Cellular & Molecular Mechanisms of Development I: Interaction and Apoptosis

Question 1

Cells have differential affinities. What does this mean?

Answer 1

certain types of cells show a clear affinity for each other, while apparently avoiding or ignoring contacts with adjacent tissues

Ex:

• mesodermal cells form a continuous sheet, while at the same time smoothly moving between the ectodermal and endodermal germ layers.
• also during formation of neural tissues, the cells of the neural plate show specific interactions that are not shared with the underlying mesoderm or the adjacent, non-neural ectoderm.

Question 2

Different cells show affinity for each other due to the presence of what?

Answer 2

specific cell adhesion molecules on their surface

*these molecules fall into 3 general classes

Question 3

What are the three general classes of adhesion molecules?

Answer 3

• calcium independent cell adhesion molecules (CAMs; Ig molecules; ex: neural cell, N-CAM, platelet endothelial cell, PE-CAM)
• calcium depenndent adhesion molcules (cadherins; neural, N-cadherin, epithelial, E-cadherin)
• sugar-mediated adhesion (protein on surface of one cell binds to polysaccharide (sugar polymer) on surface of another cell; ex: selectins, L-selectin)

Question 4

Calcium independent cell adhesion molecules

Answer 4

• these are members of the immnoglobulin superfamily of proteins
• include neural cell adhesion molecule (N-CAM) and platelet endothelial cell adhesion molecule (PE-CAM)

etc

Question 5

Ig domains are what kind of binding domains?

Answer 5

• protein-protein binding domains
• they stick light chains and heavy chains together
• two CAMs interact via their Ig domains, the two cells adhere due to many CAM interactions
• antibody molecule: different protein chains interact via the Ig domains

(proteins on two different cells that express these Ig molecules will stick together)

Question 6

Calcium dependent adhesion molecules called cadherins

Answer 6

• the calcium forms a bridge between two cadherins on different cells

calcium binding domains, calciums form bridges between different cells

cadherin proteins interact with each other via a Ca++ bridge

ex: neural cadherin (N-cadherin) and epithelial cadherin (E-cadherin)

Question 7

Sugar-mediated adhesion molecules

Answer 7

• proteins on one cell bind to polysaccharides (sugar polymers) on another cell
  ex: L-selectin (cynsitiotrophoblast, blastocysts, bound to wall of uterus which has glycoproteins on surface; it is required for implantation of the embryo into the uterine wall, like tennis ball on Velcro)

Question 8

Cell adhesion molecules may also be linked to what?

Answer 8

signaling pathways

*ex: N-CAM cytoplasmic domain can be phosphorylated and can interact with intracellular signaling molecules

Question 9

Cell adhesion molecules may also be linked to signaling pathways. What is the significance of this?

Answer 9

a cell may express different genes when bound to its identical neighbor
–> intracellular signaling molecules can lead to alteration of gene expression

Question 10

At about the 8-cell stage of development, which adhesion molecule is first expressed in the embryo?

Answer 10

E-cadherin

• the cells change from being loosely attached, to a very compact ball (more stuck and morphed to each other, compaction)
• this may be the reason that twinning can no longer occur after about the 8-cell stage; the cells are too firmly attached

Question 11

During early gastrulation, all cells of the ectodermal layer express which adhesion molecule?

Answer 11

E-cadherin

*when neural induction occurs, the future neural cells down-regulate E-cadherin and express N-cadherin

Question 12

When neural induction occurs, the future neural cells down-regulate E-cadherin and express N-cadherin. Where are neural crest cells?

Answer 12

neural crest cells at ends of N-cadherin section

Question 13

E-cadherin marks what cells? N-cadherin marks what cells?

Answer 13

E-cadhern marks ectodermal cells

N-cadherin marks neural cells

Question 14

Initially all cells ofthe neural plate express N-cadherin. However, as the neural tube starts to close, the cells at the neural crest lose N-cadherin expression. What is the consequence for these cells?

Answer 14

they are now free to migrate

*neural crest cells that finally contribute to ganglia re-express N-cadherin, so that they can interact with other neural cells

Question 15

Migrating neural crest cells express which cadherin?

Answer 15

neither

Question 16

N-cadherin expression is found in the neural tube or ectoderm?

Answer 16

neural tube

*E-cadherin expressed in ectoderm

Question 17

Remember, alterations in cell adhesion can lead to what?

Answer 17

changes in gene expression

Question 18

Cells of the inner cell mass express what?

