1

Animal Development 2

Cell Fate and Differentiation

Today’s topics:

• Principles ofDevelopment

• Determination andDifferentiation

• Examples

– Fruit fly segmentation

– Muscles

– Snakes

27 March 2009

Organogenesis

Organogenesis

Human embryo, stage 14. Somites visible Neural tube and somites

Some General Principles of Development

• Cell division and cell death

• Movement and adhesion

• Cell-cell interactions and signalling

• Determination and differentiation

Apoptosis (programed cell death)

Cell death is

tightly regulated

and results fromspecific signals.

Cell movement and attachment

Only in animals. Plant cells can’t move.

2

Cell sorting &

adhesion

Figure 47.16 Change in cellular shape during morphogenesis

Fig. 47-24a

Dorsal lip ofblastopore

Pigmented gastrula(donor embryo)

EXPERIMENT

Nonpigmented gastrula(recipient embryo)

Cell-cell interactions

Fig. 47-24b

Primary structures:

Neural tube

Secondary(induced) embryo

Notochord

Primary embryo

RESULTS

Secondary structures:

Notochord (pigmented cells)

Neural tube (mostly nonpigmented cells)

Fig. 47-21b

(b) Cell lineage analysis in a tunicate

64-cell embryos

Larvae

Blastomeresinjected with dye

Cell fate

Fig. 47-22

Mouth

Zygote

Intestine

Nervous

system,outer skin,

muscula-ture

Intestine

Hatching

Eggs Vulva

Anus

1.2 mm

ANTERIOR POSTERIOR

Muscula-ture, gonads

10

0First cell division

Germ line(future

gametes)

Musculature

Outer skin,

nervous system

Tim

e a

fte

r fe

rtil

iza

tio

n (

ho

urs

)

3

C. elegans cell fate

Determination

and

Differentiation

Stem cells

DNA

OFF OFF

OFFmRNA

mRNA mRNA mRNA mRNA

Another

transcription

factor

MyoDMuscle cell(fully differentiated)

MyoD protein

(transcription factor)Myoblast

(determined)

Embryonic

precursor cell

Myosin, other

muscle proteins,

and cell-cycle

blocking proteins

Other muscle-specific genesMaster control gene myoDNucleus

Determination. Signals from

other cells activate a master

regulatory gene, myoD,

1

Differentiation. MyoD

protein activates

other muscle-specific

transcription factors, which

in turn activate genes for

muscle proteins.

2

Determination and differentiation of muscle cells

Fig 18.16

The cell is now

ireversibly

determined

The cell is now fully

differentiated

A mutation in

bicoid leads

to tail

structures at

both ends

(bottom

larva).

Tail

Head

Normal larva

Tail Tail

Mutant larva (bicoid)

T1 T2T3

A1 A2 A3 A4 A5 A6A7

A8

A8A7 A6 A7

A8

Drosophila pattern formation

Translation of bicoid mRNAFertilization

Nurse cells Egg cell

bicoid mRNA

Developing

egg cell

Bicoid mRNA in mature

unfertilized egg

100 !m

Bicoid protein in

early embryo

Anterior end

(b) Gradients of bicoid mRNA and bicoid protein in normal egg and early embryo.

1

2

3

Hierarchy of Gene Activity in Early Drosophila Development

Maternal effect genes (egg-polarity genes)

Gap genes

Pair-rule genes

Segment polarity genes

Homeotic genes of the embryo

Other genes of the embryo

Segmentation genesof the embryo

4

Conserved from

flies to mammals

Homeotic genes

These are all transcription factors!

Fruit Fly

Mouse

The homeobox is relatively constant

because it has a precise job.Why snakes don’t have legs

http://www.wallpaperbase.com/wallpapers/animals/snakes/snake_2.jpg

http://www.flickr.com/photos/mark_leppin/3322493554/

Vestigial

Claw

Snakes evolved

from tetrapod

ancestor

Why snakes don’t have legs

Broader expression of hoxC6 in snakes results in loss of legsand more vertebrae.