Chapter 3 - modified for details

Question 1

Three behaviours require cell-cell communication via the cell surface:

Answer 1

cell adhesion
cell migration
cell signalling.

Question 2

Cell adhesion - Selective affinity

Answer 2

The surfaces of the cells have an affinity for certain other cells

Fx inner surface of ectoderm have positive affinity for mesodermal cells and negative for endoderm, causing mesoderm cells to adhere to the inside of the epidermis, but not endoderm cells. The cells (if mixed) are able to sort out into their proper embryonic positions.

Selective affinity changes during development, allowing cells to interact differently with other cell populations at specific times (extremely important for morphogenesis).

Question 3

Cell adhesion - Differential adhesion hypothesis

Answer 3

Cells interact so as to form an aggregate with the smallest interfacial free energy => cells rearrange themselves into the most thermodynamically stable pattern.

The thermodynamic differences are caused by different types of adhesion molecules.

If A-A connections are stronger than A-B or B-B connections, sorting will occur, with A cells becoming central.
If A-A =/< A-B then mix, if A-A&raquo_space; A-B then separate aggregates.

Question 4

Cell adhesion - Cadherins

Answer 4

Calcium dependent adhesion molecules
Critical for establishing and maintaining intercellular connections
Crucial for spatial segregation of cell types and to the organization of the animal form.

TM molecules, interact with other cadherins on adjacent cells.
Can also serve as signalling molecules that change a cell’s gene expression.

Question 5

Cell adhesion - Catenins

Answer 5

Catenins anchors cadherin inside the cell.

Protein complex, binds to actin (microfilament) cytoskeleton of cell.
Integrate epithelial cells into a mechanical unit and organizes cadherins, allowing them to form stable linkages

Question 6

Cell migration is a common feature of both epithelial and mesenchymal cells, differences in how?

Answer 6

Involves a broad reorganization of the actin cytoskeleton.

• Epithelia – the motive force for migration is usually provided by the cells at the edge of the sheet, and the rest of the cells follow passively.
• Mesenchymal – individual cells become polarized and migrate through the extracellular milieu.

Question 7

Cell migration - Stages of migration:

Answer 7

1. Polarization
2. Protrusion of leading edge
3. Extracellular adhesion
4. Release of adhesion

Question 8

Cell migration - Stages of migration: Polarization

Answer 8

How a cell defines its front and its back

can be directed by diffusion signals (fx chemoattractant gradients)

or by signals from the extracellular matrix (reorganize the cytoskeleton so front and back become structurally different.)

Question 9

Cell migration - Stages of migration: Protrusion of leading edge

Answer 9

polymerization of the actin microfilaments at the cell membrane, creating long parallel bundles (filopodia) / broad sheets (lamellipodia).

The actin becomes nucleated and connected to cadherins in the cell membrane.

Question 10

Cell migration - Stages of migration: Extracellular adhesion

Answer 10

the moving cell, attaches to the surrounding matrix for something to push off on.

Key proteins are integrins (span cell membrane, connecting extracellular matrix to intracellular actin).

Myosin provides motive force along actin filaments.

Question 11

Cell migration - Stages of migration: Release of adhesion

Answer 11

at rear of cell, allowing the cell to migrate in the forward direction.

It is probable that stretch-sensitive Ca2+-channels are opened and the released Ca2+ activates proteases that destroy the focal adhesion sites.

Question 12

Cell signalling - Induction

Answer 12

Cells changing the behaviour of adjacent cells

Causing them to change shape, mitotic rate or cell fate

Close range interaction between two or more cells or tissues of different histories and properties.

Inducer often uses paracrine factors

Responder must have both the receptor for the inducing factor, but also the ability to respond to the signal (competence).

Often, one induction will give a tissue the competence to respond to another inducer.

Question 13

Cell signalling - Reciprocal induction

Answer 13

Sometimes the inducer becomes the induced.

Question 14

Two major modes of inductive interaction:

Answer 14

• Instructive interaction – a signal from the inducing cell is necessary for initiating new gene expression in the responding cell
• Permissive interaction – the responding tissue has already been specified and needs only an environment that allows the expression of these traits, fx extracellular matrix

Question 15

Genetic specificity of induction is?

