cell dev. 2
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- dictyostelium life cycle
- 1. starvation 2. aggregation 3. slug 4. culmination
- starvation (slime mold)
- (refractory period) burst of gene activation: glycoproteins, cAMP receptors, adenylate cyclase, phosphodiesterase, phosphodiesterase inhibitor
- aggregation (slime mold)
- 1. Cell releases cAMP 2. When cAMP receptors are full, stops migrating 3. Posphodiesterase breaks down bound cAMP 4. Phosphodiesterase inhibitor once receptors are empty 5. Once aggregated, cells produce cellulose = slime sheath 6. Slug migrates
- pseudoplasmodium
- (slug) -establishes ant/post axis -migrates towards light/heat/low humidity -cellulose sheath (slime) -ant 20% = pre-stalk, rest pre-spore cells -regulative -cells migrate w/in slug
- differentiation in pseudoplasmodium
- -evidence that anterior responds more to & produces more cAMP -DIF (differentiation-inducing factor), small lipid, may change Ca++ response of cells
- culmination (slime mold)
- 1. slugs settle down in response to sunlight/low humidity b/c more diffusion of ammonia from slug (tested w/ activated charcoal) 2. When ammonia is depleted, culmination 3. Stalk cells change cell walls to become sturdier, spore cells climb up, stalk cells die
- polar lobe
- 1. formed on vegetal pole of snail zygotes after fertilization 2. always taken by one cells in cleavage ("trefoil" 2-cell stage) 3. contains mesodermal determinants (mosaic dev.) 4. determinants located in cortical cytoplasm (cannot be pipetted out)
- Ascidians
- chordates, tunicates/sea squirts, contain yellow crescent
- Ascidian cleavage pattern
- 1st: right/left axis 2nd: ant/post axis 3rd: a, b, A, and B pairs of blastomeres
- fate map of newly-fertilized Ascidian zygote
- clear, animal region: ectoderm grey (left vegetal): neural & ectoderm clear grey (mid-left vegetal): notochord yolky grey (center vegetal): endoderm yellow crescent (right vegetal): muscle
- Ascidian early development
- 1. (w/in 5 minutes of sperm entry) Sperm pronucleus migrates on MTs organized by sperm centriole, yellow cytoplasm also moves on these MTs 2. (over next 20 min) Yellow cortical cytoplasm is localized 3. (after 30 min) 5-sectioned fate map
- yellow crescent experiments
- 1. transplantation of yellow cytoplasm --> muscle cells 2. transplantation of nucleus in yellow crescent cell --> no muscle
- Ascaris
- -nematode -has only 4 chromosomes -undergoes chromosome diminution
- MACHO-1
- -in Ascidians -transcription factor -contained in yellow crescent cytoplasm -if its mRNA is lost --> larva missing tail muscles -if its mRNA is injected into non-yellow-crescent cells --> muscle descendants -protein turns on tbx6 genes --> code for TFs --> turn on genes for muscle actin, myosin, SNAIL
- SNAIL
- an Ascidian TF that keeps cell from becoming notochord, silences unwanted genes (turning off other fates)
- Ascaris early cleavage
- 1st: equatorial, into 100% animal cytoplasm, 100% vegetal -chromosome diminution in animal blastomere 2nd: incomplete chromosomes in 2 animal blastomeres, intact in vegetal 2 -more diminution 3rd: only most vegetal has complete chromosomes *cells with full chromosomes give rise to germline
- hydra
- gradients: head activator & head inhibitor (also foot...) -activator = long-term, has to do w/ cell identity -inhibitor needs to be continually produced to maintain gradient (by hypostome) -cells can change identity as the move through the animal
- C. elegans
- -nematode -model organism: short life span, can trace lineage of each cell, transparent, small, cheap -every wt hermaphroditic adult has exactly 959 somatic cells
- c. elegans early cleavage
- 1st: anterior, founder cell (AB) & posterior P1 stem cell (divides to give 1 differentiating cell & 1 stem cell)
- PAR proteins
- (in nematodes) partitioning, localized by MTs to determine ant/post axis
- P-granules
- (nematodes) -localized by PAR proteins into P blastomere -necessary to become a germ cell
- PIE-1
- (nematodes) -protein needed to become a germ cell -turns off gene expression, resulting in very little transcription in PGCs, preventing differentiation in early development when somatic cells are specializing
- mammalian germ cell determination
- induction using BMP4 to signal a very small group of cells to become germline
