L5: embryonic & fetal periods Flashcards
How long is the embryonic period?
Weeks 3 – 8 (embryonic period)
When should the Organogenesis be completed
Weeks 3 – 8 (embryonic period)
What does The Mesoderm differentiates to ?
Paraxial,
Intermediate,
Lateral
What is Extraembryonic cavity that form In the 2nd week?
1- YolkSac
2- Chorionic Cavity
What does Extraembryonic splits into?
- Splanchnic layer
2. Somatic layer
What is the role of the notochord?
1- Form body axes
2- Expresses a lot of genes
3- Regulates mesoderm dorsalizing
What will the The lateral Mesoderm split into?
1- One within the endoderm (splanchnic or visceral,will be in association with the developing gut)
2- one within the amniotic cavity (somatic)
What does Gastrulation establishe?
establishes the three germ layers, namely
Ectoderm,
endoderm,
mesoderm.
What is the importance of the germ layers?
form different tissues and organs (organogenesis)
what do we have at the End of the embryonic period?
1- Main organ systems are formed
2- External body form is recognizable Notochord induces (an inducer) the ectoderm to form the neural plate
What does the notochord do once it’s formed?
begins to signal to the ectoderm above it to differentiate these cells especially in the cephalic region to change into neuroectoderm (to fold)
What makes the neuroectoderm?
Cells of the neural plate make up the neuroectoderm
What happens to the neural plate during neurulation?
Begins to expand toward the primitive streak
What is the role of the primitive node during neurulation?
Organizer expresses specific genes to from the notochord
Where does the neural plate expands to ?
Toward the primitive streak
What are the Somites?
part of the mesoderm
when does somites start to appear?
During neurulation day 20
Somites will differentiate to form?
1- Sclerotome > bones&vertebra
2- Myotome > muscles
3- Dermatome > skin
When is the neural folds formed?
By the end of the 3rd week
What forms the neural plate
• the lateral edges of the neural plate
What forms the the neural groove?
The depressed mid-region
How is the neural groove formed
• Gradually the neural folds approach each other in the midline, where they fuse
Where does Fusion begins?
at cervical region (5th somite) and proceeds cranially and caudally, forming the neural tube
Simile acts a landmark for what
to tell us which stage of development we are
How does The neural tube close?
The neural tube begins to close like a zipper cephalocaudally
How do the cephalic and caudal ends of the neural tube communicate?
until fusion is complete > with the amniotic cavity with the anterior (cranial) and posterior (caudal) neuropores, respectively
when does the Closure of anterior neuropore occur?
Closure of anterior neuropore occurs about day 25
when does the Closure of posterior neuropore occur?
day28
What happens if anterior pore fails to close
Anencephaly
What happens If Posterior pore fails to close
spina bifida.
What are the differences that occur between the 25 and 28 day?
1- Increase in size
2- Umbilical cord formation
3- Closure-of neuropores
4- Otic placode give rise to eyes and ears
5- Limb ridges appear
6- Increase in somite number
What is the role of Broader cephalic portion of the neural tube?( 25 and 28 day)
forms the brain
What is the role of narrow caudal portion of the neural tube?( 25 and 28 day)
• the narrow caudal portion forms the spinal cord
What are the Derivatives of the ectodermal germ layer ?
1- The central nervous system
2- the peripheral nervous system
3- The sensory epithelium of the ear,nose and eye
4- The epidermis and its appendages
5- Subcutaneous glands
6- The mammary glands
7- The pituitary gland
8- Enamel of the teeth
When Formation and migration of Neural crest cells occur?
• This is happening during neural tube closure
How does the formation of neural crest cell happen
- At the dorsal part ,cells begin to detach and migrate from neural tube and as they detach, they change their profile from epithelial to mesenchymal
- they will give rise to structures that are associated to the CNS
What are dorsal root ganglion
A sensory gateway/door that collects info and it send to the brain
Who forms dorsal root ganglion
Neural crest cells
What are sympathetic ganglion
Autonomic part and is derived from neural crest cells developing suprarenal ganglion
preaortic ganglion
Control vasodilation and vasoconstriction
Interact ganglion
Going to GI tract, excelling GI tract to inc or dec digestion
What are the Neural crests formed from?
• edges of neural folds
What genes are important for neural crest formation?
• BMPs and FGFs
What secretes BMPs and FGFs?
secreted from ectoderm
BMPs and FGFs regulate the whole process, why?
Because they play role in ventralizing the mesoderm
so this is part of the mesoderm cells as well at this point in time, they are derived from the developing neural tube, they have the ability to shift their profile to become neural crest cell and go to that mesoderm layer and innervate the structure that they are going develop to
Why is Neural crest cells (neuroectodermal) transform from epithelial to mesenchymal form?
So they can migrate.
Where do neural crest cells migrate to
These cells migrate and enter the mesoderm
How do neural crest cells able to migrate
An Interaction with ectoderm is required
What are the Neural crest derivatives?
1- Connective tissue and bones of the skull
2- Cranial and spinal nerve ganglia
3- Septum of the heart
4- Sympathetic chain and sympathetic and parasympathetic ganglia
5- Schwann cells
6- Adrenal medulla
7- Glial cells
8- Melanocytes-in skin and other organs > pigment in skin and daughter organs
9- Odontoblasts-Teeth
What are the possible Neural tube defects?
- Anencephaly
2. B & C- Spina bifida
What causes Neural tube defects?
• Neural tube fails to close
Where does failure to close the Neural tube Occur in anencephaly?
• If it happens at the cranial region, then most of the brain fails
spina bifida?
- Below the cervical region failed closure results in spina bifida
- Spina bifida cases commonly occur in the lumbosacral region
How can can we prevent Neural tube defects?
• Folic acid
What does the mesodermal germ layer start to form?