Answer 15

Tissue may be induced to induce certain genes, but can only comply with the instructions so far as its genome permits.

Organ-type specificity is controlled by inducing mesenchyme, species specificity is controlled by responding epithelium.

Question 16

Paracrine factors: inducer molecules - Juxtacrine interactions

Answer 16

membrane proteins on one cell surface interact with receptor proteins on adjacent cell surfaces, the cell membranes are juxtaposed.

Question 17

Paracrine factors: inducer molecules - Paracrine interaction

Answer 17

proteins synthesized by one cell diffuse over a small distance (ca. 15 cell diameters) to induce changes in neighbouring cells.

Question 18

Paracrine factors: inducer molecules - Autocrine interaction

Answer 18

specific type of paracrine, the same cell secreting responds, not common fx seen on placental cytotrophoblast cells => explosive proliferation

Question 19

Signal transduction cascades

Answer 19

End point of most paracrine factor pathways either involves regulation of TFs (alter gene expression) and/or regulation of the cytoskeleton (alter shape / migration).

One paracrine factor can have several end points.
•	RTK pathway
•	JAK-STAT pathway
•	Hedgehog pathway
•	Canonical Wnt pathway
•	Noncanonical Wnt pathways
•	SMAD pathway

Question 20

Signal transduction cascades - RTK pathway

Answer 20

Paracrine factor receptors are often TM receptor tyrosine kinases (RTKs).

RTKs use ATP to phosphorylate specific tyrosine residues on particular proteins as the first step in the paracrine signalling cascade.

Each RTK can bind only one type of ligand (or one small set).

Question 21

Signal transduction cascades - JAK-STAT pathway

Answer 21

Extremely important in differentiation of blood cells, the growth of limbs and the activation of the casein gene during milk production.

Question 22

Signal transduction cascades - Canonical Wnt pathway

Answer 22

Used for sending signals to nucleus and change gene transcription

Question 23

Signal transduction cascades - Non-canonical Wnt pathway

Answer 23

Wnt proteins use these to affect calcium transport into cells as well as altering the actin and microtubule cytoskeleton.

Question 24

Signal transduction cascades - SMAD pathway

Answer 24

Results in gene transcription or repression

Question 25

Extracellular matrix as a source of developmental signals

Answer 25

Insoluble network consisting of macromolecules secreted by cells into their immediate environment

Critical region for much of animal development

Made up of the matrix protein collagen, proteoglycans and a variety of specialized glycoprotein molecules such as fibronectin and laminin.

Cell migration, cell adhesion and formation of epithelial sheets + tubes depend on the ability of cells to form attachments to extracellular matrices.

Not passive, can promote diffusion, retard diffusion or degrade paracrine factors.

Question 26

Proteoglycans play critically important roles

Answer 26

In the delivery of paracrine factors, by binding them and presenting them in a high concentration to their Rs.

Block of synthesis blocks normal cell migration, morphogenesis and differentiation.

Question 27

Glycoproteins are responsible for organizing the matrix and the cells into an ordered structure. Name two most important and general function.

Answer 27

• Fibronectin – general adhesive

* Laminin – one of the major components of basal lamina

Question 28

Integrins

Answer 28

Family of R complexes
Spans the cell membrane, integrating extracellular and intracellular scaffolds, allowing them to work together.

Movement through contracting actin filaments against fixed extracellular matrix.

Integrins can signal from the outside of the cell to the inside, altering gene expression.

Bound integrin prevents the activation of genes that promote apoptosis.

Binding of integrins to an extracellular matrix can stimulate the RTK pathway.

Question 29

Anoikis

Answer 29

“death-upon-detachment”

when focal adhesions linking an epithelial cell to its extracellular matrix are broken, the caspase-dependent apoptosis pathway is activated and the cell dies (major weapon against cancer).

Question 30

Epithelial-Mesenchymal transition

Answer 30

A polarized, stationary epithelial cell, which normally interacts with basal lamina through its basal surface, becomes a migratory mesenchymal cell that can invade tissues and form organs in new places.

