MCDB 138 Final Exam with complete
solutions graded A+ passed
What are the derivatives of the ectoderm? - correct answer ✔✔The ectoderm
derivatives include the neural tube and central nervous system (dorsal) and the
epidermis (ventral).
What are the derivatives of the endoderm? - correct answer ✔✔The endoderm
derivatives include the gut, liver, and lungs.
What are the derivatives of the mesoderm? - correct answer ✔✔The mesoderm
derivatives include the notochord and skeletal muscle (dorsal) and the smooth
muscle and blood (ventral).
What is the role of the Spemann Organizer (node) in establishing the A/P and D/V
axes of the embryo? - correct answer ✔✔The Organizer is a region of the embryo
that will complete D/V patterning of all three germ layers, organize the A/P axis,
and initiate movements of gastrulation. The most dorsal mesoderm becomes the
Organizer.
What is the Organizer? - correct answer ✔✔The Organizer is a signaling center
that produces antagonists to BMPs, which are secreted factors that are uniformly
expressed in the embryo and act as ventralizing signals. Their activity generates a
gradient of BMP activity along the DV axis that specifies D/V identity in all three
germ layers.
How do cells become determined to become neural in Xenopus? - correct answer
✔✔Fate mapping indicated that neural tissue is derived from the dorsal ectoderm
,- this was done by labelling cells in the presumptive neural ectoderm (dorsal side)
and the presumptive epidermis of the early gastrula. These cells gave rise to
neural tissue and epidermis, as expected. This experiment does not tell us when
the cells are determined.
Are the ectoderm cells already determined to become epidermis vs. neural in the
early Xenopus gastrula, before there has been extensive involution? - correct
answer ✔✔This was answered by transplanting presumptive epidermis and neural
ectoderm to regions fated to give rise to the opposite cell type. The transplanted
cells differentiated according to their surroundings. The ectoderm is thus not yet
determined to be either epidermis or neural in the early gastrula, where there are
not many gastrulation movements or involution occurring. This implies that neural
fate is induced by surrounding cells (since ventral ectoderm can become neural
tissue).
Is any part of the early amphibian gastrula determined? - correct answer ✔✔This
was answered by transplanting the dorsal lip of the blastopore (the Organizer) to a
region that gives rise to presumptive epidermis. As a result, the transplanted cells
induced a new axis, with they themselves developing mainly into derivatives of
dorsal mesoderm (e.g. notochord). Also, cells from the host embryo were
recruited into the new embryonic axis, including neural tissue. Thus, the dorsal lip
can induce neural fate in the overlying ectoderm.
What is the dorsal lip of the blastopore? - correct answer ✔✔The dorsal lip of the
blastopore is the top surface where involution is starting to happen. It is the same
as the Organizer and is therefore a signaling center.
What experiments demonstrated the role of the Nieuwkoop center in
amphibians? - correct answer ✔✔
,What experiments demonstrated the role of the posterior marginal zone in
birds/mammals? - correct answer ✔✔Transplantation eventually leads to a new
axis, and as was the case in the NC, the mesoderm and ectoderm of the induced
axis are derived from the host.
What is the posterior marginal zone in birds/mammals? - correct answer ✔✔The
posterior marginal zone in birds and mammals is analogous to the Nieuwkoop
Center in amphibians, which is also formed in a region with high B-catenin activity.
The NC produces high levels of nodal-related (Xnr), and the PMZ produces high
levels of nodal. Loss of Xnr (amphibians) or nodal (birds/mammals) leads to no
mesoderm.
What is the relationship between Nieuwkoop Center and Spemann Organizer? -
correct answer ✔✔The most dorsal mesoderm becomes the Organizer, which is
the blastopore lip or the site of involution. This marks the beginning of
gastrulation. The dorsal mesoderm is derived from the dorsal vegetal cells, which
have high B-catenin and Veg-T; this is the same as the NC.
What are the signals that are responsible for the properties of the Organizer? -
correct answer ✔✔The neural "inducer" is a set of BMP antagonists. Genes
encoding these antagonists were identified by their ability to induce a new axis
and their specific expression in the Organizer (as determined by "find it"
experiments).
