lecture 22 Flashcards
What is the central question of developmental biology?
how do we get from a single cell to a working animal
What is fertilisation?
Stage 1
Embryonic development begins with fertilisation: the joining of a haploid egg cell with a haploid sperm cell to create a diploid cell called the zygote
What is cleavage?
Stage 2
- the zygote then undergoes cleavage: a series of divisions that generate smaller and smaller cells
- after cleavage, the embryo consists of a single layer of epithelial cells surrounding a fluid filled cavity
- at this stage the embryo is called the blastula
What is gastrulation?
Stage 3
- the blastula then undergoes gastrulation: the process by which the complex body architectures of animals are established
- subsets of cells invaginate (i.e. fold inwards) to form an internal cavity which will eventually become the alimentary canal or gut
- other cells delaminate (i.e. break free of the outer layer) and move into the embryo to form a middle layer of cells
Wolpert - most important stage of your life over birth, death etc
Just about every organism does it differently
What are the three germ layers? How do they arise?
- gastrulation generates the three germ layers
- the cellular movements during gastrulation result in three distinct layers of cells:
- an outer layer called the ectoderm
- an inner layer called the endoderm
- a middle layer called the mesoderm
- these are called the primary germ layers
- each layer gives rise to different parts of the adult organism
To what different tissues to the different germ layers give rise?
Vertebrates
- Endoderm: gut, liver, lungs
- mesoderm: skeleton, muscle, kidney, heart, blood
- ectoderm: skin, nervous system
Insects
- endoderm: gut
- mesoderm: muscle, heart, blood
- ectoderm: cuticle, nervous system
What are triploblasts?
- animals with three germ layers
- triploblastic animals have bilateral symmetry and are the largest group of multicellular animals
What are the three main groups of multicellular animals?
- sponges (parazoa)
- Cnidaria/Ctenophora (diploblasts)
- Bilateria (triploblasts)
What is a popular vertebrate model system for developmental biology?
- the frog: xenopus laevis
What is neurulation?
Stage 4
- in vertebrates, the next major developmental stage is neurulation
- a mesodermal structure called the notochord induces the overlying ectoderm to form the neural plate
- this invaginates to form the neural tube of the CNS
What is origin of the notochord?
mesoderm
What is organogenesis?
Stage 5
- next stage of development is called organogenesis
- this is a general term to describe the many ongoing cell rearrangements and differentiations that generate the organs of the adult body
What is metamorphosis?
stage 6
- in some animals
- the larval form that is the end result of embryogenesis undergoes metamorphosis
- a series of radical morphological changes that give rise to the adult form
How can development be seen as a tree of cell divisions?
- zygote
- one cell divides into two cells
- those cells divide to make four cells
- eight cells
- formation of a body as a hierarchical tree
- C elegans
- this pattern of divisions is completely invariant
- can track them and say that cell gives rise to that cell etc and eventually gives rise to this particular neuron
Note: all of these cells receive a complete copy of the chromosomes i.e. they are all genetically equivalent
adult male worm has exactly 1031 cells and that’s it
- complete cell ineages available for the worm
What is the concept of cell differentiation?
- cells become more specialised as development proceeds
- this process is called cell differentiation
How do we make different types of cells, when they all have the same genetic instructions?
- certain housekeeping genes are expressed in all cells
- e.g. actin, tubulin, histone
- other henes are specific to particular types of cells: different types of cells express different subsets of genes
e. g. Twist: on in muscle, not neurons or epidermis, Snail: muslce and neurons, but not epidermis, N-cam: neurons, not muscle and epidermis, E-cadherin: epidermis, but not muscle or neurons
How can differences in 2 daughter cells be achieved?
- asymmetric division vs symmetric division
- asymmetric: segregation of intrinsic cell fate determinants
- symmetric: cells receive different external signals
What is an example of a cell fate determinant?
- asymmetric division
- drosophila neural stem cells (also called neuroblasts) divide to produce multiple daughter cells (called GMCs)
- segregation of the transcription factor Prospero to the GMC results in the GMC taking on a different cell fate than that of its parent neuroblast
- transcription, moves the nucleus and affects gene transcription of the daughter cells
- turns off genes that are to do with self renewal and turns on genes that are to do with differentiation
- in prospero mutants, GMCs, which would normally terminate cell division and differentiate , are transformed into self-renewing neural stem cells
How do cells coordinate cell fate determinants and spindle orientation?
- asymmetric localisation of a protein like prospero will only make cells different if the spindle is in the right orientation
What is the mechanism of asymmetric division in Drosophila?
This is achieved by a protein complex (aPKC/Baz/Par6) on the apical side of the cell which does two things:
- it directs the Prospero/Miranda complex to the basal cortex by phosphorylating Lgl only on the apical side, and,
- it aligns the spindle, by connecting to microtubules, through the Insc/Mud/Pins complex
very important
stem cells must divide asymmetrically
What is an example of cell differences achieved by external signals?
