Cell Fate and Differentiation Gastrulation Organogenesis

Fig. 47-UN3 Animal Development 2 Cell Fate and Differentiation Gastrulation

Today’s topics: • Gastrulation and organogenesis • Cell movement and positional signals • Development is a stepwise process of differentiation Sea urchin Frog Chicken – Fruit fly segmentation – Muscles – Snakes Details differ, but all result in the development 31 March 2010 of 3 tissue layers

The cells in the three germ layers have Organogenesis defined fates in the adult:

Fig 47.14

Some General Principles of Development Organogenesis

• Cell division and cell death • Movement and adhesion • Cell-cell interactions and signalling • Determination and differentiation

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

1 Apoptosis (programed cell death) Cell movement and attachment

Cell death is tightly regulated and results from specific signals. (Only in animals. Plant cells can’t move.)

Figure 47.16 Change in cellular shape during morphogenesis Cell sorting & adhesion

Fig. 47-24a Fig. 47-24b Cell-cell interactions RESULTS Primary embryo EXPERIMENT Dorsal lip of blastopore

Secondary (induced) embryo

Pigmented gastrula Primary structures: (donor embryo) Neural tube Nonpigmented gastrula Notochord (recipient embryo) Secondary structures: Notochord (pigmented cells) Neural tube (mostly nonpigmented cells)

2 Fig. 47-21b Fig. 47-22 Zygote 0 Mapping Cell Fate First cell division

Nervous Muscula- Outer skin, Germ line system, ture, gonads nervous system (future outer skin, gametes) 64-cell embryos musculature Musculature Time after fertilization (hours) (hours) after fertilization Time Blastomeres 10 injected with dye Hatching

Intestine

Intestine Larvae Mouth Anus Eggs Vulva

(b) Cell lineage analysis in a tunicate

ANTERIOR POSTERIOR 1.2 mm

C. elegans cell fate

Determination and Differentiation Stem cells

Determination and differentiation of muscle cells

Nucleus Master control gene myoD Other muscle-specific genes Tail Head DNA Embryonic OFF OFF precursor cell A mutation in T1 T2 A8 T3 A6 A7 1 Determination. Signals from A1 A2 A3 A4 A5 OFF bicoid leads other cells activate a master mRNA Normal larva regulatory gene, myoD, The cell is now to tail Tail Tail MyoD protein Myoblast ireversibly (determined) (transcription factor) determined structures at

2 Differentiation. MyoD both ends protein activates A8 other muscle-specific (bottom A8 transcription factors, which A7 A6 A7 in turn activate genes for Mutant larva (bicoid) muscle proteins. mRNA mRNA mRNA mRNA larva).

Myosin, other muscle proteins, MyoD Another and cell-cycle Muscle cell transcription blocking proteins (fully differentiated) The cell is now fully factor Fig 18.16 differentiated

3 Homeotic genes Fruit Fly Drosophila control segment pattern formation: identity

Stepwise expression of transcription Conserved from factors flies to mammals 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

Snakes evolved Why snakes don’t have legs from tetrapod ancestor

Vestigial Claw

Broader expression of hoxC6 in snakes results in loss of legs and more vertebrae. http://www.flickr.com/photos/mark_leppin/3322493554/