CMB: Chapter 19

How do ECM and cell interactions differ between connective and epithelial tissues?

Tissue	ECM	Cell Interactions	Mechanical Role
Connective (bone, tendon)	Extensive ECM, few cells	Cells mostly attach to ECM, few cell–cell junctions	ECM bears stress; cells sense matrix & migrate
Epithelial (skin, gut)	Thin basal lamina	Cells tightly connected; linked to basal lamina	Cytoskeleton transmits stress; maintains sheet integrity

Types of epithelial cell–cell anchoring junctions?

Junction	Cytoskeleton	Function
Adherens junctions	Actin filaments	Anchor actin to neighboring cells, transmit stress
Desmosomes	Intermediate filaments	Provide mechanical strength by linking filaments

Types of cell–matrix anchoring junctions?

Junction	Cytoskeleton	Function
Actin-linked cell–matrix junctions	Actin filaments	Connect actin cytoskeleton to ECM, sense stress
Hemidesmosomes	Intermediate filaments	Anchor intermediate filaments to ECM for stability

Other key junction types and functions?

• Tight junctions: Seal gaps near apical surface, prevent leakage.

• Gap junctions: Channels linking cytoplasms of adjacent cells for communication.

Main transmembrane adhesion proteins and their roles?

Protein	Primary Attachment	Specialization
Cadherins	Cell–cell	Adherens junctions (actin) or desmosomes (intermediate filaments)
Integrins	Cell–matrix	Actin-linked junctions (actin) or hemidesmosomes (intermediate filaments)

• Some integrins can mediate cell–cell adhesion.

How are cytoskeleton, junctions, and transmembrane proteins organized in epithelial cells?

• Apical: Tight junctions seal cells.

• Lateral: Adherens junctions + desmosomes link cytoskeleton to neighbors.

• Basal: Actin-linked junctions + hemidesmosomes connect cytoskeleton to basal lamina.

• Cytoskeleton: Transmits stress and shape signals.

• Transmembrane proteins: Cadherins (cell–cell), integrins (cell–matrix).

What is the primary role of cadherins in tissue organization?

• Mediate selective homophilic adhesion: cells of the same type stick together preferentially.

• Drive tissue segregation, not general stickiness.

• Enable specific recognition between cells, shaping tissue architecture.

What evidence shows cadherins drive selective cell adhesion?

Observation	Implication
Dissociated amphibian embryo cells reaggregate into original-like structures	Cells have selective adhesion mechanisms
L cells transfected with different cadherins → separate aggregates	Homophilic cadherin binding drives tissue segregation
L cells with different amounts of same cadherin → partial sorting	Quantitative differences in cadherin expression influence tissue organization

How does cadherin expression change during development, and what are the effects?

Stage	Cadherin Expression	Function
Neural tube formation	Neural tube: N-cadherin; ectoderm: E-cadherin	Cells segregate into distinct tissue layers
Neural crest migration	E/N-cadherins downregulated; Cadherin 7 upregulated	Loosely associated migrating cell groups
Ganglion formation	N-cadherin re-expressed	Aggregation into organized structures

• Experimental note: Overexpression of N-cadherin in neural crest → migration fails, confirming cadherin role.

What principles govern cadherin-dependent adhesion and tissue organization?

• Qualitative differences: Type of cadherin determines which cells stick together.

• Quantitative differences: Amount of cadherin fine-tunes adhesion strength.

• Cadherin switching: Allows dynamic tissue remodeling, migration, and segregation during development.

What is Epithelial–Mesenchymal Transition (EMT) and its biological significance?

• EMT: reversible process where epithelial cells become mesenchymal (motile) and mesenchymal cells become epithelial (organized epithelium).

• Normal development: neural crest formation, tissue remodeling.

• Pathology: cancer metastasis; epithelial-derived malignant cells gain migratory ability.

Key transcription factors controlling EMT and their effects?

TF	Effect on EMT
Twist	Promotes EMT by inhibiting cadherin (E-cadherin) expression
Snail	Represses E-cadherin, promotes mesenchymal traits
Slug	Represses epithelial cadherins, facilitates migration

• Twist overexpression → epithelial cells become mesenchymal-like.

• Blocking Twist → malignant cells revert to epithelial phenotype.

• E-cadherin loss/mutation → facilitates metastasis.

How do cadherins link to the cytoskeleton?

• Adherens junctions: cadherins → actin filaments.

