Interactions Between Cells and Their Environment

Overview of Cellular Interactions

• Cells interact with their environment to form tissues and organs.
• The extracellular matrix (ECM) plays a crucial role in these interactions.
• Animal cells are differentiated into specific types based on their environment.

Structure and Components of the ECM

• Extracellular Matrix (ECM):
• Composed of proteins, glycoproteins, and carbohydrates.
• Examples: Collagen, proteoglycans, fibronectin, laminin.
• Main Functions:
• Provides structural support to cells
• Facilitates cell attachment and communication.

Types of Cells in Tissues

• Epithelial Cells: Form protective layers (skin, lining gut).
• Endothelial Cells: Line blood vessels.
• Mesenchymal Cells: More mobile, can differentiate into multiple cell types.

Collagen

• Most abundant protein in the human body (25%).
• Classification:
• Fibrillar Collagens: Provide tensile strength (e.g. Type I collagen in tendons).
• Non-fibrillar Collagens: Form a scaffold in basement membranes (e.g. Type IV collagen).
• Synthesis: Requires vitamin C for hydroxyproline formation; deficiency leads to scurvy.

Proteoglycans

• Complexes of proteins and polysaccharides (GAGs).
• Functions include:
• Water retention: Forms gel-like matrix, contributing to tissue resilience.
• Signal modulation: Can bind growth factors, thus playing a role in cell signaling.

Fibronectin and Laminin

• Fibronectin:
• Dimeric protein that facilitates cell migration and ECM binding.
• Important in embryonic development and wound healing.
• Laminin:
• Key protein in the basal lamina.
• Involved in cell adhesion and migration, particularly for primordial germ cells.

Integrins

• Transmembrane receptors that connect cells to the ECM.
• Structure: Composed of alpha and beta subunits.
• Functions:
• Transmit signals from ECM to cells (outside-in signaling) and vice-versa (inside-out signaling).
• Important in focal adhesion formation, which connects integrins to actin filaments.

Cell-Cell Interactions

• Cadherins:

• Calcium-dependent glycoproteins that mediate cell-cell adhesion.

• Essential for tissue integrity and development.

• Selectins:

• Integral membrane proteins that facilitate the adhesion of leukocytes to endothelial cells during inflammation.

Intercellular Junctions

• Tight Junctions: Seal gaps between adjacent epithelial cells to prevent solute movement.
• Gap Junctions: Allow direct communication between adjacent cells through connexins.

Hemidesmosomes**:

• Connect epithelial cells to the basement membrane via intermediate filaments.
• Critical for skin integrity; defects can cause blistering.

Importance of ECM in Disease

• Cancer and Inflammation:
• Changes in ECM composition and cell adhesion properties can lead to metastasis in tumor cells.
• Leukocytes use specific adhesion molecules to migrate through blood vessel walls during inflammation.

Conclusion

• Understanding cell-ECM and cell-cell interactions is essential for studying tissue development, maintenance, and pathology.