Overview
This lecture introduces the study of tissues (histology), focusing on the structure and components of tissues, the extracellular matrix, and cell junctions in the human body.
Introduction to Tissues and Histology
- Histology is the study of the normal structure of tissues in the human body.
- A tissue consists of structurally and functionally related cells plus the extracellular environment (ECM) that holds them together for a common function.
- Tissues have two basic components: a population of cells and surrounding extracellular matrix (ECM).
The Extracellular Matrix (ECM)
- The ECM surrounds cells and can be liquid, gel-like, or solid.
- ECM functions include positioning cells, supporting the plasma membrane, regulating cell development, division, and survival.
- At the tissue level, the ECM holds tissues together, gives strength, and resists stretching and compression.
Components of the ECM
- The ECM is mainly composed of ground substance and protein fibers.
- Ground substance is primarily interstitial (extracellular) fluid containing water, nutrients, and ions.
- Three families of macromolecules found in ground substance:
- Glycosaminoglycans (GAGs): Negatively charged sugars that help establish concentration gradients for diffusion and osmosis.
- Proteoglycans: Protein cores that add firmness, resist compression, and can act as diffusion barriers.
- Cell adhesion molecules (CAMs): Hold cells together and help maintain tissue structure.
Protein Fibers in the ECM
- Collagen fibers: Thick, white fibers providing resistance to tension and stretching.
- Elastic fibers: Made mainly of elastin, allow extensibility (stretching) and elasticity (return to original length).
- Reticular fibers: Thin, short collagen fibers forming meshworks that support cells and trap foreign particles in organs like spleen and lymph nodes.
Cell Junctions
- Neighboring cells interact via junctions formed by integral proteins in plasma membranes.
- Tight junctions: Seal cells closely to create impermeable barriers; may become leaky (e.g., during immune responses).
- Desmosomes: Strong adhesive junctions at basolateral membranes, anchoring cells and providing attachment for cytoskeleton.
- Gap junctions: Protein channels creating tunnels for ions/small molecules to move between adjacent cells; important in electrically active tissues like cardiac muscle.
Key Terms & Definitions
- Histology — Study of tissues’ normal structure.
- Tissue — Group of related cells and ECM performing a common function.
- Extracellular Matrix (ECM) — Material outside cells providing support and structure.
- Glycosaminoglycans (GAGs) — Negatively charged sugars that help create concentration gradients.
- Proteoglycans — Proteins in ECM that provide firmness and block diffusion.
- Cell Adhesion Molecules (CAMs) — Proteins holding cells in place within a tissue.
- Collagen fibers — Thick, tensile protein fibers in ECM.
- Elastic fibers — Stretchable and resilient fibers in ECM.
- Reticular fibers — Mesh-like supportive fibers in ECM.
- Tight junctions — Barriers sealing adjacent cells together.
- Desmosomes — Strong junctions attaching neighboring cells and cytoskeletons.
- Gap junctions — Channels allowing material exchange between adjacent cells.
Action Items / Next Steps
- Prepare for next lecture on the specific types of tissues.
- Review definitions and properties of ECM components and cell junctions.