lecture 22

Question 1

What is the central question of developmental biology?

Answer 1

how do we get from a single cell to a working animal

Question 2

What is fertilisation?

Answer 2

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

Question 3

What is cleavage?

Answer 3

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

Question 4

What is gastrulation?

Answer 4

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

Question 5

What are the three germ layers? How do they arise?

Answer 5

• 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

Question 6

To what different tissues to the different germ layers give rise?

Answer 6

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

Question 7

What are triploblasts?

Answer 7

• animals with three germ layers

- triploblastic animals have bilateral symmetry and are the largest group of multicellular animals

Question 8

What are the three main groups of multicellular animals?

Answer 8

• sponges (parazoa)
• Cnidaria/Ctenophora (diploblasts)
• Bilateria (triploblasts)

Question 9

What is a popular vertebrate model system for developmental biology?

Answer 9

• the frog: xenopus laevis

Question 10

What is neurulation?

Answer 10

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

Question 11

What is origin of the notochord?

Answer 11

mesoderm

Question 12

What is organogenesis?

Answer 12

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

Question 13

What is metamorphosis?

Answer 13

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

Question 14

How can development be seen as a tree of cell divisions?

Answer 14

• 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

Question 15

What is the concept of cell differentiation?

Answer 15

• cells become more specialised as development proceeds

- this process is called cell differentiation

Question 16

How do we make different types of cells, when they all have the same genetic instructions?

Answer 16

• 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

Question 17

How can differences in 2 daughter cells be achieved?

Answer 17

• asymmetric division vs symmetric division
• asymmetric: segregation of intrinsic cell fate determinants
• symmetric: cells receive different external signals

Question 18

What is an example of a cell fate determinant?

Answer 18

• 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

Question 19

How do cells coordinate cell fate determinants and spindle orientation?

Answer 19

• asymmetric localisation of a protein like prospero will only make cells different if the spindle is in the right orientation

Question 20

What is the mechanism of asymmetric division in Drosophila?

Answer 20

This is achieved by a protein complex (aPKC/Baz/Par6) on the apical side of the cell which does two things:

1. it directs the Prospero/Miranda complex to the basal cortex by phosphorylating Lgl only on the apical side, and,
2. it aligns the spindle, by connecting to microtubules, through the Insc/Mud/Pins complex

very important
stem cells must divide asymmetrically

Question 21

What is an example of cell differences achieved by external signals?

Answer 21

• 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

Question 22

What is the general paradigm regarding signalling pathways allowing external signals to change gene expression?

Answer 22

• 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

Question 23

What are common developmental signalling pathways?

Answer 23

• 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

Question 24

What is a simplified view of the MAPK pathway?

Answer 24

1. unactivated FGF-receptors exist as monomers in the membrane
2. FGF dimers bind to receptors, causes them to dimerise and phosphorylate each other
3. phosphorylated residues provide docking sites for Drk, which recruits and activates the RasGTP Exchange Factor, sos
4. Sos activates Ras by catalyising GDP–>GTP exchange
5. activated Ras binds and activates the kinase Raf
6. activated Raf phosphorylates and activates the kinase Mek
7. activated Mek phosphorylates and and activates the kinase Erk
8. activated Erk phosphorylates the transcription factor, Jun
9. phospohorylated Jun binds Fos and activates transcription of target genes

amplification process - one Ras molecule can activate many Ref molecules

Question 25

What is determination vs differentiation?

Answer 25

- 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

Question 26

What is an example of determination?

Answer 26

- 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

Question 27

What is morphogenesis?

Answer 27

- 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)

Question 28

What are morphogenetic mechanisms?

Answer 28

- 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

Question 29

What is an example of collective cell behaviour?

Answer 29

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