Connecting cells in tissues

Key Terms

Apical membranes - The top surface of a cell, facing the outside environment

Adherens junctions - Connections between cells that hold them together using proteins linked to the cell’s cytoskeleton

Adhesion belts - A belt-like ring of Adherens junctions around a cell that helps keep neighbouring cells attached.

Desmosomes - Strong connections between cells that provide mechanical strength and prevents cells from pulling apart.

ECM - A network of proteins and other molecules outside cells that supports and connects them

Basal membranes - The bottom surface of a cell, usually attached to the basement membrane

Introduction

Challenges of structure and communication

  • Cells must Adhere strongly to one another and function in a coordinated manner for tissues and organs to operate effectively.

  • The support structure must remain adaptable to stress and changing conditions, allowing for repairs.

  • Effective communication among cells is vital

  • cells must relay information to fulfill their roles appropriately.


Tissues

Group of cells working collaboratively to achieve specific functions

  • specialised cells within multicellular organisms are organised into tissues

  • The organisation pattern of these is tailored to the tissues function


Types of tissues

  • adipose (fat)

  • bone

  • skin

  • skeletal muscle

  • cartilage

  • intestine


Ways Cells Connect

  • Apical membranes - The top surface of a cell, facing the outside environment

  • Adherens junctions - Connections between cells that hold them together using proteins linked to the cell’s cytoskeleton

  • Adhesion belts - A belt-like ring of Adherens junctions around a cell that helps keep neighbouring cells attached.

  • Desmosomes - Strong connections between cells that provide mechanical strength and prevents cells from pulling apart.

  • ECM - A network of proteins and other molecules outside cells that supports and connects them

  • Basal membranes - The bottom surface of a cell, usually attached to the basement membrane


Cell Junctions

Cells are connected to other cells via cell junctions

  • Most types of junctions connect cell surface molecules on neighbouring cells with component of the cytoskeleton.


Types

  • Gap junctions

  • Tight junctions

  • Desmosomes

  • Adherens Junctions


Gap Junctions

Channel proteins that align between 2 cells

  • They directly connect the cytoplasm’s of the 2 cells and allow direct communication between them.

  • they don’t connect to the cytoskeleton


Functions:

  • Exchange of Metabolites

  • Passage of communication signals

  • Adhesion between cells, providing mechanical strength to tissues


Tight Junctions

Produce a virtually impermeable barrier between cells

  • neighbouring cells are connected via cell surface proteins

    • called claudins and occludins

  • connected to actin cytoskeleton to provide strength

  • ZO complex anchor proteins connect the claudin and occludin molecules to actin filaments


Functions:

  • Regulates movement of water, ions and molecules through the space between cells

  • separates the apical (top) of the cells from the basal (bottom)


Desmosomes

Cadherins that allow similar calls to recognise each other and form connections

  • interactions between 2 identical cadherin molecules on different cells leads to the cells forming a junction between them.

  • Different types of cadherin molecules join different types of cells together.

    • They link cells through intermediate filaments


Functions:

  • strong attachment between cells

  • Help tissues withstand stretching and pulling forces

  • Prevents the cells from separating


Adherens Junctions

cadherin molecules and anchor proteins to connect cells

  • they connect to actin filaments


The Extracellular Matrix (ECM)

ECM - The scaffolding system surrounding cells that they attach to. It consists of fibres of large proteins such as collagen


  • Cells attach to the ECM through cells surface molecules called integrins.

  • ECM provides structure and both mechanical and biochemical support for tissues

  • also forms the basement membranes (Skin)


Components

  • fibrous structural proteins

    • collagen

    • elastin

  • Proteoglycans and glycosaminoglycans

  • Adhesive glycoproteins

    • Fibronectin

    • Laminin


Collagen

  • most abundant protein, in the body

Structure

Structural role - forms molecular cables to strengthen tendons and calcified tissues like bones and teeth via mineral integration.

  • composed of tightly wound chains in a triple helix configurations

Types of collagen

  • Type I: 90% of collagen; found in skin, bones, tendons, ligaments.

  • Type II: Found in elastic cartilage for joint support.

  • Type III: Present in muscles, arteries, and organs.

  • Type IV: Constitutive of basement membranes.

  • Type V: Located in cornea, skin layers, hair, and placental tissue.


Proteoglycans

  • proteins that are heavily glycosylated and contain very long chains of sugar molecules branching off of the main protein chain.

  • there are at least 43

  • they can be found on the cell surface or in the ECM

  • they lubricate the ECM

  • Modify the activity and stability of other proteins in the ECM.


Fibronectin

  • is a glycoprotein that has binding sites for multiple ECM proteins has as well as binding sites for cell surface integrins.


Integrins and ECM attachment

Integrins - cell surface molecules that act as matric receptors

  • they connect to the ECM, but also transmit signals into the cell allowing the cell to sense and respond to tis environment.

  • They consist of alpha and Beta chains and different combinations of these subunits allows them to recognise a wide range of ECM components

  • when they bind to the ECM they change shape into an open conformation.



Adhesion Complex

  • Cytoplasmic tails of integrins bind with the actin cytoskeleton via anchor or adaptor proteins, forming an adhesion complex that enhances cell stability.


Hemidesmosomes

Hemidesmosomes - connect the ECM to intermediate filaments in the cell

  • through integrin molecules

  • they provide additional mechanical strength to the cells and tissues


Fibroblasts & ECM Maintenance

  • Fibroblasts are key cells responsible for ECM production and maintenance.

  • During wound healing, fibroblasts are activated to secrete collagen, which can lead to scar formation as part of the healing process.

Multicellularity

Multicellularity exists across life forms

  • animals, plants, most fungi, and certain algae; with some organisms alternating between unicellular and multicellular stages based on conditions.


Advantages

  • Improved resource acquisition.

  • Enhanced resistance to physical and chemical stresses.

  • Greater protection from predation.

  • More effective colonization of new environments.

  • Opportunities for specialized functions across different cell types through differentiation.


Disadvantages

  • Increased energetic costs associated with the production of adhesion and communication molecules.

  • Physical constraints due to limited mobility and proliferation.

  • Vulnerability to exploitation by non-cooperative cells or “cheaters.”


Examples Algae

  • Chlamydomonas: Unicellular but forms multicellular colonies under favorable conditions.

  • Gonium: Multicellular with 4-16 cells.

  • Volvox: Forms colonies with thousands of cells, exhibiting specialized cell types for different functions.