How do we know that secreted BMP antagonists are responsible for the
properties of the Organizer? - correct answer ✔✔We can do a "move it"
experiment by injecting ventral blastomeres of an early embryo with chordin
mRNA. This causes induction of a new twinned axis and conversion of ventral
tissue to dorsal fate.
, How do we know that secreted BMP antagonists are responsible for the ability of
the Organizer to dorsalize mesoderm and ectoderm? - correct answer
✔✔Injecting morpholinos against all 3 BMP antagonists prevents neural induction
and the formation of the dorsal mesoderm ("lose it" experiment). Thus, a second
patterning mechanism is needed that involves a gradient of BMP activity for
sustained and complete patterning of the mesoderm.
What would happen if BMP signaling is completely blocked? - correct answer
✔✔This was answered by injecting morpholinos against BMPs, with the
expectation that preventing BMP signaling would lead to overproduction of dorsal
cell types in the ectoderm and mesoderm and loss of ventral cell types.
The result was an excess of dorsal tissue (neural) and a loss of ventral tissue
(epidermis).
How does Wnt activity pattern the A/P axis in Xenopus? - correct answer ✔✔low
Wnt = anterior identity, high Wnt = posterior identity; gradient presumably due to
properties of the Organizer changing in some way and thus shaping Wnt
expression (early Organizer produces the lowest levels of Wnt/highest levels of
Wnt antagonists)
What experiments demonstrate the role of the amphibian Organizer in
establishing the A/P axis? - correct answer ✔✔The establishment of the A/P axis
in Xenopus was understood by transplanting the dorsal lip (Organizer) to a host
embryo. The induced embryo had a recognizable A/P axis, in addition to its D/V
axis. Thus, the Organizer must establish the A/P axis.
What experiments demonstrated that the properties of the amphibian Organizer
change over time? - correct answer ✔✔transplant Organizers from embryos at
different stages of development: early stage Organizers could induce a complete
axis, but as the transplanted Organizer got older (and lots of involution had
happened), the induced axis was more posteriorly restricted; inductive properties
solutions graded A+ passed
What are the derivatives of the ectoderm? - correct answer ✔✔The ectoderm
derivatives include the neural tube and central nervous system (dorsal) and the
epidermis (ventral).
What are the derivatives of the endoderm? - correct answer ✔✔The endoderm
derivatives include the gut, liver, and lungs.
What are the derivatives of the mesoderm? - correct answer ✔✔The mesoderm
derivatives include the notochord and skeletal muscle (dorsal) and the smooth
muscle and blood (ventral).
What is the role of the Spemann Organizer (node) in establishing the A/P and D/V
axes of the embryo? - correct answer ✔✔The Organizer is a region of the embryo
that will complete D/V patterning of all three germ layers, organize the A/P axis,
and initiate movements of gastrulation. The most dorsal mesoderm becomes the
Organizer.
What is the Organizer? - correct answer ✔✔The Organizer is a signaling center
that produces antagonists to BMPs, which are secreted factors that are uniformly
expressed in the embryo and act as ventralizing signals. Their activity generates a
gradient of BMP activity along the DV axis that specifies D/V identity in all three
germ layers.
How do cells become determined to become neural in Xenopus? - correct answer
✔✔Fate mapping indicated that neural tissue is derived from the dorsal ectoderm
,- this was done by labelling cells in the presumptive neural ectoderm (dorsal side)
and the presumptive epidermis of the early gastrula. These cells gave rise to
neural tissue and epidermis, as expected. This experiment does not tell us when
the cells are determined.
Are the ectoderm cells already determined to become epidermis vs. neural in the
early Xenopus gastrula, before there has been extensive involution? - correct
answer ✔✔This was answered by transplanting presumptive epidermis and neural
ectoderm to regions fated to give rise to the opposite cell type. The transplanted
cells differentiated according to their surroundings. The ectoderm is thus not yet
determined to be either epidermis or neural in the early gastrula, where there are
not many gastrulation movements or involution occurring. This implies that neural
fate is induced by surrounding cells (since ventral ectoderm can become neural
tissue).
