Lecture 6 (3a) Gastrulation Flashcards
Cleavage
rapid mitotic divisions
- in Drosophila, only nucleus
• After cleavage = blastula
- blastoderm in Drosophila
Gastrulation
transforming a hollow ball into a 3-layered structure
• INVAGINATION TO FORM LAYERS
Germ layer
main division into cell types
Evtoderm makes
skin and nervous system
Mesoderm and endoderm make
inner structures
Organogenesis
different tissues and organs from
Metamorphosis
larval stage to maturity
The life of a new frog begins with the
fusion of the genetic material of the sperm and egg
fertilization
After fertilization
female frog lays eggs
Animal cap
brown part
• ectoderm in blastula
• remove = form epidermis
= some intrinsic factors that specify genes
• take from different regions - they sort themselves, reconstruct themselves
The first process after fertilization
CLEAVAGE
eg 2-cell embryo to 8-cell embryo
Somite
lateral mesoderm to spinal cord forms muscles and vertebrae
Cleavage results in formation of the
blastula
The blastula is a
hollow ball
Cavity in the blastula is called a
blastocell
Gastrulation begins with the formation of a
slit in the future dorsal side of the embryo
After gastrulation
organs and tissues are formed
• eg neural groove (to become neural tube)
The cells that generate the 3 germ layers are
present in the late blastula
Mid-blastula transition
- shortly before gastrulation
* transcription is activated in the embryo
Both endoderm and ectoderm are specified by
maternal cell fate determinants
• animal half expresses maternal factor ENDODERMIN
+ zygotic factor FOX1E
• vegetal half expresses maternal factor VEGT
+zygotic factor NODAL (mesodermal induction)
Explants from animal cap form
ectodermal derivatives
Explants from vegetal cells form
endodermal derivatives
Explants from animal cap and vegetal cells form ectoderm and endoderm derivatives
AUTONOMOUSLY
Germ layer cells reconstruct their spatial relationship
after dissociation
In Xenopus, endoderm development is initiated by the maternal transcription factors
VegT and Veg1
VegT and Veg1 found in
endoderm
One maternal factor activates a cascade of
embryonic genes
VEGT ACTIVATES THE EXPRESSION OF MANY ZYGOTIC TRANSCRIPTION FACTORS
These transcription factors factors regulate each other and there expression is maintained by
Nodal
Nodal
- activated by VegT
* is a morphogen, especially in future dorsal and lower concentration to ventral
Nodal is a morphogen and accumulates in the
endoderm
Mesoderm
induced by Nodal and at same time told what type of mesoderm it is to form
Morphogen
proteins in concentration gradient
• cells respond to different concentrations
• instruct cells to adopt different cell fates at different concentrations
The nodal gradient determines
the type of mesoderm formed
Xnr is highest in
the dorsasl region
The various mesodermal fates are specified by
the Xnr gradient
Nodal is activated by
β-catenin + VegT + Vg1 --> gradient --> different mesodermal cells • organizer of dorsal structure -- gradient ventral to dorsal
Additional factors required for mesdoerm induction:
VegT also activates the secreted factors activin and Derriere which in turn induce the formation of mesoderm by activating brachyury
Brachyury is expressed in
all mesodermal cells
In the late blastula, the cell fates of
the cell within the individual germ layers are determined
• additional signalling pathways such as Wnt and FGF are required for regional specification of the germ layers
Cells are made different
with different morphogens in different concnetration
Cell fates are determined in
the late blastula stage
The fate map varies in different vertebrae groups because
the cleavage pattern is different
Internalization of the endoderm and mesoderm occurs at
a dorsal area - the BLASTOPORE
Internalization is shown as a
slit (meso and endo)
Mesodermal and endodermal cells migrate towards the
animal pole
The ectoderm expands towards the
vegetal pole
Gastrulation varies in different groups but
the result is the same
• ectoderm
• mesoderm
• endoderm
Gastrulation in the human embryo
endoderm and mesoderm are internalized through the primitive groove
Epiblast
human embryo
• disc-like
• internalized at groove (meso and endo)
Formation of the germ layers in Drosophila melanogaster
- endoderm makes the midgut at poles
* the ectoderm forms the foregut and hindgut
In Drosophila the mesoderm is
ventral
• the endoderm is split into an anterior and posterior region
Germ cells are
NOT from germ layers
• germ cells are set aside before
(eg pole cells in Drosophila)
The mesoderm is
internalized
In vertebrates, the mesoderm forms
the muscles among others
At the same time as the mesoderm is internalized, the endoderm
invaginatese anteriorly and posteriorly
The endoderm forms the
midgut
foregut and hindgut are formed by the ectoderm
The midgut is subdivided into
4 chambers
The 3 germ layers give rise to
different organs and tissues
• the germ cells don’t arise from the 3 germ layers
The notochord
induction of the nerve cord
• transient, forms mesoderm
Derivatives of the somites
- cartilage of vertebrae and ribs
- muscles of the rib cage, limbs, abdominla wall back, tongue
- tendons that connect the muscles to the bones
- dermis of the dorsal skin
- vascular cells of aorta and intervertebral blood vessels
Functions of the mesoderm
- induces the mesodermal germ layer
- induces the formation of several mesodermal organs (notochord, heart, blood vessels)
- constructs the linings of 2 tubes within the vertebrate body - the digestive tube and the respiratory tube
The endoderm induces the formation of
several mesodermal organs
(notochord, heart, blood vessels)
Anterior endoderm BMP signalling
–> cardiogenic mesoder (heart)
&
–> hemangiogenic mesoderm (blood, blood vessels)
How is the gut endoderm regionally specified?
The areas of the digestive system are specified by regional transcription factors even before the endoderm forms a tube
• mesoderm instrutus the endoderm to different things
• boundaries in digestive system
BMP signalling
tells cells what to do
The stabilization of the boundaries results from
interactions with the mesoderm
• BMPs activate wnt signalling in intestine
• wnt inhibitors repress wnt signalling in stomach
Wnt
wingless in drosophila
• for boundaries
(engrailed for posterior border)
How are different areas initally specified in the endoderm?
• Shh gradient
• different Hox genes are expressed in response to different levels of Shh
• the mesoderm and endoderm “talk” back & forth
- induction both ways
Cleavage of the egg shell results in the formation of the
blastula
The cells that generate the 3 germ layers are present in the
late blastula
The 3 germ layers are separated during
gastrulation
• endoderm
• mesoderm
• ectoderm
The endoderm is specified by the maternal transcription factor
VegT
The endoderm induces the
mesoderm and some of its derivatives
The endoderm generates the
digestive and respirative tubes
