Vertebrate Embryology

• Definition = migration and division of cells to set up the 3
primary germ layers.
• What positions do presumptive germ layers occupy in the
blastula, before gastrulation?
– Experimental answers to this question provide our basis for
understanding cell movements that occur during gastrulation.
• FATE MAPS = diagram of blastula/blastodisc showing the
“fate” of each part.
• Fate mapping technique developed by Vogt in 1920s.
Involves …
– Marking surface of blastula with vital dyes
– Dyes retained by cells for prolonged periods, but don’t interfere
with normal cellular processes
– Follow movements of marked cells during gastrulation to
ultimate locations in later embryos
Sample fate map of frog embryo
• Micro- and Mesolecithal eggs (Amphioxus and
Amphibians) …
• Basic cell movements during gastrulation are
spreading and stretching, accompanied by cell
• 7 stages to gastrulation in micro- and
mesolecithal eggs.
Steps in Gastrulation
Prospective endodermal cells in vegetal hemisphere elongate and
move toward interior.
Base of cell remains attached to surface of blastula, this causes
strain on surface which causes an indentation to form
Area directly above the blastopore = Dorsal Lip of Blastopore. This
is where cells begin to move inward.
Invagination spreads circulolaterally enclosing yolk plug of
Continued invagination and spreading so that surface of gastrula
contains only ectoderm.
Notochord and somatic mesoderm become stretched out
longitudinally inside, neural ectoderm longitudinally on surface.
When complete:
composite inner layer involving both mesoderm and endoderm
notochordal mesoderm and neural ectoderm in parallel
blastocoel eliminated by spreading of endoderm
yolk plug has withdrawn internally so blastopore is only a narrow slit
Fig 5.7 – Gastrulation and
Neurulation in Amphioxus
Fig 5.8 – Early embryonic development in Lampreys
Fig 5.11 – Gastrulation
and Neurulation in
Amphibian Gastrulation Video
Gastrulation in Macrolecithal Eggs
• Pattern of movements is different, but the
process is basically the same.
• Elasmobranchs = basically infolding over
posterior margin of blastodisc.
• Outer surface becomes ectoderm, inner
surface endoderm, mesoderm resides in
• Entire embryo still on top of yolk.
Gastrulation in Macrolecithal Eggs
Reptiles and Birds:
Blastodisc contains cells of varying size: large, yolk-rich cells
accumulate at lower and posterior surface, small yolk-poor cells at
upper surface.
A separation gradually forms between large lower and small upper
cells (= blastocoel), forms epiblast (dorsally) and hypoblast
– Upper epiblast forms prospective ectoderm and mesoderm, lower
hypoblast forms prospective endoderm.
Hypoblast spreads anterolaterally to form endoderm layer.
Mesoderm infolding occurs in central region of embryo.
– Infolding begins at posterior end of blastodisc; progresses anteriorly
forming a groove bounded by parallel ridges = primitive streak
– At anterior end of this groove, there exists a raised node of tissue with
a pit (primitive pit) extending down and forward beneath it. Node =
Henson’s Node – equivalent of dorsal lip of blastopore in micro- and
mesolecithal eggs.
Mesodermal tissue migrates downward along primitive streak
expanding laterally beneath ectoderm and above endoderm.
Fig 5.13 – Gastrulation in the
Bird embryo
Chick Gastrulation Video
Gastrulation in Macrolecithal Eggs
• Hypoblast may exert causal influence on
primitive streak:
• If hypoblast separated from epiblast and
reoriented, the axis of the primitive streak
is similarly reoriented.
• If hypoblast removed, primitive streak fails
to form
• These experiments suggest that the
hypoblast forms a necessary substratum for
orientation and deployment of prospective
Gastrulation in Mammals
• Cavities form above and below ICM,
which expand (as amnion and yolk sac,
respectively), leaving a flat 2-layered
plate of cells.
• Primitive streak forms as in birds and
reptiles to produce primary germ layers.
Figs 5.15 & 5.16 – Gastrulation in Mammals
Major Tissue Regions after Gastrulation
Skin ectoderm
Neural ectoderm
Notochordal mesoderm
Lateral mesoderm
Folding up of tissue at junction of future skin
ectoderm and neural ectoderm areas; the two
tissues separate as this fold forms
Skin ectoderm grows over the top of neural
Beneath “skin,” lateral margins of neural ectoderm
grow upward and together to form tube
Tube first closes at midpoint, progresses anteriorly
and posteriorly. Anterior end opens to surface as
neuropore, posterior end forms common opening
with blastopore (becomes anus) forming
neurenteric canal.
Fig 5.7 – Gastrulation and
Neurulation in Amphioxus
1. Formation of neural folds along margins of
skin-neural ectoderm
2. Mid-dorsal meeting of folds, simultaneous
with joining of skin ectoderm
3. During folding, high crests of tissue are
formed on either side = neural crest cells
Fig 5.16 – Gastrulation
and Neurulation in
• Majority of body structures are mesodermal in
• Notochordal Mesoderm rapidly rounds up and
separates from lateral mesoderm, forming a
discrete cylinder = notochord.
