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Principles of

developmental biology

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

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