cell fate specification 2

processes underlying cellular specification

• occur during and after gastrulation

  • proliferation, specification, interaction, movement

• cell movements mean that cells are exposed to different environments after gastrulation, including different signals from other cells

nuclear transplantation experiments

• diploid nucleus transferred from adult frog to unfertilised egg which has had its own haploid nucleus destroyed by UV

• a tadpole is able to develop from the egg

genetic info is not lost as cell differentiate

internal vs external info

• cloning experiments demonstrate that genetic info is maintained throughout development

• instead, cells begin to turn on or off different sets of genes, leading to cell specification and differentiation

  • differential gene expression

cortical rotation

re-organisation of the cytoplasm in the newly fertilised frog egg. gastrulation begins in the grey crescent

origins of asymmetry in frog egg

• when the egg is divided along the plane of first cleavage into two blastomeres, each gets half of the grey crescent, and when experimentally separated, each develops into a normal embryo

• when only one of the blastomeres receives the entire grey crescent, it alone forms a normal embryo

grey crescent

• required to establish a DV axis

• gives rise to cells that initiate gastrulation

• cells form the dorsal lip of the blastopore

  • initial involution of cells, moving inside embryo to form initial pore

organisation of a secondary axis by dorsal blastopore lip tissue

• dorsal lip tissue from an early gastrula is transplanted into another gastrula in the region that normally becomes ventral epidermis (opposite side to normal)

• donor tissue invaginates and forms a second archenteron, then a second embryonic axis

• both donor and host tissues are seen in the new neural tube, notochord, and somites

• eventually a second embryo forms, joined to the host

does organiser splitting explain conjoined twins

• main theory- incomplete separation of a fertilised egg in a twinning event

• the node in birds and mammals is the functional homologue of the organiser

• if a second chick node is transplanted, the two primitive streaks (point at which gastrulation initiates) can fuse together

  • if a second region of the marginal zone expresses a key signalling protein (Nodal) a second axis can be generated

• since humans are thought to have the same modular pathway for making the primitive streak, it would seem possible that conjoined twins could be made by having two areas of the margin produce Nodal

where does the organiser come from

• fertilised xenopus egg has asymmetrically distributed factors

• sperm entry initiates cortical rotation, which relocates wnt signalling components to set up DV axis

• to specify organiser, you need a dorsal signal (beta catenin) and a vegetal signal (vg1/VegT)

induction of the organiser

• at last blastula stages, vg1 and VegT found in vegetal hemisphere, and beta catenin in dorsal

• act together to activate xenopus nodal-related genes (Xnr) to create a Xnr gradient, highest in dorsal region

high Xnr → dorsal mesodermal

medium Xnr → lateral mesoderm

low Xnr → lateral medoderm

model of organiser function in xenopus

• the organiser consists of pharyngeal endoderm, head mesoderm, notochord, and dorsal blastopore lip

• organiser functions by secreting proteins that block BMP signal that would otherwise ventralise mesoderm and activate epidermal genes in ectoderm

• in the head region, an additional set of proteins block the wnt signal from the ventral and lateral mesoderm