2b - ECM and Adhesions

Page 1: Introduction to the ECM and Cytoskeleton

Overview of Extracellular Matrix (ECM): The ECM is a complex network of proteins and carbohydrates that provides structural and biochemical support to surrounding cells. It plays a critical role in tissue and organ structure, influencing cell communication and behavior.
Importance of Cytoskeleton Extensions: The cytoskeleton, consisting of microtubules, intermediate filaments, and actin filaments, provides shape, mechanical support, and facilitates cell movement. Extensions of the cytoskeleton enable dynamic interactions with the ECM, crucial for maintaining cellular integrity and facilitating intracellular transport.

Phospholipid Bilayer: The bilayer structure is formed by amphipathic phospholipids that arrange themselves with hydrophilic heads facing outward and hydrophobic tails facing inward, creating a semi-permeable membrane that controls the passage of substances.
Fluid Mosaic Model: This model illustrates the membrane as a fluid entity composed of diverse proteins that float in or on the fluid lipid bilayer, allowing for flexibility and movement.
Components:
- Embedded Proteins: Integral and peripheral proteins serve various functions, including transport and signaling.
- Glycoproteins and Glycolipids: These carbohydrates attached to proteins and lipids aid in cell recognition and signaling processes.
- Cholesterol: Interspersed within the membrane, cholesterol molecules help modulate fluidity and stability.
Intercellular Connections: Multicellular organisms rely on tight junctions, desmosomes, and gap junctions for communication and structural integrity, linking cells to each other and to the ECM seamlessly.

Comparison of Plant and Animal Cells: In contrast to animal cells, which rely on ECM for structural support, plant cells have rigid cell walls composed of cellulose, contributing to their shape and resistance to pressure. Understanding these differences reveals insights into the evolutionary adaptations of various life forms.

Plant Cell Walls: Composed of various polymers and proteins, they provide strength and rigidity. The diversity in composition among different plant species influences their growth patterns and environmental interactions.
Primary Cell Wall: Rich in pectin, it allows flexibility during cell growth. The primary wall is critical in the initial stages of cell division and expansion.
Secondary Cell Wall: Formed after growth, mainly consisting of cellulose, it provides additional strength and resistance to physical stress, essential for woody plants.
Other Organisms:
- Bacteria: Have walls made of peptidoglycan, essential for their structural integrity.
- Fungi: Chitin in their cell walls offers similar protection but differs chemically from cellulose.
- Protists: May have cells walls made of cellulose or silica, varying by species.
- Archaea: Utilize pseudopeptidoglycan, showcasing the diversity of wall structures across domains of life.
Absence in Animals: Animal cells do not have cell walls, relying instead on the ECM for support, allowing for greater flexibility and mobility in tissues.

Main Functions:
- Mechanical Strength: The ECM withstands tensile and compressive forces, maintaining tissue integrity during stress.
- Surface for Cell Migration: The ECM serves as a substrate for cells to migrate during development and healing processes, providing cues for directional movement.
- Facilitates Communication Between Cells: Through signaling molecules and receptors, the ECM participates in regulating cell behavior, differentiation, and tissue repair.
Composition:
- Proteoglycans: These molecules are critical for hydration, cushioning, and creating a gel-like matrix around cells.
- Collagen: A predominant structural protein, collagen fibers provide tensile strength and framework for tissues.
- Matrix Proteins: Diverse proteins that organize and connect ECM components, essential for tissue architecture.
- Elastin: Crucial for tissue elasticity, allowing structures like blood vessels to stretch.

Definition of Tissue: Tissues are groups of similar cells that work together with their ECM to achieve a common function, essential for the organization of complex organisms.