cell fate specification 1

fate maps

dye injection into cells allows you to follow the labelled cells and find out what each cell will contribute to the final organism

germ layers and regions within them can be established

are predictable - all early embryos of the same species have the same fate map

fate mapping with transgenic DNA

• transplant a region of neural tube from a transgenic chick labelled with GFP into the same place in an unlabelled embryo

• follow the fates of the neural crest cells from this region

tracing cell lineages today

• modern lineage tracing is very precise. cells can be labelled via viral vectors or genetic methods and tracked via computer

• e.g. brainbow - random fixation of cells with distinct colours, allowing descendants to be tracked

what fate maps can tell us

• show us where tissues of each germ layer come from

• allow the description of anatomical changes in terms of changes in cell behaviour, which convert the egg into the fully formed organism

what fate maps can’t tell us

• states of cell determination

• when cells become determined (if they do)

• what molecular factors are responsible for the cell’s fate

glass needle experiment

• used a hot needle to kill one cell in a frog embryo at the 2 cell stage

• blastula formed only a half embryo on one side

autonomous specification

• specification by differential acquisition of certain factors present in egg cytoplasm

• invariant cleavages produce same lineages in each embryo, and cell type specification precedes any large-scale cell migration

• results in mosaic development

• predominates in most invertebrates

regulative development experiment

• separating at 4 cell stage gave 4 small larvae

• at these stages, blastomeres are able to give rise to entire embryonic pattern (totipotent)

natural examples of regulative development

• monozygotic twins from a single embryo, which split into two cells after fertilisation

• slight differences in appearance are due to non-genetic factors

human mechanisms of twinning

• dizygotic twins result from implantation of two fertilised eggs

• if split (in monozygotes) is at 2 cell stage, they will have separate membrane

• if split occurs after trophoblast formation, but before amnion formation, twins have own amniotic sacs, but one chorion

• if splitting occurs after amnion formation they will share an amniotic sac- usually not survivable

conditional specification

• predominates in vertebrates

• specification by interactions amongst cells

  • cell positions relative to each other are key

• capacity for regulative development allows cells to acquire different functions as a result of interactions with neighbouring cells

• variable cleavages

cell commitment experiments

• as development proceeds, blastomeres lose totipotency and become committed to particular developmental fates

• commitment is a restriction of developmental potential and a first step towards differentiation

• found when an 8 cell sea urchin embryo was split into 4 call animal half and vegetal half, each producing an incorrectly developed embryo

cell fate determination

• two differentially positioned blastula cells are specified to become distinct muscle and neuronal cells when places in isolation

• when placed together in culture, a cell that is specified to form muscle cell can adopt a neuronal fate due to interactions with neighbouring cells

• this will not happen with a determined cell

regulative development

• some early embryos have considerable capacity for regulation (high developmental plasticity)

• during development, most cells in embryo become more restricted in developmental potential

syncytial specification

transcription factors form gradients within a large cell that contains many nuclei. the nuclei express genes depending on the ratios of those factors

• predominates in most insect classes

• specifications of body regions by interactions between cytoplasmic regions prior to cellularisation of the blastoderm

• variable cleavage produced no rigid cell fates for particular nuclei

• after cellularisation, both autonomous and conditional specification are seen