KN

Unit 08 Pt2

Introduction to Adherence Junctions

  • Adherence junctions require a variety of actin-associated proteins for assembly and maintenance.
  • Important for cellular movement, especially in epithelial tissues such as the intestine, which is crucial for peristalsis, aiding digestion.

Forms and Functions of Adherence Junctions

  • Adherence junctions are organized in a continuous adhesion belt beneath the apical surface of epithelial cells, also called zonular adherence.
  • Becomes significant during tissue remodeling and development (morphogenesis), such as neurotube formation.

Characteristics of Desmosomes

  • Composed of cadherins linked to intermediate filaments, providing tensile strength, found in stress-prone tissues.
  • Keratin is the key intermediate filament in epithelial tissues, while desmin is found in heart muscle tissue.
  • Disruption can lead to inherited disorders and autoimmune conditions, such as skin blistering.

Overview of Tight Junctions

  • Tight junctions, also known as occluding junctions, play a crucial role in forming a selectively permeable barrier between epithelial cells.
  • They help maintain cell polarity by preventing apical and basolateral proteins from diffusing into incorrect spaces.
  • Have selective permeability that may vary based on cell type and allows for paracellular transport of small molecules between cells.

Structural Features of Tight Junctions

  • Formed by strands of proteins, primarily claudins, occludins, and tricellulin, that encircle the epithelial cells.
  • Claudins are essential for the formation and function; mutants lacking claudins die shortly after birth.
  • Occludins, while not essential for structure, influence permeability, with different claudin members contributing to tissue specificity.

Experimental Evidence of Tight Junctions

  • Electron microscopy allows visualization of tight junctions by showing their impermeability to specific markers when cells are soaked in heavy metals or other visualizing agents.
  • Important for understanding the structural arrangement and function of tight junctions in cell biology.

Pathological Insights Regarding Tight Junctions

  • Certain infectious agents, like Clostridium perfringens, utilize toxins that bind to claudins, disrupting tight junctions and leading to gastrointestinal symptoms.
  • Insight into the pathology can guide therapeutic interventions; paradoxically, toxins that increase permeability can also aid drug delivery across barriers.

Gap Junctions Overview

  • Gap junctions create direct channels between adjacent cells, allowing for communication and are essential for electrical signaling.
  • Made up of connexins (in vertebrates) and innexins (in invertebrates), forming hemichannels.
  • Allow passage of small molecules, including ions and nutrients, enhancing coordinate cellular responses, especially in muscle tissues.

Dynamic Nature of Gap Junctions

  • The functionality of gap junctions can vary based on tissue type; they can switch between open and closed states in response to various stimuli (voltage, pH changes, etc.).
  • Gap junctions are dynamic, with continual assembly and disassembly influenced by cellular activity, impacting how signals are transmitted between cells.

Summary

  • Understanding the roles of adherence junctions, tight junctions, and gap junctions provides insight into their function in health and disease, highlighting their significance in cellular communication, structural integrity, and tissue function.