Cell Junctions

Epithelial Tissue

• Epithelial cells form a tight seal at the apical end, appearing fused or tightly apposed under an electron microscope.
• The zonula occludens (tight junction) forms a seal between adjacent cells.
• The zonula adherens (belt desmosomes) encircles the cell immediately under the tight junction.
• Macula adherens (desmosomes) are disc-shaped structures that match identical structures on adjacent cell surfaces.
• Gap junctions (nexus) facilitate intercellular communication rather than adhesion or occlusion.

Glandular Epithelium

• Glands are groups of cells that synthesize substances for release into the bloodstream or body cavities.
• Types of glands:
  • Unicellular glands (e.g., Goblet cells)
  • Multicellular glands (e.g., Salivary glands)
  • Endocrine glands (e.g., Thyroid, Adrenal glands) with no ducts
  • Exocrine glands (e.g., Salivary, Sweat glands) with ducts
  • Apocrine glands (e.g., secretion involves cell disintegration)
  • Merocrine glands (e.g., secretion via exocytosis)
  • Holocrine glands (e.g., entire cell disintegrates as secretion)
  • Serous glands
  • Mucous glands
  • Mixed seromucous glands

Connective Tissue

• Fibrous components of connective tissue are formed from proteins secreted by fibroblasts.
• Main types of fibers:
  • Collagen fibers
  • Reticular fibers
  • Elastic fibers

Epithelial Surface Specializations

• Free or apical surface epithelial cells show projections:
  • Microvilli (increase apical surface area for absorption)
  • Cilia (move mucous in the lumen)
  • Stereocilia (long microvilli for absorption and signal generation)

Lateral Surfaces Specializations

• Intercellular junctions:
  • Collagen and reticular fibers
  • Elastic fibers

Connective Tissue Fibers

• Collagen fibers:
  • Form various extracellular fibers, sheets, and networks
  • Extremely strong and resistant to normal shearing and tearing forces
  • Represent 30% of the human body's dry weight
  • Produced by fibroblasts and other cell types
• Reticular fibers:
  • Found in delicate connective tissue of many organs
  • Consist mainly of collagen type III
  • Form an extensive network for supporting cells
  • Invisible in H&E and require silver stain
• Elastic fibers:
  • Thinner than collagen fibers
  • Form sparse networks interspersed with collagen bundles
  • Have rubber-like properties, allowing tissue to be stretched and return to its original shape

The Ground Substance (Matrix)

• A highly hydrated, transparent, complex mixture of:
  • Glycosaminoglycans (GAGs)
  • Proteoglycans
  • Glycoproteins

Mucoid Tissue

• Found in the fetal umbilical cord (Wharton's jelly)
• Gelatinous, with sparse collagen fibers and scattered fibroblasts
• Abundant ground substance composed chiefly of hyaluronan

Adipose Tissue

• White adipose tissue:
  • Specialized for long-term energy storage
  • Adipocytes are spherical when isolated and polyhedral when closely packed
  • Contain single large droplets of triglycerides (unilocular adipocyte)
• Brown adipose tissue:
  • Constitutes 2%-5% of newborn body weight
  • Found mainly in the back, neck, and shoulders, but reduces during childhood and adolescence
  • Contains many small lipid inclusions (multilocular adipocyte)

Specialized Connective Tissue

• Cartilage:
  • Consists of cells called chondrocytes embedded in the ECM
  • Chondrocytes synthesize and maintain ECM components
  • Semi-rigid consistency due to water bound to hyaluronan and GAG chains
  • High content of bound water allows cartilage to serve as a shock absorber
  • Lacks vascular supplies, with chondrocytes receiving nutrients by diffusion from capillaries in surrounding connective tissue
  • Main types of cartilage:
    • Hyaline cartilage
    • Elastic cartilage
    • Fibrocartilage
• Bone:
  • A specialized connective tissue composed of calcified extracellular material and three major cell types:
    • Osteocytes and osteoblasts
    • Osteoclasts (giant, multinucleated cells)
  • Types of bone:
    • Compact (cortical) bone
    • Cancellous bone