• Desmosomes: cadherins → intermediate filaments.

• Cytoskeletal linkage essential for mechanical integrity and adhesion stability.

Key catenin/adaptor proteins and their roles in adherens junctions?

Protein	Function
β-catenin	Binds cadherin cytoplasmic tail; recruits α-catenin
α-catenin	Links β-catenin to actin filaments; organizes actin network
p120-catenin	Stabilizes cadherin on plasma membrane; regulates adhesion strength
Actin regulators	Modulate filament assembly for junction clustering and dynamics

• Hundreds–thousands of cadherins per junction.

• Extracellular cadherins → adhesion; cytoplasmic catenins + actin → strong, dynamic linkage.

What are the key principles of EMT and cadherin–catenin-mediated adhesion?

• EMT is transcription-factor controlled via cadherin regulation.

• Cadherins alone cannot stabilize adhesion; catenins link them to cytoskeleton.

• Adhesion is dynamic and reversible, enabling tissue remodeling, migration, and cancer metastasis

How are adherens junctions linked to the cytoskeleton?

• Connected to actin filaments and non-muscle myosin II.

• Contractile forces are critical for junction assembly and maintenance.

• Force balance between cells maintains tissue integrity.

• Myosin disruption → junction disassembly.

How do adherens junctions sense and respond to mechanical forces?

• Junctions act as tension sensors (mechanotransduction).

• Increased contractility in one cell → junction enlarges, neighbor increases contractility → balanced forces.

• Allows cells to adjust junction strength to mechanical stress.

Molecular mechanism of force sensing at adherens junctions?

• α-Catenin unfolds under tension → exposes binding site for vinculin.

• Vinculin recruits more actin → junction reinforced.

• Moderate forces → strengthen junction.

• Excessive forces → may weaken or peel apart junctions, enabling tissue remodeling.

Key principles of adherens junction force sensing?

• Junctions sense/respond to mechanical stress → maintain tissue integrity.

• Force balance prevents tissue disruption.

• α-Catenin + vinculin translate tension into structural reinforcement.

• High contractile forces reduce adhesion → allow tissue remodeling during development.

How do adherens junctions contribute to tissue remodeling?

• Link actin filaments of neighboring cells → enable coordinated actin contraction.

• Shape multicellular structures during development and morphogenesis.

• Forms:

  • Small punctate/linear junctions → cortical actin in nonepithelial tissues.

  • Continuous adhesion belts (zonula adherens) → encircle epithelial cells beneath apical face.

  • Specialized junctions in heart muscle → anchor contractile actin bundles, work with desmosomes.

How do adherens junctions coordinate with the actin–myosin network?

• Actin–myosin bundles lie parallel to membrane, tethered via cadherins + adaptor proteins.

• Bundles across cells form a transcellular network.

• Coordinated contraction drives:

  • Folding of epithelial sheets into tubes/vesicles.

  • Morphogenetic movements in embryos.

Example: Drosophila germ-band extension – mechanism?

• Early epithelium elongates along anterior–posterior axis.

• Actin-dependent contraction along specific cell boundaries.

• Loss of specific adherens junctions → allows cell intercalation.

• β-Catenin regulation: localized phosphorylation → targeted degradation → selective junction loss.

Key principles of tissue remodeling via adherens junctions?

• Adherens junctions + actin contractility = dynamic machinery for morphogenesis.

• Continuous adhesion belts → coordinated contraction in epithelial sheets.

• Selective junction remodeling → enables cells to move, intercalate, elongate tissues.

• Spatial regulation of β-catenin is crucial for localized junction disassembly during development.

What are desmosomes and their main function?

• Specialized cell–cell junctions similar to adherens junctions.

• Link cadherins to intermediate filaments (not actin).

• Provide mechanical strength to tissues under stress.

• Appear as buttonlike spots (“rivets”) on the lateral membrane.

• Found in mature vertebrate epithelia (skin, heart muscle); absent in Drosophila.

How are desmosomes linked to the cytoskeleton?

• Connected to intermediate filaments (cell-type specific).

• Examples: Keratin (epidermis), Desmin (heart).

• Filaments interconnect desmosomes of neighboring cells → tissue-wide tensile strength.

• Anchored filaments form a structural framework resisting stretching and mechanical stress.

What are the main proteins in desmosomes?

• Desmosomal cadherins span the plasma membrane.

• Adaptor proteins link cadherins to intermediate filaments.

