Cell interactions and cell movements in development focusing on gastrulation

Question 1

What is the generation of form called?

Answer 1

Morphogenesis - this means literally shape creation.

Question 2

What are the key cellular properties involved in generating animal embryonic form?

Answer 2

Cell adhesiveness, cell shape changes and cell motility.

Question 3

What is cell adhesiveness?

Answer 3

When animal cells adhere to one another and the ECM (extracellular matrix) through interactions involving cell surface proteins known as adhesion molecules. Changes in adhesion molecules can have effects on the tissue.

Question 4

What are cell shape changes?

Answer 4

Cells can change shape by internal contractions and constrictions - this is caused by cytoskeletal rearrangements and is crucial in many developmental processes such as folding or rolling of a cell sheet.

Question 5

What is cell motility?

Answer 5

This is also called cell migration - this is the ability of many types of cell to move as individuals or as groups from one location to another. It is crucial to development as many processes involve the guided coordinated movement of cells from their origin to their final location.

Question 6

What are the two groups that cells in embryos can be classified as and how are they classified?

Answer 6

Epithelial cells and mesenchymal cells due to properties of cell adhesion and motility.

Question 7

What are epithelial cells?

Answer 7

A sheet of cells, each joined to it’s neighbour by cell to cell adhesion. They are usually organised into sheets or tubules that are attached to an underlying basement membrane.

Question 8

What are mesenchymal cells?

Answer 8

Scattered cells, embedded in loose extracellular matrix (ECM). There is little to no contact with adjacent cells and it fills up much of the embryos and later forms the fibroblasts, adipose tissue, smooth muscle and skeletal muscle.

Question 9

Where are epithelial cells found?

Answer 9

In all 3 germ layers.

Question 10

Where are mesenchymal cells found?

Answer 10

In the ectoderm and mesoderm.

Question 11

What are epithelia held together by?

Answer 11

Cell junctions - a specialised site on a cel. at which it is attached to another cell.

Question 12

What is a cell junction?

Answer 12

A specialised site on a cell at which it is attached to another cell.

Question 13

What are basal epithelia?

Answer 13

Cells joined to the matrix.

Question 14

What are apical epithelia?

Answer 14

Cells to joined to other epithelial cells.

Question 15

What is apico-basal polarity?

Answer 15

The idea that epithelia have polarity due to the presence of junctions on their apical surface and interacting with the basal membrane on the basal surface.

Question 16

What are cadherins?

Answer 16

Calcium dependent transmembrane proteins that protrude from the cell. In the presence of Ca2+ ions, caherins on adjacent cells will stick together in a zipper like fashion.

Question 17

What are integrins?

Answer 17

A large family of proteins found in the extracellular matrix. They bind to matrix molecules (collagen, fibronectin, laminin, proteoglycans) and have two subunits - alpha and beta.

Question 18

What is an adherens junction made up of?

Answer 18

Actin filaments, cadherin, alpha catenin and beta catenin. The cadherens are linked to intracellular proteins called catenins that link to the actin cytoskeleton. Adherens junctions are present in many tissues.

Question 19

What is a focal adhesion junction made up of?

Answer 19

Actin filaments, integrin and focal adhesion kinase. Focal adhesion kinase (FAK) links integrins to the actin cytoskeleton.

Question 20

What is a desmosome made up?

Answer 20

Intermediate filaments, caherin, plakoglobin and desmoplankin. The cadherins are linked to the plakoglobin (intracellular protein) that is connected to the intermediate filament proteins (such as keratin). Desmosomes are mainly in epithelia.

Question 21

What is a hemidesmosome made up of?

Answer 21

Intermediate filaments, integrin and dystonin. The dystonin links integrins to intermediate filaments.

Question 22

What structures are important in cell to cell adhesion?

Answer 22

Adherens junctions and desmosomes. Both are made up of cadherin.

Question 23

What structures are important in cell to matrix adhesion?

Answer 23

Focal adhesion and hemidesmosomes - both contain integrins.

Question 24

What are apical junctions?

Answer 24

Both adherens junctions and desmosomes. They mechanically link all of the cells within the epithelial sheet.

Question 25

What are calcium independent cell adhesion molecules?

Answer 25

Members of the large immunoglobulin superfamily.

Question 26

What is an example of a calcium independent cell adhesion molecule?

Answer 26

N-CAM - the neural cell adhesion molecule.

Question 27

What are the binding features of calcium independent cell adhesion molecules?

Answer 27

Some mediate homophilic binding (bind to each other) whereas others are heterphilic - which bind to integrins.

Question 28

What are basal junctions?

Answer 28

Focal adhesions and hemi-desmosomes. They allow integrins to transmit information about the extracellular environment to the cell and these signals can affect cell shape, motility, differentiation and metabolism.

Question 29

What can changes in cell-adhesion molecules on the cell surface cause?

Answer 29

They can alter the strength of cell-cell interactions and specificity.

Question 30

How have differences in cell adhesiveness been proven?

Answer 30

Experiments involving different embryonic tissue being confronted with each other in an artificial setting.

Question 31

Give an example of an experiment to prove cell adhesiveness.

Answer 31

Pieces of late ectoderm and endoderm were placed together. They initially fused to form a smooth sphere, but in time separated until there is only a narrow bridge of tissue connecting the two together.

Question 32

What happens if you try and fuse early ectoderm and mesoderm and why?

Answer 32

The tissues do not separate as they normally adhere to each other in the blastula. The mesoderm usually envelops the ectoderm as they express complimentary adhesion moleculesso can interact with one another.

