Gastrulation Flashcards
When is the embryonic period?
Weeks 3-8 inclusive
Why is the embryonic period significant?
- It’s the period of greatest change
- Most perilous
What happens during the embryonic period?
All major and structures and systems are formed
What are germ layers?
Rudimentary lineages from which all others will arise
What does the bilaminar disc consist of?
NAME?
What happens to the epiblast in the embryonic period?
It undergoes radical development process
What is the result of the radical development of the epiblast?
3 germ layers established
What is gastrulation?
The process establishing the trilaminar disc, which consists of the 3 germ layers, and hence origin of all tissues of the body
What are the set axis observed in adults?
- Anterior / posterior
- Dorsal / ventral
- Right / left
What marks the onset of gastrulation?
The appearance of the primitive streak
What is the primitive streak?
The process thats driving and coordinating the gastrulation process
What is associated with the primitive streak?
The primitive node and the primitive pit
Where does the primitive streak have major significance?
In terms of the legal ramifications of the study of embryos
What happens once the primitive streak has formed?
Cellular rearrangement
Where does cellular rearrangement occur?
Within the epiblast of the bilaminar disc
In what ways to the cells rearrange?
NAME?
Is migration controlled?
Yes, highly
How far to cells migrate?
Widely
What happens in invagination?
The cels interdigitate themselves within other layers
What does the epiblast look like at the end of the second week?
Featureless, all cells look exactly the same
When does the primitive streak appear?
~day 16
Where does the primitive streak appear?
The dorsal surface of the epiblast
Describe the primitive streak
NAME?
Where does the primitive streak occur on the disk?
At one edge, but can be anywhere along the edge
Where is the primitive node located?
At the cranial end of the stalk
What is the cranial end of the stalk?
The end furtherest away from the outside of the epiblast
Where is the function of the primitive node?
It’s the co-odinating centre, co-ordinating and regulating all of the cells signals, and responsible for causing migration
Where is the primitive pit located?
At the centre of the node
What happens as gastrulation proceeds?
We see rapid morphological changes
What morphological changes do we see during gastrulation?
The establishment of 3 germ layers
What happens to the primitive streak once the 3 germ layers are established?
It regresses
What is the clinical significance of the regression of the primitive streak?
Occasionally it doesn’t regress, which has consequences
In what direction does development proceed?
Cranial → caudal
What is the result of the direction of development?
The head is always more developed than the feet, until fully grown
What happens to the epiblast of the bilaminar disc in gastrulation?
The cells migrate
What happens as the cells of the epiblast migrate?
They divide and differentiate as they do
What is the result of the migration of the cells of the epiblast?
Forms a new layer
How does the migration of the cells of the epiblast cause the production of a new layer?
The cells start to pile up on either side of the primitive streak, and when they get to the edges of the streak, they push through the epiblast later and spread throughout the embryonic disc, which displaces the hypoblast and produces a new layer
Is the hypoblast involved in the formation of the new layer?
No
Why is the hypoblast not involved in the formation of a new layer?
It has already done it’s job of lining the primitive yolk sac and forming the secondary yolk sac
What happens to the hypoblast by the end of the gastrulation process?
It has regressed entirely
What are the 3 layers produced by gastrulation?
NAME?
What is the name of the differentiated top layer?
Ectoderm
What is the name of the new middle layer?
Mesoderm
What is the name of the layer that is the replacement for the displaced hypoblast?
Ectoderm
What happens as more and more cells migrate through the streak?
They spread laterally and cephelad (towards the head)
What is the result of the spreading of the migrating cells?
The mesoderm spreads out between the ectoderm and endoderm (like the filling of a sandwich)
What is the exception to the spreading of mesoderm between the other layers?
Two places where there are gaps in the mesoderm, and therefore the ectoderm and endoderm touch
What is going to develop where the ectoderm and endoderm touch?
Future mouth and anus
Where is the places where the ectoderm and endoderm touch important?
When looking at the development of the GI, urinary and reproductive tracts
What does the ultimate fate of the invaginating epiblast depend on?
Where in the streak or node they invaginate
What does the presence of a notocord define?
The phylum Chordata
What is the notocord the basis for?
The axial skeleton
What is meant by the notocord being the basis for the axial skeleton?
The axial skeleton forms around it
What does the notocord drive?
Neurolation
What does neurolation do?
Establishes the CNS
How is the notocord formed?
NAME?
