Study Questions – Lecture 15
1. Draw a cross section of a typical amniote embryo (following gastrulation but before
the onset of organogenesis) showing the neural tube, and the location of the four
types of mesoderm.
2. What are the fates of the four mesoderm types?
3. What is a somite? What mesoderm type are somites derived from? Into what
regions is the somite eventually compartmentalized and what structure/tissues are
derived from these compartments?
A somite is a segment/ball of mesoderm derived from paraxial mesoderm adjacent to the
notochord. It will initially differentiate into the sclerotome and the dermamyotome, the
latter becomes the dermatome and myotome. Somites will form the axial skeleton, all
skeletal muscles, dorsal dermis, tendons, joints, and dorsal aortic cells.
4. What antagonist signalling protein is involved in establishing the paraxial
mesoderm? Paraxial mesoderm is set up Noggin, a BMP antagonist which blocks BMP signalling.
5. The presomitic mesoderm shows an oscillating pattern of gene expression with
respect to the c-hairy1 (as well as some other genes) in the chick embryo. What
experiments demonstrate that the oscillations represent an intrinsic property of the
presomitic paraxial mesoderm and do not arise from a propagating signal?
Experiments were shown that waves of c-hairy1 expression corresponded to the timing of
somite formation. However, it was confirmed through cell labelling experiments that
cells did not move to generate these pattern differences and surgical experiments showed
that there was no propagation of any activating signals. Thus, it was concluded that this
rhythmic expression had to be a property of paraxial mesoderm.
6. Which fundamental signalling pathway is involved in establishing the somite
boundaries? What type of signalling is involved (paracrine/juxtacrine/autocrine)?
The Notch signalling pathway is involved in establishing somite boundaries by separating
them into anterior and posterior regions. This is considered juxtacrine signalling.
7. How were chimeric embryos useful in establishing the mechanism by which somite
boundaries are established? What type of chimerics were used? Briefly describe the
experimental system and the reported observations?
Chimeric embryos were useful in establishing the mechanism by which somite
boundaries are established because it proved that the same method existed in other
species. In particular, quail and chicken chimeras were used.
The presumptive boundary cells of quail were transplanted into unsegmented paraxial
mesoderm of a chicken and a boundary was induced. However, when non-boundary cells
were inserted into the same type of area, no boundary emerged.
Notch signalling was determined to be responsible when it was expressed ectopically,
and it induced a boundary to form.
8. The segmentation of the presomitic mesoderm also involves two other signalling
molecules. What are these molecules and where are they produced? How are these
molecules established as two opposing gradients?
The other two molecules responsible for presomitic mesoderm segmentation are FGF8
and retinoic acid. FGF8 is produced by the cells in and around the node, forming a
gradient that is high at the node but decreasing towards the anterior. Retinoic acid is an
antagonist of FGF signalling and is found in a gradient opposite to that of FGF since it is