Question 33

What causes differential adhesiveness?

Answer 33

Differences in the types and numbers of adhesion molecules they have on their surfaces.

Question 34

How is cadherin cell type specific? Give an example.

Answer 34

N-cadherin is expressed in cardiac muscle cells, VE-cadherin is expressed in endothelial cells. If two cells are expressing different cadherins or different amounts of the same cadherin are mixed, they organise into like.

Question 35

What happens if different tissues are disaggregated into single cells and the cells are mixed together to allow reaggregation?

Answer 35

Like cells preferentially associate with like cells.

Question 36

What happens if ectoderm and mesoderm from amphibian blastulas are disaggregated into single cells and mixed back together?

Answer 36

They sort out with mesoderm cells on the outside.

Question 37

How are changes in cell shape performed?

Answer 37

Constrictions and contractions of the cytoskeleton.

Question 38

What is the cytoskeleton?

Answer 38

An intracellular protein framework made up of actin filaments, microtubules and intermediate filaments.

Question 39

What is characteristic about actin filaments and microtubules?

Answer 39

They are dynamic structures that polymerise and depolymerise depending to the cells requirements.

Question 40

What is characteristic about intermediate filaments?

Answer 40

They are more stable, forming rope-like structures that transmit mechanical forces, spread mechanical stress and provide mechanical stability to the cell.

Question 41

What can actin filaments do with myosin?

Answer 41

It can assemble into contractile structures that act as mini muscles.

Question 42

What is the contractile ring?

Answer 42

Bundles of actomyosin. The contraction of this pinches the cell into two.

Question 43

What is apical constriction?

Answer 43

The contraction of a ring of actomyosin arranged around the apical end of a cell that leads to apical constriction and elongation of the cell.

Question 44

How does cell migration occur?

Answer 44

Cells extend a thin sheet-like layer of cytoplasm (lamellipodium, pushed outwards from the cell by assembly of actin filaments. Contraction of the actomyosin assembly at the front of the cell then draws the cell forward.

Question 45

What are the basic steps in cell motility?

Answer 45

Protrusion of the leading edge, adhesion at the leading edge, deadhesion at the trailing edge and movement of the cell body.

Question 46

How does adhesion at the leading edge occur in cell motility?

Answer 46

Actin rearrangement allow the cell to send out lamellipodia at the leading edge where it makes new attachments with the substratum. The cell is stretching and the cytoskeleton is under tension.

Question 47

How is tension released in cell motility?

Answer 47

At the rear end there is deadhesion.

Question 48

What morphogenetic processes underlie diverse forms of embryo?

Answer 48

Involution and invagination, cavitation, epithelial to mesenchymal transition (EMT), convergent extension, epiboly, delamination and ingression.

Question 49

What are morphogenetic processes controlled by in the embryo?

Answer 49

Inductive signals and regulatory genes.

Question 50

What is involution?

Answer 50

A layer of cells “roll in” to form two separate lines.

Question 51

What is invagination?

Answer 51

When a layer of cells creates an indent to form a bend in the structure.

Question 52

What can involution and invagination result in?

Answer 52

A multilayered structure from a single epithelium.

Question 53

How does involution and invagination arise?

Answer 53

Changes in cell shape caused by contraction of cytoskeletal filaments. There is localised contraction in actomyosin filaments in a few cells that changes their shape. Because all of the cells are mechanically linked, the whole sheet is drawn in at the point of the original contraction.

Question 54

Give an example of when involution occurs.

Answer 54

Involution occurs during gastrulation of the frog embryo. Cells in the marginal zone undergo apical constriction of the cytoskeletal network which causes them to elongate. This in turn draws the sheet in causing it to involute.

Question 55

What is cavitation?

Answer 55

The formation of a hollow ball or tube - this is an alternative to invagination.

Question 56

How does cavitation occur?

Answer 56

Fluid filling or apoptosis of cells in a solid mass.

Question 57

Give an example of cavitation.

Answer 57

Blastocyst formation. Sodium ions are actively pumped across the basolateral surface of the outer layer of the trophectoderm cells. Blastocoel compartment then inflates as water moves in through osmosis. Water is prevented from escaping by the presence of tight junctions in the apical regions of the cells. This creates a hydrostatis pressure that is one of the forces that gives the blastocyst its spherical shape.

Question 58

What is EMT?

Answer 58

Epithelial-to-mesenchymal transition. It is the conversion of an epithelium into a more loosely connected mass of mesenchyme or into individual mesenchymal cells that can migrate - this is frequent in early development.

Question 59

What occurs in EMT?

Answer 59

There is dissolution of adherens junction between epithelial cells, migration of cells out of an epithelium.

Question 60

What is MET?

Answer 60

Mesenchymal-to-epithelial transition. It results in the polarisation of motile cells, it establishes cell-cell contacts through formation of adherens juncrtions and creates an organised epithelium.

Question 61

What is convergent extension?

Answer 61

When a sheet of cells changes shape due to active rearrangement of cells. The axes must already be defined. Cells become elongated in direction of right angles to the anterior-posterior direction. THey also become aligned parallel to one another in a direction perpendicular to the direction of tissue extension. Each end of these bipolar cells form filopodia, allowing them to exert traction of the neighbouring cells and on the underlying substratum and to shuffle in between each other.

Question 62

What is epiboly?

Answer 62

Expansion of a sheet of cells to surround and enclose another population. Cells spread and think out, and involves several changes that include cell divison, alternation in cell shape and intercalation of cells.