LECTURE 1: Early embryogenesis, Early experiemnts, fate maps and how to clone a frog

3 major things

1. Proliferation

2. Differentiaona

3. Pattern formation

   • where and when

Two important questions to answer

1. Cell differentiation→ how are genes switched on and off in different cell types

2. How does the right cell form at the right plance, how do patterns of cells form?

Example of cell differentiation

1. Cell of frog somite

(tissue that gives rise to muscle and other strucutures)

• express the gene for muscle myosin

but

2. Immediate neighbours in the skin and neural tube do not

Examples of pattern formation

1. Cyanobacterium Anabaena

   • Forms heterocysts

   • specialised for fixation of nitrogen

   • How is this spacing controlled?

2. Hydra coelenterate

   • Head with tentacles and a mouth for ingesting prey organisms

   • AT OTHER END

   • foot for attaching to the substrate

2. what happens to hydra is cut in half?

• Cut face of the foot end regenerates a head

• The Cut foot of the face ende regernetaes a foot

→ Cell programmes are reset after cutting

• makes the missing structure

BUT: how do cell ‘know’ where they are in the 2 halves

• and form appropriate new strucutures?

Embryo develpooment in different species

• sequence and pattern is very regular

Model organism egg developent

African frog→ Xenopus

• divides to produce a ball of cells

  → called a blastula

How to follow the fate of single cells?

Fate maps

1. inject cell in frog blastula with fluorescent dye

2. find out which cells are labelled in the developing frog

3. e.g cell C3 and its progeny appear to have contributed to muscle and skin

Fate maps: Why can they be made?

• development is a regular process

• can use many experiments to build up a complete picture of what each cell contribute to the final structure

  → easily map it back onto the early embryo

What a fate map does?

• predicts the developmental fate

• of each cell in the early embryo

E.g fate map for frogs helps to

• predict the upper dorsal part of blastula→ future nervous system

Experiment to find out how cells know this: Wilhelm Roux 1

1. Hot pin to kill one of two cells formed in the frog’s first division

→ Result:

• If LH killed→ only the right half of the embryo developed

1 Roux conclusion

• Information necessary to make embryo was partitioned at each cell division

→ shown here as only right hand side destroyed

1 Flaw in the experiment and conclusions?

• Could not remove the dead cell

  → Blocked normal development of the embryo on that side

Experiement 2: Hans Driesh

1. Separated the first two cells formed in sea urchin

2. cultured them individually

→ RESULT: any of these cells on its won was capable of forming an entire sea urchin embryo and larva

2 Conclusions from this

• Embryo has self organising system

• Could organise and recognise its own construction

  → even if it was cut in half

→ This is what developmental biologists seek to understand

Does information (DNA) get lost as the embryo develops?

• e.g do skin cells speicialse becaus they lose genetically encoded info to make other cell types?

How answer this question?

1. Take a nucleus of a differentiated cell

   • adult skin cell or tadpole gut epitheial cell

2. Ask it to programme whole development

   → putting it back into enucleated egg cell

→ Nuclear transplantation

PLEASE LOOK AT STEPS→ keeps cytoplasm in tact

how to clone a frog→ Blastula

• Albino nucelus in wild type ooctye

3 Experiement: Sir John Gurdon conclusions

• nucleus is capable of orchestrating the complete developmet of a tadpole

• even if it is differentiated

THEREFORE: Genetic encoded info is clearly not lost as cells differentiate

Then waht does happen to this information?

Not lost

• Undertake specialised programmed of gene expression

With this techniques you can…

Clones genetically identical frogs→ cow

THEREFORE: no permanent loss of genetically encoded infor from differentiated cells

Cell differentiation therefore depends on…

• the selective expression of parts

• of a full set of genes

→ How do cells ‘know’ which genes to express?

Additional information comes from two sources

1. Intrinsic: From inside

   • Asymmetric distribution of factors (determinants) as cells divide

2. Extrinsic: from outside

   • environmental cues

     1. cells signal to each other

     2. environmental signals influence cells