- fly PGC migration
- 1. passive movement into mid-gut region 2. migrate from there by chemotaxis: repulsed by signal from mid-gut, attracted by signals from gonads
- zebra fish PGC migration
- 1. VASA protein marks PGCs (only 4 cells at 1,000-cell stage) 2. PGCs form 4 clusters and follow gradient of sdf1 secreted by gonad
- frog PGC migration
- -cells migrate w/ single large filopodium along oriented ECM fibrils (= contact guidance) -also gradient of fibronectin = evidence of haptotaxis
- mammalian PGC migration
- 1. PGCs express protein STELLA 2. chemotaxis towards sdf1 from gonad 3. stimulated to divide while migrating by surface-bound growth factor on cell substrate -if a migrating PGC gets lost, can give rise to teratocarcinoma, can contain many different tissues
- sdf1
- chemoattractant produced by gonads in both mammals and zebrafish during PGC migration
- bird PGC migration
- 1. PGCs formed in germinal crescent region of embryo 2. embryo makes circulatory system 3. PGCs can enter blood vessels via diapedesis 4. evidence for chemotaxis; evidence that gonads have more sticky blood vessel walls to PGCs -these signals are not species-specific
- amphibian cytoplasmic organization
- 1. sperm enters through animal hemisphere (animal hemi. contains cortical pigment granules) 2. sperm centriole organizes MTs into transient parallel array at interface between vegetal cortex and inner cytoplasm 3. cortical rotation: 30 degrees towards point of sperm entry 4. few pigment granules left behind by animal hemisphere form grey crescent
- grey crescent
- -seen in amphibian zygotes -future dorsal side -region where gastrulation is initiated -in many species, there is no visible crescent even though the cortical rotation takes place
- frog early cleavage
- -holoblastic, but lots of yolk slows cleavage furrow formation, 2nd cleavage starts before 1st is complete -vegetal blastomeres are larger, yolkier -in 1st 12 cleavages, almost no transcription (eggs come with lots of mRNA, all proteins made at this stage are from maternal DNA) -transcription starts at mid-blastula transition -blastula has small animal blastocoel and both surface and deep cells -blastocoel separates animal and vegetal blastomeres and provides site for cell migration & movement
- amphibian gastrulation
- -all endoderm is internalized and the blastocoel is displaced -invagination, involution, convergent extension, epiboly
- amphibian gastrulation- Involution
- (2nd step) -initiated by bottle cells -cells turn a corner and migrate back on the underside of their former position -provides new cell-cell contacts -gets mesodermal cells in place
- amphibian gastrulation- Convergent extension
- (3rd step) -cells change adhesive and migratory properties to change overall shape of tissue from short/round/squat to long/thin -changes in cadherin expression -Ca++ flux changes actin dynamics and affects cell migratory properties
- amphibian gastrulation- Invagination
- (1st step) -Bending and buckling of sheets of surface cells -caused by certain cells changing shape and adhesive properties to become bottle cells and form the blastopore
- Spemann- separate 2-cell stage frog embryo
- twins, if each contains grey crescent
- Spemann- separate 2-cell stage frog embryo artifically made so that 1 cell contains all the grey crescent material
- cell with no grey crescent gives rise to a "belly piece" (no mesoderm); cell with grey crescent gives a normal tadpole
- Spemann- transplant frog dorsal lip cells in early-mid gastrulation
- transplant induces/organizes 2nd embryonic axis --> conjoined twins
- Spemann- transplant frog presumptive neural tissue into host gut region at early-mid gastrulation
- donor tissue becomes epidermis
- Spemann- transplant frog presumptive neural tissue into host gut region at late gastrulation
- neural tube develops on belly
- presumptive dermal layers in frog blastula
- animal cap cells: ectoderm marginal zone cells: mesoderm vegetal cells: endoderm
- Nieuwkoop- put animal cap on vegetal chunk
- mesodermal cells formed at animal/vegetal contact region (orientation of mesoderm determined by orientation of vegetal chunk)
- amphibian gastrulation- Epiboly
- (4th step) -ectodermal cell sheet spreads over entire embryo -caused by cell division and flattening -covers yolk plug