4 Development Of Bilaminar And Trilaminar Germ Discs Flashcards
Why is the amniotic cavity opened?
To examine dorsal side of epiblast
True or false. The epiblast and hypoblast are not in contact with each other.
False. The hypoblast and epiblast are in contact with each other
Layer of cuboidal cells adjacent to blastocyst cavity
Hypoblast
Layer of columnar cells adjacent to amniotic cavity
Epiblast
Amnioblasts
Epiblast cells adjacent to cytotrophoblasts
Whats the structure that implanted in the uterine?
Blastocyst
Whats the blastocyst composed of?
Inner cell mass that will develop into embryo and cyto and syncytiotrophoblast will give rise to supporting cells of embryo
What will the trophoblast give rise to?
Future placenta
When are the three germ layers formed?
During gastrulation
What is the bilaminar disc mainly composed of ?
Epiblast and hypoblast
Where does fertilization occur?
Ampulla of uterine tube / fallopian tube
Where does implntation occur?
Body of uterus
What part of the blastocyst will attach to the uterine wall?
The outer layer of the trophoblast
When does cleavage of the zygote occur?
After fertilization
Inner cell mass aka
Embryoblast
Embryoblast pulls away from trophoblast and forms
Hollow amniotic cavity
What happens in the 2nd week of development?
Trophoblast-> cyto + syncyto
Extraembryonic meso -> splanchnic + somatic
Embryoblast -> epi + hypo
2 cavities form -> amniotic + primary ys
Hypoblast cells start to migrate and form
Primary yolk sac
Part of the hypoblast will make
Extraembryonic mesoderm ( somatic pleura + splanchnic pleura)
What happens at the end of the 2nd week?
Primitive streak appearance
Functions of primitive streak
Marking early stages of gastrulation
Where does primitive streak appear?
In caudomedial region of embryo
Whats the source of the primitive streak?
Epiblast cells
Whats the function of cavities?
Nourish the embryo
What does the extraembryonic mesoderm form?
Important in connecting stalk which will give rise to umbilical cord
Where does the primitive streak form a shallow groove?
In the caudal region of the germ disc
When does Formation of the primitive streak occur?
3rd week
Where does the primitive node appear?
at the cephalic end of the streak
When does Appearance of primitive node at the cephalic end of the streak occur?
3rd week
When does Migration of the epiblast cells toward the primitive streak occur?
3rd week
Where do epiblast cells migrate to?
Toward primitive streak
When does Detachment and invagination (ingression) of epiblast cells in the region of the
primitive streak occur?
3rd week
When does Formation of embryonic endoderm and the mesoderm from the migrating cells of
the epiblast occur?
3rd week
Formation of embryonic endoderm and the mesoderm from
the migrating cells of
the epiblast
When does Formation of the ectoderm from the remaining cells of the epiblast occur?
3rd week
Formation of the ectoderm from the
remaining cells of the epiblast
Main events during third week of development:
Gastrulation and formation of trilaminar germ disc
Gastrulation and formation of trilaminar germ disc step by step
Formation of the primitive streak
• Appearance of primitive node at the cephalic end of the streak
• Migration of the epiblast cells toward the primitive streak
• Detachment and invagination (ingression) of epiblast cells in the region of the
primitive streak
• Formation of embryonic endoderm and the mesoderm from the migrating cells of
the epiblast
• Formation of the ectoderm from the remaining cells of the epiblast
Orophrangyeal membrane is very early evidence of what?
Our mouth
Cloacal membrane is early evidence of what?
Of the anus
Prenotochordal cells
Will migrate to cephalic region and give rise to notochord
Which side of the embryo will develop first?
The left side
Where will the primitive node appear?
At cephalic end of primitive streak
Function of primitive node
Organizes migration of epiblast cells to form three germ layers
Ensured what transcription factors or genes are expressed at what time ( ensure gastrulation)
When does formation of notochord happen?
Day 17
Cells that will make up the notochord are ..
Very close to the primitive streak
Function of notochord
Axis for development
Source of notochord
Mesoderm cells that attach to endoderm ( forming notochordal plate) then detach from it
What are the two main types of mesoderms?
Extraembryonic and intraembryonic
The intraembryonic mesoderm will further differentiate into
Paraxial lateral and intermediate mesoderms
Paraxial mesoderm will give rise to
Somites that develop into muscles and bones
When does Establishment of the body axes occur?
Week 2 and 3
What must happen in order for Establishment of the body axes to occur?
Notochord must form
The notochord will regulate gene expressions and transcription factors to ensure that development occurs _________ and __________
Cephalocaudally and left to right
Up and down
Dorsal and ventral
North east and south west
Right and left
Left and right
Cranial and caudal
The anteroposterior axis is signaled by cells at
anterior (cranial) margin of the embryonic disc, the Anterior Visceral Endoderm (AVE)
Cells in the AVE express genes essential for … and secreted factors cerebrus …
for head formation
contribute to head development and establish the cephalic region.
