BIOL2010 LT3 Positional Information and Morphogens
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Course
UCL BIOL2010 Developmental Biology
Institution
University College London (UCL)
Covering whole lecture from Developmental Biology: Introduction to the concept of morphogens and how they convey positional information - also with examples from Drosophila. Extra reading with sources cited.
Cell fate depends on cell position within the overall pattern
Cells have ability to respond differently to different concentrations of biological
molecules in their environment
Morphogen: diffusible biochemical molecule that can determine the fate of the cell by
its concentration (specifies cells in a quantitative. graded manner) (positional
information given by the concentration gradient)
- Can be transcription factors produced within cells or paracrine factors produced
in one group of cells then travel to another population of cells, specifying the
target cells differentially according to concentration of morphogen
Lewis Wolpert’s French flag model (1996)
- All multipotential cells – a morphogen gradient from source to sink – cells sensing
highest concentrations turn blue
- There is a threshold morphogen concentration below which cells turn white
- When morphogen falls to another threshold, turn red
Crick 1970 – morphogen diffuses from its site of synthesis (source) to its site of
degradation (sink)
- Drop in concentration can be due to simple diffusion, to the cells binding the
morphogen and thus “using it up”
- Or to a combination of a source synthesising a morphogen and an environment
containing an enzyme that degrades it
Morphogens have been part of many models, experimental evidence from polarities
of embryos, specification of cell identity in specific tissues (limb appendages and
nervous system of both vertebrates and Drosophila) (Tabata and Takei, 2004)
Many mechanisitic issues – how a signal is perceived and interpreted in a graded
manner by receiving cells has received less consideration how the signal is
perceived and interpreted in a graded manner, how does graded signalling
accommodate fluctuations in biological conditions to achieve necessary robustness
required for accurate developmental patterning?
- A morphogen at a minimum by definition should direct generation of at least 2
different cell types at different concentrations – theoretical analysis raises
possibility of 30 thresholds (Lewis et al., 1977), empirically 3-7 thresholds (DV
axes of Drosophila embryos – Dorsal establishes 4-7, activing in Xenopus
blastula 5 states, Shh also 5 in neural tube signalling)
- Issues of maintaining accuracy and error correction mechanisms, how gradients
are scaled to accommodate variability in tissue sizes during development (Ashe
and Briscoe, 2006)
,Syncytial Specification
Uses elements of autonomous and conditional
specification
Early embryos of insects, nuclei divide within the egg
– but cell does not divide
Cytoplasm that contains many nuclei = syncytium
Insect egg cytoplasm is not uniform – nuclei in
anterior part of the cell will be exposed to
cytoplasmic transcription factors not present in
posterior part of the cell and vice versa
Interactions of nuclei and transcription factors –
eventually resulting in cell specification take place in a
common cytoplasm
Early Drosophila Development
Oogenesis: germ cell divides 4 times = single oocyte and 15
nurse cells
Cells surrounded by follicle cells (form the future egg
shell) – ovarian cells of somatic origin
Sperm enters an egg that is already activated
- Egg activation accomplished at ovulation, few minutes
before fertilisation
- Drosophila oocyte squeezes through a narrow orifice,
ion channels open, allowing calcium ions to flow through it
- Oocyte then resumes meiotic divisions and cytoplasmic mRNAs become
translated without fertilisation (Horner and Wolfer, 2008)
Nurse cells export large amounts of maternal RNA and protein into oocyte –
required for first few hours of embryonic development
- Anterior-posterior axis is established by localising specific mRNAs to either
pole (eg. bicoid, localised in anterior pole, promotes head development)
- First dorsal-ventral patterning also occurs – reciprocal signalling between oocyte
and follicle cells
- Yolk proteins into oocyte via haemolymph
Fertilisation: only one site (micropyle) at the future dorsal anterior region of the
embryo, where sperm can enter embryo
Micropyle is a tunnel in the chorion (hardshell) which prevents polyspermy
By the time the sperm enters, the egg has already begun to specify axes
, After fertilisation, 1st 12 cleavage divisions not accompanied by cytokinesis, nuclei
therefore part of a syncytium
By 9th division - ~5 nuclei become enclosed by cell membranes and generate pole
cells that give rise to gametes of an adult
Most insect eggs also undergo superficial cleavage – a large mass of centrally
located yolk confines cleavage to cytoplasmic rim of the egg – forms a syncytial
blastoderm
Nuclei divide within common cytoplasm but cytoplasm is not uniform – Karr and
Alberts 1986, have shown that each nucleus within the syncytial blastoderm is
contained within the territory of cytoskeletal proteins
- When nuclei reach the periphery of the egg during the tenth cleavage cycle,
each nucleus becomes surrounded by microtubules and microfilaments
- Nuclei and their associated cytoplasmic islands = energids
Following division cycle 13, oocyte plasma membrane folds inward between the
nuclei, eventually partitioning off each somatic nucleus into a single cell – process
creates cellular blastoderm
- Membrane movements, nuclear elongation and actin polymerisation each appear
to be coordinated by microtubules (Riparbelli et al., 1007)
- First phase of blastoderm cellularisation is characterised by the invagination o
cell membranes between the nuclei to form furrow canals *process can be
inhibited by drugs that block microtubules)
- After furrow canals have passed the level of the nuclei, second phase of
cellularisation occurs
- Rate of invagination increases and the actin-membrane complex begins to
constrict at the future basal end of the cell (Foe et al., 1993)
- In Drosohila, cellular blastoderm consists of ~6000 cells and formed within 4
hours of fertilisation
Mid-blastula transition: slowdown of nuclear division, cellularisation, concomitant
increase in new RNA transcription
- At this stage – maternally provided mRNAs are degraded and hand over control
of development of the zygotic genome (Bradnt et al., 2006)
- This transition seen in embryos of numerous vertebrate and invertebrate phyla
- Coordination of mid-blastula transition and maternal-to-zygotic transition
controlled by several factors – ratio of chromatin to cytoplasm (due to
increasing amount of DNA while cytoplasm remains constant – Edgar et al., 1986
– compared development of wild-type embryos to mutant haploid embryos which
have less DNA; underwent an extra 14th division before cellularisation), Smaug
protein (RNA binding protein often involved in repressing translation – in mid-
blastula transition targets the materal mRNAs for destruction Tadros et al.,
2007, mutants disrupt slowing down of nuclear division, prevent cellularisation,
thward increase in zygotic genome transcription), cell cycle regulators (as
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