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Development generally involves:
- pattern formation
- morphogenesis
- cell differentiation
- growth
Principles:
- progressive development / epigenesis
- starts with a single cell
- progressive restriction in developmental potential
- balance between proliferation and apoptosis
- oriented / asymmetric cell division
- fields and boundaries
- cell-cell signalling and induction
- differential cell affinities
Result of evolution:
- non-optimal design features
- redundant mechanism
in face of fluctuations
Development is progressive and the fate of cells becomes determined at different times
reliability of development:
Internal fluctuations:
- mutation in genes
- changes in molecule concentrations
Development may involve metamorphosis:
External fluctuations:
- temperature
- environmental chemicals
Solutions:
- apparent redundancy of mechanism
- negative feeadback
many different states of gene activity
Devico Shi
Morphogen:
a chemical whose concentration varies and which is involve in pattern formation
6. Developmental fields and boundaries
2. Development starts with a single cell
4-D problem
- Fields result in boundaries
1. Progressive development
- Patterning involves interpretation of positioning value
- spatial arrangement
- temporal development
a generative rather than descriptive program
Pattern formation
the process by which cellular activity is organised in space and time so that a well-ordered structure develops within the embryo
Epigenesis:
Body axes:
anterior (head); prosterior (tail)
dorsal (back); ventral (front)
body axes show polarity
- Axis formation
- different germ layers
Different germ layers
ectoderm: epidermis of skin, nervous system
mesodem: skeleton, muscle, kidney, heart, blood
endoderm: gut, liver, lungs
Morphogenesis
gastrulation
involve extensive cell migration
the theory that an embryo develops progressively from an undifferentiated egg cell
7. dependence on cell-cell signaling
9. Cell-cell affinities affect final shape
cell sorting, cell shape and morphogenesis can be achieved by differential cell affinities
Different signals can activate a particular gene at different stages of development
the same signal can be used to elicit different responses in different cells
Two types of inductions:
- permissive
- instructive
The response to inductive signals depends on the state of the cell: competent to respond, limited number of possible responses
8. Developmental induction
Different modes of cell-cell signaling
Induction:
3. Progressive restriction in developmental potential
There are many times in development where a signal from one group of cells influences the development of an adjacent group of cells
All cells in an organism have the same genetic information
Cells become progressively restricted in developmental potential
Cells influence each other's fate by producing and responding to signals
- diffusible signal (long range)
- diffusible signal (short range)
- signaling by cell contact
- signaling by cytoplasmic connection
Most cells differentiate
A few remain less committed
- Totipotent: capable of giving rise to any cell type or a complete embryo
By their response to signals, cell generate the physical forces for morphogenesis
- Pluripotent: give rise to all of the cell types that make up the body, except those that form a placenta or embryo
- Multipotent: develop into more than one cell type, but limited to its ability to differentiate
Cytonemes:
5. Oriented cell division
thin cytoplasmic extensions that allow contact across several cell diameters
4. balance between proliferation and cell death
apoptosis
can generate pattern and shape
precise control: correct body proportions
- asymmetric division
- localisation of cytoplasmic determinants
- control of spindle position
- random events