14.2 Cell Differentiation I

Question 1

All of the different cell types in multicellular organisms are derived from single fertilized egg through ______?

Answer 1

differentiation

Question 2

Differentiation is the process by?

Answer 2

Which an unspecialized cell becomes specialized

Question 3

Differentiation can involve changes in??

Answer 3

Cell size,
shape,
polarity,
metabolic activity, responsiveness to signals, and gene expression profiles

Question 4

During differentiation what can happen to genes?

Answer 4

certain genes are turned on while other genes become inactivated

Question 5

Cells become different from one another because ___?

Answer 5

b/c of their genes and they synthesize different sets of RNA and proteins (cells have pretty much the same DNA, with minor exceptions) so genes make them different.

Question 6

Since cells synthesize different sets of RNA and proteins, a differentiated cell will thus develop ____?

Answer 6

specific structures and perform specific functions distinct from the cell of origin

Question 7

Cell differentiation depends on…?

Answer 7

changes in gene expression and not on changes in the nucleotide sequence of cell’s genome.

Question 8

In early development, what are the three layers of tissue animals are made up of?

Answer 8

Ectoderm - for the outside

Endoderm - on the inside

mesoderm - for everything in the middle.

Question 9

Mammalian embryos like humans start out as a group of cells organized into what?

Answer 9

three different layers (ecto, endo, meso)

Question 10

Basic mechanisms of development are the same for?

Answer 10

vertebrates and invertebrates

Question 11

Homologous proteins are interchangeable between?

Answer 11

different species

50% of genes present in C. elegans (nematodes worm), Drosophila (fly), and humans have
clearly recognizable homologs in one or both of the other two species. This is especially true for
genes that regulate cell interactions and for gene regulation (ie., transcription factors).

Question 12

How is it possible for early genes in different animals to be the same yet produce widely different end products such as wing on an insect and arms on a mouse or human?

Answer 12

b/c the development of any final structure is really the result of a series of cascading genes that get activated in sequence.

Ex: gene 1 is the master control of a developmental program that will tell tissues to make an appendage, while genes 2 and 3 are more specific as to what kind of appendage is going to get made.Further along there will be other genes that will specify what sorts of proteins willbe expressed in each cell in the appendage and so on.The process gets more specific over time

Question 13

Which comes first, cell differentiation or morphological changes?

Answer 13

cell differentiation occurs before morphological changes

Question 14

What is determination?

Answer 14

the process a cell goes through when it becomes destined to become a particular cell type

Question 15

What can a cell do before determination? what about after?

Answer 15

Before a cell is determined, it can respond to environmental cues and change the final outcome of its developmental program.

once a cell is determined, it will follow a single development path no matter where it is placed in the body.

Question 16

What are transplantation experiments? What occurs if cells are transplanted later in development

Answer 16

where tissues are moved from one place to another in early development and take on the characteristics of the destination site.

If cells are transplanted later in development there are different results, where the cells that are moved retain the developmental program that was present at the initial site or their site of origin

Question 17

How do cells become different (cell specification)?

Answer 17

By segregating molecules

By exposure to different environments

Positive feedback

Question 18

How can two different cells be produced from a single cell?

Answer 18

First, it’s possible there is an unequal distribution of some factor that’s already been programmed into the cell before it divides, such that one cell gets more or less of the factor after the cell divide. Once a cell has become segregated in some fashion, the unequal
distribution of molecules can then become a determining factor that will leave one cell to adopt a
different cell fate than the other.

Ex: egg cells are generally not just homogenous masses and they are often produce w/in the female w/different distributions of RNA molecules and other morphogenic factors.

The second possibility is that the cell responds to different local environmental cues. Many cells use this approach.
EX: if a skin stem cell divides, it gives rise to one other stem cell, plus another cell that will go on to differentiate into a mature adult skin cell.

Question 19

What controls limb development in vertebrates?

Answer 19

a morphogenic gradient of Sonic Hedgehog

Question 20

What are morphogenic gradiets?

Answer 20

a common mechanism used by developing systems that allow finely graded or transitional changes from one place of an organism to another. It is an alternative to binary distinctions

• signals from cells outside of group alters developmental/differentiation pathway
• the signal is limited in time and space so that only a subset of cells take on induced character
• these are longer-range signals that can diffuse through the extracellular medium

Morphogens are short and long-range inducers that can exert graded effects

Question 21

What are common signaling elements used during development ?

Answer 21

Wnt, notch, and hedgehog signaling pathways and others that would operate by interacting w/receptor tyrosine kinases

Question 22

How can morphogen diffusion be limited in space and time?

Answer 22

Two mechanisms
1) a morphogen can just be produced in a restricted area. Cells close to the source receive a high dose of morphogen and others further away receive lower amounts, but if the production of morphogen occurs over a very lengthy amount of time, it’s likely that even distal tissues will eventually receive a high concentration of the morphogen.

Thus, there are antagonist for most morphogens.

2) Create morphogen gradients

Gradients are often the result of a process that inhibits or destroys the morphogen at more distant
sites. Thus there are antagonists for most morphogens. On the right hand side of the slide is an
example where an initially uniform expression of the morphogen TGFβ is altered by an inhibitor
called chordin, which itself is expressed as a gradient. The result is a TGFβ gradient.

(ex: morphogen TGFbeta is altered by an inhibitor called chordin, which itself is expressed as a gradient. The result is a TGFbeta gradient. )

Question 23

Gradients are often the result of … ?

