1. SoS, Dept. of Biology, Lautoka Campus
BIO508 Cell Biology
Slide Design: Copyright © McGraw-Hill Global Education Holdings, LLC.
Lecturer: Dr.Ramesh Subramani
Topic 10: Cells in development
(Cell junctions, adhesions and ECM)

2. Extracellular components and connections between cells help coordinate cellular activities
Most cells synthesize and secrete materials that are external to the plasma membrane
These extracellular structures include
Cell walls of plants
The extracellular matrix (ECM) of animal cells
Intercellular junctions

3. Cell Walls of Plants
The cell wall is an extracellular structure that distinguishes plant cells from animal cells
Prokaryotes, fungi, and some protists also have cell walls
The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water
Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein

4. Plant cell walls may have multiple layers
Primary cell wall: relatively thin and flexible
Middle lamella: thin layer between primary walls of adjacent cells
Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall
Plasmodesmata are channels between adjacent plant cells

5. Plant cell walls Secondary cell wall Primary cell wall Middle lamella
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
Plasmodesmata

6. The Extracellular Matrix (ECM) of Animal Cells
Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM)
The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin
ECM proteins bind to receptor proteins in the plasma membrane called integrins

7. Extracellular matrix (ECM) of an animal cell
Collagen
EXTRACELLULAR FLUID
Polysaccharide molecule
Proteoglycan complex
Carbo- hydrates
Fibronectin
Core
protein
Integrins
Proteoglycan molecule
Plasma membrane
Proteoglycan complex
Micro-
filaments
CYTOPLASM

8. Functions of the ECM
Support
Adhesion
Movement
Regulation

9. Collagen provides structural support to tissues
The principal function of collagens is to provide structural support to tissues.
Collagens are a family of over 20 different extracellular matrix proteins.
Together they are the most abundant proteins in the animal kingdom.

10. Collagen subunits are:
All collagens are organized into triple helical, coiled-coil “collagen subunits.”
They are composed of three separate collagen polypeptides.
Collagen subunits are:
secreted from cells
then assembled into larger fibrils and fibers in the extracellular space

11. Mutations of collagen genes can lead to a wide range of diseases, from mild wrinkling to brittle bones to fatal blistering of the skin.

12. Proteoglycans provide hydration to tissues
Proteoglycans consist of a central protein “core” to which long, linear chains of disaccharides, called glycosaminoglycans (GAGs), are attached.
GAG chains on proteoglycans are negatively charged.
This gives the proteoglycans a rodlike, bristly shape due to charge repulsion.

13. Proteoglycans attract water to form gels that:
The GAG bristles act as filters to limit the diffusion of viruses and bacteria in tissues.
Proteoglycans attract water to form gels that:
keep cells hydrated
cushion tissues against hydrostatic pressure

14. Expression of proteoglycans is:
Proteoglycans can bind to a variety of extracellular matrix components, including:
growth factors
structural proteins
cell surface receptors
Expression of proteoglycans is:
cell type specific
developmentally regulated

15. Fibronectins connect cells to collagenous matrices
The principal function of the extracellular matrix protein fibronectin is to connect cells to matrices that contain fibrillar collagen.
At least 20 different forms of fibronectin have been identified.
All of them arise from alternative splicing of a single fibronectin gene.

16. The soluble forms of fibronectin are found in tissue fluids.
The insoluble forms are organized into fibers in the extracellular matrix.

17. Fibronectin proteins contain six structural regions.
Fibronectin fibers consist of crosslinked polymers of fibronectin homodimers.
Fibronectin proteins contain six structural regions.
Each has a series of repeating units.

18. Fibrin, heparan sulfate proteoglycan, and collagen:
bind to distinct regions in fibronectin
integrate fibronectin fibers into the extracellular matrix network
Some cells express integrin receptors that bind to the Arg-Gly-Asp (RGD) sequence of fibronectin.

