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what is development?
single cell (zygote) cleaves and grows into multicellular organism with organized body plan with differentiated cell, tissues, and organs (1 cell to a human)
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morphogenesis?
formation of specific 3D structures (tissues and organs)
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what was Haekel's philosophy of development?
linear evolution, those less developed forms stop developing earlier in the line
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Ernst von Baer philosophy?
- 1) more specialized thing like hair occurs later in development
- 2) more specific traits are modifications of more general ones
- 3) the embryos of a particular species diverge more and more from those of other animals as development proceeds
- 4) the young embryo of a "higher" animal is not like the adult of a "lower" one
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What is differentiation?
development of specialized cell types that have specialized gene proteins
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What are the 2 stages of commitment?
- 1) specification - cells can differentiate autonomously when placed in a neutral environment (can still change their mind if put in different environment)
- 2) determination - cells can differentiate autonomously even when placed in another region of an embryo
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what happens before differentiation?
commitment
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how can determination occur?
- 1) segregation of cytoplasmic determinants (morphogenetic determinants) ... assymetric distribution = when the cell divides you get 2 different types of cells because of uneven distribution of granules
- 2) cell - cell interactions
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what is it called when a cell - cell interaction affects cell fate?
- induction
- - diffusible factors, cell bound molecules, extracellular matrix
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what is a morphogen? morphogen gradient? source and sink?
- cell fates determined by how much morphogen it is exposed to
- soluble molecule = morphogen
- source = where morphogen comes from
- sink = low concentration
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what happens if you put Activin into presumptive ectoderm from animal hemisphere? what is it an example of?
- you get mesoderm
- morphogen gradient
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what is a stem cell?
what can it do?
- a cell with the capacity to divide indefinitely and can give rise to more specified cells
- - fx: replace tissue, can grow or can freeze
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what can stem cells do that progenitors cells cant do?
freeze in time.
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what is the pathway from pluripotent stem cell to differentiated cell?
pluripotent - committed stem cell - progenitor cell - differentiated cell
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totipotent? pluripotent? multi? uni?
- T - can give rise to an entire organism
- P - all 3 germ layers
- M - committed to 1 germ layer (ex: all diff kinds of blood cells)
- U - committed stem cells, committed to one fate, it can still freeze in time though
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what will happen if you mix presumptive epidermal cells and neural plate cells together? what is this an example of?
- they will spontaneously reaggregate, epidermal cells on outside and neural crest on inside b/c they display selective affinities for each other
- DIFFERENTIAL CELL AFFINITY
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Townes and Holtfreter experiments:
(related to select affinity)
- 1) reaggregated cells become spatially segregated
- 2) final positions of reaggregated cells reflect their embryonic position
- 3) tissues display selective affinities
- 4) selective affinities change during development
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What is the Differential Adhesion Hypothesis?
- Tissues form hierarchies of adhesiveness
- whatever sticks the strongest to itself (high surface tension) will be in the middle
- has to do with thermodynamics
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molecular basis for cell - cell adhesion?
- adherins junction:
- noncovalent interaction between 2 cadherins on 2 diff epithelial cells, creates zipper band
- homotypic binding
- calcium dpt
- inside cell membrane: catenins bound to actin cytoskeleton, phosphate
- outside CM: calcium binding sites and adhesive recognition site
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how do cadherins relate to differential cell adhesion?
cells regulate the: type of cadherin, amount of cadherin, and the timing
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what are the 2 main classes of cells in the embryo?
- epithelial (connected, adherens, sheats or tubes, cell junctions)
- mesenchymal
- - epithelial mesenchymal transitions (transition from one form to another)
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what affects morphogenesis?
cell divisions, shape, movement, growth, death
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what happens if mesenchymal cells condense?
they will form epithelia cells
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dispersal?
delamination?
intercalation (cell migration)?
- entire epithelia become mesenchyme (no epithelia left)
- epithelia stays intact but some of the cells in epithelia leave and become mesenchymal
- rows of epithelia merge to form fewer rows
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how do genomic equivalence and the differential gene expression concepts relate to each other?
- all cells of organism have the same genome (except for b and t cells)
- differentiation is about EXPRESSING different genes
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how is cell differentiation regulated by?
- differential gene transcription
- RNA processing
- mRNA translation
- protein modification
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what is necessary for induction to happen?
- signal (ligand)
- cell tissue needs a receptor to be expressed (competence)
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