Lecture 5 - Tissue Patterning

Question 1

define cell differentiation

Answer 1

the acquisition of specialised cell functions (cell fates) via differential genome expression

Question 2

ectoderm

Answer 2

epidermis and nervous system

Question 3

mesoderm

Answer 3

muscles, connective tissue, bones, blood, kidneys, etc

Question 4

endoderm

Answer 4

gut, lungs, pancreas, liver, etc

Question 5

what is a lineage tree and what does it show?

Answer 5

it is a diagram that shows you the cells that you get after a zygote is formed as a function of time

Question 6

what is required for different cell fates?

Answer 6

differential genome expression

Question 7

how is differential genome expression achieved?

Answer 7

through cis regulatory elements - this is what transcription factors bind to, controlling transcription

Question 8

state two ways in which cell fate can be acquired

Answer 8

1. asymmetric division
2. symmetric division, then perception of a signal

Question 9

asymmetric vs symmetric division

Answer 9

asymmetric - child cells are born with different fates
symmetric - child cells are born the same, but acquire different fates as a result of influences acting on them

Question 10

asymmetric division —– partitions cell fate determinants

Answer 10

unevenly

Question 11

describe asymmetric division

Answer 11

• some specific cell fate markers are unevenly distributed before division
• after division, one child cell will inherit more of this cell fate marker than the other

Question 12

what does asymmetric division require?

Answer 12

correct spindle alignment and cytokinesis

Question 13

are most proteins and cellular components distributed evenly or unevenly?

Answer 13

evenly

Question 14

what can symmetric and asymmetric divisions create?

Answer 14

tissue patterns

Question 15

describe how asymmetric and symmetric divisions can work together to create functional organisms

Answer 15

• some cells divide symmetrically to create a field of similar cells
• other cells continue divide asymmetrically to create several different cell types

Question 16

in a cell division graph, what do different colours and black lines mean?

Answer 16

different colours = different types of cells
black lines = sister cells

Question 17

after symmetric division, what are three ways in which cell fate may be acquired?

Answer 17

1. lateral inhibition
2. induction by diffusible signals
3. other mechanisms

Question 18

define stochastic

Answer 18

temporary

Question 19

describe lateral inhibition

Answer 19

1. both cells might start the same
2. some tiny or short-lived difference between the two cells tips the balance
3. molecular mechanisms will amplify these differences
4. the cells will acquire different fates

Question 20

what results from lateral inhibition?

Answer 20

creates a pattern of isolated differentiated cells in a field of relatively undifferentiated cells

Question 21

give an example of lateral inhibition

Answer 21

some fly epidermal cells will develop into sensory bristles

Question 22

describe Delta and Notch as an example of lateral inhibition

Answer 22

Delta activates Notch signals; Notch signals inhibit Delta expression:
1. at the start, both the top and bottom cells have Delta, activating Notch in each other.
2. Notch inhibits cell specialisation and Delta expression.
3. Eventually, one cell will win out - it will have active Delta and will specialise, whilst activating Notch on the neighbouring cells and causing them to remain unspecialised.

Question 23

describe how induction by diffusible signals happens

Answer 23

• one or more cells in the organiser tissue secrete a morphogen
• morphogens are diffusible signals that can affect cell differentiation; they diffuse and act on nearby cells
• cells respond to the morphogenetic by taking on a new cell fate
• this creates a pattern of bands or rings of different cell fates around the source

Question 24

why is it that morphogens have such diverse impacts?

Answer 24

cells can respond differently to the same morphogenetic given at different concentrations

Question 25

what does the diffusion range of the morphogen depend on?

Answer 25

- how much morphogenetic is made and for how long - diffusion rate of the signal - stability of the signal

Question 26

are morphogens always active?

Answer 26

no; morphogens cannot be indefinitely active

Question 27

how were morphogens discovered?

Answer 27

through transplant experiments

Question 28

describe the transplant experiments that were conducted on amphibian embryos to identify morphogens

Answer 28

- two copies of organiser tissue created two dorsal sides and a shared ventral side - the organiser tissue induced structures that the host would not normally make (eg an amphibian with two backs) - however, if the opposite (ventral) piece was transplanted, nothing was observed

Question 29

why is it that we did not observe anything unusual when repeating the transplant experiment with a different tissue?

Answer 29

the ventral cells are not organiser cells, so they don't make a difference on the development of the tissue

Question 30

totipotent cells

Answer 30

can become any cell type (usually only cells very early after fertilisation)

Question 31

pluripotent cells

Answer 31

can become any adult cell type

Question 32

give an example of pluripotent cells and explain why they are not totipotent

Answer 32

blastomeres; they lose the ability to form ALL extra-embryonic tissues.

Question 33

multipoint cells

Answer 33

can become multiple cell types

Question 34

give two examples of multipotent cells

Answer 34

mesoderm, ectoderm

Question 35

what are two ways in which multiple signals can be combined during cell fate determination?

Answer 35

- cell memory/sequential signaling - combinatorial signaling

Question 36

distinguish between cell memory/sequential signaling and combinatorial signaling

Answer 36

cell memory/sequential signaling: a cell’s past exposures to signals change how it responds to later signals. combinatorial signaling: a cell’s response depends on multiple signals received at the same time.

Question 37

what is a possible effect of sequential induction and how does this work?

Answer 37

sequential induction can generated regulatory hierarchies: - the same morphogenetic produced by cell type C can have different effects on cell types A and B - this can divide the body into segments

Question 38

what do regulatory hierarchies consist of?

Answer 38

gene regulation affecting other gene regulation affecting other gene regulation

Question 39

give an example of how multiple signals can overlap to create complex patterns

Answer 39

different transcription factor gradients can produce really complex patterns and combinations

Question 40

what is the impact of segmentation on the animal body?

Answer 40

it creates repeats with variation in the animal body

Question 41

describe how transcription factors act in a regulatory hierarchy in drosophila

Answer 41

1. anterior-posterior 2. define head, thorax, abdomen 3. segments within the heat, thorax, and abdomen 4. polarity within each segment 5. body parts develop from segments due to Hox proteins

Question 42

what is the function of Hox genes?

Answer 42

they determine which body parts will develop from a segment

Question 43

how is Hox gene expression organised?

Answer 43

- Hox genes are expressed in different segments and specify different body parts - Hox genes are organised on chromosomes in order of expression

Question 44

Hox genes are organised into a ---- ------ on chromosomes

Answer 44

Hox complex

Question 45

define a gene complex

Answer 45

genes located in a nearby segment of DNA

Question 46

how many copies of the Hox complex do humans have? how are these similar to each other?

Answer 46

4; the order of Hox genes on the chromosome remains the same

Question 47

what do Hox genes encode?

Answer 47

transcription factors

Question 48

how do Hox genes partly explain different segmentation in different organisms?

Answer 48

the downstream targets that Hox proteins activate differ between organisms

Question 49

what is the impact of Hox gene mutations?

Answer 49

they alter which body parts develop in a segment

Question 50

loss or gain of function Hox gene mutants cause

Answer 50

body parts to develop in a segment where they should not normally be