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PROFESSOR JOHN TORDAY:
Vertebrate ontogeny,
the short history of evolution.
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Ontogeny,
or embryonic development,
is the short history
of the organism
in contrast to phylogeny,
which is the long
history of the organism.
Beginning with the fertilization
of the egg by the sperm
forming the zygote,
cell-cell interactions dictate
the patterning of the embryo
to generate form and function.
The cell-cell interactions
are mediated
by solute-secreted
growth factors
signaling to nearby cells
that have specific
cognate receptors
for those growth factors.
For example,
fibroblast growth factor
binds to the fibroblast
growth factor receptor.
The receptor then generates
a second messenger
that affects the growth
and differentiation
of the target cell.
The target cell then
generates a growth factor
that acts on other cells
within the region
and so on and so on,
until the entire organism
is formed.
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Here we see the fertilized
egg or zygote on the left,
progressively dividing
and differentiating
to form a two-layered
blastula
and then a three-layered
gastrula
composed of an ectoderm,
endoderm,
and mesoderm.
From this stage on,
the three germ layers
form the embryo
through the cell-cell
growth factor signal.
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On the left are depicted
the interactions
between the various components
of developing bone,
round chondrocytes,
columnar chondrocytes,
prehypertropic chondrocytes,
hypertrophic chondrocytes,
terminal hypertrophic
chondrocytes,
and trabecular bone.
These different bone cell types
interact through signaling,
mediated by PTHrP,
interacting with Indian hedgehog
fibroblast growth factor,
bone morphogenetic protein,
notch, and the wingless
int or Wnt pathway.
