mesoderm column is continuous with…
intermediate mesoderm
Intermediate mesoderm is continuous with the extraembryonic mesoderm, now called…
lateral mesoderm
Paraxial mesoderm gives rise to…
somites
Intermediate mesoderm gives rise to…
urogenital system
Lateral mesoderm + ectoderm gives rise to…
intraembryonic somatic mesoderm (anterior body wall)
Lateral mesoderm + endoderm gives rise to…
intraembryonic splanchic mesoderm (digestive tube wall)
Around day 19-20, the paraxial mesoderm further differentiates and condenses into columns called…
somitomeres (“rolled-out dough”)
Somitomeres are segmented into spheres along the notochord called…
somites (“gnocchi”)
An embryo can have as many as … somite pairs
42-44
Some somites will degenerate for an end result of … somite pairs
31 somite pairs
The 31 somite pairs will give rise to…
31 spinal nerve pairs
Notochord will give rise to…
the spinal cord
Are there somites in the cranial region?
No, there are no somites in the cranial region, only somitomeres (will participate in pharyngeal arches)
Role of somites (3)
- Gives rise to (most of) the axial skeleton
- Gives rise to muscles of the axial skeleton
- Gives rise to associated dermis
During the 4th and 5th week, the number of somites allows us to estimate…
the embryo’s age
Define neurulation
Formation of the neural tube
What is the neural tube?
It is the developing brain and spinal cord (CNS)
Name the 3 steps of neurulation
- Notochord induces the overlying ectoderm to thicken, elongate and form the neural plate
- The neural plate invaginates along its central axis
- Neural folds begin to fuse to form the neural tube
Describe the position of the neural plate
Neural plate is cranial to the primitive node; extends as far as the oropharyngeal membrane
The neuroectoderm of the neural plate gives rise to…
the brain and spinal cord
On each side of the neural groove are the…followed by the … (irregular mass).
On each side of the neural groove are the NEURAL FOLDS followed by the NEURAL CRESTS (irregular mass).
What is the first sign of brain development?
Neural folds become more prominent on the cranial end.
Neural crest cells give rise to… (2)
- spinal and autonomic nervous system ganglia
- suprarenal medulla, neurons and other organs and tissues
Neurulation is completed by the end of the … week.
4th
Intraembryonic coelom formation
- Coelomic spaces appear in the lateral intraembryonic mesoderm
- Coelomic spaces fuse and form a single “horseshoe-shaped” cavity: intraembryonic coelom
The intraembryonic coelom separates the lateral mesoderm into two (name and what it gives rise to):
- Intraembryonic somatic (parietal) mesoderm: w/ectoderm forms anterior body wall (aka somatopleure)
- Intraembryonic splanchic (visceral) mesoderm: w/endoderm form digestive tube wall (aka splanchnopleure)
Distrubance of neurulation may result in…
severe birth defects of brain and spinal cord
Neurulation: anterior neuropore misclosure causes…
anencephaly
Neurulation: posterior neuropore misclosure causes…
spina bifida
