3. TWO-GERM-LAYER STAGE
Epiblast and Hypoblast Formation
Just before the embryo implants early in the
second week, cells of the inner cell mass become
rearranged into an epithelial configuration.
(embryonic shield).
The main upper layer (epiblast), and the lower
layer (hypoblast, or primitive endoderm).
Hypoblast Formation In Mouse
Expression of the transcription factor nanog
(precursors of the epiblast) and Gata6(the
hypoblast) by cells of the inner cell mass.
“time inside–time outside” hypothesis
those cells that enter the inner cell mass
earliestare biased to express nanog, which
perpetuates their pluripotency.
Possibly because of the influence of (FGF-4),
secreted by these first arrivals to the innercell
mass, later immigrants are then biased to express
Gata 6.
4. Summary of major genes involved in various stages of early embryonic development. A,
Preprimitive streak (sagittal section). B, Early formation of the primitive streak. C, Gastrulation (period of germ layer
formation). D, Late gastrulation and neural induction. The molecules in red are signaling molecules, and the molecules in
blue are transcription factors. Names of specific molecules (bold) are placed by the structures in which they are
expressed.
5. PRIMITIVE STREAK AND THREE BODY AXES FORMATION
about day fifteen.
a thickening containing a midline groove
forms along the midsagittal plane.
elongates to occupy about half the
length of the embryonic disc.
the primitive groove,becomes deeper
and more defined.
The cranial end of the primitive streak is
expanded (primitive node).
Formation of the primitive streak
defines all major body axes.
These consist of the cranial-caudal (or
head-tail) axis, the dorsal-ventral (or
back-belly) axis, the medial-lateral axis,
and the left-right axis.
The movement of cells through the
primitive streak and into the interior of
the embryo is called ingression.
7. CELLULAR BASIS OF PRIMITIVE STREAK FORMATION
Four major processes
are involved:
cell migration
oriented cell division
progressive delamination
from the epiblast
convergent extension
8. FORMATION OF DEFINITIVE ENDODERM
On day sixteen epiblast cells lateral
to the primitive streak begin to move
into the primitive streak.
They undergo (EMT).
During EMT: epiblast cells often
elongate and become flask or bottle
shaped. Then detaching from their
neighbors as they extend footlike
processes called pseudopodia (as
well as thinner processes called
filopodia and flattened processes
called lamellipodia).
The first ingressing epiblast cells
invade the hypoblast and displace its
cells, so that the hypoblast eventually
is completely replaced by a new layer
of cells—the definitive endoderm.
9. FORMATION OF INTRAEMBRYONIC
MESODERM
Starting on day sixteen, some
epiblast migrating cells diverge into
the space between epiblast and
nascent definitive endoderm to form a
the intraembryonic mesoderm.
Shortly thereafter, the mat
reorganizes to form four main
subdivisions of intraembryonic
mesoderm: cardiogenic mesoderm,
paraxial mesoderm, intermediate
mesoderm (also called
nephrotome), and lateral plate
mesoderm
a fifth population of mesodermal cells
migrates cranially from the primitive
node at the midline to form a thick-
walled midline tube called the
notochordal process.
10. Paths of migration of mesoderm during gastrulation.Cells of the primitive node migrate cranially at the
midline to form the notochordal process (not shown, as it occurs later). Cells that ingress more caudally
through the primitive streak migrate to form the mesoderm lying on either side of the midline. The most
cranially migrating of these cells form the cardiogenic mesoderm, which moves cranial to the future position
of the oropharyngeal membrane (cranial oval structure). The more laterally migrating of these cells form the
paraxial, intermediate, and lateral plate mesoderm.
11. OROPHARYNGEAL MEMBRANE, AND CLOACAL MEMBRANE
FORMATION
During the third week,two faint
depressions form in the ectoderm:
one at the cranial end of the
embryo overlying the prechordal
plate and the other at the caudal
end behind the primitive streak.
Late in the third week, the
ectoderm in these areas fuses
tightly with the underlying
endoderm, excluding the
mesoderm and forming bilaminar
membranes. The cranial
membrane is called the
oropharyngeal membrane, and
the caudal membrane is the
cloacal membrane.
12. FORMATION OF ECTODERM
Once formation of the definitive
endoderm and intraembryonic
mesoderm is complete, epiblast
cells no longer move toward and
ingress.
The remaining epiblast now
constitutes the ectoderm, which
quickly differentiates into the
central neural plate and the
peripheral surface ectoderm.
the embryo develops in a cranial-
to-caudal sequence, so that once
epiblast is no longer present
cranially, for some time it still will
be present caudally where cells
undergo ingression
13.
14. CELLULAR BASIS OF GASTRULATION
During gastrulation, cells undergo four
types of coordinated group movements,
called:morphogenetic movements.
epiboly (spreading of an epithelial
sheet),
emboly (internalization)
Convergence (movement toward
the midline)
Extension (lengthening in the
cranial-caudal plane).
Convergent extension involves
cell rearrangement to narrow the
medial-lateral extent of a
population of cells and
concomitantly increase its cranial-
caudal extent.
