Model Organisms: Vertebrates
In the formation of oocytes, the timing of meiosis (and the formation
of polar bodies) varies in different organisms,
and is completed in some organisms only after fertilization.
All vertebrate embryos undergo a similar pattern of development.
1) fertilization
2) cleavage
3) blastulation
4) gastrulation (where ectoderm covers embryo, endoderm and mesoderm
are inside).
The Phylotypic stage
At the end of gastrulation all embryos appear to be similar and is
thus called the phylotypic stage.
Structures that are common to the phylotypic stage of the vertebrates
are
1) the notochord (an early mesoderm structure along A/P),
2) the somites (blocks of mesoderm on either side of notochord which
form the muscles of the trunk & limbs),
3) the neural tube- ectoderm above notochord form a tube (brain and
spinal cord).
Xenopus laevis : fertilization and early growth
1. one sperm enters animal region
2. completes meiosis
3. egg and sperm nuclei fuse
4. vitelline membrane lifts
5. yolk rotates to down (15 mins)
6. cortical rotation (60 mins). Cortex is layer below plasma membrane
-rotation determine future dorsal region (opposite sperm entry site)
7. 1st cleavage (90 mins) A/V
8. 2nd cleavage (110 mins) A/V 90 degrees to 1st
9. 3rd cleavage (130 mins) equatorial (4 small animal and 4 large vegetal=
8 blastomeres)
Xenopus laevis: blastulation &
gastrulation
The blastula (after 12 divisions, thousand's cells) has radial symmetry.
The marginal zone will become the internalized mesoderm and endoderm.
Internalization of the mesoderm and endoderm starts at the blastopore.
1) mesoderm and endoderm converge and begin to move inwards at dorsal
lip of the blastopore
2) this extends inwards along A/P axis
3) ectoderm spreads to cover embryo= EPIBOLY
4) dorsal endoderm separates mesoderm from the space between the yolk
cells = ARCHENTERON (future gut cavity)
5) lateral mesoderm spread ventrally to cover inside of archenteron.
Xenopus laevis: late gastrulation
By the end of gastrulation...
1) dorsal mesoderm is beneath dorsal ectoderm
2) mesoderm spread to cover gut
3) epiboly- ectoderm covers embryo
4) yolk cells are internalized (food source)
5) dorsal mesoderm develops into a) notochord (rod along dorsal midline)
and b) somites (segmented blocks of mesoderm along notochord).
Xenopus laevis: Neuralation
Neuralation or neural tube formation
1) The neural plate is the ectoderm located above notochord and somites.
2) The edge of the neural plate forms neural folds which rise towards
midline.
3) The folds fuse to form neural tube.
4) The neural tube sinks below epidermis.
The anterior neural tube becomes brain. Mid and posterior neural
tube becomes spinal cord.
The somites...
The dorsal part of somites become dermatome (future dermis).
The rest of each somite becomes vertebrae and trunk muscles (and limbs).
Lateral plate mesoderm becomes heart, kidney, gonads and gut muscles.
Ventral mesoderm blood-forming tissues.
Also at this stage, the endoderm gives rise to the lining of the gut, liver & lungs.
Xenopus laevis: early tail bud stage
After gastrulation, the early tail bud stage
occurs.
In the anterior embryo,
a) the brain is divided,
b) eyes and ears form,
c) 3 branchial arches form (anterior arch later becomes the jaw).
In the posterior embryo, the tail formed last from dorsal lip of blastopore
by extension of notochord, somites and neural tube.
Xenopus laevis : Neural Crest Cells
Neural crest cells come from the edges of the neural folds after neural
tube fusion.
They detach and migrate as single cells between the mesodermal tissues.
to become:
1) sensory and autonomic nervous systems
2) skull
3) pigment cells
4) cartilage
Chick embryo: the blastodisc
The blastodisc arises through cleavage and is formed within 20 hours
of fertilization.
The chick blastodisc can be divided into
two areas:
1) the area pellucida (a light area) surrounded by
2) the area opaca (a dark ring).
The posterior marginal zone (PMZ) forms at the junction of the
area
pellucida and the area opaca and defines the dorsal side and
posterior end of the embryo.
The hypoblast (the source of extra-embryonic tissues) develops
as a layer on top of yolk and develops from cells from the posterior marginal
layer and the overlying cells of the blastoderm.
Chick embryo: the primitive streak
The primitive streak is a slit or line on the disk which lays
down the A/P axis.
This structure begins to form from the posterior marginal zone and
extend to a point in the central region of the disk.
Cells move towards the streak and mesoderm & endoderm internalize
at this site.
When the primitive streak reaches its greatest length, the anterior
end begins to regress back to the posterior end.
The anterior end of the regressing streak is known as Hensen's Node.
Chick embryo: gastrulation
As Hensen's Node moves toward the posterior,
several structures form behind it.