• Dense intracellular plaque anchors filaments.

Why are desmosomes functionally important and clinically relevant?

• Essential for mechanical integrity of tissues under stress.

• Clinical example: Pemphigus → autoantibodies target desmosomal cadherins → adhesion disrupted → skin blistering and fluid leakage.

What are the main functions of tight junctions in epithelia?

1. Seal (Barrier): Prevents free passage of molecules between adjacent cells.

2. Fence (Polarity): Maintains apical–basal polarity by restricting diffusion of membrane proteins.
   Essential for selective permeability.

Where are tight junctions located in epithelial cells?

• Near the apical region at the apical–lateral border.

• Basal side attached to basal lamina, apical side faces lumen/extracellular fluid.

• Ensures transcellular transport works efficiently and prevents paracellular leakage.

How do tight junctions act as a barrier?

• Seal prevents molecules from passing between cells.

• Example: In small intestine, nutrients move from lumen to extracellular fluid without backflow.

• Experimental demonstration: Low-molecular-weight tracers cannot pass tight junctions.

How do tight junctions act as a fence?

• Restrict diffusion of apical vs. basolateral membrane proteins.

• Maintain proper localization of transporters for nutrient absorption.

How does tight junction permeability vary by tissue?

• Impermeable to macromolecules.

• Ion/small molecule permeability differs:

  • Small intestine: high (e.g., Na⁺).

  • Urinary bladder: very low.

• Determined by composition of tight junction proteins.

How are tight junctions structurally organized?

• Form a branching network of sealing strands encircling the apical end of epithelial cells.

• Strands are made of transmembrane homophilic adhesion proteins.

• Extracellular domains bind directly to each other, occluding the intercellular space.

What are the main transmembrane proteins in tight junctions and their functions?

• Claudins: Essential for formation & ion selectivity; ~24 types; tissue-specific; form selective pores.

• Occludin: Limits permeability; not essential for assembly.

• Tricellulin: Seals tricellular junctions; prevents leakage at three-cell contacts.

Give an example showing the importance of claudins.

• Claudin-1 deficiency in mice: no epidermal tight junctions → water loss → death shortly after birth.

• Ectopic claudin expression in fibroblasts → induces tight-junction-like connections.

What are the functional roles of transmembrane proteins in tight junctions?

• Sealing function: Prevents paracellular leakage.

• Permeability control: Claudin combinations determine selective ion passage.

• Tissue specificity: Different epithelia express distinct claudins for specialized function.

What is the role of scaffold proteins in tight junctions?

• Organize and stabilize tight junctions by linking transmembrane adhesion proteins (claudins, occludins) to the actin cytoskeleton.

• Key proteins: ZO-1, ZO-2, ZO-3 (Zonula Occludens proteins).

What are the key features of ZO proteins?

• Type: Scaffold proteins.

• Domains: Multiple PDZ domains (~80 aa).

• Binding partners: Claudins, occludins, actin cytoskeleton, other ZO molecules.

• Function: Organize intracellular mat that positions and stabilizes tight junction strands.

What is a junctional complex and its organization?

• Assembly of tight junctions, adherens junctions, and desmosomes at the apical region of epithelial cells.

• Tight junctions are apical to adherens and desmosomes.

• Formation is interdependent: blocking adherens junctions also disrupts tight junctions.

• Provides mechanical cohesion and barrier function to epithelial sheets.

What is the main function of gap junctions?

Create direct cytoplasmic channels between adjacent cells for ion and small molecule exchange, enabling electrical and metabolic coupling.

What are the roles of gap junctions in excitable vs non-excitable tissues?

• Excitable tissues (heart, smooth muscle): rapid electrical coupling for synchronized action potentials.

• Non-excitable tissues (connective tissue, epithelia): metabolic coupling to share small metabolites and coordinate cellular activity.

Describe the structure of gap junctions.

• Gap width: 2–4 nm.

• Channel proteins: Connexins (vertebrates, 21 isoforms) or Innexins (invertebrates).

• Pore size: ~1.4 nm, allows ions & small molecules, not macromolecules.

• Appearance: patches of closely spaced membranes bridged by channels.

Give examples of tissue-specific gap junction functions.

• Heart muscle: synchronize contractions.

• Smooth muscle: coordinate peristalsis.

• Connective tissues/epithelia: metabolic cooperation via small molecule sharing.

How do gap junctions differ from tight junctions?

