4.1 to 4.4

4.1 The Four Tissue Types

• Histology = study of tissues.

• Four main tissue types:

1. Epithelial tissue → covers surfaces, lines passageways, forms glands.

2. Connective tissue → fills spaces, provides support, stores energy.

3. Muscle tissue → contracts for movement.

4. Neural tissue → conducts electrical impulses, carries info.

• Tissues = collections of specialized cells + cell products that perform limited functions.

• Organs = multiple tissues working together → organ systems.

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4.2 Epithelial Tissue

Characteristics

• Tightly packed cells.

• Apical (free) surface exposed to environment or passageway.

• Attached to basement membrane.

• Avascular (no blood vessels).

• High regeneration rate.

Functions

1. Physical protection.

2. Control permeability.

3. Provide sensation (sensory receptors).

4. Produce secretions (glands).

Secretions

• Exocrine: onto surfaces (sweat, enzymes, milk).

• Endocrine: into blood/tissue fluids (hormones).

Cell Junctions

• Tight junctions – seal; prevent water/solute passage.

• Gap junctions – connexons form channels for ions/small molecules (fast communication).

• Desmosomes – durable, tie cells together; resist stress.

• Spot desmosomes → cell-to-cell.

• Hemidesmosomes → cell-to-basement membrane.

Epithelial Surface

• Microvilli – ↑ surface area (absorption/secretion).

• Cilia – movement of materials (e.g., respiratory tract).

Basement Membrane

• Anchors epithelium to connective tissue.

• Provides strength, barrier for large molecules.

Renewal/Repair

• Short life span (1–2 days).

• Replaced by stem (germinative) cells near basement membrane.

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4.3 Classification of Epithelia

By Layers

• Simple – single layer; thin, fragile, absorption/secretion.

• Stratified – multiple layers; protection against stress.

By Shape

1. Squamous – thin, flat.

2. Cuboidal – cube-like.

3. Columnar – tall, slender.

Types

• Simple squamous – absorption/diffusion; lungs, kidneys, vessels.

• Simple cuboidal – secretion/absorption; glands, kidney tubules.

• Simple columnar – protection, absorption, secretion; stomach, intestines.

• Stratified squamous – protects; skin, mouth, tongue, esophagus, anus.

• Stratified cuboidal – rare; ducts of sweat/mammary glands.

• Stratified columnar – rare; pharynx, urethra, anus.

• Pseudostratified columnar – appears layered; often ciliated; trachea, nasal cavity.

• Transitional – stretches/recoils; bladder, ureters.

Glandular Epithelium

• Exocrine glands → ducts; mucous or serous.

• Endocrine glands → ductless; hormones.

• Mechanisms of Secretion:

• Merocrine → exocytosis (saliva, sweat, mucus, milk).

• Apocrine → cytoplasm + secretion lost (milk, viscous sweat).

• Holocrine → cell bursts (sebaceous glands → oils).

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Clinical Note – Exfoliative Cytology

• Study of epithelial cells removed/shed.

• Used to detect cancer/genetic abnormalities.

• Pap test → cervical epithelial cells → cervical cancer screening.

• Amniocentesis → amniotic epithelial cells → detect fetal abnormalities (e.g., Down’s syndrome).

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4.4 Connective Tissue

Functions

• Support/protection.

• Transport (blood, lymph).

• Energy storage (fat).

• Defense (antibodies, immune cells).

Components

• Specialized cells.

• Protein fibers (collagen, elastic, reticular).

• Ground substance (fluid → with fibers forms matrix).

Types

1. Connective tissue proper

• Loose (packing materials):

• Areolar → cushions, permits movement.

• Adipose → padding, insulation, energy storage.

• Reticular → framework for organs (liver, spleen, lymph nodes).

• Dense:

• Regular → parallel collagen (tendons, ligaments).

• Irregular → meshwork; strength in many directions (dermis, capsules).

2. Fluid connective tissues

• Blood → plasma, RBCs, WBCs, platelets.

• Lymph → returns to cardiovascular system; immune defense.

3. Supporting connective tissues

• Cartilage (avascular; poor healing; chondrocytes in lacunae):

• Hyaline → stiff, flexible; reduces friction (ribs, joints, trachea).

• Elastic → flexible, resilient (ear, epiglottis).

• Fibrocartilage → tough, resists compression (intervertebral discs, knee).

• Bone:

• Matrix = calcium salts + collagen.

• Osteons with osteocytes in lacunae, connected by canaliculi.

• Covered by periosteum.

• Constant remodeling → heals well.

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Clinical Notes – Connective Tissue

1. Marfan Syndrome

• Defect in fibrillin (elastic fibers).

• Tall, long limbs, fingers.

• Major risk: aortic rupture → fatal bleeding.

2. Adipose Tissue & Weight Control

• Fat cells shrink in weight loss but don’t die → easy to regain.

• Excess nutrients → new fat cells form.

• Liposuction = temporary solution.

3. Cartilage & Joint Injuries

• Cartilage = avascular → poor healing.

• Injuries (esp. fibrocartilage in knee) → arthritis, mobility loss.

• Surgery often temporary.