Histology: The Study of Tissues

The Four Primary Tissue Types

Tissues are groups of similar cells working together to perform specific functions. The human body contains four primary tissue types, each with distinct roles:

• Epithelial Tissue: Covers and lines body surfaces, protects, secretes, and absorbs.

• Connective Tissue: Supports, binds, protects, and stores substances.

• Muscle Tissue: Contracts to produce movement.

• Nervous Tissue: Facilitates communication via electrical signals.

Extracellular Matrix (ECM)

The ECM is the material outside the cells, providing structural support, holding cells together, helping tissues withstand stress, and regulating cell behavior. It consists of:

• Ground Substance: Gel-like material filling spaces between cells and fibers.

• Protein Fibers:

  • Collagen fibers: Strong, resist pulling forces.

  • Elastic fibers: Stretch and recoil.

  • Reticular fibers: Form thin, supporting networks.

Cell Junctions

Cell junctions connect adjacent cells and determine tissue function:

• Tight Junctions: Seal cells together, preventing passage of substances between them.

• Gap Junctions: Channels allowing ions and small molecules to pass for cell-to-cell communication.

• Desmosomes: Strong attachments anchoring cells together, preventing separation under stress.

Epithelial Tissue

Locations and Characteristics

• Covers body surfaces, lines internal cavities and hollow organs, and forms glands.

• Key characteristics:

  1. Cellularity: Mostly cells, little ECM.

  2. Polarity: Has apical (top) and basal (bottom) surfaces.

  3. Attachment: Anchored to a basement membrane.

  4. Avascular: Lacks direct blood supply.

  5. Regeneration: Rapidly replaces damaged or dead cells.

Functions (PASS)

• Protection

• Absorption

• Secretion

• Sensation

Basement Membrane

• Basal lamina: Closest to epithelial cells.

• Reticular lamina: Deeper, associated with connective tissue.

• Functions: Anchors epithelium, provides support, separates from underlying tissue.

Regeneration

• Necessary due to constant exposure to friction, chemicals, pathogens, and physical damage.

• New cells produced by stem/mitotically active cells.

Glands

• Endocrine Glands: Ductless, secrete hormones into tissue fluid/blood.

• Exocrine Glands: Have ducts, release secretions onto epithelial surfaces (e.g., sweat, saliva, oil).

Connective Tissue

General Characteristics

• Fewer cells, abundant ECM, variable blood supply.

• Matrix = fibers + ground substance

Functions

• Support, protection, binding, storage, transportation, defense.

Important Cells

• Fibroblasts: Produce fibers and ground substance.

• Phagocytes: Engulf pathogens/debris.

• Adipocytes: Store fat, insulate, cushion.

• Mast cells: Release inflammatory substances.

Connective Tissue Fibers

• Collagen: Strength, resists tension.

• Elastic: Stretch and recoil.

• Reticular: Delicate supporting framework.

Ground Substance

• Gel-like material filling spaces between cells and fibers.

Cartilage

• Physical properties determined by ECM (fibers and ground substance).

• Perichondrium: Connective tissue layer surrounding most cartilage; supports, nourishes, and aids repair.

Muscle Tissue

Myocytes and Excitability

• Myocytes: Muscle cells, specialized for contraction.

• Excitable: Respond to stimuli by changing electrical state.

Nervous Tissue

Cell Types

• Neurons: Transmit electrical signals.

• Neuroglia (glial cells): Support and protect neurons.

Neuron Structure

• Cell body (soma): Contains nucleus.

• Dendrites: Receive signals (toward cell body).

• Axon: Carries signals away from cell body.

Membranes

• Mucous Membrane (Mucosa): Lines cavities open to the exterior (e.g., digestive, respiratory tracts).

• Serous Membrane: Lines closed internal cavities, covers organs, produces serous fluid to reduce friction.

Tissue Repair

• Regeneration: Replacement with same tissue type.

• Fibrosis: Replacement with fibrous connective tissue (scar tissue).

Integumentary System

Cutaneous Membrane (Skin)

• Epidermis: Outer, epithelial layer.

• Dermis: Deeper, connective tissue layer.

Accessory Structures

• Hair, nails, sweat glands, sebaceous glands.

Major Functions of Skin

• Protection, temperature regulation, sensation, vitamin D production, excretion, blood reservoir, immune defense.

Epidermis

• Keratinocyte: Most abundant cell.

• Thick skin: Palms/soles, 5 layers (includes stratum lucidum).

• Thin skin: Most of body, 4 layers (no stratum lucidum).

Melanocytes & Melanin

• Melanocytes: Produce melanin pigment.

• Melanin: Contributes to skin color, protects DNA from UV radiation.

Dermis

• Papillary Layer: Superficial, loose connective tissue, contains dermal papillae.

• Reticular Layer: Deeper, dense irregular connective tissue, provides strength and elasticity.

Subcutaneous Layer (Hypodermis)

• Contains adipose tissue and blood vessels; common site for injections.

Hair Types

• Vellus Hair: Fine, soft, light.