– Notochord is much reduced or obliterated in most
adult vertebrates, but forms the center around which
vertebral formation occurs.
• Lateral Mesoderm – Amphioxus
– Mesoderm forms paired series of segmentally
arranged blocks = somites.
– From their initiation, somites have a cavity inside =
coelomic cavity.
Fig 5.7 (b) – Notochordal and lateral mesoderm in Amphioxus
• Lateral Mesoderm – Vertebrates
• Initially there is no segmentation of mesoderm; instead
forms as a continuous sheet without a central cavity.
• Mesodermal differentiation occurs from dorsal midline
outward into 3 divisions, each extending the entire length
of the body trunk.
• Differentiation always occurs head-to-tail.
• The 3 divisions are:
Next to neural tube and notochord = Epimere (somites).
Thicken and subdivide on either side to form longitudinal rows
of blocks. This is the first indication of segmentation in
vertebrate embryos. Proliferation and differentiation occurs
within somite forming:
• Sclerotome = portion surrounding notochord and neural tube
• Dermatome = outermost portion near skin ectoderm
• Myotome = middle portion between and ventral to sclerotome and
2. Lateral and ventral to somites is a relatively
small region of mesoderm, known as
intermediate mesoderm or Mesomere. This
may show segmentation similar to somites.
3. Beyond mesomere region, extending
ventrolaterally is a sheet of mesoderm known as
lateral plate mesoderm or Hypomere.
– Apart from cyclostomes, there is no segmentation in
this region. Coelomic cavity forms within lateral plate
mesoderm, dividing it into:
• Somatopleure = external mesoderm + ectoderm
• Splanchnopleure = internal mesoderm + endoderm
Figs 5.11 & 5.16
Mesoderm divisions
in Amphibians and
• Once the mesoderm divisions are set up, then
ontogenetic development proceeds to
embryonic differentiation to adult body.
• What causes this differentiation?
• Induction = process by which developmental
fate of cells is determined
• Classic Experiments of Spemann (1920s) – won Nobel Prize
• Took piece of presumptive neural plate from early gastrula
stage, transplanted to ventral region of another early gastrula
 normal development, prospective neural ectoderm
becomes skin ectoderm.
• Similar experiment with late gastrula  transplanted neural
ectoderm becomes neural ectoderm regardless of transplant
• Transplant dorsal lip (prospective notochordal mesoderm) of
early gastrula to ventral region in another embryo  induces
formation of a secondary embryo involving both host and
transplanted tissues.
• Conclusions:
– Some change took place between early and late gastrula that
determined fate of cells
– Dorsal lip responsible for inducing shift in direction of differentiation
of host tissue. Dorsal lip plays a central role in determining the
craniocaudal axis of the embryo = Primary Organizer.
Spemann Expt - Transplanted dorsal lip (prospective notochordal mesoderm) of
early gastrula to ventral region in another embryo  induces formation of a
secondary embryo involving both host and transplanted tissues.
Primary Induction
• Definition = induction with primary
importance in determining cranio-caudal axis,
also the first of many inductive interactions.
• Two types of inductive interactions:
– Instructive = directs differentiation of cells along a
certain path
– Permissive = allows differentiation when given a
proper stimulus
What is the Mechanism of Induction?
• Classic Triturus (Salamander) experiment …
• Dorsal Lip (Inducer) cultured for 24 hr across
nitrocellulose membrane with pores of specific
diameter from undifferentiated ectoderm →
Ectoderm differentiates to become neural tissues.
• If undifferentiated ectoderm cultured in the
absence of dorsal lip, it develops into
unspecialized skin ectoderm.
• EM analysis of membrane after culture showed
no cellular processes between dorsal lip and
• Conclusion = a diffusable substance responsible
for induction.
What is the Mechanism of Induction?
• Chemical nature of diffusable substance?
– Not known with certainty
• Purified active ingredients from various
inducers turn out to be proteins or
• Also some evidence that changes in ratio of
bound/free ions within cells of early gastrula
may influence induction.
What is the Mechanism of Induction?
• Other inductive interactions between cells can
result from …
– Cell-to-cell direct physical interactions – usually
between molecules located on the cell surface
– Cell-to-cell communication of signals through gap
• For induction via diffusable substances or
direct physical interactions, cell membrane
receptors on the induced cells are required
• The full developmental pathway is dependent
upon the genetic and biochemical capacities
of the induced cells + the full inductive
capabilities of the inducing tissue.
• Many inductive interactions occur throughout
development and influence gene expression
and the production of specific gene products
and cell migration, among other processes
• Precise mechanisms remain uncertain in most

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