early development of invertebrates

An Introduction to Early Developmental

Processes

The Early Development of Sea Urchins

The Early Development of Snails

Early Development in Tunicates

Early Development of the Nematode

Caenorhabditis elegans

Cleavage

rapid cell division

no overall growth

no increase in volume

Cleavage

Cell Cycle Control

Initially, factors stored in the egg control cleavage (the

cell cycle);

e.g. stored mRNAs

stored proteins

….. initiate the cell division

Mitosis Promoting Factor (MPF) stimulates the cell cycle

Cleavage-Stage Cell Cycle -blastomeres

Cleavage-Stage Cell Cycle-

blastomeres

MPF – mitosis promoting factor:

-Cyclin B – controls cdc2 activity

-cdc2 = cyclin dependent Kinase

CDK phosphorylates histones,

etc.

#Cyclin B degrades; cell division

stops

Cyclin B presence/degradation

controlled by egg cytoplasmic

proteins

Mid Blastula Transition (MBT)

-the rate of cleavage decreases, the blastomeres

become motile, and nuclear genes begin to be

transcribed.

#Cleavage begins soon after fertilization and ends shortly

after the stage when the embryo achieves a new balance

between nucleus and cytoplasm.

Post Cleavage Cell

Cycle

Post MBT:

• Cell cycle adds two

G phases

• New mRNA

transcription

• Cell division

becomes

asynchronous

Cytoskeletal Mechanisms

karyokinesis & cytokinesis

Cleavage Patterns

Cleavage

Rapid cell divisions

Divisions of fertilized egg into many cells

*What influences the pattern of cleavage in a particular organism?

Gastrulation

the process of highly coordinated cell and tissue movements whereby the

cells of the blastula are dramatically rearranged

Axis Formation

1. The Early Development of Sea

Urchins

Sea Urchin Cleavage

-holoblastic radial cleavage

Sea Urchin Development

-What characterizes the blastula stage?

expanding blastocoel

cilla develops

embryo rotates in fertilization membrane

formation of vegetal plate

formation of hatching enzyme

Germ Layers / Fate Maps

Germ Layers / Fate Maps

inner = endoderm

middle = mesoderm

outer = ectoderm

Sea Urchin Zygote

Sea Urchin Cell Fate

Cells are specified by either: Asymmetric distribution of patterning molecules

into particular cells or cell-cell interactions

Mechanisms for establishing asymmetry:

1. Patterning molecules bound to egg cytoskeleton

2. Molecules actively transported along the cytoskeleton

3. Molecules become associated with one centrosome, and then follow that

centrosome into one of the two mitotic sister cells

Once asymmetry is established, one cell can specify another (and

participate in reciprocal inductions)


At what stage are the fates of individual

cells determined?


How does gastrulation

begin?


What appears to be responsible for the ingression of primary

mesenchyme?


What appears to be responsible for the initial

invagination that occurs during gastrulation?


What happens during later stages of

invagination?

The Early Development of Snails

What kind of cleavage pattern characterizes these

animals?

-Spiral holoblastic

Snail Development

Orientation of cleavage

plane determines right or

left coiling snails

Snail Development

What appears to be responsible for the mosaic

development seen in molluscs?

Snail Development

What is the polar lobe and why is it important?

Snail Development

Why does removal of the D blastomere or its first

or second derivatives result in incomplete

larvae?

If D blastomeres don’t directly contribute cells to

formation of many structures why are they so

important to the formation of the same

structures?

Snail Development

How does gastrulation take place in snails?

Early Development in Tunicates

What type of cleavage

pattern do they have?

-bilateral holoblastic

Tunicate Development

In what way does the pigmentation in Styela partita provide

developmental information?


What evidence is there of autonomous

specification in tunicate blastomeres?

-transplant experiments

-RNA hybridization experiments

-altering β-catenin levels in cells


What evidence is there for conditional specifiction?

-BMP signal from endoderm induces anterior cell to

become notocord-

-works through activation of Brachury gene

-FGF signal induces posterior cell to become

mesenchyme


When are the embryonic

axes established?

