The Fate of the Mesoderm Flashcards
What has happened by the end of the third week post fertilisation?
NAME?
What ist he most plastic of the germ layers?
Mesoderm
What is meant by the mesoderm being the most plastic?
It gives rise to the biggest number of tissues
What is the notochord responsible for?
Releasing signals to surrounding ectoderm
What does the notochord undergo?
Neurulation
What forms the nervous system?
Notochord-driven induction of the ectoderm
What happens to the neural plate?
It folds up to form the tube which develops into the brain and spinal cord
What is the tube that develops into the brain and spinal cord called?
The neural tube
What is the formation of the neural tube key to?
Normal development of the CNS
What can result in problems with neural tube formation?
Spinabifida
When does neural tube formation occur?
23 days post fertilisation
What is the structure of the notochord during neurulation?
Solid rod of cells running in midline
What is the role of the notochord in neurulation?
Signalling
Where does the notochord run?
Between ectoderm and endoderm
What is the result of the notochord running between the ectoderm and endoderm?
Signals can reach the ectoderm
What does the notochord direct?
Conversion of overlying ectoderm to neuroectoderm
How does ectoderm become neuroectoderm?
Differentiation
What is neuroectoderm?
A type of tissue that goes on to give the CNS
What do notochord signals cause?
Overlying ectoderm to thicken
What does the thickening of overlying ectoderm lead to?
Slipper-shaped neural plate
How is the neural tube formed for the neural plate?
The edges elevate out of the plane of the disk, and curl towards each other
What causes the edges of the plate to rise up?
The cells grow faster at the edge
What gives rise to the majority of tissues?
Mesoderm
What is required for the mesoderm to give rise to the majority of tissue?
Rapid differentiation
What does mesoderm have lots of?
Discreet zones
What does each zone of the mesoderm have?
Separate tasks
What are the 4 types of mesoderm?
- Paraxial
- Intermediate
- Somatic
- Splanchnic
Where is paraxial mesoderm found?
Either side of the axis
What is the somatic mesoderm to do with?
Body, and body structures, including skeletal muscle
What is the splanchnic mesoderm to do with?
Viscera and organs
What exists in additional to the 4 types of mesoderm?
Intraembyronic coalom
What is intraembryonic coalom?
Spaces inside the embryo that form a new cavity
How is paraxial mesoderm organised?
Into segments
When do the first pair of somites appear?
Day 20
Where do the first pair of somites appear?
In the occipital region
What is the advantage of the first pair of somites being a very predictable presentation?
It can be used to age foetus
Where do more somites appear following the first?
The craniocaudal sequence
How quickly do more somites appear?
3 pairs a day
How many somites pairs are eventually present?
42 to 44
What happens to some of the somite pairs?
They disappear
What is the ultimate number of somite pairs?
31
How do somites appear?
As regular block of mesoderm cells arranged around a small cavity
What is the appearance of somites followed by?
‘Organised degradation’
What happens in organised degradation?
The ventral wall of somite breaks down
What does organised degradation lead to?
The formation of sclerotome
What does organised degradation represent?
A loss of a clearly defined block structure, but still kept very ordered
What does further organisation of dorsal portion form?
The dorsal dermomyotome
How does the dermomyotome differ from the sclerotome?
It is even more organised
What does the dorsal dermomyotome contribute to?
Formation of skin and muscular formation
What does the myotome do?
Proliferates and migrates
What does the dermotome do?
Disperses
What does the myotome and dermatome remain affiliated to?
The parent somite
What are somite derivatives the beginning of?
Development of musculoskeletal system
What is the dermatome?
‘Skin section’
What is the myotome?
‘Muscle section’
What is the sclerotome?
‘Hard tissue’ section
What is the implication of segmentation?
- Organisation of mesoderm into somites give rise to repeating structures
- Guides innervation
What repeating structures rise form somites?
- Vertebrae
- Ribs
- Intercostal muscles
- Spinal cord segments
What is the epimere innervated by?
Dorsal branch of spinal nerve for that segment
What is the hypomere innervated by?
Vental branch
What is the significance of there being 31 somites?
Means there a 31 segments and therefore 31 pairs of spinal nerves
Developmentally, what is the dermatome?
The part of the somite that gives rise to the dermis
Developmentally, what is the myotome?
Gives rise to the muscle
Clinically, what is the dermatome?
A strip of skin supplied by a single spinal nerve
Clinically,
A muscle/group of muscles supplied by a single spinal nerve
Clinically, what is the dermatome and myotome the basis of?
Examination of musculoskeletal and nervous systems
What does mesoderm differentiate into?
- Notochord
- Paraxial
- Intermediate
- Somatic
- Splanchnic
What does the paraxial mesoderm develop into?
- The axial skeleton
- Dermis
- Muscles of A/L body wall
- Some limb muscles
What does the axial skeleton consist of?
Vertebral column and ribs
What does the intermediate mesoderm develop into?
Urogenital system
What does the urogenital system consist of?
- Kidneys
- Ureters
- Gonads
What does the somatic mesoderm develop into?
Connective tissue of limbs
What does the splanchnic mesoderm develop into?
- Smooth musculature
- Connective tissue
- Vasculature of gut
What will the buccopharyngeal membrane become?
The mouth
What will the cardiogenic area become?
The heart
Where on the embryonic disc is the cardiogenic zone of mesoderm?
Cranial end
What kind of folding does the embryonic disc undergo?
NAME?
What is meant by cephalocaudal folding?
Folding at cranial (head) end and tail
What is cephalocaudal folding driven by?
Size of the neural tube
What is meant by lateral folding?
Folding of sides
What is lateral folding driven by?
Size of developing somite
What does folding create?
A pocket, with an opening into the umbilical cord
What is the result of of cephalocaudal folding?
A layer of mesoderm between ectoderm and endoderm, with future pericardial cavity next to it
What causes an expansion of the ectoderm?
The size of the neural tube
What does the expansion of the ectoderm do?
Pushes the head and tail under the layer of ectoderm, endoderm and mesoderm, leading to little pockets of yolk sac being pinched up into the embryonic body
What does the expansion of the amniotic sac downwards mean?
The whole embryo is wrapped in the amniotic sac, making sure whatever is facing outwards is ecroderm
What is the result of cephalocaudal folding on the heart?
It is now in the right place
What happens as the amniotic sac moves down?
- Get two ‘leaves’ at the lateral edges
- Somites forming in paraxial mesoderm
What are the two ‘leaves’ at the lateral edges?
Somatic and splanchnic
What happens to the lateral edges of the disk?
They are forced down by growth of somites
What opens up between somatic and splanchnic layers?
Increasing space
What happens to the somatic and splanchnic ‘leaves’?
They develop downward, eventually meeting
What happens when the leaves meet?
They pinch off a bit of yolk sac
What is the result of the pinching off of yolk sac?
Results in cavity inside the embryo from the yolk sac
What does the cavity from the yolk sac allow for?
Development of systems
What does folding achieve?
- Draws together margins of disk
- Creates ventral body wall
- Pulling amniotic membrane around disc, so embryo becomes suspended in it
- Pulling connective stalk ventrally
- Creates the primordium of the gut
- Puts heart and primordium of diaphragm in right place
- Creates new cavity within embryo
Is the creation the ventral body wall an active or automatic process?
Active
What is the primordium of the gut created from?
The yolk sac