• Terminal Hair: Thick, coarse, pigmented.

Skin Glands

• Sebaceous Glands: Produce sebum (oil).

• Eccrine Sweat Glands: Watery sweat for temperature regulation.

• Apocrine Sweat Glands: Active at puberty, found in armpits/groin.

Bones & Bone Tissue

Functions of the Skeletal System

• Support, protection, movement, mineral storage, blood cell production.

Bone Marrow

• Red marrow: Produces blood cells (hematopoiesis).

• Yellow marrow: Stores fat.

Compact vs. Spongy Bone

• Compact bone: Dense, strong, organized into osteons.

• Spongy bone: Lattice-like, contains trabeculae, lighter.

Bone Matrix

• Organic component: Mostly collagen, provides flexibility/tensile strength.

• Inorganic component: Calcium salts, provides hardness/compressive strength.

Bone Cells

• Osteoblast: Builds bone.

• Osteocyte: Mature cell, maintains matrix.

• Osteoclast: Breaks down bone (osteolysis).

Osteoid

• Unmineralized organic bone matrix secreted by osteoblasts.

Osteon (Haversian System)

• Basic structural unit of compact bone.

• Contains central canal (blood vessels/nerves), concentric lamellae, lacunae (with osteocytes), canaliculi (tiny channels).

Trabeculae

• Thin plates/columns forming spongy bone framework.

Periosteum & Endosteum

• Periosteum: Covers outside of bone (except joints); important for growth, repair, tendon/ligament attachment.

• Endosteum: Lines internal bone surfaces.

Ossification

• Intramembranous: Bone develops from mesenchymal tissue (e.g., flat bones).

• Endochondral: Bone replaces hyaline cartilage model (most bones).

Bone Growth

• Longitudinal: At epiphyseal (growth) plate; increases length.

• Appositional: Adds bone to surface; increases width/thickness.

Activity & Bone Mass

• Mechanical stress increases bone strength; inactivity decreases bone mass.

Calcium Homeostasis

• Parathyroid Hormone (PTH): Raises blood calcium.

• Calcitonin: Lowers blood calcium.

Key equations:

Fracture Repair

1. Fracture hematoma (blood clot)

2. Soft callus (fibrocartilaginous)

3. Hard callus (bony)

4. Remodeling (reshaping/strengthening)

Articulations (Joints)

Definition and Functions

• Joint/articulation: Where two bones meet.

• Functions: Movement, stability, holding bones together.

Functional Classifications

Structural Classifications

• Fibrous: Dense fibrous connective tissue; no joint cavity.

• Cartilaginous: Bones joined by cartilage; no joint cavity.

• Synovial: Bones separated by joint cavity; freely movable.

Fibrous Joints

• Sutures: Skull.

• Gomphosis: Tooth in socket.

• Syndesmosis: Bones connected by ligament/membrane (e.g., tibia-fibula).

Cartilaginous Joints

• Synchondrosis: Hyaline cartilage (e.g., epiphyseal plate).

• Symphysis: Fibrocartilage (e.g., pubic symphysis).

Synovial Joints

• Freely movable (diarthroses).

• Key structures: joint cavity, articular cartilage, synovial fluid, joint capsule.

Support Structures

• Ligament: Bone to bone.

• Tendon: Muscle to bone.

• Bursa: Fluid-filled sac reducing friction.

• Tendon sheath: Elongated bursa around tendons.

Types of Synovial Joints

1. Plane: Sliding/gliding.

2. Hinge: Movement in one plane (e.g., elbow).

3. Pivot: Rotation.

4. Condylar: Movement in two planes.

5. Saddle: Wide range of movement (e.g., thumb).

6. Ball-and-Socket: Greatest range of motion (e.g., shoulder, hip).

High-Priority Memorization List

• Four tissue types: epithelial, connective, muscle, nervous

• Junctions: tight (seal), gap (communication), desmosome (anchor)

• Fibers: collagen (strength), elastic (stretch/recoil), reticular (framework)

• Neuron structure: dendrite (toward cell body), axon (away)

• Keratinocyte: most abundant epidermal cell

• Thick skin: palms/soles + stratum lucidum

• Epidermal layers: Basale → Spinosum → Granulosum → Lucidum → Corneum

• Melanocyte → melanin → UV protection

• Red marrow = blood cells; yellow marrow = fat

• Osteoblast = builds; osteocyte = maintains; osteoclast = breaks down

• Osteon = compact bone unit; lacuna = contains osteocyte; trabeculae = spongy bone

• Periosteum = outside; endosteum = inside

• Epiphyseal plate = length; appositional growth = width

• PTH ↑ blood Ca²⁺; calcitonin ↓ blood Ca²⁺

• Fracture repair: hematoma → soft callus → hard callus → remodeling

• Synarthrosis = immovable; amphiarthrosis = slightly movable; diarthrosis = freely movable

• Suture = skull; gomphosis = tooth/socket

• Ligament = bone-to-bone; tendon = muscle-to-bone

• Six synovial joints: plane, hinge, pivot, condylar, saddle, ball-and-socket