• Gap junctions: bridge cytoplasms for communication.

• Tight junctions: seal gaps, prevent passage of molecules.

What is a connexin and a connexon?

• Connexin: Four-pass transmembrane protein; basic subunit of a connexon.

• Connexon (hemichannel): Hexamer of 6 connexins forming a hemichannel in one cell membrane.

How is a full gap junction channel formed?

A: Two connexons from adjacent cells align and dock, forming a continuous aqueous channel for direct cytoplasmic communication.

Q: What are gap-junction plaques?

A: Clusters of multiple connexons (from a few to thousands) arranged in parallel, providing robust intercellular connections.

Q: What are the main functions of connexons?

• Electrical & metabolic coupling (ions & small molecules).

• Supplemental adhesion with cadherins and claudins.

• Tissue specificity via different connexin types.

• Heteromeric: mixed connexins in one hemichannel.

• Heterotypic: different connexins in adjacent cells forming channels with distinct properties.

Q: How are gap junction channels regulated?

A: Open/close in response to:

• Voltage difference between cells.

• Membrane potential of each cell.

• Cytoplasmic factors (pH, Ca²⁺).

• Extracellular signals (neurotransmitters).
  They do not remain open constantly.

Q: How do connexons undergo dynamic turnover?

• New connexons added at plaque periphery via exocytosis.

• Old connexons removed from center and degraded.

• Connexin half-life: a few hours.

• Unpaired hemichannels normally closed, open under certain conditions to release small signaling molecul

Q: What are plasmodesmata and their general function?

A: Cytoplasmic channels bridging plant cell walls, enabling direct cell–cell communication, analogous to animal gap junctions.

Q: What is the structural context of plasmodesmata in plant cells?

• Plant cells have thick cell walls (~0.1 μm+).

• Plasmodesmata traverse these walls.

• Diameter: 20–40 nm.

• Found in virtually all living plant cells.

Q: What are the main structural components of plasmodesmata?

• Plasma membrane: continuous between connected cells.

• Desmotubule: central tube, continuous with smooth ER of both cells.

• Cytosolic annulus: space between desmotubule & plasma membrane for small molecule passage.

• Pit fields: clusters of plasmodesmata inserted through existing walls.

Q: How are plasmodesmata formed?

• During cytokinesis: around smooth ER trapped in developing cell plate.

• De novo: inserted through preexisting cell walls.

Q: What are the transport properties and regulation of plasmodesmata?

• Allows molecules < ~800 Da (similar to gap junction cutoff).

• Transport can be restricted in specific cells or regions; mechanisms not fully known.

• Provides direct cytoplasmic continuity and regulated exchange of small molecules, similar to gap junctions.

Q: What is the primary function of selectins?

A: Mediate transient cell–cell adhesion in the bloodstream, especially for white blood cell trafficking into lymphoid organs and inflamed tissues.

Q: What type of adhesion and Ca²⁺ dependence do selectins have?

• Heterophilic adhesion: binds carbohydrate ligands on other cells.

• Ca²⁺-dependent, like cadherins and integrins.

Q: What are the main structural features and types of selectins?

• Transmembrane protein with conserved lectin domain that binds oligosaccharides.

• Types:

  • L-selectin: white blood cells (lymphocytes)

  • P-selectin: platelets & activated endothelial cells

  • E-selectin: activated endothelial cells

Q: Describe the mechanism of selectin-mediated adhesion.

1. Initial adhesion: Weak selectin–carbohydrate binding → WBC rolling.

2. Integrin activation: Rolling triggers integrins to switch to high-affinity state.

3. Strong adhesion: Integrins bind endothelial Ig-family proteins → firm attachment.

4. Extravasation: WBC crawls between endothelial cells to enter tissue.

Q: What are the key concepts of selectin function?

• Transient adhesion: enables rolling without permanent sticking.

• Heterophilic binding: selectins bind carbohydrates, integrins bind Ig proteins.

• Sequential action: selectins initiate rolling → integrins mediate firm adhesion → tissue entry.

Q: What is the primary function of Ig-superfamily adhesion proteins?

A: Mediate cell–cell adhesion independent of Ca²⁺, contributing to fine-tuning adhesion, development, regeneration, and specialized interactions.

Q: What types of binding do Ig-superfamily proteins perform?

• Homophilic: binds same-type proteins (e.g., NCAM).

• Heterophilic: binds different proteins, such as integrins (e.g., ICAMs, VCAMs).