• Cells in the AVE express genes essential for head formation, including
the transcription factors OTX2, LIM1, and HESX1
What secreted factors contribute to head development and establish the cephalic region?
Cerebrus
What happens once the streak is formed and gastrulation is progressing?
BMP4 is secreted throughout the bilaminar disc and acts with FGF to ventralize mesoderm into intermediate and lateral plate mesoderm
Where is goosecoid expressed?
In the node
Goosecoid function
regulates chordin expression,
What is Goosecoid?
Gene product
Function of goosecoid together with noggin and follistatin?
inhibits the activity of BMP4, dorsalizing mesoderm into notochord and paraxial mesoderm for the head region.
expression of the Brachyury (T) gene antagonizes BMP4 to do what?
to dorsalize mesoderm into notochord and paraxial mesoderm in caudal region of the embryo.
What antagonizes BMP4?
Brachyury (T) gene
What genes are important in head formation?
OTX2, LIM1 , HESX1, secreted factor cerebrus
What will activating BMP4 with FGF result in?
Ventralizing ( lateral and intermediate)
What will blocking BMP4 and FGF ( using goosecoid , noggin and follistatin together) result in?
Dorsalizing ( paraxial mesoderm and notochord.)
Why was goosecoid used to dorsalize the embryo/ whats the purpose of dorsalizing the embryo?
So that the notochord is positioned in the back of the baby not the stomach
_____secreted by the node and primitive streak, establishes ______,
FGF8
Nodal
What secretes FGF8?
Node and primitive streak
FGF8 secreted by the node and primitive streak, establishes Nodal and then nodal proteins …
accumulates on the left side of
the node.
What is Nodal?
a member of TGF-β superfamily
Why are they called “LEFTY-1” and “ LEFTY-2” ?
Because they can only be found on the left side
As the neural plate starts to form, neurotransmitter serotonin (5HT)
increases in concentration on the left side
As the neural plate starts to form, neurotransmitter serotonin (5HT) increases in concentration on the left side. Why?
because of its metabolism by MAO on the right.
What happens under the influence of 5HT?
FGF8 induces expression of Nodal and LEFTY-2 in the lateral
plate mesoderm, whereas LEFTY-1 is expressed on left side of the ventral aspect of the neural tube
What also participates in induction of Nodal, LEFTY-1, and LEFTY-2?
Products of Brachyury (T), expressed in the notochord
Expression of Nodal and LEFTY-2 regulates…
expression of the transcription factor PITX 2, which, through further downstream effectors, establishes left-sidedness.
What establishes left sidedness through further downstream effectors?
PITX2
where is SHH expressed?
In the notochord
SHH possibly serves as a
as a midline barrier and also represses expression of left-sided genes on the right.
Expression of the transcription factor Snail may regulate
downstream genes important for establishing right-sidedness.
What germ layer is the nervous system derived from?
Ectoderm
What germ layer is the sensory epithelium of eye , ear, nose derived from?
Ectoderm
What germ layer is the epidermis , haur, and nails derived from?
Ectoderm
What germ layer is the mammary and cutaneous glands derived from?
Ectoderm
What germ layer is the epithelium of sinuses oral and nasal cavities , intraoral glands derived from?
Ectoderm
What germ layer is the tooth enamel derived from?
Ectoderm
What germ layer are the muscles derived from?
Mesoderm
What germ layer are the CT derivatives : bone cartilage, blood,dentin, pulp, cementum, periodontal ligament derived from?
Mesoderm
In reference of the notochord whats the most lateral epiblast cell going to give rise to?
Extraembryonic mesoderm
In reference of the notochord whats the most medial epiblast cell going to give rise to?
Paraxial mesoderm
In reference to notochord list the fate map of the epiblast cells from most medially to most laterally.
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
Extraembryonic mesoderm
What germ layer are the GI tract epithelium and associated glands derived from?
Endoderm
Malformation of head region
Can be duplication, malformation, or conjoined twins
Cause of malformation of head region
Over / under expression of goosecoid which activates BMP4 inhibtors
Malformation of lumbosacral region 2 egs
Sirenomelia ( caudal disgenesis) and sacrococcygeal teratoma
Range of defects of sirenomelia
Hypoplasia
Fusion of lower limbs
Vertebral abnormalities
Renal agenesis
Anomalies of genital organs and imperforate anus
Cause of sirenomelia
Insufficient mesoderm formation in caudomost region of embryo/ loss of mesoderm in lumbosacral region
What is associated with sirenomelia?
Maternal diabetes
Sacrococcygeal teratoma
Tumors may become malignant and are most common in female fetuses
Cause of sacrococcygeal teratoma
Reminants of primitive streak that remained in sacrococcygeal region will become clusters of pluripotent cells that divide and form and result in sacrococcygeal teratom
Failure of establishment of normal L-R asymmetry ( situs inversus)
Failure to properly establish L-R axis
Situs solitus
Normal positioning of internal organs
What is important for establishing laterality?
Neurotransmitter serotonin (5HT)
What do laterality defects cause?
Complete reversal of organs
