Lecture 15: How Cells Become Different

Question 1

gastrulation in a nutshell

Answer 1

zygote -> cleavage (morula) -> blastula -> gastrula

intially totipotent -> later become less potent, more specified

Question 2

how do cells become different

Answer 2

step-wise restriction of “cell fate potential”
-cell intrinsic (cell autonomous) - making cells different, asymmetric cell divsion
-cell extrinsic (non-cell autonomous) - cell-cell communuication w/ outside env. (signals)

Question 3

what makes cells different

Answer 3

differential gene expression

terminally differentiated: potential is reached, cannot become anything else

Question 4

cell fate

Answer 4

describes what cells will normally develop into

differentiated cell:
cell thst has become specialized (e.g. liver)
usually refers to terminal state of a cell but can also refer to intermediate states (mesoderm, endoderm, etc.)

Question 5

commitment

Answer 5

cells may have become committed to a particular fate even though they may not look differentiated

the process of commitment can be divided into two phases:
specification: commitment to a particular fate, but in a lanile or reversible manner
determination: irreversible commitment to a particular fate

Question 6

test of specification (isolation experiment)

Answer 6

put the cell or tissue in isolation in a neutral environment (e.g. tissue culture) what does the cell become?

1. if it expresses a different fate than it would normally, it was NOT specified and NOT determined
2. if it expresses its normal fate, it is specified but may or may not be determined.

must do peturbation to know if differentiated

Question 7

in vitro (in a dish) test for determination

Answer 7

put the cell or tissue in a new environment. what does the cell become?

1. if it still expresses its normal fate, then it is determined
2. if not, then it is not determined

Question 8

in vivo test for determination

Answer 8

cells are DETERMINED if transplantation from their normal position to a new position in the embryo does not change their normal fate

ask tutor for help

Question 9

commitment to a particular fate is progressive

Answer 9

each step is irreversible -> cells have memory
cell fate becomes increasingly restricted during development

Question 10

is the genome changing during development?

test: nuclear transplantation

Answer 10

Question 11

first animal cloning experiment

Answer 11

Gurdon (!962) showed that the nucleus from fully differentiated tadpole skin cell can direct normal frog development

clone frogs look like donor frogs
nucleus contains all info needed to develop

Question 12

differential gene expression theory

Answer 12

1. the genome is contant in all somatic cells
2. only a small proprotion of the gneome in any cell type is expressed
3. different cell types express different groups of genes that confer unique cellular characteristics
4. unused genes that are not transcribed are not mutated or destroyed; they retain potential to be expressed (skin cells can be reactivated/reprogrammed)

Question 13

differential gene expression makes cells different

Answer 13

ask tutor to explain

Question 14

cell autonomous (ASYMMETRIC cell divsion)

Answer 14

cell intrinsic mechanism
sister cells are born different
asymmetric cell divsion

1 cell -> 2 different cells

Question 15

non-cell autonomous

Answer 15

cell extrinsic mechanism
involves “cell signaling”
sister cells become different as a result of influences acting on them after their birth

inductive signaling; lateral inhibition

Question 16

cell signaling is …

Answer 16

a major cell EXTRINSIC mechanism of cell diversification and organismal development

Question 17

extracellular signaling molecule examples

Answer 17

proteins, peptides, amino acids, nucleotides, steroids, fatty acid derivatives, gases

signaling molecule could be: part of the extracellular space, attached to another cell, flowing through the circulatory system

Question 18

cell signaling diagram

Answer 18

intracellular: relay, amplify, integrate, distribute

effector may alter: gene expression, metabolism, cytoskeleton

response: cell division, growth, survival, migration, secretion, contraction, differentiation

ask tutor

Question 19

inductive signaling

Answer 19

some inductive signals function in an all-or-none manner

some inductive signals function in a concentration-dependent manner- morphogens

Question 20

signaling molecules can function as morphogens (what are they?)

Answer 20

a morphogen is…
a diffusable molecule secreted from a source (Signaling cell or cells)

forms a graded distribution (concnetration gradient: higher near the ource and lower farther away)

produces specific cellular responses depending on its local concentration

provides a mechanism to specify fates in a reproducible pattern

Question 21

example of a morphogen gradient

Answer 21

inappropriate expression of Shh or Shh signaling causes extra digit form

ask tutor

Question 22

lateral inhibition

Answer 22

cells born as equivalent cells can adopt different fates in the absence of tissue asymmetries ( due to cell-cell communication)

both cells produce the same amount of X, and inhibit the production of X in their neighbor equally

a transient increase in X produced by cell 1 causes a stronger inhibition of X production by cell 2

a decrease in X produced by cell 2, allows cell 1 to make more X

Question 23

how does lateral inhibtion work?

Answer 23

group of equivalent cells, cells compete by inhibiting neighbors, some cells predominate

lateral inhibtion works between two neighboring cells that are directly adjacent to each other

Question 24

in sum - how do cels become different?

Answer 24

Question 25

cell-cell communication in animal development is mediated by only a small number of conserved cell-cell signaling pathways, including

Answer 25

transforming growth factor-B (TGFB)
Wnt
Hedgehog (Hh)
Notch
Receptor tyrosine kinase (RTKs)

yet, there is enormous cell diversity in animals

Question 26

the same inductive signal can…

Answer 26

generate different responses

initally identical but receive diff. signals due to location

Question 27

sequential induction

Answer 27

initially 2 groups of cells that can generate a lot of cell diversity from signals