Q: Name key Ig-superfamily members and their roles.

• ICAMs: endothelial cells, heterophilic with integrins → white blood cell adhesion.

• VCAMs: endothelial cells, heterophilic with integrins → immune trafficking.

• NCAM: nerve cells, homophilic → development/migration; polysialic acid reduces adhesion.

• Other members: various tissues, specialized adhesion and signaling.

Q: How do Ig-superfamily proteins differ from cadherins?

• Cadherins: strong, Ca²⁺-dependent adhesion; maintain tissue integrity and segregation.

• Ig-superfamily: weaker, modulatory adhesion; Ca²⁺-independent; allows flexibility and dynamic interactions.

Q: What is the functional significance of polysialylated NCAM?

A: Long chains of sialic acid reduce adhesion via charge repulsion, enabling cell migration and preventing excessive sticking.

Q: Key concept distinguishing Ig-superfamily adhesion?

A: Ca²⁺-independent adhesion, mainly modulatory and signaling, not primary mechanical integrity like cadherins or selectins.

What is the ECM?

• Complex network of macromolecules secreted by cells.

• Closely associated with cell surfaces.

• Functions:

  • Structural scaffold

  • Mechanical properties (tensile strength, elasticity, calcification, transparency, jelly-like, protective shell)

  • Regulates cell behavior: survival, migration, proliferation, differentiation, shape, function

ECM Production and Organization

• Produced by: Local cells (fibroblasts in connective tissue, chondroblasts in cartilage, osteoblasts in bone)

• Orientation control: Cytoskeleton of producing cell guides ECM orientation

• Tissue specificity: ECM composition varies → tailored mechanical/biochemical properties

Major Classes of ECM Macromolecules

Class	Composition	Function / Notes
Glycosaminoglycans (GAGs)	Long, charged polysaccharides (often in proteoglycans)	Hydrated gel → resists compression, allows diffusion of nutrients/hormones
Fibrous proteins	Mainly collagen; also elastin	Collagen → tensile strength & matrix organization; Elastin → elasticity & resilience
Non-collagen glycoproteins	Proteins with N-linked oligosaccharides	Mediate adhesion, migration, differentiation; can form multimers

ECM Composition in Mammals

• ~300 matrix proteins

• ~36 proteoglycans

• ~40 collagens

• ~200 glycoproteins (multidomain, multimeric)

• Includes matrix-associated enzymes → cross-linking, remodeling, degradation

ECM Organization

• Proteoglycans → hydrated ground substance

• Collagen fibers → strength & structural organization

• Glycoproteins → guide cell migration/differentiation

• Elastin fibers → flexibility & resilience

Functional Summary of ECM

• Mechanical support: tensile strength, compression resistance, elasticity

• Structural diversity: bone, cartilage, cornea, tendons, jelly-like tissues, protective shells

• Cell regulation: guides survival, migration, proliferation, differentiation

• Transport facilitation: hydrated gel allows diffusion of small molecules between blood and cells

General Features of GAGs

• Structure: Unbranched polysaccharide chains of repeating disaccharides

  • Sugar 1: amino sugar (N-acetylglucosamine or N-acetylgalactosamine), often sulfated

  • Sugar 2: uronic acid (glucuronic or iduronic)

• Charge: Highly negative → most anionic molecules produced by animal cells

• Conformation: Stiff, extended chains occupying large volume relative to mass

• Hydration: Form hydrated gels even at low concentrations

Major Types of GAGs

GAG Type	Sugar Composition	Features / Function
Hyaluronan	N-acetylglucosamine + glucuronic acid	Large, non-sulfated; space-filling & lubrication
Chondroitin sulfate & Dermatan sulfate	N-acetylgalactosamine + uronic acids	Sulfated; cartilage & connective tissue support
Heparan sulfate	Glucosamine + uronic acids	Highly sulfated; regulates cell signaling & adhesion
Keratan sulfate	Galactose + N-acetylglucosamine	Sulfated; found in cornea, cartilage, bone

Physical Properties and ECM Role

• Hydration & Gel Formation:

  • Negative charges attract cations (Na⁺) → osmotic water influx → swelling pressure

• Mechanical Role:

  • Supports compressive forces (e.g., cartilage)

  • Works with collagen (tensile resistance) to maintain tissue integrity

Biological Importance

• Space-filling: GAG chains occupy large ECM volume despite low mass (<10% of connective tissue protein)

• Defects & Disease:

  • Example: Dermatan sulfate deficiency → short stature, premature aging, skin/joint/muscle/bone defects

Summary of Functional Properties

Property	Function
Negative charge	Attracts cations → water retention
Hydration	Forms gel → enables nutrient/metabolite diffusion
Extended conformation	Occupies ECM volume, resists compression
Interaction with collagen	Provides compressive support, complements tensile strength

Q: What is the structure and unique features of hyaluronan?

