Mammals:
Mus musculus
Homo sapiens
Close Genomic
Relationships
2.5Gb, 20 chromosomes
~20,500 genes
Eukaryotic Branch of the Tree of Life
3Gb, 23 chromosomes
~20,500 genes
The Mouse Trap
(Trans-species
Enhancer Trapping)
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Zona Pellucida
Fertilization
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Pronuclei
Vitelline
Layer
ART Cinema
Cleavage
PGD
Morula
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Embryo
Sheds the
ZP
Compaction
Implantation
of the
Blastocyst
(Blastula)
Epiblast develops
into embryonic body
Amniotic cavity develops here
Gastrulation
First Endoderm
Then Mesoderm
Leaving Ectoderm
"It is not birth, marriage, or
death, but gastrulation,
which is truly the most
important time in your life.”
- Lewis Wolpert (1986)
cilia, microfilaments and nuclei
Hensen’s Node
The Layered Embryo
Regulatory Conversations
Fibroblast Growth Factors
(FGF) pathway
Mesoderm differentiation,
axonal projections, blood vessel
formation
Hedgehog pathway
Patterning of the neural tube,
the L-R axis and the gut
Notch pathway
Somitogenesis and other
patterning events
Wnt pathway
Induction of somitic muscle,
limb polarity, the urogenetal
system
Transforming Growth Factor
(TGF)- Beta pathway
Regulate bone formation (via
Bone Morphogenetic Proteins
or BMPs) and cell division
Forming the
Vertebrate:
Neurulation and
Somitogenesis
Hoxb1
Hoxb4
Hoxb9
Hox Expression
Patterns the
A-P axis
Morphogenesis
at the Movies
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Mouse
Human
Flip-book
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Pharyngula
Stage
Human
Ontogeny
recapitulates
Phylogeny
Human FGF
Signaling
Mutations
Limb
Development
Flip-book
Apical ectodermal ridge (FGF)
Progress zone (RA)
Zone of polarizing activity (SSH)
RA
Hox genes
Segment identity
Shaping
Limbs
Shaping the
Skeleton
Stem
Cells
Embryonic
Stem Cells
Somatic Cell
Nuclear
Transfer
(“Cloning”)
Hello Dolly!
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Reprogramming
Development
A Few Questions for Thought:
• In what ways does comparative genomics offer us a
window onto understanding life?
• Discuss the roles of stem cells in development and
maintenance of multicellular organisms (plants as
well as animals)
• Compare and contrast mammalian gastrulation with
the development of the other model metazoa that we
have considered.
• Describe, in a “big picture” way, the process of
neurulation in vertebrates.
• Recapitulate the complex interplay of mosaic and
regulatory mechanisms in metazoan development.
• How has evolution morphed simpler embryos into
more complex body plans by enriching the content of
regulomes and signal transduction pathways?
Coda
I thank all of you for the effort you continue to
put into studying biological systems, into
seeing the inner beauty of the cells and
organisms with whom we share this planet. It
has been my special privilege to be your
guide. May what you have learned in this text
push you to question, to pursue new
knowledge and understanding, and to find the
answers to life’s most interesting questions.
And always remember to take time to smell
the flowers!
- Karl Johnson, Professor of Biology, Haverford College