Answer 23

a process that inhibits or destroys the morphogen at more distant sites.

Question 24

What is a caenorhabditis elegans (C. elegans) ?

Answer 24

a worm used to study development of cells since it’s transparent. Although simple, this worm still has muscles, nerves, epithelia, digestive, and reproductive tissues that are homologous to similar structures found in vertebrates.

Question 25

What is the most interesting aspects of C. elegans development?

Answer 25

In c. elegans, cell lineage is fully known. The organism is simple enough that every cell in its body has a predictable outcome and can be traced directly back to its origin.

Question 26

In c. elegans, unequal cell divisions are produced by activity of what?

Answer 26

activity of Par (partitioning defective) proteins

Question 27

What are p granules?

Answer 27

P granules contain proteins and RNA molecules that will be used to differentiate the germ line of cells. They are prepackaged into the egg during egg cell production by the female, but they are initially randomly distributed.

p-granules segregates into a single cell at each of the first four cell divisions in c. elegans.

Question 28

What redistributes p granules and is considered an example of egg cell responding to en environmental cue to create differences w/in itself ?

Answer 28

the entry of the sperm into the egg creates a polarity that’s used by the cell to redistribute P granules and other developmental factors

Question 29

What is the par protein and what does mutations in the par protein leads to ?

Answer 29

Par protein helps reorganize p-granules in cell to the opposite side of cell entry. Even when the cell has not yet divided the p-granules can be seen congregating on the opposite side of sperm entry. After the first cell division, all p-granules end up in one cell only and this occurs until the last cell division, a total of 16 cells and only one has p-granules.

Defects in egg cell partitioning

Question 30

P cell is responsible for?

Answer 30

p-cell determines which cell becomes germ cells and has p-granules

Question 31

The AB cell lineage in c. elegans will eventually become?

Answer 31

the outer layer of cells AKA ectoderm.

AB cells also define the axis of the embryo to further divide into ABa for anterior and ABp for posterior.

Question 32

C cells in c.elegans give rise to?

Answer 32

secondary ectoderm: skin, muscle and nerve cells.

C arise after the second cell division from a p-cell

Question 33

What is the fate of EMS cells?

Answer 33

EMS cells divide into two additional cells (E and MS) E eventually become the endoderm (gut) and MS will produces mesoderm and stomodeum muscle and mouth parts.

Question 34

The fate of Abp, MS and E cells in e. elegans are determined by what signals?

Answer 34

signals that come from the P2 cells which contains p granules.

P2 produces delta which activates notch receptor on the Abp cell. This causes it to become different from the Aba cell and ends up determining the anterior posterior axis of the embryo.

P2 produces wnt which binds to the frizzled receptor on EMS cell. Wnt signal polarizes EMS cell, controlling orientation of mitotic spindle. EMS cell then divides to generate two daughter cells. E (future gut) and MS (muscle and other body parts)

Question 35

P2 acts in much the same way as what in drosophila embryo?

Answer 35

bicoid, which distinguishes the axis of the drosophila embryo

Question 36

The posterior P2 cell uses what signaling pathway?

Answer 36

The posterior P2 cell uses the Notch/Delta signaling pathway to send a signal to Abp cell.

Wnt/frizzled to differentiate EMS into E and MS

Question 37

P2 produces Delta which binds to?

Answer 37

P2 produces Delta which binds to Notch receptor on Abp cell and to activate Notch-responsive
genes.

Question 38

P2 produces Wnt, which binds to

Answer 38

P2 also produces Wnt, which binds to Frizzled receptor on EMS cell. Wnt signal polarizes
EMS cell, controlling orientation of mitotic spindle. EMS cell then divides to generate two
daughter cells, one of which becomes future gut founder cell and one of which becomes
committed to form muscle and other body parts.

Question 39

Cells closes to P2 in Wnt/Frizzled becomes?

Answer 39

the E cell and then the embryonic gut

Question 40

Cells further away from the P2 in wnt/frizzled cell becomes?

Answer 40

the MS cells

Question 41

How does apoptosis play a role in development of c. elegans? Which proteins play a role?

Answer 41

-C. elegans generates 1031 somatic cell nuclei; 131 of these die by apoptosis.

they undergo apoptosis under control of proteins like caspases and bcl2 but in c.elegans most of these proteins are called ced proteins, which stands for cell death abnormal.

Question 42

ced 3 does what? What is it homologous to?

Answer 42

Apoptosis in c.elegans and homologous to caspase

Question 43

ced4 does what? what is it homologous to?

Answer 43

Apoptosis, homologous to Apaf1

Question 44

egl-1 stands for? what is it homologous to?

Answer 44

Egg laying defective protein and it is homologous to Bad (suppresses Bcl2 activity or ced-9)

Question 45

Ced-9 does what? what is it homologous to?

Answer 45

ced 9 represses apoptosis and it is homologous to Bcl2

Question 46

The small differences between daughter cells (Binary distinctions) that might be created by unequal distributions of
morphogenic factors are enforced by?

Answer 46

feedback systems.
Example: a cell with more X’s protein instructs the cell with fewer not to make any more X protein. The result is the fully differentiated pair of cells. This is exactly the type of system that plays a role in the differentiation of the neural epithelium by the Notch/Delta signaling system. This type of developmental regulation generates a binary or an on/off distinction
between two cells.