• Structure: Linear GAG of repeating disaccharides (up to 25,000 units)

• Unique Properties:

  • Non-sulfated

  • Identical disaccharide units

  • Not covalently linked to core proteins (unlike other GAGs)

  • Synthesized at the plasma membrane by embedded enzymes

• Location: All tissues & body fluids; abundant in early embryos

• Synthesis: Spun out directly from cell surface (vs. other GAGs made intracellularly & secreted via exocytosis)

Q: What are the main functions of Hyaluronan?

• Space filler during tissue morphogenesis: Expands with water, forces tissue shape changes

• Facilitates cell migration: Creates temporary cell-free spaces beneath epithelia

• Embryonic development: Drives formation of structures like heart valves and septa

• Wound healing: Accumulates locally to support tissue repair and migration

• Lubrication: Major constituent of joint fluid, resists compressive forces

Q: What are the mechanical and biological properties of Hyaluronan?

• Resists compressive forces

• Can deform tissue locally due to swelling with water

• Transient ECM component: degraded by hyaluronidase after its role in development or repair is complete

How can Hyaluronan be summarized?

• Acts as a high-volume, hydrated matrix component

• Provides mechanical support, space for morphogenetic movements, and facilitates tissue repair

• Unique among GAGs: lacks sulfation, very large, synthesized extracellularly

Q: What are proteoglycans and how are they synthesized?

• Proteins with one or more covalently attached GAG chains (except hyaluronan)

• Core protein made on membrane-bound ribosomes → enters ER lumen

• GAG chains assembled in Golgi:

  • Linkage tetrasaccharide attached to serine

  • Sugars added one at a time by glycosyltransferases

  • Sugars modified by epimerization & sulfation → increases negative charge

• Up to 95% carbohydrate by weight, mostly long unbranched GAG chains (~80 sugars each)

• Diversity: number/type of GAG chains vary; disaccharides sulfated in complex patterns; core proteins have conserved domains (e.g., LINK)

Q: What are key examples of proteoglycans and their features?

• Aggrecan: Very large (~3 × 10⁶ Da), >100 GAG chains; major cartilage component, forms aggregates with hyaluronan

• Decorin: Small, 1 GAG chain; binds collagen fibrils, regulates fibril assembly & diameter; mice lacking decorin → fragile skin

Q: How are proteoglycans organized in the ECM and what is their mechanical role?

• Form large polymeric complexes with GAGs + hyaluronan (size ~ bacterium)

• Interact with fibrous proteins (collagen) and basal lamina proteins → complex composites

• Contribute to ECM hydration, compressive resistance, structural organization

Q: What are membrane proteoglycans and their functions?

• Syndecans: Membrane-spanning or GPI-anchored core protein

  • Interact with actin cytoskeleton & signaling proteins

  • Modulate integrin function via fibronectin interaction

  • Influence growth/proliferation via binding soluble growth factors

• Other proteoglycans can act as cell-surface signaling platforms

Q: How can proteoglycans be summarized?

• Central ECM macromolecules providing hydration, spacing, compressive strength

• Organize matrix architecture via interactions with fibrous proteins & hyaluronan

• Participate in cell signaling, migration, adhesion

• Highly diverse → structure & function tailored to tissue needs

General Features of Collagens

• Family: Fibrous proteins, present in all multicellular animals

• Abundance: Most abundant protein in mammals (~25% of total protein mass)

• Source:

  • Large amounts: connective tissue cells

  • Small amounts: many other cell types

• Structure: Triple-stranded helix (superhelix) of 3 α-chains, rich in glycine & proline

