Cell Junctions

Study Notes

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

Description

This quiz covers different types of cell junctions, including tight junctions, adherens junctions, desmosomes, and gap junctions. It describes their structure and function in intercellular communication and adhesion.