Embryology - embryonic phase - gastrulation (week 3) Flashcards
Which weeks make up the embryonic period?
Weeks 3-8
What happens during the embryonic period?
It is the period of greatest change.
All major structures and systems are formed.
The most perilous stage for the developing child - many congenital defects have their root in this time.
Why is week 3 called the week of threes?
- 3 cavities - chorionic cavity est by end of second week
2. 3 germ layers - rudimentary lineages from which all others will rise
What is gastrulation?
The process establishing three germ layers and hence the origins of all tissues in the body.
A trilaminar disk is formed and axes set which are observed in the adult.
Which three axes are set in gastrulation which are observed in the adult body?
- anterior/ posterior
- dorsal/ventral
- right/left
What indicates the beginning of gastrulation?
The primitive streak appears
When do researchers have to stop studying embryos?
When the primitive streak appears
What cellular rearrangement occurs during gastrulation?
Migration and invagination
What is the amnion?
The amnion is a membrane that when first formed, closely covers the embryo. It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo.
Where does the primitive streak, pit and node first appear?
On the dorsal surface of the epiblast.
What does the primitive streak look like?
Narrow groove with bulging edges.
Where is the primitive node located?
Cranial end of primitive streak (end furthest away from the edge of the disk).
Where is the primitive pit located?
At the centre of the primitive node.
What is the role of the primitive node?
It is the control centre, coordinating all movements.
What happens to the primitive streak once the three germ layers are established and gastrulation proceeds?
It regresses. If it doesn’t regress there are consequences.
What is an important rule of thumb for the direction of development?
It proceeds in a cranial to caudal direction.
In which direction does the primitive streak develop?
Cranial to caudal direction.
Where do the epiblast cells surrounding the primitive streak migrate and invaginate to?
They displace the hypoblast creating a third layer (in the middle).
What two layers make up the bilaminar disk?
Epiblast and hypoblast,
What three layers make up the trilaminar disk?
Ectoderm, mesoderm and endoderm.
As more and more cells migrate through the streak, which directions do they spread?
Laterally and cephalad (towards the head).
Which are the two locations where the mesoderm does not spread and therefore the ectoderm is in direct contact with the endoderm?
Future mouth and future anus.
What determines the ultimate fate of the invaginating epiblast?
Where in the streak or node they invaginate
The presence or absence of which structure in the embryo determines whether a species is in the phylum Chordata or not?
Notochord
What is the notochord a basis for?
Axial skeleton - it drives formation of the nervous system.
Where in primitive streak/pit do prenotochordal cells invaginate?
Cranial part of primitive pit.
Where do prenotochordal cells migrate?
They form a solid rod of cells running in the midline.
What is the significance of the notochord?
- Defines the midline
- Axial skeleton forms around it
- Drives neuralation
What is the vestigial remenant of the notochord in adults?
Nucleus pulposa of intervertebral discs
Anterior refers to which direction in an embryo?
Head not belly, like in adults
Posterior refers to which direction in an embryo?
Tail not back, like in adults
Which direction does rostral/cephalic/anterior describe?
Towards the head.
Which direciton does caudal/posterior describe?
Towards the feet.
Which direction does dorsal describe?
Towards the back.
Which direction does ventral describe?
Towards the belly.
Which direction does left describe?
45 degrees direction between rostral and ventral.
Which direction does right describe?
45 degrees direction between caudal and dorsal.
What is the first thing to set the axes in a developing embryo?
The primitive streak defines rostral and causal.
The presence of what ensures correct dorso-ventral and left-right development?
Molecular signals emanating from the primitive node ensure symmetry of limbs and assymetry of viscera.
When does gastrulation occur?
Week 3
Why does gastrulation happen?
To ensure the correct placement of precursor tissue to allow subsequent morphogenesis to take place.
How does gastrulation happen?
Primitive streak appears in the caudal epiblast, leading to migration and invagination of epiblast cells.
What is the fate of differentiated cells?
They are committed to one fate and one function.
Does cross-over between the cell types in ectoderm, mesoderm or endoderm occur?
No
Which three cell lineages does epiblast differentiate into?
Ectoderm, mesoderm and endoderm.
What does ectoderm differentiate into?
Organs and structures that maintain contact with the outside world e.g. nervous system, epidermis.
What does mesoderm differentiate into?
This is the most plastic of the three layers.
It differentiates into supporting tissues e.g. muscle, cartilage, vascular system (including heart and vessels).
What does endoderm differentiate into?
Internal structures e.g. epithelial lining of GI tract, respiratory tract; parenchyma of glands (functional elements of glands e.g. secretory cells).
Give some examples of left-right asymmetry found in the body
Liver on the right, stomach and spleen on the left etc…
Left lung has two lobes, right lung has three.
How does the primitive node create left-right differences?
By the action of ciliated cells at the node, a left-ward flow of signalling molecules is created. This creates a side-specific signalling cascade to be initiated.
Presence of signalling molecules = LHS signals
Absence of signalling molecules= RHS signals
Describe situs inversus
It is an occurence, that is not that rare, of a complete mirror image of viscera in the body. It is commonly the result of immotile cilia and normally has no associated morbidity. Problems arise if there is normal and mirror-image disposition because this is a sign of more complex and compound congenital problems.
What is the difference between monozygotic and dizygotic twins?
Mono- from a single fertilised oocyte
Di- from two fertilised oocytes
When can splitting occur in monozygotic twins?
At many different times.
- If embryo splits after first cleavage, both embryos have their own placenta.
- If embryo splits by duplication of inner cell mass - there will be two embryos sharing the same placenta.
- Splitting can occur very late with duplication of the primitive streak. - two embryos share a placenta and the same amniotic sac.
What happens if separation of embryos is not complete?
Conjoined twins
What is a teratogen?
A substance which interferes with normal development.
Define teratogenesis
The process through which normal embryonic development is disrupted.
Which weeks are the most sensitive to teratogenic insult?
The embryonic phase - weeks 3 to 8. Each organ system will have a particular ‘sensitive window’ depending on the time at which most of its development occurs.
List some teratogenic agents
A large number of chemical and infectious agents are known to cause developmental defects:
Thalidomide - interferes with limb development
Rubella - interferes with head and neck development.
Alcohol - no safe threshold for use in pregnancy
Certain therapeutic drugs - paradine and warfarin (cross placenta and cause problems).