• Genes: 42 distinct α-chain genes → tissue-specific expression

• Diversity: ~40 distinct collagen molecules

Collagen Types & Functions

Collagen Type	Classification	Location / Structure	Function / Notes
I	Fibrillar	Skin, bone	Principal collagen; forms fibrils → fibers; tensile strength
II	Fibrillar	Cartilage	Forms fibrils in cartilage
III	Fibrillar	Skin, blood vessels	Flexible fibrils; often co-assembles with type I
IX	Fibril-associated	Cartilage	Links fibrils to ECM
XII	Fibril-associated	Tendon, ligament	Decorates & links fibrils to ECM
IV	Network-forming	Basal lamina	Forms mesh-like sheets supporting cells
VII	Anchoring fibril	Basal lamina (skin)	Anchors basal lamina to connective tissue
XVII	Transmembrane	Hemidesmosomes	Connects cells to ECM
XVIII	Proteoglycan core	Basal lamina	Collagen-like segment in ECM proteoglycan

Structural Assembly of Collagen

• Fibrillar collagens: Assemble into fibrils (10–300 nm diameter; hundreds μm long) → fibers

• Fibril-associated collagens: Decorate fibrils, link to ECM

• Network-forming collagens: Form sheets (e.g., basal lamina)

• Anchoring fibrils: Attach basal lamina to connective tissue

Molecular Features of Collagen

• α-chain genes: Large (~44 kb), 50 exons

• Exon pattern: Mostly 54 nucleotides → encode Gly-X-Y repeats

• Evolution: Repetitive gene duplication → collagen diversity

Key Takeaways of Collagen

• Collagens provide tensile strength, structural support, and organization

• Tissue specificity:

  • Skin/bone → type I

  • Cartilage → type II, IX

  • Basal lamina → type IV

  • Skin anchoring → type VII

Collagen Fibril Function & Tissue Arrangement

• Function: Resist tensile forces (complement GAGs, which resist compression)

• Tissue-specific arrangements:

  • Skin: Wickerwork → resists multi-directional stress

  • Tendons: Parallel bundles → aligned with tension axis

  • Bone & cornea: Plywood-like layers → fibrils in each layer parallel; adjacent layers nearly perpendicular

  • Tadpole skin: Similar layered arrangement

Role of Connective Tissue Cells

• Determine size and arrangement of collagen fibrils

• Mechanisms:

  • Express specific fibrillar collagen genes

  • Guide fibril formation near plasma membrane

  • Secrete matrix proteins (e.g., fibronectin) to organize fibrils

Fibril-Associated Collagens (FACs)

Feature	Fibrillar Collagen	FAC (Type IX, XII)
Helical structure	Continuous triple helix	Interrupted with short nonhelical domains → more flexible
Fibril formation	Forms fibrils	Binds fibril surfaces; does not form fibrils
Tissue examples	Type I: skin, tendon; Type II: cartilage	Type IX: cartilage, cornea, vitreous; Type XII: tendon, type I tissues
Function	Tensile strength	Organizes fibrils; mediates fibril–fibril & fibril–matrix interactions

Mechanism of Collagen Organization

• FACs bind periodically along fibrils

• Mediate:

  • Fibril–fibril interactions

  • Fibril–matrix interactions

• Influence tissue-specific fibril arrangements

• Cells + FACs + fibronectin → finely tuned collagen architecture

Key Points

• FACs do not form fibrils but guide fibril organization

• Type IX: cartilage, cornea, vitreous

• Type XII: tendon, type I collagen tissues

• Collagen organization relies on cells, FACs, and matrix proteins for mechanical strength and proper tissue architecture

Mechanical Interaction of Cells with ECM

• Cells interact with ECM mechanically and chemically

• Fibroblasts exert tension on collagen fibrils → influence tissue architecture

Observations in Culture

• Fibroblasts in collagen gels:

  • Random fibrils drawn together by fibroblast traction → gel contracts

• Fibroblast clusters:

  • Create densely packed, circumferentially oriented fibers

• Between tissue explants:

  • Collagen aligns into compact fiber bands

  • Fibroblasts migrate along these aligned fibers

• Feedback loop: Fibroblasts → collagen alignment → guides fibroblast distribution

Role of Fibroblasts In Vivo

• Synthesize collagen fibrils and deposit in proper orientation

• Crawl and tug on matrix to shape tissue architecture:

  • Tendons

  • Ligaments

  • Dense connective tissue layers around organs

Stepwise Mechanism of Collagen Organization

Step	Action
1	Collagen fibrils synthesized & secreted by fibroblasts
2	Fibroblasts attach via cell-matrix adhesions
3	Fibroblasts pull fibrils → align & compact matrix
4	Aligned collagen guides fibroblast migration & tissue shaping
5	Functional tissue architecture established