Gastrulation and Morphogenesis Flashcards
What is gastrulation?
The process which the bilaminar embryo is transformed into one that has 3 germ layers
What are the three germ layers that form in gastrulation?
Ectoderm, mesoderm, endoderm
What do the germ layers form?
All the tissue of the baby
What tissue types form from ectoderm?
The epithelium of the skin and nervous system
What tissue types form from mesoderm?
Bones, muscles and mesenteries (tissue that covers internal organs and attaches organs to cavity walls)
What tissue types form from endoderm?
Lining of the gut and large glands such as liver and pancreas
What is the first sign of gastrulation?
The formation of a primitive streak
What does the primitive streak define in the embryo?
Both the midline and caudal end of the embryo
What is the purpose of forming a primitive streak?
It creates symmetry in the organism
How does the primitive streak form?
When the cells in the epiblast start migrating downward from the midline at the tail end of the epiblast then down towards the hypoblast. More cells join movement towards midline further from the tail end and the primitive streak elongates rostrally (towards the head)
As the embryo develops, what happens to the primitive streak? Why?
It shortens as the tip of the primitive streak moves back towards the tail Fewer cells are actively migrating
What kind of structure is the primitive streak? What makes it visible?
It isn’t a structure Cells migrating from epiblast towards hypoblasts makes it visible
As the primitive streak grows and shrinks, what impact does this have on the cells?
The cells that are closer to the head undergo gastrulation later on and for a shorter period of time than the cells at the tail end
What is the end of the gastrulation where new cells are migrating downwards called?
The node
What do the epiblast cells need to do to migrate?
They need to lose all their intercellular connections and form a shape which can allow them to migrate
What do the epiblast cells do as they’re about to migrate?
They detach from the epithelium of the amion cells and form a ball shape
What do the epiblasts do as they are migrating?
They push down into the hypoblast layer and displace the hypoblast layer
Why is the hypoblast displaced by the epiblast?
In order to form the inner lining of the yolk sac
What do the epiblasts that displace the hypoblasts give rise to?
The endoderm
Once the epiblasts have migrated into the hypoblast layer, where do the epiblast cells continue to migrate to?
They migrate into the space between the epiblasts and endoderm/hypoblast
What germ layer do the epiblasts between the original top layers of epiblast and the endoderm give rise to?
The mesoderm
How is the mesoderm arranged as it develops?
Starts off as loosely packed then comes together forming well defined structures
What happens to the remaining epiblast on the top layer once gastrulation has finished?
It turns into ectoderm
Because the node moves from the tail to head end, how is gastrulation different in the head?
It happens later than in the tail
What does the gastrulation process create?
A three layered embryo
What are mesenchyme?
They are the mesoderm in a loosely packed state
What is a mesenchymal transformation?
When the epiblasts lose their epithelial morphology and form loosely packed mesenchyme cells
What structures do the mesenchyme form once it condenses? Where are they in the mesoderm layer?
Form: notochord, middle Somites, one on each side of the notochord Intermediate mesoderm, a rod of mesoderm tissue on each side of the somites Lateral plate mesoderm, folded over plates on each side of the intermediate mesoderm
How do the mesoderm paths during gastrulation impact the tissue development?
The direction the cells take determines what kind of tissue is formed
What kind of tissue is formed from mesoderm cells that go forward, i.e. the first stream of cells during gastrulation (towards the head)? What main structure does this form?
Axial mesoderm Notochord
What kind of tissue is formed from the mesoderm cells that go out in the second stream during gastrulation (e.g. to the side a little bit)? What structure does this form?
Medial paraxial mesoderm Forms the medial side of the somites (part of the somite closest to the notochord)
What kind of tissue is formed from the mesoderm cells that go out in the third stream during gastrulation (further out from the second team)? What structure does this form?
Lateral paraxial mesoderm Forms the lateral side of the somite (part of the somite furtherest from the notochord)
What kind of tissue is formed from the mesoderm cells that go out in the fourth stream during gastrulation (go backwards from the node)?
Forms the intermediate and lateral plate mesoderm
What other process is occurring at the same time as gastrulation?
Convergence extension
What happens during convergence extension?
The embryo becomes longer between the head and tail and thinner on the sides, becoming oblong
How is the head determined on the embryo?
It is always opposite from the starting position of the primitive streak
How are the different cells within an embryo told what kind and how to form the tissues required of the baby?
Differentiation signals (tissue type, e.g. muscle, bone…) Global signals (tissue specialisation, e.g. pectoral muscle, compact bone…)
What system is used to determine the body plan of the embryo?
Tail organiser and head organiser
What forms the tail organiser?
The node on the primitive streak
What forms the head organiser? Hows is this component formed?
The anterior visceral endoderm which is when the hypoblast gets pushed to the head end as the primitive streak moves along
What do organisers produce?
They secrete morphogens
What do morphogens do? How can it impact cell function?
Determines what the cell does, stimulates expression of hox genes Concentration and timing of cells exposed to it
How does cells recognise if they are at the tail or head end of the embryo with morphogens? What do they do with this information?
Tail and head organisers produce tail and head morphogens, the amount of morphogens of each kind determines how close the cell is to the head/tail (e.g. if receiving lots of tail but not many head morphogens then it is closer to the tail end etc.) With this positional info they cell can then determine if it should produce more or less head/tail structures
What are the two kinds of actions morphogens can have? What do these actions do?
Agonist - stimulates something to happen Antagonists - prevents something from happen
Generally, what kind of actions do head morphogens do?
They are generally antagonist blocking the functions of tail organisers
What are functions of hox genes?
They are the global signals that indicate what genes are expressed to create specific body parts
What tissues does the hox gene influence?
It affects all three embryonic tissues: endoderm, mesoderm ectoderm
Where are limb buds generated during embryonic development? What are limb buds?
At the borders of the hox genes
They are the points on an orgnaism where the limbs will grow from
What is transverse folding?
The process which the three embryonic layers fold over through the MIDDLE AXIS RUNNING FROM THE HEAD TO THE TAIL to form the structures for the intestines (inside), skin (outside) and muscle (middle)
Describe the structure of the embryo before transverse folding
What happens to the amniotic cavity as the embryo undergoes transverse folding?
It folds over the embyro until it covers the entire embryo forming a amniotic sac
When does the endoerm close up?
It never does, there is always a slight gap where there is contact with the yolk sac
What is longitudinal bending?
The process where the head and the tail end curve into a C shape
What tissue is folding over to form the C shape during longitudinal bending?
The neural tube
What happens to the endoderm and ectoderm as the embryo undergoes longitudinal bending?
The endoderm folds underneath the neural tube and the ectoderm folds over the top of the neural tube
What axis does longitudinal bending occur?
The rostro-caudal axis
What happens as the head and tail move downwards during longitudinal bending? What does this form?
The endoderm moves toward the middle forming a tube with blind ends that will eventually become the gut
What happens as the head and tail move downwards during transverse folding?
The lateral edges of the trilaminar embryo (it has 3 layers, ectoderm, mesoderm and endoderm FYI) move downwards and meet along the midline so the embryo is transformed into a tube (eg folds over into the tube)
What is the end result of longitudinal and transverse folding? What is the exception to what has formed?
Embryo becomes a closed tube
except for the small opening that connects the inside of the gut to the yolk sac
Describe the struture of an embryo cross-section after transverse folding and longitudinal bending
How long after fertilisation does the embryo begin to resemble the structure of a typical vertebrate?
One month
What does the ectoderm develop into?
the epithelium of the skin