• initially the mesodermal germ layer form a thin sheet of loosely woven tissue on each side of the midline
what induces the mesoderm to divide into 3 parts?
Notochord
When does cells close to the midline?
On 17th day, cells close to the midline
what is the lateral plate?
• On 19th day, more laterally the mesoderm layer remains thin and is known as lateral plate
What does the Intermediate mesoderm connect?
connects paraxial and lateral plate mesoderm
What divide the lateral plate into two layers?
• Cavities develop in the lateral plate
What are the divide layers of the lateral plate?
• it is divided into two layers: somatic (parietal)&splanchnic (visceral)layers
What are the derivatives of the Somatic mesoderm?
• Somatic mesoderm forms dermis of the skin in the body wall & limbs and bones and connective tissue
What are the derivatives of the Splanchnic mesoderm?
Splanchnic mesoderm forms the muscles of the gut
What are the derivatives of the Intermediate mesoderm?
Intermediate mesoderm contributes to the formation of the urinary and reproductive
Systems (gonads) > Except bladder and urethra
Blood cells and blood vessels arise from
mesoderm
What is somitomeres?
paraxial mesoderm begins to be organized as segments
when does start to appear somitomeres?
beginning of the 3rd week
where do somitomeres first appear?
first appear in the cephalic region and their formation proceeds cephalocaudally
what do we mean with somites?
When Somitomeres further organize they form somites
How many somites develop By the end of 5th?
By the end of 5th week 42-44 somites develop
What is the location of somites?
- occipital,
- 8 cervical,
- 12 thoracic,
- 5 lumbar,
- 5 sacral
- 8-10 coccygeal pairs
What is the fate of the 1st occipital and last 5-7 coccygeal somites?
The 1st occipital and last 5-7 coccygeal somites later disappear
Most ventral somite is known as
Sclerotome
Dorsal medial & ventral lateral are known as
Myotome
Middorsal
dermatome
Where are mesoderm cells arranged?
around the center of each somite
What are the derivates of somites?
- The somites form the axial skeleton (vertebrae, skull, sternum, ribs)
- body wall muscles
- dermis of the skin
What is the Transcription factor required for the ventral lateral part of the somite? > muscle cells
MYOD
What regulates MYOD expression?
- WNT from the ectoderm
* lateral plate mesoderm will secrete BMP4 to express MYOD as well
What is the Transcription factor required for the middorsal part of the somite? > dermis
and What regulates that expression?
PAX3
- NT- 3
Where does NT- 3 come from?
coming from dorsal side of N.T
What is the Transcription factor required for the dorsal medial part? > muscle cells
MYF
What regulates MYF expression?
WNT coming from the dorsal side of N.T in addition to SHH from notochord and ventral
part of N.t
What transcription factor is required in order differentiate somite’s into sclerotome?
PAX1
What induces PAX1 expression?
SHH and Noggin
Where is the SHH and Noggin coming from?
Ventral part of the neural tube and notochord
Describe the visceral mesoderm layer ?
This layer is attached to the endoderm and incorporated within the developing fetus
What does the endoderm germ layer covers?
the ventral surface of the embryo > forms the roof of the yolk sac
what happens Between the 17th and 28th days?
growth of the brain vesicles and lengthening of the neural tube
growth of the brain vesicles and lengthening of the neural tube leads to what
causes both the head and tail regions to move ventrally forming cranial and caudal body
folds and the embryo curves into the fetal position > the embryo lies within the amniotic
cavity
why does the umbilical region the ventral body wall does not close completely
to leave a connection between the yolk sac and the gut
What are the Derivatives of the endodermal germ layer?
- the epithelium of primitive gut and the respiratory tract
- thyroid, parathyroid, liver, pancreas, tonsils and thymus
- urinary bladder and urethra , tympanic cavity and auditory tube.
When does the membrane breaks?
During the 4th week
How is the the foregut separated from the stomodeum?
At the cranial end the foregut is separated from the stomodeum (primitive mouth) by the
oropharyngeal membrane
How is the hindgut separated from exterior?
At the caudal end, the cloacal membrane separates the hindgut from exterior
When does the the cloacal membrane breaks?
breaks during 7th week à these two events establish continuity with exterior (amniotic
cavity)
Gonads develop from
the intermediate mesoderm
Bladder and
urethra develop from
endoderm
Why do external appearance changes occur?
due to increase in head size and formation of the limbs, face, ears, nose and eyes (End of 8th week)
When does the limbs appear?
- 5th week- limbs appear
* first the upper limbs and then the lower limbs
what happens in 2nd month?
increase in head size, face, limbs, ears, nose and eyes
When are Most organs and organ systems formed?
during the embryonic period
Why is the embryonic period considered as critical period for development?
Because Most organs and organ systems formed
Embryonic period ends in
8th week > (2-8)
When does the Fetal period begin?
From 9th week to birth
What is the main characteristic of the Fetal period?
All systems are in place and completed development but now are maturing and increasing in size
what happens to the head during the fetal period
Slowdown of head growth compared to rest of the body 3rd Month
Second half of intra-uterine life
4th and 5th months
Why is the skin of the fetus is reddish?
Due to high level of angiogenesis (high vascularity)
Why are babies that are Born at 6 month-may not survive?
The lung development is not complete and alveolar collapse will happen
Time of birth
266 days/38 weeks after fertilization
Doctor calculates the date of delivery as
280 days/40 weeks after last period
what can misguide the doctor
Implantation bleeding
Irregular menstrual period
The size of the fetus has to be determined to
manage delivery and growth of fetus
is intra-embryonic mesoderm inside the baby?
Yes Intra-embryonic mesoderm will give rise to Paraxial Intermediate Lateral
When lateral splits it will give rise to
lining of the gut and membranes around body wall
