Lecture 3

Question 1

What is differentiation?

Answer 1

Process by which a cell develops specialized structure elements as specialized morphology and performs d’octobre functions.

And these differentiated cells will acquire gene expression patterns that are unique to them.

Question 2

What is commitment?

Answer 2

Stage that precedes differentiation. It’s ai this stage that a cell is assigned its fate.

Question 3

What are the two stages of commitment? Specify its reversibility.

Answer 3

1. Specification (reversible)
2. Determination (irreversible)

Question 4

How to determine whether a cell is specified or determined?

Answer 4

Need to produce a fate map

Question 5

When is a cell specified?

Answer 5

When it is able to differentiate autonomously.

Question 6

Why is specification reversible (labile)?

Answer 6

Because the fate of the cell is determined by its environment.

As in, even if fate map indicates that cell becomes muscle cell (for exemple). When put in pétri dish with neurons, then initial cell will become neuron.

Question 7

When is a cell determined?

Answer 7

When it is able to differentiate independently (and stick to fate) even when put in middle of other types of cells.

Question 8

Why is determination irreversible?

Answer 8

Because it sticks to its fate even when exposed to other types of cells.

Question 9

3 modes of specifications

Answer 9

-autonomous specification

-conditional specification

-syncytial specification

Question 10

What is autonomous specification?

Answer 10

The cell knows what its fate is without having to interact with other cells. Cell fate determination is earlier in embryo development.

Question 11

What is conditional specification?

Answer 11

The cell learns its fate by interaction with other cells. Cell fate determination is later in embryo development.

The early blastomeres of vertebrate embryos are conditionally specified.

Question 12

What is syncytial specification?

Answer 12

Important in insect development, uses elements of both autonomous and conditional specification.

Nuclei divide rapidly in a shared cytoplasm. Specification of presumptive cells within a syncytium.

Question 13

Three major techniques to study cell specification

Answer 13

-Defect experiment

-Isolation experiment

-Transplantation experiment

Question 14

What is defect experiment?

Answer 14

destroy a portion of the embryo and observe the development of the impaired embryo

Question 15

What is isolation experiment?

Answer 15

remove a portion of the embryo and observe the development of the partial embryo and the isolated part

Question 16

What is transplantation experiment?

Answer 16

parts of the embryo are moved around and transplanted onto different embryos or different regions of the same embryo

Question 17

Autonomous specification

How is cell fate determined?

Answer 17

Determined by cytoplasmic determinants found in blastomeres of the early embryos.

In oocyte, cytoplasmic determinants (proteins, such as transcription factors) segregate in different sections of egg. So, when egg divides (mitosis), daughter cells contain different transcription factors, which tells them their fate.

Question 18

Autonomous specification

How to determine if cell specified autonomously?

Answer 18

Remove early blastomeres and isolate them in a petri dish to see if they develop into differentiated tissues autonomously.

Question 19

Autonomous specification in the tunicate Styela partita embryo — defect experiment

Answer 19

The B4.1 blastomeres are destined to become the tail muscle and always contain yellow cytoplasm.

When B4.1 cells are removed from the 8-cell embryo, the larva has no tail muscles, indicating that no other cell in the tunicate can replace the B4.1 cells. Only B4.1 cells have the capacity to become tail muscle cells.

This is a defect experiment and shows that the tail muscles are specified autonomously.

Question 20

Autonomous specification in the tunicate Styela partita embryo — isolation experiment

Answer 20

Additionally, each blastomere will form most of its respective cell types even when separated from the remainder of the embryo. This is an isolation experiment. Most cells in the tunicate are determined by autonomous specification.

Question 21

Autonomous specification

Wilhelm Roux’s experiments on frog embryos

Answer 21

When he killed one of the cells in a 2-cell embryo, the other half of the larva developed normally. This shows that the specification is autonomous (each cell in the 2-cell embryo contained cytoplasmic determinants that specified their fates).

Question 22

Types of cell interactions (3)

Answer 22

-Cell-cell interactions (Juxtacrine factors)

-secreted signals (paracrine factors)

-the physical properties of the cell’s environment (mechanical stress)

Question 23

Conditional specification

What happens to blastomeres when they’re isolated?