- cell secreting wingless (wg)
- adjacent cell divides to produce 2 equivalent cells
- wg binds cell receptor Frizzled –> signalling cascade –> armadillo gets turned on –> transcription of engrailed, hedgehog
- only cell A (adjacent) is close enough to receive wg signal
- A receives the external signal (i.e. Wg protein) which activates Wg pathway, leading to expression of target genes (e.g. engrailed) and a different cell fate
What is the general paradigm regarding signalling pathways allowing external signals to change gene expression?
- ligand binds to receptor which induces some change in the receptor (e.g. conformation)
- this triggers a sequence of intracellular events, which eventually leads to transcription factors entering the nucleus and regulating gene expression
What are common developmental signalling pathways?
- a small set of evolutionarily conserved signalling pathways are used over and over again in developmental processes
- TGF-beta –> receptor becomes phosphorylated –> these phosphorylated kinase receptors, phosphorylate SmadN –> bind to Smad 4 –> transcription
- Delta Jagged (ligand that is also a transmembrane protein on other cell) –> Notch cleaved when bound by Delta Jagged –> intracellular domain, NICD, moves to nucleus –> NICD/RPDJ –> transcription
Wnt/Wingless
- Wnt/Wg –> LrP/Frizzled –> Beta-cat normally degraded, activated pathway stops it from being degraded –> Beta-cat + LEF –> transcription
- key pathway that gets misregulated in colon cancer
Hedgehog
- hedgehog –> Smo/Patched –> stuff –> Ci
MAPK cascade
FDF, EGF –> receptor tyrosine kinase –> blah blah blah –> Jun –> Fos
What is a simplified view of the MAPK pathway?
- unactivated FGF-receptors exist as monomers in the membrane
- FGF dimers bind to receptors, causes them to dimerise and phosphorylate each other
- phosphorylated residues provide docking sites for Drk, which recruits and activates the RasGTP Exchange Factor, sos
- Sos activates Ras by catalyising GDP–>GTP exchange
- activated Ras binds and activates the kinase Raf
- activated Raf phosphorylates and activates the kinase Mek
- activated Mek phosphorylates and and activates the kinase Erk
- activated Erk phosphorylates the transcription factor, Jun
- phospohorylated Jun binds Fos and activates transcription of target genes
amplification process - one Ras molecule can activate many Ref molecules
What is determination vs differentiation?
- consider two regions in the early embryo, A and B
- cells in A eventually differentiation into one type of cell
- cells in B differentiate into a different type of cell
- what if a cell from region B is transplanted into region A?
- if you transplant it and it changes ‘fates’, it was not determined
- if it does not change ‘fates’ it was determined
- so determination means the cell has aready decided what it’s going to do
- at a molecular level that means different genes are being expressed despite same outward appearance
What is an example of determination?
- cells in the presumptive eye region are fated to form the eye
- they ARE NOT determined at the gastrula stage
- but they ARE determined by the neurula stage
What is morphogenesis?
- the early embryo is topologically simple, and is comprised of cells with similar appearance
- during development cells arrange themselves into a complex body architecture, and differentiate into a variety of mature cell types
- this process is called morphogenesis, and is achieved by a range of cellular behaviours/mechanisms (e.g. division, apoptosis, migration, shape change etc)
What are morphogenetic mechanisms?
- cells are created by cell division
- cleavage: rapid early divisions that divide the zygote into smaller and smaller cells [no G1 and G2 phases]
- growth: proliferation of similar sized cells, so tissue gets bigger [normal M/G1/S/G2 phases]
- cells can be eliminated by a genetically controlled cell death called apoptosis
- this can be an important morphogenetic mechanism
- e.g. vertebrate limb bud development
- cell shape changes e.g. round/columnar, cell growth, protrusions
- transitions between epithelial and mesenchymal cell types
- Epithelial to mesenchymal transitions (EMTs)
- mesenchymal to epithelial transitions (METs)
- cell migrations
- cell fusion
What is an example of collective cell behaviour?
epithelial folding
- apical actin-filament bundles contract, narrowing the cells at the apices
- apical actin-filament bundles
convergent extension
- e.g. frog gastrulation
- lamellipodia attempt to crawl on surfaces of neighbouring cells, pulling them inward
epithelial sheet migration
- e.g. dorsal closure of drosophila embryo
- bunch of epithelial cells perceive a gap, trigger migratory phenotype
- i.e. partial transition to migratory behaviour
epithelial branching
- partial transition to migratory behaviour
- signal in extracellular world triggers migratory patterns causing branches of epithelial structures
