Lecture 11- Somite formation
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What is the site of gastrulation in chicks?
Henson’s Node
In chicks, what is associated with the formation of the primitive streak?
A/P axis establishment
What happens to cells over time during development?
They are progressively restricted and loose pluripotency
What is formed as a result of gastrulation?
The mesoderm
Describe the movement of cells during gastrulation
• The epiblast cells migrate and converge towards the centre, roll over the edge of the primitive streak, involute and emerge on the inside as either mesoderm or prospective endoderm • During this process, the cells undergo an epithelial mesenchymal transitions (EMT) • Once they reside inside they will migrate laterally within the embryo
What was identified from injecting fluorescent due along the primitive streak (fate mapping)?
Identified that cells were specified to various regions of the mesoderm depending on where they engraved within the primitive streak along the A/P axis
What type of tissue do cells become within Henson’s node?
Axial mesoderm which forms the prechordal mesoderm and notochord
What type of tissues do cells become slightly posterior to Henson’s node?
Paraxial mesoderm which give rise to somites
What type of tissues do cells become very posterior to Henson’s node?
Intermediate mesoderm which form the kidneys and gonads
What type of tissues do cells become at the most posterior end of the primitive streak?
Lateral mesoderm
What provides the earliest evidence for segmentation in vertebrates?
Somites
What are somites?
Transient structures that are laid down during embryonic development along the developing spinal cord
What gene expression illustrates how well segmentation is evolutionarily conserved?
The repeating expression pattern of engrailed 1
What dictates the number of vertebrae?
Somite number
Explain how somites form in pair from the paraxial mesoderm
• Paraxial mesoderm forms in a continuous manner until the proper somite number is reached • Primitive streak is the site of mesoderm formation and is to be present until somites no longer form • As long as the primitive streak is present, cells will exit the primitive streak by forming and adding new mesodermal cells through the posterior part of the embryo • This process leads to an elongation that proceeds from the anterior to the posterior end of the embryo • In the anterior part of the mesoderm, segmentation occurs from the most anterior end to the more posterior end so that the anterior part of the mesoderm is the location for the most newly formed somites
How does the number and timing of somites vary within a species>
The number of somites is fixed for a given species and the timing of somite formation remains constant within the species
How do somites form from the pre-somatic mesoderm?
Somites bud off a band of unsegmented mesoderm
What must cells within the pre-somatic mesoderm respond to?
• Positional information • Mechanisms that coordinates left and right somites (for every somite on the right, there is a somite forming on its left) • Mechanisms that generates anterior boundary • Mechanisms that generate posterior boundary • Formation of cleft
What model explains how the periodicity of somite formation is established?
The clock and wavefront model
Briefly explain the clock and wavefront model
Predicts that a clock ticks in the posterior pre-somatic mesoderm and drives a molecular oscillator that dictates the periodicity and size of somites Where cells hit the travelling wavefront, an abrupt change of property occurs leading to the decision to form somites
What is c-hairy1 in fish and mice?
Fish = Her Mice = Hes
How was c-hairy1 identified?
Insitu hybridisation
What conclusions were made regarding the expression pattern of c-hairy1?
Expression pattern is not continuous but instead, oscillates over time
What are 3 characteristics of hairy/hes/her?
• Are target genes for notch signalling • Are bLHL transcriptional repressors • Are very unstable