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classifications by germ layer

basal phyla: 2 germ layers

bilateriens: 3 germ layers

4 major branches of metazoans: PDPD

Porifera (sponges)

diploblasts;

protostomes; 3 germ layers

deuterostomes; 3 germ layers chordates choosing

OVERVIEW: cleavage, gastrulation, axis formation

cleavage

summary

series of mitotic division that the divides the enormous egg cytoplasm volume into smaller nucleated cells called blastomeres

no gaps (G1 and G2) for embryonic cell division

very fast 50,000 cells in 12 hours for frog

patterns of embryonic cleavage; 2 major parameters

amount and distribution of yolk protein within cytoplasm

yolk inhibits cleavage

factors in the egg cytoplasm that influence the angle of the mitotic spindle and the timing of its formation

holoblastic (complete) cleavage

isolecithal = even spread of yolk

mesolecithal = moderate spread og yolk

meroblastic (incomplete) cleavage- only a portion of cytoplasm is cleaved because too much yolk. the cleavage furrow doesn't penetrate the yolk portion of the cytoplasm

telolecithal = too much yolk

centrolecithal = yolk concentration in the middle; cell divides on edge around it.

mid-blastula transition (MBT)

mitosis promoting factors trigger mitosis in somatic cells

prevelant in M phase; undetectable in S phase

gap stages are added to cell cycle

synchronicity of cell division is lost

critical timing of transition from maternal to zygotic stages. new mRNA are transcribed

cytoskeletal mechanisms of mitosis

2 coordinated processes:

karyokinesis . (mitosis "M" stage)-PMAT

(mitotic) nucleus division

intitial step of cell division

sometimes followed by cytokinesis

mechanical agent: mitotic spindle

major protein composition: tubulin MT

location: central cytoplasm

major disruptive drug:

cytokinesis

cytoplasmic division into 2 complete cells

final step of cell division

can't happen without karyokinesis, occurs after "M" phase

mechanical agent: contractile ring

major protein composition: actin MF

location: cortical cytoplasm: cortical cytoplasm

major disruptive drug:

GASTRULATION- the most important time of your life

a combination of movements: types:

invagination: cell move inward..infold of cell sheet into the embryo. (sea urchin)

involution: in turning of cell sheet over the basal surface "inverlution"

ingression: migration of individual cell in embryo

delamination: splitting of one cell sheet over another (de= breakdown)

epiboly: epithelial sheets spread as a unit to enclose/coover deep layers of embro. how can it happen?

cells diving

cells changing shape

several layers of cells intercalating into fewer layers

during it, cells are given new positions and new neighbors ---> and the multilayered body plan of the organism is established

3 germ layers are produced

outer ectoderm

inner endoderm

interstitial mesoderm

what marks gastrulation?

extensive cell movement (migration)

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cell specification and axis formation

3 crucial axes of a bilaterally symmetrical animal "axes are rad"

Right-left axis

Anterior-posterior axis

Dorsal-ventral axis

4 invertebrates

sea urchin

snails

tunicates

c. elegans

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