19. Most integrins are receptors for extracellular matrix proteins
Virtually all animal cells express integrins.
They are the most abundant and widely expressed class of extracellular matrix protein receptors.
Some integrins associate with other transmembrane proteins.

20. Integrins are composed of two distinct subunits, known as α and β chains.
The extracellular portions of both chains bind to extracellular matrix proteins
The cytoplasmic portions bind to cytoskeletal and signaling proteins.

21. In vertebrates, there are many α and β integrin subunits.
These combine to form at least 24 different αβ heterodimeric receptors.
Most cells express more than one type of integrin receptor.
The types of receptor expressed by a cell can change:
over time or
in response to different environmental conditions

22. Integrin receptors bind to specific amino acid sequences in a variety of extracellular matrix proteins.
All of the known sequences contain at least one acidic amino acid.

23. Integrin receptors participate in cell signaling
Integrins are signaling receptors that control both:
cell binding to extracellular matrix proteins
intracellular responses following adhesion
Integrins have no enzymatic activity of their own.
Instead, they interact with adaptor proteins that link them to signaling proteins.

24. Cell Junctions
Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact
Intercellular junctions facilitate this contact
There are several types of intercellular junctions
Plasmodesmata
Tight junctions
Desmosomes
Gap junctions

25. Plasmodesmata in Plant Cells
Plasmodesmata are channels that perforate plant cell walls
Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell

26. Plasmodesmata between plant cells
Cell walls
Interior of cell
Interior of cell
Figure 6.31.
0.5 m
Plasmodesmata
Plasma membranes

27. Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells
At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid
Desmosomes (anchoring junctions) fasten cells together into strong sheets
Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells

28. Cell Junctions in Animal Tissues
Tight junctions prevent fluid from moving across a layer of cells
Tight junction
TEM
0.5 m
Tight junction
Intermediate filaments
Cell Junctions in Animal Tissues
Desmosome
TEM
1 m
Gap junction
Ions or small molecules
Space between cells
TEM
Extracellular matrix
Plasma membranes of adjacent cells
0.1 m

29. Malignant cancer cells from the breast (See the ABNORMAL “crab” shape of the cells.)

30. Mutation and Cancer Mutation Hyperactive Ras protein
Growth factor
Mutation
Hyperactive Ras protein
(product of oncogene)
issues signals on its own
Nucleus
G protein
Cell cycle-stimulating
Pathway
Cell cycle inhibiting pathway
Receptor Protein kinases
(phosphorylation
cascade)
Transcription
factor (activator
DNA
Gene expression
Protein that
Stimulates the cell cycle
Protein kinases
UV Active
Light form of P53
Damaged DNA
DNA
Mutation: missing
transcription
factor, such as p53, cannot activate
Protein that inhibits the cell cycle

31. Multiple signal breakdowns
Cell growth and division is such an important process that it is under tight control with many checks and balances.
But even so, cell communication can break down.
The result is uncontrolled cell growth, often leading to cancer. Cancer can occur in many ways, but it always requires multiple signaling breakdowns.

32. Often, cancer begins when a cell gains the ability to grow and divide even in the absence of a signal. Ordinarily this unregulated growth triggers a signal for self-destruction.
But when the cell also loses the ability to respond to death signals, it divides out of control, forming a tumor.
Later cell communication events cause blood vessels to grow into the tumor, enabling it to grow larger. Additional signals allow the cancer to spread to other parts of the body.

33. Acknowledgements… Any Questions??
The teaching material used in this lecture is taken from: JB Reece, LA Urry, ML Cain, SA Wasserman, PV Minorsky and RB Jackson Campbell Biology (9th Edition), Publisher Pearson is gratefully acknowledged.
Some information presented in this power point lecture presentation is collected from various sources including Google, Wikipedia, research articles and some book chapters from various biology books. Material and figures used in this presentation are gratefully acknowledged. This material is collected and presented only for teaching purpose.
Any Questions??
Dr.Ramesh Subramani, Assistant Professor in Biology