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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)
formation of specific 3D structures (tissues and organs)
what was Haekel's philosophy of development?
linear evolution, those less developed forms stop developing earlier in the line
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
What is differentiation?
development of specialized cell types that have specialized gene proteins
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
what happens before differentiation?
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
what is it called when a cell - cell interaction affects cell fate?
- - diffusible factors, cell bound molecules, extracellular matrix
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
what happens if you put Activin into presumptive ectoderm from animal hemisphere? what is it an example of?
- you get mesoderm
- morphogen gradient
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
what can stem cells do that progenitors cells cant do?
freeze in time.
what is the pathway from pluripotent stem cell to differentiated cell?
pluripotent - committed stem cell - progenitor cell - differentiated cell
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
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
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
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
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
how do cadherins relate to differential cell adhesion?
cells regulate the: type of cadherin, amount of cadherin, and the timing
what are the 2 main classes of cells in the embryo?
- epithelial (connected, adherens, sheats or tubes, cell junctions)
- - epithelial mesenchymal transitions (transition from one form to another)
what affects morphogenesis?
cell divisions, shape, movement, growth, death
what happens if mesenchymal cells condense?
they will form epithelia cells
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
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
how is cell differentiation regulated by?
- differential gene transcription
- RNA processing
- mRNA translation
- protein modification
what is necessary for induction to happen?
- signal (ligand)
- cell tissue needs a receptor to be expressed (competence)