Early invertebrate development
A.
B.
C.
D.
E.
F.
G.
Cleavage
Midblastula transition
Cell movements/ gastrulation
Axis formation
Sea urchin
Snail/ maternal contribution
C. elegans
A.CLEAVAGE
Cleavage is the early embryonic stage during which cells rapidly divide, but the
embryo shows little increase in size.
• Involves a specialized rapid cell cycle—no G1 or G2 phases, just DNA
synthesis and mitosis.
• There is little gene transcription. The egg is filled with stored products
needed to start embryogenesis, including regulatory factors. These gene
products are provided by the mother during oogenesis.
The type of cleavage is influenced by the amount of yolk in the egg. Yolk is thick
and difficult for the cleavage furrow to penetrate.
Definitions:
Holoblastic--cleavage through entire egg –occurs in species with little yolk.
Meroblastic--cleavage part way through egg (not holoblastic) –occurs in species
with a lot of yolk.
Animal
Mammal
Sea urchin
Frog
Yolk
Little
Little
Moderate
Cleavage pattern
holoblastic
Holoblastic
Holoblastic
Bird
Fruit fly
(Drosophila)
Dense yolk
Dense yolk in
center of egg
Meroblastic
Meroblastic
Strategy
Placental dev
Feeding larva
Some yolk/ form
tadpole
Feed off yolk
Feed off yolk/
feeding larva
B.MID-BLASTULA TRANSITION
The MBT is characterized by 2 changes:
The dramatic upregulation of embryonic transcription.
The lengthening of the cell cycle.
Onset of embryonic transcription
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At the MBT, there is a transition from maternal control of development to
embryonic control.
The time at which embryonic transcription begins is different in different species.
Very often, a few genes are transcribed early, and then at a later time (the midblastula transition) embryonic transcription is dramatically increased.
Lengthening of the cell cycle
The cell cycle begins to include growth phases (G1 and G2). At this point, there
is significant cell growth between divisions. As such, the embryo begins to
increase in size.
C. GASTRULATION
The first morphogenetic movement—large scale, orchestrated cellular
movements and rearrangements.
Generates the three primary germ layers, ectoderm, mesoderm and
endoderm. The prospective endoderm and mesoderm are internalized.
!
Ectoderm will give rise to epidermis and nervous system
!
Mesoderm gives rise to skeleton, muscles and blood
!
Endoderm gives rise to the gut
The pre-gastrulation embryo is called a blastula. It is basically a hollow sphere of
cells. The hollow cavity is called the blastocoel. It functions to provide room for
internalization of endodermal and mesodermal cells during gastrulation and to
prevent premature cell-cell contacts.
A dimple forms and protrudes into the hollow interior space. This forms a tube
that extends through the blastocoel and attaches to the roof on the opposite
side. The pore formed by the inward protrusion of the dimple is called the
blastopore. The tube is called the archenteron, and represents the primitive
gut.
In protostomes (mollusks and worms) the blastopore forms the mouth. In
deuterostomes (chordates and echinoderms) the blastopore becomes the anus.
Several types of active cell movements are involved. (Fig 8.5)
Invagination:
cells change shape at apical and basal surfaces
Involution:
expanding epithelium folds and forms new layers
Ingression:
epithelial to mesenchyme transition
Delamination:
splitting of one sheet into two sheets
Epiboly:
epithelial sheets flatten; lateral expansion of the entire sheet
Convergent extension:convergence of cell layers leads to extension (Fig 8.23)
Cell movements involve cytoskeletal activity within the cells.
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D. Axis formation
During early embryogenesis, need to set up the major axes, anterior-posterior
(A-P), dorsal-ventral (D-V), and sometimes left right.
Variable timing and mechanisms. In some phyla, axes are established during
oogenesis, prior to fertilization, in others they are established during gastrulation.
E. Early SEA URCHIN Development
Cleavage results in the formation of macromeres, mesomeres, micromeres. (Fig
8.7)
Axis formation: The anterior-posterior axis of the sea urchin embryo is
determined prior to fertilization. The vegetal pole (where the micromeres will
form) will form posterior structures. Cytoplasmic determinants deposited in
the oocyte by the mother are localized at the vegetal pole and will specify
micromere identity.
See Fig 8.11.
Fate map
Induction:
• Micromeres induce posterior axis
• Micromeres induce endodermal cell fates
Potency
Gastrulation
sea urchin blastula is about 1000 cells—in an epithelial sheet
1.Ingression of primary mesenchyme (Fig 8.18) transition of epithelial cells to
mesenchyme
! They lose affinity for the hyaline layer surrounding the embryo and for
their neighbors.
! Gain affinity for ECM of the blastocoel
! Cells at the vegetal pole extend filopodia & interact with extracellular
matrix in blastocoel.
! Attraction to ECM cues guides their migration.
2.Archenteron invagination(Fig. 8.20)
Movement of a sheet of cells, driven by cell-shape changes and by modifications
in the ECM/ hyaline layer. Invagination allows the vegetal plate to reach ~1/3 of
the way into the blastocoel.
3.Archenteron convergent extension.(Fig. 8.22)
4.Filopodia reach for the roof of the animal hemisphere and complete formation
of the intestinal tube (fig 8.24).
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F. SNAILS/ MATERNAL CONTRIBUTIONS
Snail shells contain spiral patterns.
The “handedness” of the spiral is determined by the cleavage pattern. The
spirals can be left-handed or right-handed.
Controlled by a “maternal” gene.
D confers right handedness
d confers left handedness.
DD or Dd females have right handed offspring, and dd females have left handed
offspring, regardless of the genotype of the male parent. So consider the mating
of a Dd female with a dd male: all the offspring will have right handed spiral
shells, even though half will have the dd genotype. Similarly, a dd female mated
with a DD male will produce all left handed offspring even though all the
offspring carry a Dd genotype. The phenotype of the offspring is
determined by the genotype of the mother ≡ ”maternal gene”.
G. ELEGANS
Specifying the A/P axis:
The egg is oval. The A/P axis will be the long axis of the oval. The sperm entry
point specifies the posterior end. The sperm nucleus centriole nucleates
microtubule reorganizations.
Blastomeres show both autonomous and conditional specification.
If AB and P1 are separated, P develops autonomously to produce a posterior half
animal. However AB only produces a subset of the cell types it normally would. P
cell derivatives are required to induce certain cell fates.
Autonomous cell specification—asymmetric segregation of cytoplasmic
determinants
E.g. P-granules: specify germ cell fate. Are asymmetrically segregated at each
mitotic division of early embryogenesis.
Conditional (non-cell-autonomous) specification of certain cell fates in C. elegans.
P2 induces EMS to form E. If separate, EMS cell generates 2 MS daughter cells.
P2 necessary for E cell fate. Not sufficient for E fate because contact with ABp
cell does not induce E fate.
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