Answer 23

Isolated = no interactions

Blastomeres can this become almost anything. They can also change fate is a component is deleted to compensate.

Question 24

Conditional specification

Transplantation experiment

Answer 24

If you transplant fate-mapped back cells from the blastula into a region fate-mapped to become the belly, the “back” cells become belly tissue because their environment has changed.

Question 25

Conditional specification Defect experiment

Answer 25

If you remove cells from the dorsal region, the remaining cells can compensate for the missing part.

Question 26

Conditional specification in the sea urchin Isolation experiment

Answer 26

Each blastomere from the 4-cell embryo regulated its own development to produce a complete organism, rather than self-differentiating into its future embryonic part.

Question 27

Conditional specification in the sea urchin Defect experiment

Answer 27

Driesch also experimentally removed cells, which changed the context for all other remaining cells. This changed the fate of all cells and the embryo developed as normal. Cell fates were changed to suit the conditions.

Question 28

Conditional specification Driesch’s findings on the sea urchin embryo were momentous for embryology 2 findings

Answer 28

1. He demonstrated that the prospective potency of an isolated blastomere is greater than its actual prospective fate (the cell types it would normally give rise to over the course of unaltered development). •In his experiments, a single isolated blastomere gave rise to an entire organism. In unaltered development, that blastomere would have only given rise to specific cell types. 2. He demonstrated that cell interaction is critical for normal development. If the prospective potency of an isolated blastomere is greater than its actual prospective fate, then cell interactions in the unaltered embryo must be involved in restricting and specifying its cell fate.

Question 29

What is blastomeres biopsy?

Answer 29

Blastomere biopsies involved the removal of one or two blastomeres from the 8-cell embryo to test for genetic defects (ploidy defects, mutations).

Question 30

Blastomere biopsy for in vitro fertilization

Answer 30

-Embryos could develop normally following the removal of one or two blastomeres  conditional specification. -Trophoectoderm biopsies are now the most common method of embryo screening. This involves removing cells that will not become part of the fetus but do become part of the placenta.

Question 31

Syncytial specification What is a syncytium?

Answer 31

Cytoplasm with multiple nuclei. There is no membrane separating the nuclei.

Question 32

Syncytial specification in Drosophila Why is there no membrane between the nuclei?

Answer 32

For the first 13 cycles of cell division, Drosophila nuclei divide without any cytoplasmic cleavage. Cellularization (formation of membranes) occurs after nuclear cycle 13, just prior to gastrulation.

Question 33

Syncytial specification in Drosophila How are cell fates determined to become the head, thorax, abdomen, and tail before cellularization? (2)

Answer 33

1. Nuclei in the syncytium obtain their identity from their position relative to neighboring nuclei 2. Determination factors are localized in gradients throughout the anterior-posterior axis of the embryo

Question 34

Syncytial specification in Drosophila How do nuclei maintain their position?

Answer 34

Microtubules keep nuclei in place. These microtubules are reestablished between each division.

Question 35

Syncytial specification in Drosophila Two types of transcription factors to determine anterior-posterior ends.

Answer 35

-bicoid -caudal

Question 36

Syncytial specification in Drosophila How does concentration gradients of transcription factors determine anterior-posterior ends?

Answer 36

-High concentration of Bicoid at the anterior end = head -High concentrations of Caudal at the posterior end = tail -Medium amounts of both in the middle = thorax/abdomen

Question 37

Bicoid What is it? Who identified it first?

Answer 37

-A morphogen -Bicoid is a transcription factor that contains the homeobox DNA sequence found in genes that control -Christiane Nüsslein-Volhard.

Question 38

What is a morphogen?

Answer 38

A morphogen is a molecule that determines the fate of a group of cells through a concentration gradient.

Question 39

How to localization Bicoid mRNA in the embryo

Answer 39

- in situ hybridization - labeled DNA probes bind complementary RNA targets -Immunofluorescence/immunostaining -Embryos or cells must be “fixed” with formaldehyde -This process freezes all proteins and RNA in place. -This is unlike the creation of transgenic animals with GFP transgenes (seen in lecture 2). Transgenes allow you to visualize proteins in live cells or organisms, but the process of creating transgenic cells is more difficult and takes longer.