Histology of Epithelial and Cardiac Tissues

Epithelial Tissue Part 3

• Epithelial cells have modifications on their basolateral, lateral, and basal surfaces, and also in glands.
• Basolateral surface specialization includes lateral folds and basal folds.
• Lateral folds are prominent in cells rapidly transporting fluids. Infoldings increase lateral surface. This is found in intestines and kidney tubules. Water enters apically (at the top) and leaves laterally.
• Basal folds are characteristic of cells actively transporting molecules. The folds and mitochondria are oriented together, providing energy for active transport. This is seen in proximal and distal tubules of kidneys and salivary gland ducts.
• The folds form a "striated" appearance in the basal aspect of cells, such as in striated ducts of salivary glands.
• Microscopic images of examples like proximal tubules show basolateral folds with mitochondria.

Cell-to-Cell Adhesions

• Cell-to-cell adhesions involve terminal bars, and a junctional complex with three components: occluding (zonula occludens/tight junctions), anchoring junctions, and communicating junctions.
• Occluding junctions (zonula occludens/tight junctions) form a ring around cells to prevent diffusion. They are composed of proteins like occludin and claudins. ZO proteins interact with cytoskeletal actin filaments.
• Anchoring junctions include zonula adherens and macula adherens/desmosomes.
• Zonula adherens are formed by cadherin proteins, which bind to catenin proteins. They are calcium-dependent and involved in cell-cell adhesion.
• Macula adherens/desmosomes are spot adhesions used for strong attachment between cells. They contain cadherin family proteins like desmogleins and desmocollins that bind with plakoglobins and desmoplakins, which then attach to intermediate filaments like keratins (tonofilaments).
• The zonula adherens of cardiac muscle is called fascia adherens.

Gap Junctions

• Gap junctions are channels on the lateral surfaces of adjoining cells allowing the movement of signaling molecules.
• Specialized transmembrane proteins called connexins form the channels (connexons). Six of these form each connexon.
• They allow communication between cells, such as the exchange of ions.

Basal Surface Anchoring Junctions

• Hemidesmosomes are half-desmosomes located on the basal surface of cells.
• These have intracellular attachment plaques and are attached to basal lamina via transmembrane proteins such as integrins and BP180/type XVII collagen.
• Focal adhesions include proteins for connecting cells to the extracellular matrix, connecting cytoskeleton and sensing and transmitting extracellular signals.

Epithelial Glands

• Glands are an organized collection of secretory epithelial cells that project into underlying connective tissue.
• Exocrine glands maintain direct continuity with the surface through ducts, while endocrine glands do not. Glands form by proliferation of epithelial cells.
• Unicellular glands are rare, such as goblet cells in intestines and respiratory epithelium.
• Multicellular glands with their ducts are classified based on shapes, structures and secretion mechanisms. Several forms of secretion mechanisms include merocrine, holocrine, and apocrine.
• Based on secretion: Serous (protein), mucous (mucin), or mixed (both).

Useful Definitions

• Parenchyma: Functional tissue of an organ
• Stroma: Supporting tissue composed of connective tissue, blood vessels and nerve fibers

Test your knowledge on the structure and function of different cell junctions and gland types in epithelial and cardiac tissues. This quiz covers topics such as zonula adherens, secretion modes, and key characteristics of gland types. Perfect for students studying histology or related fields.