Overview
This lecture covers the tissue level of organization, focusing on tissue types, their embryonic origins, and the structure and function of different cell junctions.
Tissue Organization
- Tissues are groups of similar cells with shared structure and function.
- The four adult tissue types are epithelial, connective, muscle, and nervous tissue.
- All tissue types arise from embryonic germ layers.
Germ Layers and Tissue Differentiation
- Three germ layers: ectoderm (outer), mesoderm (middle), and endoderm (inner).
- Ectoderm gives rise to epithelial and nervous tissues.
- Mesoderm forms muscle, most epithelial, and connective tissues.
- Endoderm develops into epithelial tissues.
- Epithelial tissue can originate from all three germ layers.
Cell Junctions: Structure and Function
- Cell junctions hold tissue cells together and enable communication.
- Four main types of cell junctions: tight junctions, desmosomes, hemidesmosomes, and gap junctions.
Tight Junctions
- Form seals between cells using transmembrane proteins.
- Prevent material from passing between adjacent cells (e.g., in the stomach lining).
Desmosomes
- Provide tissue strength during stretching and twisting.
- Use cadherins (locking transmembrane proteins) to anchor cells together.
- Found in cardiac muscle and skin (keratinized stratified squamous epithelium).
Hemidesmosomes
- Anchor the base of epithelial cells to the basement membrane.
- Use integrins (a type of transmembrane protein).
- Only found at the interface between epithelial cells and the basement membrane.
Gap Junctions
- Allow immediate communication between adjacent cells.
- Use connexons (hollow protein tubes) for the transfer of ions and signals.
- Found in epithelial, muscle, and nervous tissue.
Key Terms & Definitions
- Tissue — group of similar cells performing a specific function.
- Germ layer — layer of embryonic cells (ectoderm, mesoderm, endoderm) from which tissues develop.
- Cell junction — structure connecting adjacent cells for support or communication.
- Tight junction — junction sealing cells to prevent material passage.
- Desmosome — junction providing strength through cadherins.
- Hemidesmosome — structure anchoring cells to the basement membrane using integrins.
- Gap junction — junction enabling direct communication via connexons.
Action Items / Next Steps
- Review the four tissue types and their embryonic origins.
- Memorize the functions and locations of the main cell junctions.