Answer 18

PE-CAM

*first, multiple independent cells in the blastocyst express PE-CAM. These cells move inside to form the ICM

Question 19

What is intercalation?

Answer 19

• means to “insert between two others”
• this movement functions to elongate embryonic tissues and involves both cell movement and changes in cell adhesion

Ex: lengthening of the notochord along the cranio-caudal axis by intercalation

Question 20

Describe the intercalation of the notochord

Answer 20

Initially it is fairly fat and disorganized, and then they become organized and a long, thin structure

Question 21

Epithelial-Mesenchymal Transition (EMT)

Answer 21

• at a number of times during early development, cells lose their adhesive properties, leave a specific cell layer, and become migratory
  i. e. they become mesenchymal
• EMT is required for gastrulation, neural crest migration, and heart valve formation

Question 22

What is EMT required for?

Answer 22

• gastrulation
• neural crest migration
• heart valve formation

Question 23

What two transcription factors regulate the migratory phenotype?

Answer 23

snail and slug

Question 24

Expression of snail and slug in tumors is associated with ________ (decreased/increased) EMT and metastasis.

Answer 24

increased

Question 25

Epithelial cells express intermediate filament proteins of the keratin family. The keratin filaments connect to other epithelial cells through cell/cell contacts.

Answer 25

don’t need to worry about this

Question 26

Vimentin is a marker of what?

Answer 26

mesenchymal cells

• when cells undergo EMT, keratin proteins are down-regulated and new intermediate filament protein, vimentin, is expressed
• this happens at the same time that cell adhesion protein (specifically E-cadherin) is switched off

Question 27

Vimentin is also a marker of invasive and metastatic tumor cells. Explain.

Answer 27

metastatic tumor cells lose their epithelial organization and express vimentin

Question 28

What is vimentin a marker for?

Answer 28

• mesenchymal cells

- invasive and metastatic tumor cells

Question 29

Epithelial wedging - describe and give an example

Answer 29

Cells in an epithelium can constrict either their basal or apical surfaces and change shape

• in many cases a cuboidal cell takes on a wedge shape
• coordinated shape change in the cells of an epithelium brings about a change in the shape of the tissue
  e. g. neural tube folding (at base of groove, apical membrane constricted, at tops of hill of neural groove on either side, basal membrane constricted)

Question 30

Apoptosis

Answer 30

During development some cells undergo programmed cell death. This is an integral part of certain developmental programs. It is not to be confused with necrosis, which is normal cell death due to infection or injury.
Apoptosis involves a complex series of protein degradation and DNA degradation events. In fact, the chromosomes are completely fragmented so that the cell has no chance of recovery.

Question 31

If we did not have apoptosis, what would happen to our limbs (hands and feet)?

Answer 31

they would basically be flippers

• cells in the limb bud selectively apoptose to create the separations between the digits
• chicks have individual toes, duck have webbed feet

Question 32

Human disease Syndactyly?

Answer 32

• webbed fingers or toes

- about 1 in 2000 births

Question 33

How does apoptosis relate to heart valve formation?

Answer 33

• separating the ventricle and the atrium, apoptosis helps to shape the valve leaflets by eliminating unwanted cells
• cushion formation, tissue is removed –> mature valves formed

Question 34

How does apoptosis relate to the opening of the oral cavity?

Answer 34

removes cells to make a clear opening

Question 35

Treacher-Collins Syndrome

Answer 35

• facial abnormalities
• neural crest defect leading to facial alterations
• loss of one copy of the gene causes neural crest cells to apoptose
• excessive apoptosis of cranial neural crest cells

Question 36

How does apoptosis relate to cancer?

Answer 36

• critical role as protection against tumors (after embryonic development)
• it is likely that many spontaneous tumors arise in our bodies during our normal lifetimes, but the vast majority of these are destroyed by apoptosis
• only those cells that escape apoptosis can go on to cause cancer
• mutations in genes of the apoptosis pathway commonly occur in tumors
• apoptosis genes function as tumor suppressor genes

Question 37

Apoptosis genes act as what in relation to cancer?

Answer 37

tumor-suppressor genes

Question 38

What is L-selectin?

Answer 38

• a sugar-mediated adhesion molecule; specifically selectin
• expressed on surface of the blastocyst and binds to polysaccharides on the surface of cells of the uterine wall, beginning the process of implantation

Question 39

Apoptosis of neuromuscular junctions

Answer 39

during formation of neuromuscular junction, only those neurons that make useful connections will survive; other neurons destroyed by poptosis