Introduction to Human Physiology: Tissue Types and the Extracellular Matrix

Human Physiology: Tissue Types and the Extracellular Matrix

Overview of Tissue Levels in Human Physiology

Human physiology is organized into hierarchical levels, from the cellular level to the organism level. Understanding tissues is essential for grasping how organs and systems function.

• Cellular Level: The basic unit of life, such as a squamous epithelial cell.

• Tissue Level: Groups of similar cells performing a common function, e.g., stratified squamous epithelium.

• Organ Level: Structures composed of multiple tissue types, such as the esophagus.

• Organ System Level: Groups of organs working together, e.g., the digestive system.

• Organism Level: The complete human body, integrating all systems.

Types of Body Tissues

The human body is composed of four primary tissue types, each with distinct structures and functions.

• Connective Tissue: Provides structural support, binds tissues together, and stores energy (e.g., bone, cartilage, adipose tissue).

• Muscle Tissue: Responsible for movement and force generation (e.g., skeletal, cardiac, and smooth muscle).

• Nervous Tissue: Facilitates communication and control via electrical impulses (e.g., neurons, glial cells).

• Epithelial Tissue: Covers body surfaces, lines cavities, and forms glands.

Structure of Tissues

Tissues are made up of cells that are related in structure and function. The surrounding material, known as the extracellular matrix (ECM), plays a crucial role in tissue characteristics.

• Cells: The living units performing specialized functions.

• Extracellular Matrix (ECM): Non-cellular material that provides structural and biochemical support to cells.

• Proportion of Cells to ECM: This ratio varies by tissue type and determines tissue properties.

Extracellular Matrix (ECM)

The ECM consists of substances in liquid, gel, or solid form that surround cells. It is essential for tissue strength, organization, and cell survival.

• Tensile Strength: ECM resists stretching forces.

• Compressive Forces: ECM withstands pressure.

• Cell Positioning: ECM holds cells in place and maintains tissue structure.

• Cell Survival: ECM supports proper function and survival of cells.

Main Components of the ECM:

• Ground Substance: Amorphous material made up of water, ions, and organic molecules.

• Protein Fibers: Provide structural support and elasticity.

Detailed Components of the ECM

The ECM is composed of several key elements that contribute to its function and structure.

• Ground Substance:

  • Contains water, ions, and glycosaminoglycans (GAGs), which are long polysaccharide chains.

  • Proteoglycans (protein + polysaccharide) add viscosity and resilience.

  • Cell Adhesion Molecules (CAMs): Specialized proteins that anchor cells to each other and to the ECM.

• Protein Fibers:

  • Collagen Fibers: Provide tensile strength and resistance to stretching.

  • Elastic Fibers: Allow tissues to stretch and recoil.

  • Reticular Fibers: Form supportive networks for cells and ground substance.

Summary Table: ECM Components and Functions

Component	Main Function	Example
Ground Substance	Hydration, support, diffusion of nutrients	Glycosaminoglycans in cartilage
Collagen Fibers	Tensile strength	Tendons, ligaments
Elastic Fibers	Elasticity, recoil	Skin, lungs
Reticular Fibers	Supportive meshwork	Lymph nodes, spleen
Cell Adhesion Molecules (CAMs)	Cell anchoring and communication	Integrins in epithelial tissue

Key Terms and Definitions

• Extracellular Matrix (ECM): The non-cellular component present within all tissues and organs, providing essential physical scaffolding for the cellular constituents.

• Glycosaminoglycans (GAGs): Long, unbranched polysaccharides consisting of repeating disaccharide units that contribute to the viscosity of the ground substance.

• Proteoglycans: Proteins that are heavily glycosylated, forming a major component of the ground substance.

• Collagen: The most abundant protein in the ECM, providing strength and structure.

• Elastic Fibers: Composed of elastin, allowing tissues to resume their shape after stretching or contracting.

• Reticular Fibers: Thin fibers forming a supportive mesh in soft tissues.

• Cell Adhesion Molecules (CAMs): Proteins located on the cell surface involved in binding with other cells or with the ECM.

Example Application

In connective tissue such as tendons, the ECM is rich in collagen fibers, providing the strength needed to withstand pulling forces during movement.

Additional info: The ECM is also involved in cell signaling, tissue repair, and pathological processes such as fibrosis and cancer metastasis.