Biology · Cell Architecture, Growth & Division

Cell adhesion, extracellular matrices and tissue organisation

A sheet of cells lining a cavity must withstand forces, keep useful differences between its two sides and coordinate replacement of its cells. Its organisation depends on connections within and between cells and on material those cells have placed outside themselves.

The cells chapter introduces membranes and compartments; the DNA chapter distinguishes a DNA molecule from a chromosome and its replicated sister chromatids. Gene expression and signalling explain how proteins and regulatory responses arise. Here, nuclear DNA counts exclude organelle DNA. Every numerical movement, chromosome set and comparison case is an original teaching model, rather than a measured specimen.

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By the end, you can

Connect cellular contacts, matrix links and polarity to tissue function without treating tissues as passive piles of cells.

Adhesion links structures across a boundary

Cell adhesion is attachment between cells or between a cell and extracellular material. Adhesion proteins and associated complexes connect cellular surfaces to structural and signalling systems. In animal tissues, cadherin-based contacts commonly link neighbouring cells, while integrins participate in many cell–matrix attachments. These are different relationships, not two names for one universal junction.

A typical integrin spans the plasma membrane, binds a suitable extracellular ligand and connects through intracellular proteins to the cytoskeleton. Many such links involve actin; other specialised attachments involve intermediate filaments. The connection can transmit force and support signalling, so attachment changes can affect cell shape, movement or behaviour.

Adhesion is regulated and can be remodelled. Moving cells need contacts they can form and release, while a tissue resisting stress can distribute forces through maintained connections. Stronger attachment everywhere is not automatically better organisation: location, partners and dynamics all matter.

Extracellular material contributes to tissue architecture

An extracellular matrix is organised material outside cells. Animal matrices can contain collagen fibres, glycoproteins and hydrated carbohydrate-rich components. Cells produce and remodel this material, and its mechanical and chemical properties influence their surroundings. Matrix is therefore part of tissue organisation, not simply empty space between cells.

An epithelial sheet has specialised surfaces. In the illustrated example, the apical side faces a lumen, the cavity enclosed by the tissue, while the basal side attaches to a basement-membrane matrix. Junctions between neighbours contribute to mechanical continuity and regulated between-cell passage. A tight-junction barrier can be selective; it is not an absolute seal against every molecule.

Plant cell walls are specialised extracellular matrices with a different composition and organisation from animal collagen-rich examples. Cellulose-containing walls constrain expansion, while cellular connections and wall remodelling contribute to plant tissue function. Animal junction names and the illustrated basement membrane should not be silently assigned to a plant tissue.

Neighbour links and matrix links organise different relationships

Original selected animal epithelial architecture, with no dimensional scale. Not every integrin links to actin, no particular ligand pair is experimentally identified, and animal junction/basement-membrane names are not assigned to plant tissues.

Read the diagram in words

Three animal epithelial cells form a continuous sheet with an apical lumen-facing surface above and basal matrix below. Side-to-side contacts connect neighbours; selected tight junctions regulate the between-cell route near the apical boundary. Basal integrin-containing complexes span the plasma membrane and link through intracellular adaptor proteins to actin, with extracellular attachment to basement matrix. A zoomed link inset explicitly separates cytoskeleton, adaptors, membrane-spanning integrin and extracellular ligand.

Structure locations in the epithelial model
Identifier	Structure or state	Location or facing side
lumen	Lumen	Above the epithelial sheet
apical	Apical surface	Facing the lumen
neighbour	Cell–cell contact	Between neighbouring cells
barrier	Selective between-cell barrier	Near an apical neighbour contact
basal	Basal surface	Facing the matrix
matrix	Basement-membrane matrix	Beneath the sheet
actin	Actin cytoskeleton	Inside the cell
adaptors	Intracellular adaptor proteins	Cytosolic face of the attachment
integrin	Selected integrin complex	Spanning the plasma membrane
ligand	Extracellular matrix ligand	Outside the cell

Structural and membership relationships
First structure identifier	First structure	Second structure identifier	Second structure	Relationship kind	Relationship meaning
actin	Actin cytoskeleton	adaptors	Intracellular adaptor proteins	Structural connection	Intracellular structural link
adaptors	Intracellular adaptor proteins	integrin	Selected integrin complex	Structural connection	Link to membrane complex
integrin	Selected integrin complex	ligand	Extracellular matrix ligand	Attachment	Bind suitable extracellular partner
ligand	Extracellular matrix ligand	matrix	Basement-membrane matrix	Membership	Component of matrix

Diagram terms and qualifications
Label identifier	Meaning in this model
cell	Epithelial cell
membrane	Plasma membrane
orientation	Apical and basal surfaces have different relationships
force	Selected connection can transmit force and signals
selective	Selective barrier does not mean absolute seal

A tissue is maintained by coordinated changes

Tissue organisation emerges from cell identity, polarity, contacts, matrix, signals and the distribution of growth and loss. Division can add cells, cell enlargement can change dimensions, and removal can alter a local arrangement. A tissue can remain functional while its molecules and some of its cells turn over.

Cell division is not an isolated improvement in every setting: daughters must be positioned, connected and supplied appropriately. Likewise, removing a cell is not necessarily failure. Development and maintenance can involve regulated removal alongside survival and replacement. The preceding lessons supply mechanisms whose effects must be considered in a particular tissue.

To evaluate a change, identify the connection and the level of measurement. A disrupted cell–matrix link, a changed between-cell barrier and an altered cell number are different observations. They can interact, but one does not automatically prove all the others. This keeps a tissue-level explanation tied to evidence rather than to a diagram's tidy appearance.

Worked example

Two attachments can give different observations

In a constructed epithelial model, cells remain joined to their neighbours but attach poorly to the underlying matrix. A possible explanation concerns their cell–matrix connections, even though the cell–cell contacts are still present. The diagram shows those links in different places so one observation cannot substitute for the other.

Measure the relevant attachment, cytoskeletal connection and cellular response before assigning a cause. A change in matrix composition or signalling could also contribute, so weak attachment alone does not identify one defective integrin. The example separates useful hypotheses without claiming a real tissue diagnosis or a direct measurement of barrier permeability.

Check your understanding

Must intact cell–cell adhesion mean that a cell's attachment to the underlying matrix is also intact?
Show reasoning
No. The partners and molecular connections differ. They can interact, but each attachment requires its own evidence.
Is an animal extracellular matrix simply an inert empty gap?
Show reasoning
No. It is organised material produced and remodelled by cells, with mechanical and chemical properties that influence tissue organisation.
Why can normal tissue maintenance include cell death as well as division?
Show reasoning
Function can depend on coordinated removal, replacement and survival. The consequence of each event depends on its location, timing and connection to the tissue's organisation.

What to carry forward

Tissue function depends on organised links and coordinated cell behaviour, with cell–cell contacts, matrix attachment and selective barriers distinguished.

References

Resources supporting this lesson’s explanations and examples.