EMT also important in adults, needed for wound healing, and unfortunately used in cancer metastasis

Question 31

5 types of cadherins:

Answer 31

• E-cadherin – expressed on all early mammalian embryonic cells, later restricted to epithelial tissues
• P-cadherin – predominantly on placenta, helps it stick to the uterus
• N-cadherin – highly expressed on cells of the developing central nervous system, may mediate neural signals
• R-cadherin – critical in retina formation
• Protocadherins – lack the attachment to the actin skeleton through catenin. Expression of similar keeps migrating epithelial cells together, dissimilar is an important way to separate tissues.

Question 32

Differences in cell surface tension and tendency of cells to bind together depends on strength of cadherin interactions, achieved through: (2)

Answer 32

• Quantitatively – more cadherins on opposing surfaces = tighter adhesion
• Qualitatively – some binds to different types, some don’t

Question 33

What is focal adhesion?

Answer 33

The point of connection between actin and integrin on cell membrane where it contacts the extracellular matrix.

Question 34

Does a structure/organ need to be fully differentiated to have a function?

Answer 34

Before a tissue has its “adult” functions, it can have critically important transient functions in building the organs of the embryo.

Question 35

What is a cilia?

Answer 35

Extensions of the cell membrane made by microtubules

Question 36

EMT done how?

Answer 36

Usually initiated by paracrine factors from neighbouring cells, instructing the target cells to:
• Downregulate cadherins
• Release attachment to laminin + other basal components
• Rearrange their actin cytoskeleton
• Secrete new extracellular matrix molecules characteristic of mesenchymal cells

Question 37

The induction of numerous organs is effected by a relatively small set of paracrine factors, grouped based on structure:

Answer 37

• The fibroblast growth factor (FGF) family - Functions through RTK and JAK-STAT pathways.
• The Hedgehog family - Works through Hedgehog pathway.
• The Wnt family
• The TGF-beta superfamily
• Some paracrine factor proteins have no or few close relatives and can’t be grouped but still have important roles during development. Fx epidermal growth factor, hepatocyte growth factor, neurotrophins and stem cell factor.

Question 38

Paracrine factors - The Wnt family

Answer 38

Family of cysteine rich glycoproteins

Critical in:

• Formation of fly wings
• Establishing the polarity of insect and vertebrate limbs
• Promoting the proliferation of stem cells
• Several steps of the mammalian urogenital system development

Uses conventional or nonconventional Wnt pathway.

Question 39

Paracrine factors - The TGF-beta superfamily

Answer 39

TGF = transforming growth factor

Regulates some of the most important interactions in development.
(fx formation of extracellular matrix, cell division, migration and differentiation, bone formation, apoptosis, specifying left and right sides of vertebrate body axis and the different regions of the mesoderm).

Includes:
TGF-beta family
Nodal and activin families
bone morphogenetic proteins (BMPs)

Uses SMAD pathway.

Question 40

Signal transduction cascades - Hedgehog pathway

Answer 40

Extremely important in vertebrate limb development, neural differentiation and facial morphogenesis.

Often used by embryo to induce particular cell types and to create boundaries between tissues.

Question 41

Notch proteins:

Answer 41

Binds to a family of ligands (Delta, Jagged or Serrate proteins)

Uses Notch pathway.

Involved in the formation of numerous vertebrate organs (kidney, pancreas, heart, eye)

Extremely important Rs in the nervous system (binding of Notch to Delta = do not become neuron).

Question 42

Juxtacrine signalling, two of the most widely used families of juxtacrine factors:

Answer 42

• Notch proteins

* Eph receptors

Question 43

Eph Rs

Answer 43

Ligand = ephrin

Binding of ligand and R gives rise to signal in both cells involved. Often attraction or repulsion signals.

Ephrins are often seen where cells are told where to migrate or where boundaries are forming, fx in formation of blood vessels and neurons.

Question 44

Signal transduction cascades - RTK pathway

Answer 44

Paracrine factor receptors are often TM receptor tyrosine kinases (RTKs).

RTKs use ATP to phosphorylate specific tyrosine residues on particular proteins as the first step in the paracrine signalling cascade.

Each RTK can bind only one type of ligand (or one small set).