15. Morphogenetic movements are generated by
a combination of changes in cell behaviors. These
behaviors include changes in cell shape, size, position,
number and cell-to-cell or cell-toextracellular matrix
adhesion.
Both epiboly and emboly are involved in
human gastrulation as cells move toward,
into, and through the primitive streak.
16. EPITHELIAL-TO-MESENCHYMAL TRANSFORMATION (EMT).
EMT involves changes in both cell-to-cell adhesion and cell shape.
During EMT, epiblast cells within the primitive streak shift their predominant adhesive activity
from cell-to-cell to cell-to-substratum(basement membranes and extracellular matrix).
One gene responsible for repressing epithelial characteristics in the mesenchymal cells is
snail (a zinc-finger transcription factor).
Under its influence, expression of certain cell-to-cell adhesion molecules such as E-cadherin
ceases, whereas expression of cytoskeletal proteins, such as vimentin, is induced.
The cytoskeleton is altered by expression of members of the Rho family of GTPases such as
RhoA and Rac1.
EMT is required to regulate actin organization and the development of lamellipodia of
gastrulating cells within the primitive streak.
When GTPases are disrupted, cells accumulate and die within the space between epiblast and
hypoblast.
loss-of-function mutations in N-cadherin, a cell-cell adhesion molecule, and β-catenin, a
cytoplasmic component of the cadherin/catenin adhesion complex, as well as afadin, an actin
filament–binding protein.
Fgf signaling plays a role in EMT. In loss-of-function mutations Fgfr1, involuting cells lose
their ability to ingress and, as a consequence, accumulate within the primitive streak
17. FATE MAPPING AND CELL LINEAGE
Definition: Demenestration the sites of origin of
epiblast cellsthat give rise to various subdivisions of
ectoderm, endoderm,and mesoderm.
In fate mapping, groups of cells
are marked in some manner (often with fluorescent
dyes) and then are followed over time.
In cell lineage studies, individual cells are marked
(often genetically with reporter genes), rather than
groups of cells, and their descendants are then
followed over time.
18. FATE OF EPIBLAST CELLS
the early primitive streak
stage:
Prospective gut (definitive)
endoderm: from the epiblast
surrounding the cranial half of the
primitive streak
prospective prechordal plate:
within the cranial end of the
primitive streak.
Prospective cardiogenic
mesoderm from the epiblast
moves into the middle part of the
primitive streak and then migrates
cranially.
Prospective extraembryonic
mesoderm moves from the
epiblast into the caudal end of the
primitive streak
19. FATE OF EPIBLAST CELLS
The mid-primitive streak
stage:
Prospective notochord migrates
cranially at the midline.
prospective head mesoderm
more caudally, and in cranial-to-
caudal succession.
Prospective somites move into
and through the primitive streak.
Prospective intermediate
mesoderm moves into and
through the primitive.
prospective lateral plate
mesoderm moves into and through
the primitive streak
20. FATE OF EPIBLAST CELLS
The fully elongated primitive
streak stage:
prospective neural plate is
located cranial and lateral to the
cranial end of the primitive
streak.
Prospective neural crest cells
flank the lateral sides of the
neural plate.
prospective placodal
ectoderm, a horseshoeshaped
area that lies peripheral to the
craniolateral borders of the
neural plate.
Prospective surface ectoderm
constitutes the remaining areas
of the ectoderm.
21. NOTOCHORD FORMATION
begins with cranial midline extension from
the primitive node of a hollow tube, the
notochordal process.
This tube grows in length as primitive node
cells are added to its proximal end,
concomitant with regression of the primitive
streak.
several morphogenetic transformations
in notochordal process:
1) the ventral floor of the tube fuses with the
underlying endoderm and the two layers
break down, leaving behind the flattened
notochordal plate.
2) The notochordal plate then completely
detaches from the endoderm, and its free
ends fuse as it rolls up into the
mesoderm,changing as it does so into a
solid rod called the notochord
23. CELLULAR BASIS OF CONVERGENT EXTENSION
Formation of the notochordal plate involves convergent
extension :
The coordinated narrowing of a cluster of node-derived cells in the medial-lateral
plane and concomitant lengthening in the cranial-caudal plane as the notochordal
plate forms.
Convergent extension of the notochord is driven by:
cell-to-cell intercalation: medial-lateral interdigitation of cells.
oriented cell division:
Mitotic division planes (i.e., metaphase plates) are positioned in dividing
notochordal cells to separate daughter cells preferentially in the cranial-caudal
plane, rather than in the medial-lateral plane.
24. PARAXIAL MESODERM DIFFERS IN HEAD
AND TRUNK
In the future head region:
bands of cells that remain
unsegmented as the head
mesoderm.
more dispersed to loosely fill the
developing head as the head
mesenchyme.
neural crest cells start to
migrate.
So the head mesenchyme is
derived from both head
mesoderm and ectodermal
neural crest cellls and
prechordal plate at midline .
Derivatives:
T he striated muscles of the face,
jaw, and throat.
