Principles In Animal Development Flashcards
How do cells know what to become and
- how do cells know where to go
- is this fundamentally the same q?
Positional fate mapping
Interactions in order to generate an adult plan, as well as progression of developmental shapes
Process of cellular “becoming”
- Fate and lineage
- commitment (specification and determination) and differentiation
- organelle induction
- positional information, morphine and self-organisation
How do cells know where to go?
Morphogenesis via cell migration and motility
Fate and lineage
Label a blastomere and observe
Fate
What a cell will (or has) become
Lineage
Where/who a cell ce from
Xenopus photoconvertible methodology
- inject protein @ zygote stage; integrated into every cell
- UV
Lineage tracing
- label one cell and follow all descendants to create a Family Tree
- v difficult
- new system of inquiry
- facilitated by intestinal organoid microscopy
Specification
Reversible
Determination
Irreversible
Differentiation
- When specific cells types are formed (e.g. muscle cells, neurons)
- an be inferred from morphology/ T profiles
- often come post-mitotic
Committed?
- an experimental embryology approach
- do they do in culture what they would in vivo?
- yes: committed
Specified/determined
- transplant!
- do they do what they would do in vivo in donor?
- yes: determined
- no: specified
Induce
Change the fate
Pattern
Generate an organised set of structures
Induction
- tested using transplanted cells
- eye lens, heart
- the embryonic process in which one group cells, the inducing tissue, directs the development of another group of cells, the responding tissue
- first shown in newts
When under the influence of a transplant, are surrounding cells
Induced to become something different
Investigating induction
- interspecies transplants by dissection, under the prospective ectoderm of the blastopore lip in newts
- does the foetus develop finto in host/transplanted tissues?
Newts
- tissue colour varies by species (light/pigmented)
Induction results
- 2nd invagination site
- secondary axis with somites, neuronal plates
Organisers
A region, or a group of cells in an embryo that can both induce, and cause pattern, adjacent embryonic structures
Blastopore lip
An organiser in amphibian embryos, affects neighbour behaviour
Experimental organisation of blatopore lip results in
Newts conjoined @ belly
Hensen’s node
- avian amniote organiser
- forms from primitive streak?
- analogous to blastopore lip
- grafting from quail embryo to chick host results in new axis induction
Pattern formation
The process by which distinct spatial territories are specified in a population of cells in a developing tissue
Territories
- defined by different molecular / T identities
- tend to precede morphological change ; embryonic complexity
Drosophila territories
Segment gene hierarchy
Gap
- broad, overlapping domains
- all happens in the blastoderm, before morphological appearance
Positional information
Proposes that cells acquire positional values as in a co-ordinate system, which they interpret by developing specific ways to give rise to spatial patterns
Questions regarding positional information
- how is it set up?
- how is it recorded?
- how is it interpreted? Cells read concentrations of a given signal?
The French Flag model
- morphogen conc changes by distance from source
- e.g. bcd
- graded signals across a field or cells would be read and interpreted
- adjust according to position in gradient
- size variation doesn’t affect proportions: scalable
Morphogens
Signalling molecules that emanate from a restricted tissue region, spreading away from their source to form a conc gradient
Fate and morphogens
As the fate of each cell in the field depends on the morphogen concentration, the gradient prefigures developmental patterning ; initial system conditions dependent on
Morphogen action
- act upstream
- affect pattern of genes in a concentration-dependent manner
- dynamic domains shift anteriorly over developmental time (unaccounted for by positional information; explains behaviour)
- measurable
GRNs
- gene interactions to interpret spatial inputs provided by morphogens, to output concentrations of different genes spatiotemporwlly
- explains why gene don’t overlap
- e.g. bistable switch
Changes between interactions have
- shaped evolution
- gap gene pattern changed across time in Diptera
- evolve GRNs for pattern generation; diverse phenotypes
Self-organising patterns
- found @ all levels
- not everything is directed by positional info/morphogens
In self-organising systems
1) pattern emerges at the global level from local interactions amoung components @ a lower level in the system’s organisation
2) without intervention by external directing influences
- emergent property of system
- can work in combination
Reaction-Diffusion model
- two molecular sp.
- diffusion is actively driving the formation of a pattern
- autoactivation, activation and inhibition
- inhibitor diffused faster than
Reaction-Diffusion process
- start with a homogeneous by fluctuating field of cells
- little fluctuations are large enough to generate peaks in auto-activation
- pattern propagation w cyclical dynamic
- stripes/dots
- you can get pattern from homogeneity
- you don’t pattern to generate pattern
Turing patterns
- changing the interaction parameters: stronger/weaker
- diffusion can be longer/shorter
- generated different patterns (larger/smaller dots; more/less stripes)
- tweaks pattern by tweaking global parameters
Turing patterns in nature
- skin cells in lizards
- feathers in birds
Combinatorial systems
- tweaking parameters of underlying Turing system to change patterning from same origin
- e.g. 2 fin patterning in dogfish
- e.g.2. Digit patterning in mice
Turing models
- produce a “consistent wavelength” controlled by global parameters
- wavelength of system depends on proximal-distal position across fin/limb bud
- increase distal, finger coherence
Fin
- only a “Turing space” in the middle
- no digits, just dots
- racpitulated by Sox9
Morphogenesis by cell migration and motility
- e.g. neural crest migration in zebrafish
Animal cells are motile
- animal embryos rely on cell motility and migration for the generation of form
Tissues are patterned
As they develop and grow
Pattern formation is an emergent property of
- GRNs (positional info and self-organisation)
- Morphogenesis
- coupled dynamics
Diversity can emerge from
The evolutionary tinkering of pattern formation
