Trilaminar Disc Formation Flashcards
What day does gastrulation commence?
day 15
What is gastrulation?
Why must it be correct?
Development from two layers into three layers
(blastula to gastrula)
It must be correct as all body systems develop from the trilaminar disc
What is the first stage in gastrulation?
Formation of the primitive streak
This consists of the primitive pit, node and groove
What is established after the primitive streak forms?
Where does the streak develop?
A right and left side of the embryo
The primitive streak forms on the dorsal face of the developing embryo, toward the caudal end
What is situs inversus?
The right to left asymmetry of the body is reversed
This can be partial or complete
How is complete situs inversus usually found?
Why?
It is usually an incidental finding
There are still all the normal connections between organs, so there aren’t usually any issues
What is Kartagener syndrome and how does it arise?
It affects the cilia, which sweep the correct cells to the right or left sides
If the cilia do not sweep the cells correctly, some organs may be reversed
What types of mutations usually affect the cilia?
Mutations may be in different genes that encode proteins that are important to the structure and function of the cilia
This may cause them to become immotile or dysmotile
Why is Kartagener syndrome more problematic than complete situs inversus?
Some organs have been reversed but others haven’t
This leads to abnormal connections in the body
What happens on day 15 of development?
formation of the definitive endoderm
What is the first stage in development of the definitive endoderm?
cells of the epiblast begin to move towards the primitive streak
What is the second stage in the development of the definitive endoderm?
Epiblasts migrate through the primitive streak, from the dorsal side, and displace the hypoblast cells
Why are no body systems formed from the hypoblast layer?
All systems are formed from the epiblast cells
The epiblast cells replace the hypoblast cells during formation of the definitive endoderm
What is the definitive endoderm?
The layer of epiblast cells that completely replaces the hypoblast
How is the mesoderm formed?
Epiblast cells continue to migrate through the primitive streak
They form a middle layer between the epiblast and definitive endoderm
On what day does the mesoderm form?
16
How is the ectoderm formed?
The remaining epiblast cells do not migrate and form the ectoderm
What is complete after the ectoderm has formed?
Gastrulation
Three distinct layers that make up the trilaminar disc have been formed
What are the two types of mesoderm in the trilaminar disc?
Intraembryonic mesoderm that is continuous with extraembryonic mesoderm
What structures are formed from the endoderm?
- epithelium of the digestive system and respiratory system
2. organs associated with digestive system e.g. pancreas and liver
What structures are formed from the mesoderm?
- musculoskeletal system
- cardiovascular system
- dermis of the skin
What structures are formed from the ectoderm?
It is involved in development of the nervous system, neural crest and the epidermis of the skin
On what day does the mesoderm become highly organised?
What is it organised in to?
day 17
- paraxial mesoderm
- intermediate mesoderm
- lateral plate mesoderm
What does paraxial mesoderm form?
This is mesoderm parallel to the midline
It forms the majority of the skeleton, skeletal muscle and dermis of the skin
What does the intermediate mesoderm form?
Gonads, reproductive tract and the kidneys
What does the lateral plate mesoderm form?
Lining of body cavities
How does the paraxial mesoderm undergo further organisation?
On what day does this occur?
It begins to divide into blocks called somites
This occurs on day 20
At what rate are somites formed?
How many somites are formed in total?
They are formed in a cranial-caudal fashion at a rate of 3 or 4 pairs per day
This continues until 42-44 pairs of somites have been formed
Why can somite formation be used to date an embryo?
It is a very highly regulated and predictable process
What systems is mesoderm important in developing?
Musculoskeletal and genitourinary system
They cannot form properly if there is not enough mesoderm
What is sirenomelia?
“mermaid syndrome”
There is insufficient mesoderm in the causal region of the embryo
What are the 2 reasons why sirenomelia might result?
- insufficient numbers of epiblasts have migrated through the primitive streak
- the primitive streak has regressed too early
What are the consequences of sirenomelia?
Abnormalities of the urogenital system and lower limbs
What is a sacrococcygeal teratoma?
The remnants of the primitive streak may persist and give rise to a tumour
What is the tumour in sacrococcygeal teratoma made from?
The abundance of mesoderm forms a large mass
Why does sacrococcygeal teratoma occur?
Too many epiblast cells have migrated through the primitive streak
or the primitive streak has regressed too late
What is the prognosis after resection of a sacroccocygeal teratoma?
Prognosis tends to be good as most sacroccocygeal teratomas diagnosed in newborns are benign
Why is the primitive streak described as a ‘transient structure’?
It is only temporary
It begins to regress as soon as sufficient amounts of epiblast cells have migrated
What happens as the primitive streak regresses?
It gets progressively shorter and lays down a layer of cells beneath it
the primitive node forms the notochordal process
What happens to the notochordal process as the primitive streak regresses further?
the notochordal process is a hollow tube which gets longer as the streak regresses
On day 18, what does the notochordal process transiently fuse with?
It transiently fuses with the endoderm to form the flattened notochordal plate
What happens to the notochordal plate once it is formed?
It separates from the endoderm and fuses to form a solid notochord
What is the role of the notochord?
It is a signalling centre
It starts to signal and cause components of the body to begin to develop
How is neurulation initiated?
What is this process called?
Signals from the notochord that cause the ectoderm to thicken and form the neural plate
This is induction
How does the notochord cause the ectoderm to thicken?
What is significant about this process?
It signals to cause the ectoderm to start proliferating
the notochord induces formation of the CNS by signalling for the ectoderm to thicken
What is neurulation?
the first event in the development of the central nervous system
How does the notochord influence differentiation of the somites?
It induces the differentiation of the somites into 3 different structures:
- sclerotome
- myotome
- dermatome
What does the sclerotome form?
bone and cartilage
What does the myotome form?
skeletal muscle
What does the dermatome form?
dermis of the skin
How does the notochord further organise the sclerotome?
It further organises the sclerotome to form vertebral bodies
How are vertebral bodies and vertebral arches formed?
Sclerotome cells surround the notochord to form the vertebral body
Sclerotome cells surround the neural tube to form the vertebral arch
What is the fate of the notochord?
The majority of the notochord degenerates as bodies of vertebrae form
The remnants of the notochord form the nucleus pulposus
What is the purpose of the intervertebral discs?
They are shock absorbers that prevent bone from clashing and becoming damaged
What are the 2 components of the intervertebral disc?
Strong outer rim is the annulus fibrosis
The jelly-like centre is the nucleus pulposus
Why do spinal defects occur?
Abnormal induction of the sclerotomes
This means that vertebral bodies will not form correctly
What is the least severe form of spina bifida?
How is it identified?
Spina bifida occulta
Identified by dimpled skin and a tuft of hair
What is the process behind spina bifida occulta?
The vertebral arches have failed to form
The spinal cord is only protected by a layer of subcutaneous fat and skin
What is meningocele?
It involves the 3 meninges that surround the CNS
Failure to form vertebral arches leads to meninges protruding through the gap
The meninges sac is filled with CSF
What is myelomeningocele?
This is where failure to form the vertebral arches leads to the meninges and part of the spinal cord protruding through the gap
Why is myelomeningocele dangerous?
How can it be detected?
some of the spinal cord is open to the external environment
It can be detected on ultrasound and in utero surgery can fix it