25. THE PARAXIAL MESODERM IN THE FUTURE TRUNK REGION,
Bands of segmented cells : somites
. The first pair of somites : about day twenty at the head-
trunk border.
The remainder: in cranial-caudal progression ‚three or
four a day, finishing on about day thirty.
Approximately 42 to 44 pairs of somites, flanking the
notochord from the occipital (skull base) region to the tip
of the embryonic tail.
The first four pairs of somites form in the occipital region.
Derivatives: the occipital part of the skull‚ bones that
form around the nose, eyes, and inner ears‚ extrinsic
ocular muscles and muscles of the tongue.
The next eight pairs of somites form in the presumptive
cervical region.
Derivatives: the occipital bone, the cervical vertebrae
and associated muscles, part of the dermis of the neck.
The next twelve pairs, the thoracic somites,
Derivatives: thoracic vertebrae‚ the musculature and
bones of the thoracic wall; the thoracic dermis; and part
of the abdominal wall.
The five lumbar somites.
Derivatives: abdominal dermis, abdominal muscles,
and lumbar vertebrae,
The five sacral somites.
Derivatives: the sacrum with its associated dermis and
musculatur.
26.
27. INTERMEDIATE AND LATERAL PLATE
MESODERM
This two other subdivisions of mesoderm form in the trunk only.
The mesoderm lying immediately lateral to each somite also segments and
forms a small cylindrical condensation: the intermediate mesoderm.
Derivatives: the urinary system and parts of the genital system.
Lateral to the intermediate mesoderm, unsegmented and flattened sheet
lateral plate mesoderm.
Starting on day 17, the lateral plate mesoderm splits into two layers: a ventral
layer associated with the endoderm (splanchnic mesoderm) and a dorsal layer
associated with the ectoderm (somatic mesoderm).
Derivatives:
Splancnic L: the mesothelial covering of the visceral organs (viscera), part of
the wall of the viscera.
Somatic L: the inner lining of the body wall and to parts of the limbs.
28.
29. FORMATION OF NEURAL PLATE
(NEURAL INDUCTION)
On day eighteen.
A thickened neural plate in the ectoderm
just cranial to the primitive node
(induced by the primitive node
quivalent of the organizere in human )
Differentiation into a thick plate of
pseudostratified, columnar
neuroepithelialcells (neuroectoderm).
Neural plate forms in a cranial-to-
caudal direction. broad cranially and is
tapered caudally.
Folds during the fourth week to form a
neural tube.
The lateral lips of the neural plate also
give rise to neural crest cells.
The notochord lies at the midline just
deep to the neural plate. It extends
cranially from the primitive node to end
near the future juncture between
forebrain and midbrain.
30. PRIMARY VERSUS SECONDARY BODY
DEVELOPMENT
By day 22, the primitive streak represents about 10% to 20% of the embryo's length, and
by day 26, it seems to disappear.
On about day 20 remnants of the primitive streak swell to produce a caudal midline
mass of mesoderm called the tail bud or caudal eminence.
Derivatives: A: The most caudal structures of the body‚ B: a reservoir of cells that
allow the embryo to extend caudally during formation of its rudimentary and transient
tail‚ C: contributes cells to the caudal end of the neural tube and neural crest cells
(sacral and coccygeal), as well as the caudal somites.
The notochord of the tail extends into this region from more cranial levels, rather
than forming from the tail bud.
Gastrulation occurs during a period of development called primary body development
Formation of the rudimentary tail occurs after gastrulation is complete, during a period of
development called secondary body development.
In contrast to primary body development, secondary body development involves
the direct formation of organ rudiments from the tail bud without the prior
formation of distinct germ layers.
32. The mechanosensory model of nodal flow. A, Model showing that nodal flow, generated
by motile monocilia in cells expressing Lrd, stimulates calcium flux in cells (i.e., crown cells)
containing non-motile cilia that sense flow on the left side. B, Mouse node viewed from its
endodermal side showing calcium signaling predominantly at the left side of the node.
Arrows indicate motile cilia expressing Lrd fused with a fluorescent reporter gene.
34. Experiments on the role of the
organizer.
A, Donor blastopore (i.e., the
organizer) grafted onto a host
frog embryo induces formation
of a complete secondary body
axis, resulting in the formation
of “conjoined” twins.
B, A frog embryo was irradiated
with ultraviolet light to abolish
“organizer” activity, and then two
blastomeres were injected with
goosecoid mRNA, resulting in
induction of two embryonic axes
35. The molecular basis of the clock and wavefront model. Diagrams of the caudal end of
chick embryos during two rounds of somitogenesis. Retinoic acid (blue) and Fgf8 (gray)
gradients move caudally as the embryo elongates (axis extension) during somitogenesis.
In chick, a somite pair forms every ninety minutes, which constitutes the length of the clock
cycle. Expression of cycling genes (red) extends from caudal to cranial, and when
expression of these genes spreads cranially to cross the threshold level of Fgf8 signaling
(called the determination wavefront; diagonal line), somites are established (indicated by
expression of Mesp genes; purple)
