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How Do Cells Attach?

Published in Cell Adhesion 4 mins read

Cells attach to each other and to their surrounding environment through specialized structures called cell junctions, which are crucial for tissue formation, strength, and function.

The Foundations of Cellular Adhesion

Cell attachment is fundamental for the organization of multicellular organisms, allowing cells to form cohesive tissues like skin, muscle, and organs. This intricate process relies on various types of cell junctions, each designed for specific roles, from providing structural integrity to forming impermeable barriers. These junctions are primarily composed of protein complexes that link cells together or connect them to the extracellular matrix.

Key Types of Cell Junctions

The primary mechanisms by which cells attach involve two main categories: anchoring junctions and occluding junctions. Each type serves distinct functions, ensuring the stability and functionality of tissues.

Anchoring Junctions: Maintaining Cellular Cohesion

Anchoring junctions are critical for maintaining cells together and strengthening the contact between them, providing mechanical stability to tissues that experience significant stress, such as skin and heart muscle. They are characterized by their ability to link the cytoskeletons of adjacent cells or cells to the extracellular matrix.

Adherens Junctions

Adherens junctions primarily function to maintain cells together and strengthen contact between them, often forming continuous belts around cells in epithelial layers.

  • Structure: These junctions link the actin cytoskeleton of adjacent cells. They typically involve transmembrane proteins called cadherins, which bind to similar cadherins on neighboring cells. Inside the cell, cadherins are connected to the actin filaments via adapter proteins like catenins.
  • Role: They are essential for tissue morphogenesis during embryonic development and for maintaining the integrity of epithelial tissues. Their contractile nature can also contribute to tissue shaping.
  • Example: Found in the tight packing of epithelial cells lining the intestine, helping them form a cohesive layer.

Desmosomes

Desmosomes are spot-like adhesive junctions that are highly effective at maintaining cells together and strengthening contact by providing robust mechanical links.

  • Structure: Similar to adherens junctions, desmosomes use cadherin proteins (specifically desmoglein and desmocollin) but link to the intermediate filaments of the cytoskeleton (e.g., keratin filaments in epithelial cells). These cadherins attach to dense plaques of adapter proteins within the cytoplasm, which then connect to the intermediate filaments.
  • Role: They are abundant in tissues that face significant mechanical stress, preventing cells from being pulled apart.
  • Example: Prevalent in skin, heart muscle, and other tissues requiring strong, stable cell-cell adhesion. Think of them as cellular rivets holding tissues together.

Hemidesmosomes

Unlike adherens junctions and desmosomes, hemidesmosomes are specialized junctions that strengthen contact by attaching cells to the extracellular matrix (ECM), rather than to other cells.

  • Structure: These junctions utilize transmembrane proteins called integrins, which bind to components of the extracellular matrix like laminin. Inside the cell, integrins connect to intermediate filaments (similar to desmosomes) via adaptor proteins.
  • Role: They are vital for anchoring epithelial cells firmly to the underlying basement membrane, providing stability and preventing tissue detachment.
  • Example: Found at the base of epithelial cells, securing them to the connective tissue beneath, crucial for the integrity of skin and mucous membranes.

Occluding Junctions: Sealing Gaps and Controlling Diffusion

Occluding junctions, specifically tight junctions, are responsible for forming barriers that regulate the passage of substances between cells.

Tight Junctions

Tight junctions, also known as zonula occludens, are unique in their ability to seal gaps between cells through cell–cell contact, making an impermeable barrier for diffusion.

  • Structure: They are formed by a network of claudin and occludin proteins in the plasma membranes of adjacent cells, which essentially "stitch" the cells together. These protein strands create a tight seal that prevents molecules from passing between cells (paracellular pathway).
  • Role: Their primary function is to regulate paracellular transport, ensuring that substances must pass through cells rather than around them, allowing for selective absorption or secretion. They also maintain cell polarity by preventing the lateral diffusion of membrane proteins.
  • Example: Crucial in the lining of the intestine, bladder, and blood-brain barrier, where they prevent leakage and control the movement of ions and molecules. For instance, in the gut, they ensure that nutrients are absorbed specifically by intestinal cells rather than passively leaking between them.

Summary of Cell Junctions

The following table summarizes the main types of cell junctions and their key roles in cell attachment:

Junction Type Primary Function Key Transmembrane Proteins Cytoskeletal Linkage Cell-Cell or Cell-ECM
Adherens Junctions Maintain cells together, strengthen contact Cadherins Actin Filaments Cell-Cell
Desmosomes Maintain cells together, strengthen contact Cadherins (Desmoglein, Desmocollin) Intermediate Filaments Cell-Cell
Hemidesmosomes Attach cells to extracellular matrix, strengthen contact Integrins Intermediate Filaments Cell-ECM
Tight Junctions Seal gaps between cells, impermeable barrier for diffusion Claudins, Occludins Actin Filaments (indirect) Cell-Cell