1) The head fold (from ectoderm and endoderm)
2) The notochord and somites (from mesoderm)
3) The neural tube forms above the notochord
(from ectoderm)
(The anterior structures are formed first while the posterior structures
are completed last.)
4) Neural folds fuse at the dorsal midline and neural crest cells migrate
away
5) Finally, the head fold separates, gut
forms and heart pieces fuse to form heart.
__________________________________________
Mouse embryo: blastocyst
The morula (~32 cell stage) has 2 cell fates: inner 8 cells
(Inner Cell Mass) and outer ~20 cells (trophectoderm).
In the blastocyst (~3&1/2 days), the trophectoderm and ICM are
established.
Fluid is pumped in to expand cavity and increase the size of the blastocyst.
The preimplantation blastocyst (3&1/2 - 4&1/2 days)
The surface of ICM will become the primitive endoderm while the remaining
becomes primitive ectoderm (= epiblast).
Implantation occurs and the zona pellucida is discarded and blastocyst
attaches to uterine wall.
Mouse embryo: post-implantation
In the first two days post-implantation, the mural trophectoderm
(cells that are not in contact with the ICM) gives rise to polyploid trophoblast
giant cells.
The rest of trophectoderm becomes the ectoplacental cone and
the extra-embryonic ectoderm which give rise to the placenta.
Primitive endoderm migrates ...
1) to cover inner surface of mural trophectoderm to become the parietal
endoderm and
2) to cover egg cylinder and epiblast to become the viseral endoderm
By six days after fertilization, the epiblast is cup-shaped (~1000
cells).
Mouse embryo: gastrulation
By 6&1/2 days after fertilization ...
The primitive streak forms at the
start of gastrulation at the future posterior end! (Inside cup is future
dorsal side)
Cells move through the streak and spread forward and laterally between
the ectoderm and the visceral endoderm to form the mesoderm.
Later, the definitive endoderm (from epiblast) will replace the visceral
endoderm.
The primitive steak first elongates, then at the anterior tip of the
primitive streak, the node forms.
Then notochord and somites form anterior to the node.
Cells migrate through mesoderm to form endoderm (gut).
Mouse embryo: late embryogenesis
By 8 &1/2 days after fertilization,
1) the neural folds form at anterior and dorsal and
2) the embryonic endoderm internalizes to form the gut.
Between 8&1/2 and 9&1/2 days, the mouse embryo undergoes a
complex conformational change and turns to be
completely enclosed in the protective amnion and amniotic fluid.
Finally, by 9 days after fertilization is gastrulation is complete.
Drosophila melanogaster: embryogenesis
By 13 mitoses the membranes sprout to surround the nuclei to form cells
(cellular blastoderm).
~15 cells at posterior (pole cells) are sequestered and become the
germline.
During first ~3 hrs. large molecules such as proteins can move between
nuclei until the cellularization occurs.
Single layer of cells give rise to all tissues.
Gastrulation starts at ~3 hrs.
1) Mesoderm forms from ventral tissue.
2) Midgut from endoderm at the anterior and posterior ends.
3) Ectoderm remains on outside.
Drosophila melanogaster: gastrulation
The mesodermal tube forms from ventral tissue then cells separate
& move to internal locations under the ectoderm.
The mesoderm becomes muscle and connections tissues.
In insects, the nerve cord lies ventrally (vertebrates: dorsal).
Neuroblasts form a layer between mesoderm and outer ectoderm.
The midgut (anterior & posterior) grow from threads and
fuse.
= anterior and posterior midgut
Ectoderm becomes epidermis.
No cell division occurs during gastrulation but division restarts
afterward.
Drosophila melanogaster: segmentation
The germband (ventral blastoderm) is main trunk region.
The process of germ band extension pushes the posterior end
over dorsal side.
The first signs of segmentation grooves appear to outline parasegments
which give rise to segments.
Segments are formed from the posterior of one parasegment and the anterior
of the next.
There are14 parasegments: 3 mouth, 3 thorax, 8 abdominal.
Drosophila melanogaster: larvae
The larvae hatch at 24 hrs post-fertilization.
The larval structures of note include ...
The anterior end is the acron.
The posterior end is the telson.
Along with the head, the larvae has 3 thoractic segments and 8 abdominal
segments.
The ventral side of the larvae has denticle belts, alternating
patches of denticle hairs and cuticle on each segment, used for locomotion.
Drosophila melanogaster: metamorphosis
Three instar stages of larval life are separated by molts.
1st instar -(Molt)-> 2nd instar -(Molt)-> 3rd instar
Third instar larvae forms pupae (pupation) to undergo metamorphosis.
The adult tissues arise from imaginal discs and histoblasts.
The imaginal discs are small sheets of epidermis (~40 cells each of
cellular blastoderm) which grow throughout larval life.
6 leg, 2 wing, 2 haltere, 2 eye-antenna, plus genital, head discs and
~10 histoblasts (nest of cells in the abdomen which give rise to the abdominal
segments).
