Download Gastrulation 2 Somite Formation

What’s going on….at cellular level.
G10
W8
Changes in individual cell shape
(apical constriction) added
together produce tissue
deformation (dimpling,
invagination)….
But there’s more…..
W8
ECM (fibronectin)
migration substrate
Cell migration leads cells to crawl over the underside of the
blastocoel roof, on an adhesive carpet of fibronectin matrix.
But there’s more……
W3,8
brachyury
The new ability of mesoderm cells to migrate, with a net direction
toward the midline, plus a general loosening (but not loss) of cellcell adhesion, allows cells of lateral origin to intercalate between
more medial cells. As a mass, the tissue becomes narrower but
lengthens: this is called convergent extension.
But there’s more…….
Gastrulation stage
mitotic marker
G10
W8
The cells of the animal hemisphere—fated to be ectoderm–
increase in number by mitosis and decrease in number of cell
layers (by radial intercalation). They may also become
individually more flattened. The net result of this is this layer
increases in area. This also drives the ectoderm to “take over” all
the superficial layer, and helps drive internalisation of other cell
types (mesoderm, endoderm). This type of morphogenetic
movememt is called epiboly.
The total outcome of invagination+cell
migration+convergent extension+epiboly
internalises the cells defined as endoderm and
mesoderm, with the latter surrounding the
former. These two are completely surrounded
by the ectodermal cell layer. The relative
importance of these processes seems to vary
between species, but the overall outcome (3
layers) is in principal similar. This then places
tissues together in new ways so that new
inductive interactions are possible.
EARLY AMNIOTE VERTEBRATE DEVELOPMENT
The chick embryo starts as a thin disk on top of an immense yolk sphere
This thin disk of “normal” oocyte
cytoplasm houses the oocyte pronucleus,
and after fertilization, the zygotic nuclei.
W2
Initially, complete cell division lags behind nuclear proliferation, but soon catches up.
The single layer (epiblast) seeds cells to form another layer beneath (hypoblast),
separated by a fluid-filled space….just like a squashed version of a frog blastula.
G11
W2
Gastrulation:
Some epiblast cells now
invaginate. Process is
very similar to
gastrulation in frogs, but
the role of ‘individual”
cell migration is clearer.
End result: specific
epiblast cells are
internalised to produce a
middle layer
(mesoderm).
Gastrulation in birds and
mammals is an example of
the Epithelial-Mesenchymal
Transition (EMT).
EMT is used repeatedly in
development where cells
start to move around. It also
occurs when carcinoma
cells start to invade.
G11
The site of invagination– the primitive streak –
persists for some hours but the cells in it change
continuously.
The primitive streak gets longer then shorter.
The end –Hensen’s Node--becomes a major
signalling centre equivalent to the Spemann
Organizer/ dorsal lip of the blastopore.
G11
At the end of gastrulation, a large
number of cells have been lost from
the epiblast layer: the depleted top
layer is then called the ectoderm.
The invaginated cells form the
mesoderm layer, but some go
further and intercalate into the
hypoblast (now called endoderm)
Patterning genes expressed in frog blastulas (hollow
spherical) are also found in positionally relevant
places in the blastoderms of birds (flat blastoderm)
and mice (flat but cup-shaped blastoderm).
And the relevant regions (eg. chick Hensen’s Node)
can signal across species barriers (eg. Like frog DLB),
indicating that function is conserved, as well as
gene/protein sequence and spatiotemporal expression
pattern.
W4
Segmentation of the mesoderm layer
where and when do segments form?
what is the mechanism of segmentation?
what do the segments turn into?
The segments (somites) timing is programmed well before overt segmentation
about the time when the pre-somitic cells are “spun out” of Hensen’s Node
W4
One segment carved off every 90 min…..
Pattern of Hairy1 mRNA expression
W4
Scheme in Pre-Somitic Mesoderm:
G6
•FGF-8 at Hensen’s node represses Lunatic Fringe
•As cells displaced further anterior to HN, Lun Fringe activated
•Lunatic fringe potentiates Notch signalling
•Notch signals induce a repressor of Notch gene
•Repressor has a short half-life.
•Notch appears in an oscillating pattern.
•Notch signals to form new boundary, presumably by…...
•Notch regulates many genes inc. Hairy
Candidate genes for transcriptional control by hairy1 is Ephrin and Eph
receptors.
The Eph-Ephrin system are cell surface molecules signalling cellular
repulsion. Given a segmental striping of expression of these genes, his
would be an ideal mechanism for separating tissue blocks.
The segments (somites) type is programmed well before overt segmentation
about the time when the pre-somitic cells are “spun out” of Hensen’s Node
W4
The somites are initially
rosettes of epithelial cells.
They appear relatively uniform
in all axes.
But soon regional differences
appear.
The role of adjacent tissue in
inducing this regionalisation of
the somites can be tested by
grafting.
W4
The somite (Pax3 +ve) is target of
numerous known signals from all its
adjacent tissues, that sub-divide it into
regions expressing distinct gene
signatures. These regions are specified
for distinct fates.
W4
G14
Signalling pathways in these [and other]
developmental events seem to be
unnecessarily complex [BMP], apparently
sloppy [Wnt], or indirect [Hedgehog]!
G6
L-R Patterning--Not included 2007