-dorsal-ventral – prior to first

cleavage

-anterior-posterior – prior to first

cleavage

-left-right – first cleavage


What is gastrulation like in

these organisms?

Early Development of the Nematode Caenorhabditiselegans

What does C. elegans look like?

C. elegans

What pattern of cleavage is seen in this

nematode?

Axis formation

Anterior-posterior axis formation

C. elegans

When does gastrulation

begin in this organism?

SUMMARY

1. During cleavage, most cells do not grow. Rather, the volume of

the oocyte is cleaved into numerous cells. The major exceptions

to this rule are mammals.

2. The blastomere cell cycle is governed by the synthesis and

degradation of cyclin. Cyclin synthesis promotes the formation of

MPF, and MPF promotes mitosis. Degradation of cyclin

brings the cell back to the S phase. The G phases are added at

the midblastula transition.

3. "Blast" vocabulary: A blastomere is a cell derived from cleavage

in an early embryo. A blastula is an embryonic structure composed

of blastomeres. The cavity in the blastula is the blastocoel. If the

blastula lacks a blastocoel, it is a stereo blastula. A mammalian

blastula is called a blastocyst (in Chapter 11), and the invagination

where gastrulation begins is the blastopore.

4. The movements of gastrulation include invagination, involution, ingression,

delamination, and epiboly.

5. Three axes are the foundations of the body: the anterior-posterior axis

(head to tail or mouth to anus), the dorsal-ventral axis (back to belly), and the

right-left axis (between the two lateral sides of the body).

6. In all four invertebrates described here, cleavage is holoblastic. In the sea

urchin, cleavage is radial; in the snail, spiral; in the tunicate, bilateral; and in

the nematode, rotational.

7. In the tunicate, snail, and nematode, gastrulation occurs when there are

relatively few cells, and the blastopore becomes the mouth. This is the

protostome mode of gastrulation.

8. Body axes in these species are established in different ways. In some,

such as the sea urchin and tunicate, the axes are established at fertilization

through determinants in the egg cytoplasm. In other species, such as the

nematode and snail, the axes are established by cell interactions later

in development.

9. In the sea urchin, gastrulation occurs only after thousands of cells have formed, and the blastopore becomes the anus. This is the deuterostome mode of gastrulation, and

is characteristic only of echinoderms and chordates.

10. In sea urchins, cell fates are determined by signaling. The micromeres constitute a

major signaling center. β-catenin is important for the inducing capacity of the

micromeres.

11. Differential cell adhesion is important in regulating sea urchin gastrulation. The

micromeres delaminate first from the vegetal plate. They form the primary

mesenchyme which becomes the skeletal rods of the pluteus larva. The vegetal plate

invaginates to form the endodermal archenteron, with a tip of secondary mesenchyme cells. The archenteron elongates by convergent extension and is guided

to the future mouth region by the secondary mesenchyme.

12. Snails exhibit spiral cleavage and form stereoblastulae, having no blastocoels. The

direction of the spiral cleavage is regulated by a factor encoded by the mother and

placed into the oocyte. Spiral cleavage can be modified by evolution, and

adaptations of spiral cleavage have allowed some molluscs to survive in otherwise harsh conditions.

14. The tunicate fate map is identical on its right and left sides. The yellow

cytoplasm contains muscle-forming determinants; these act autonomously.

The nervous system of tunicates is formed conditionally, by interactions

between blastomeres.

15. The soil nematode Caenorhabditis elegans was chosen as a model

organism because it has a small number of cells, a small genome, is easily

bred and maintained, has a short lifespan, can be genetically manipulated,

and has a cuticle through which one can see cell movements.

16. In the early divisions of the C. elegans zygote, one daughter cell becomes

a founder cell (producing differentiated descendants) and the other becomes

a stem cell (producing other founder cells and the germ line).

17. Blastomere identity in C. elegans is regulated by both autonomous and

conditional specification.

Thank you!!!

early development of invertebrates

Science