Is any part of the early amphibian gastrula determined? - correct answer ✔✔This
was answered by transplanting the dorsal lip of the blastopore (the Organizer) to a
region that gives rise to presumptive epidermis. As a result, the transplanted cells
induced a new axis, with they themselves developing mainly into derivatives of
dorsal mesoderm (e.g. notochord). Also, cells from the host embryo were
recruited into the new embryonic axis, including neural tissue. Thus, the dorsal lip
can induce neural fate in the overlying ectoderm.
What is the dorsal lip of the blastopore? - correct answer ✔✔The dorsal lip of the
blastopore is the top surface where involution is starting to happen. It is the same
as the Organizer and is therefore a signaling center.
What experiments demonstrated the role of the Nieuwkoop center in
amphibians? - correct answer ✔✔
,What experiments demonstrated the role of the posterior marginal zone in
birds/mammals? - correct answer ✔✔Transplantation eventually leads to a new
axis, and as was the case in the NC, the mesoderm and ectoderm of the induced
axis are derived from the host.
What is the posterior marginal zone in birds/mammals? - correct answer ✔✔The
posterior marginal zone in birds and mammals is analogous to the Nieuwkoop
Center in amphibians, which is also formed in a region with high B-catenin activity.
The NC produces high levels of nodal-related (Xnr), and the PMZ produces high
levels of nodal. Loss of Xnr (amphibians) or nodal (birds/mammals) leads to no
mesoderm.
What is the relationship between Nieuwkoop Center and Spemann Organizer? -
correct answer ✔✔The most dorsal mesoderm becomes the Organizer, which is
the blastopore lip or the site of involution. This marks the beginning of
gastrulation. The dorsal mesoderm is derived from the dorsal vegetal cells, which
have high B-catenin and Veg-T; this is the same as the NC.
What are the signals that are responsible for the properties of the Organizer? -
correct answer ✔✔The neural "inducer" is a set of BMP antagonists. Genes
encoding these antagonists were identified by their ability to induce a new axis
and their specific expression in the Organizer (as determined by "find it"
experiments).
How do we know that secreted BMP antagonists are responsible for the
properties of the Organizer? - correct answer ✔✔We can do a "move it"
experiment by injecting ventral blastomeres of an early embryo with chordin
mRNA. This causes induction of a new twinned axis and conversion of ventral
tissue to dorsal fate.
, How do we know that secreted BMP antagonists are responsible for the ability of
the Organizer to dorsalize mesoderm and ectoderm? - correct answer
✔✔Injecting morpholinos against all 3 BMP antagonists prevents neural induction
and the formation of the dorsal mesoderm ("lose it" experiment). Thus, a second
patterning mechanism is needed that involves a gradient of BMP activity for
sustained and complete patterning of the mesoderm.
What would happen if BMP signaling is completely blocked? - correct answer
✔✔This was answered by injecting morpholinos against BMPs, with the
expectation that preventing BMP signaling would lead to overproduction of dorsal
cell types in the ectoderm and mesoderm and loss of ventral cell types.
The result was an excess of dorsal tissue (neural) and a loss of ventral tissue
(epidermis).
How does Wnt activity pattern the A/P axis in Xenopus? - correct answer ✔✔low
Wnt = anterior identity, high Wnt = posterior identity; gradient presumably due to
properties of the Organizer changing in some way and thus shaping Wnt
expression (early Organizer produces the lowest levels of Wnt/highest levels of
Wnt antagonists)
What experiments demonstrate the role of the amphibian Organizer in
establishing the A/P axis? - correct answer ✔✔The establishment of the A/P axis
in Xenopus was understood by transplanting the dorsal lip (Organizer) to a host
embryo. The induced embryo had a recognizable A/P axis, in addition to its D/V
axis. Thus, the Organizer must establish the A/P axis.
What experiments demonstrated that the properties of the amphibian Organizer
change over time? - correct answer ✔✔transplant Organizers from embryos at
different stages of development: early stage Organizers could induce a complete
axis, but as the transplanted Organizer got older (and lots of involution had
happened), the induced axis was more posteriorly restricted; inductive properties
