What data indicate that all three germ layers are specified in the blastula? What are the differences between dorsal and ventral mesodermal derivatives and what cellular interactions are required for their specification? Solution Three germ layers of amphibians are specified in the blastula is determined by isolating these tissues in vitro, and they are able to form into specific germ layers. The animal pole cap cells make ectoderm, marginal region cells make mesoderm, and vegetal cells make endoderm. Theyare specified but not determined. It is also interesting to note that if animal cap cells are place co-cultured with vegetal cells, the animal cap cells with become mesoderm. This indicates that vegetal cells induce other cells to form mesoderm. Experiment paired animal cap cells in 4 different sections of vegetal blastomeres to see if they induce different dorsal-ventral mesodermal fates. Result showed that different sections of vegetal blastomere have specific inductive capacities, which is crucial for dorsal-ventral mesoderm determination. Difference between dorsal and ventral mesoderm derivatives: dorsal mesoderm is the notochord and somite. In all bilaterian animals, the mesoderm is one of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and endoderm (inside layer), with the mesoderm as the middle layer between them. The mesoderm forms mesenchyme, mesothelium, non-epithelial blood cells and coelomocytes. Some of the mesoderm derivatives include the muscle (smooth, cardiac and skeletal), the muscles of the tongue (occipital somites), the pharyngeal arches muscle (muscles of mastication, muscles of facial expressions), connective tissue, dermis and subcutaneous layer of the skin At mid-blastula two signaling centers are present on the dorsal side: The prospective neuroectoderm expresses bone morphogenetic protein (BMP) antagonists, and the future dorsal endoderm secretes Nodal-related mesoderm-inducing factors. When dorsal mesoderm is formed at gastrula, a cocktail of growth factor antagonists is secreted by the Spemann organizer and further patterns the embryo. A ventral gastrula signaling center opposes the actions of the dorsal organizer, and another set of secreted antagonists is produced ventrally under the control of BMP4. The early dorsal -Catenin signal inhibits BMP expression at the transcriptional level and promotes expression of secreted BMP antagonists in the prospective central nervous system (CNS). In the absence of mesoderm, expression of Chordin and Noggin in ectoderm is required for anterior CNS formation. FGF (fibroblast growth factor) and IGF (insulin-like growth factor) signals are also potent neural inducers. Neural induction by anti-BMPs such as Chordin requires mitogen-activated protein kinase (MAPK) activation mediated by FGF and IGF. These multiple signals can be integrated at the level of Smad1. Phosphorylation by BMP receptor stimulates Smad1 transcrip.

1. What data indicate that all three germ layers are specified in the blastula? What are the
differences between dorsal and ventral mesodermal derivatives and what cellular interactions are
required for their specification?
Solution
Three germ layers of amphibians are specified in the blastula is determined by isolating these
tissues in vitro, and they are able to form into specific germ layers. The animal pole cap cells
make ectoderm, marginal region cells make mesoderm, and vegetal cells make endoderm.
Theyare specified but not determined. It is also interesting to note that if animal cap cells are
place co-cultured with vegetal cells, the animal cap cells with become mesoderm. This indicates
that vegetal cells induce other cells to form mesoderm. Experiment paired animal cap cells in 4
different sections of vegetal blastomeres to see if they induce different dorsal-ventral
mesodermal fates. Result showed that different sections of vegetal blastomere have specific
inductive capacities, which is crucial for dorsal-ventral mesoderm determination. Difference
between dorsal and ventral mesoderm derivatives: dorsal mesoderm is the notochord and somite.
In all bilaterian animals, the mesoderm is one of the three primary germ layers in the very early
embryo. The other two layers are the ectoderm (outside layer) and endoderm (inside layer), with
the mesoderm as the middle layer between them.
The mesoderm forms mesenchyme, mesothelium, non-epithelial blood cells and coelomocytes.
Some of the mesoderm derivatives include the muscle (smooth, cardiac and skeletal), the
muscles of the tongue (occipital somites), the pharyngeal arches muscle (muscles of mastication,
muscles of facial expressions), connective tissue, dermis and subcutaneous layer of the skin
At mid-blastula two signaling centers are present on the dorsal side: The prospective
neuroectoderm expresses bone morphogenetic protein (BMP) antagonists, and the future dorsal
endoderm secretes Nodal-related mesoderm-inducing factors. When dorsal mesoderm is formed
at gastrula, a cocktail of growth factor antagonists is secreted by the Spemann organizer and
further patterns the embryo. A ventral gastrula signaling center opposes the actions of the dorsal
organizer, and another set of secreted antagonists is produced ventrally under the control of
BMP4. The early dorsal -Catenin signal inhibits BMP expression at the transcriptional level and
promotes expression of secreted BMP antagonists in the prospective central nervous system
(CNS). In the absence of mesoderm, expression of Chordin and Noggin in ectoderm is required
for anterior CNS formation. FGF (fibroblast growth factor) and IGF (insulin-like growth factor)
signals are also potent neural inducers. Neural induction by anti-BMPs such as Chordin requires
mitogen-activated protein kinase (MAPK) activation mediated by FGF and IGF. These multiple

2. signals can be integrated at the level of Smad1. Phosphorylation by BMP receptor stimulates
Smad1 transcriptional activity, whereas phosphorylation by MAPK has the opposite effect.
Neural tissue is formed only at very low levels of activity of BMP-transducing Smads, which
require the combination of both low BMP levels and high MAPK signals. Many of the molecular
players that regulate D-V patterning via regulation of BMP signaling.
During the early development of the B lineage, the anterior daughter of B, B.a, generates eight
cells. These cells migrate to form four pairs of cells that flank the developing cloaca (ventral,
dorsal, and two identical lateral pairs). For each pair, the more anterior cell produces a distinct
lineage ('anterior fate') from the posterior cell ('posterior fate'). For the ventral and dorsal
pairs, either cell can migrate to the anterior position and produce the anterior lineage, and the
other cell migrates posterior and produces the posterior lineage . The migration is variable,
although the resultant fate pattern is invariant. In the two lateral pairs, both the migration and fate
pattern are invariant. Using a laser microbeam to selectively ablate neighboring cells we have
found that the cells of the lateral pair also respond to positional cues. For all four pairs other
male-specific blast cells provide extracellular cues. In general, F and U promote anterior fates, Y
promotes some posterior fates, and the B.a progeny promote posterior fates. Several of these
cues are redundant. By ablating combinations of cells we have deduced how these signals may
act in concert to specify the fates of the B.a progeny. We propose that fate specification in these
pairs depends on three general classes of extracellular cues: positional cues, modulators of
positional cues, and lateral signals. The B lineage thus provides an opportunity to study with
single cell resolution the integration of multiple intercellular signals.