How do the cells of the notocord differ from all others in gastrulation?
They are instructed to form a solid rod, whereas the others are instructed to invaginate and spread widely
What is the role of the notocord?
Important signalling role
What does the notocord define?
The midline
Why does the notocord define the midline?
Because prior to this, we had the primitive streak at one end, but no midline
What happens to the notocord once it has performed its roll?
Regresses
What is the vestigial remnant of the notocord in adults?
The nucleus pulposus of intervertebral discs
What is the clinical significance of the nucleus pulposus of intervertebral discs?
It is the component that herniates when you ‘slip a disc’
Are humans amorphous?
No, we have clearly defined regions
Are humans symmetrical?
NAME?
What does the fact that there is visceral asymmetry mean?
There must be a process by which symmetry and asymmetry is coordinated
What does medics terminology rely on?
Universal use of the anatomical position
On the human body, what is meant by superior and inferior?
NAME?
In the human body, what is meant by anterior and posterior?
NAME?
Are the axis set early or late in an embryo?
Early
What is meant by rostral/cephalic in an embryo?
Head
What is meant by cadual in an embryo?
Bottom
What is meant by ventral in an embryo?
Front
What is meant by dorsal in an embryo?
Back
What does the primitive streak define?
Front and back
How does the primitive streak define front and back?
Because it appears at one end of the bilaminar disc
What ensures correct dorso-ventral and left-right development?
Molecular signals emanating from the primitive streak
What is derived from the ectoderm?
Organs and structures that maintain contact with the outside, for example the nervous system and the epidermis
What is derived from the mesoderm?
Supporting tissues, e.g. muscle, cartilage, bone, vascular system (including heart and vessels)
What is derived from the endoderm?
Internal structures, e.g. epithelial lining of GI tract, respiratory tract, parenchyma of glands
In terms of symmetry, what could be said prior to gastrulation?
The embryonic disc is bilaterally symmetrical
In terms of symmetry, how does the embryonic disc differ from the neonate?
Clear left vs. right differences
Where does the body show asymmetry?
In the thoracic and abdominal viscera there is asymmetrical arrangement of the tissues and systems
Give 3 examples of asymmetry in the body
- Liver on right
- Spleen on left
- Left lung has 2 lobes, right lung has 3
What action is left-right asymmetry developed by?
The way ciliated cells beat at the node
How do ciliated cells lead to left-right asymmetry?
- Beating of ciliated cells at the node results in left-ward flow of signalling molecule
- This initiates the side-specific signalling cascades
What signals lead to left sidedness?
If the signal for left is present
What signal leads to right sidedness?
Absence of left signal
What is situs inversus?
Complete mirror-image viscera
What does mirror inversus commonly result from?
Immobile cilia
Is there associated morbidity with situs inversus?
Not usually
When do problems arise with situs inversus?
If there is both normal and mirror-image disposition
What is indicated when there is both normally and mirror image disposition?
More serious congenital defects
Where are the three germ layers in place by the end of week 3?
Anteriorly
When is gastrulation complete?
Week 4
Why is gastrulation not complete until week 4?
Because development proceeds cephalocaudally, it’s not complete in the caudal region until later
What are the types of twinning?
NAME?
How do you get dizygotic twins?
When 2 oocytes are fertilised
How do you get monozygotic twins?
When a single fertilised oocyte gives rise to 2 identical infants
How can a single fertilised oocyte give 2 infants?
NAME?
What is the result when the embryo splits after the first division?
2 embryo, each with own placenta
What is the result when the inner cell mass is duplicated?
2 embryos, sharing a placenta
What is it called when the inner cell mass is duplicated?
Splitting
When does splitting occur?
Can be very late, with the duplication of the primitive streak
What do the embryos produced by splitting share?
NAME?
What is the clinical importance of splitting?
Sometimes the separation is not complete, leading to conjoined twins
What is teratogenesis?
The process by which normal embryonic development is disrupted
What period of development is most susceptible to teratogenic insult?
Weeks 3-8
What does each organ system have?
A particular ‘sensitive window’
What does an organ systems sensitive window depend on?
The time at which most of its development occurs
What kind of agents can be teratogenic?
Chemical and infectious
Give 4 teratogenic agents known to cause developmental defects
- Thalidomide
- Rubella
- Alcohol
- Certain therapeutic drugs
Why can alcohol act as a teratogenic agent?
It’s a small molecule, so can penetrate the placenta
