Chapter 4 – Histology

4.1 Introduction to Tissues

Tissues are groups of similar cells and extracellular matrix (ECM) that perform a specific function. Understanding tissue types is foundational for studying organ structure and function.

• Histology: The study of tissues.

• Levels of Organization: Cells → Tissues → Organs → Organ Systems.

• Tissues: Groups of similar cells + ECM performing a function.

• 4 Primary Tissue Types:

  • Epithelial: Covers, lines, forms glands.

  • Connective: Supports, binds, protects, transports.

  • Muscle: Movement, contraction.

  • Nervous: Communication, rapid signaling.

• Extracellular Matrix (ECM): Material outside cells, provides structure and function.

  • Ground Substance: Water, ions, nutrients, glycoproteins, proteoglycans.

  • Protein Fibers:

    • Collagen: Strength, resist stretch.

    • Elastic: Stretch and recoil.

    • Reticular: Thin, web-like, support.

• Cell Junctions:

  • Tight junctions: Seal, prevent leakage (e.g., bladder).

  • Desmosomes: "Rivets," strong adhesion (e.g., skin).

  • Gap junctions: Channels for ions/molecules (e.g., cardiac muscle).

4.2 Epithelial Tissue

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is specialized for protection, secretion, and absorption.

• General Features:

  • Polarity (apical, basal surfaces).

  • Avascular (no blood vessels).

  • Regenerative.

  • Supported by connective tissue basement membrane.

• Classifications (by layers + shape):

  • Simple squamous: Thin, diffusion (lungs, vessels).

  • Simple cuboidal: Secretion/absorption (kidney tubules).

  • Simple columnar: Absorption/secretion (GI tract).

  • Pseudostratified columnar: Cilia, mucus (respiratory tract).

  • Stratified squamous: Protection, keratinized (skin) vs. non-keratinized (mouth, esophagus).

  • Stratified cuboidal/columnar: Rare, glands.

  • Transitional: Stretch, urinary bladder.

• Glands:

  • Exocrine (ducts): Sweat, salivary.

  • Endocrine (ductless): Hormones into blood.

  • Exocrine Types:

    • Merocrine: Exocytosis.

    • Apocrine: Part of cell pinched off.

    • Holocrine: Whole cell bursts.

4.3 Connective Tissue

Connective tissue is characterized by abundant ECM and few cells. It provides support, protection, transport, storage, and insulation.

• General Characteristics: Few cells, abundant ECM.

• Functions: Support, protection, transport, storage, insulation.

• Cell Types:

  • Fibroblasts: Make fibers.

  • Macrophages: Phagocytosis.

  • Adipocytes: Fat storage.

  • Chondrocytes: Cartilage.

  • Osteocytes: Bone.

  • RBCs/WBCs: Blood.

• Types of CT:

  • Loose CT:

    • Areolar: Supports organs.

    • Adipose: Fat storage, insulation.

    • Reticular: Lymph nodes, spleen.

  • Dense CT:

    • Dense regular: Tendons, ligaments.

    • Dense irregular: Dermis.

    • Elastic: Arteries.

  • Cartilage (avascular):

    • Hyaline: Most common, trachea, joints.

    • Elastic: Ear.

    • Fibrocartilage: Discs, knees.

  • Bone: Compact & spongy.

  • Blood: Plasma + cells.

• Fibrosis: Scar tissue (dense irregular CT).

• Nervous & Cardiac Repair: Poor regeneration.

4.4 Muscle Tissue

Muscle tissue is specialized for contraction and movement.

• Skeletal: Voluntary, striated, multinucleated, movement.

• Cardiac: Involuntary, striated, intercalated discs, pumps blood.

• Smooth: Involuntary, non-striated, hollow organs.

4.5 Nervous Tissue

Nervous tissue is responsible for communication and control in the body.

• Neurons: Excitable, transmit impulses.

• Neuroglia: Support, insulate, protect neurons.

4.6 Membranes

Body membranes are sheets of tissue that cover or line surfaces and cavities.

• Serous: Line ventral cavities, reduce friction (pleura, pericardium).

• Mucous: Line passages open to outside (digestive, respiratory).

• Cutaneous: Skin.

• Synovial: Joints, secrete synovial fluid.

Chapter 5 – Integumentary System

5.1 Overview

The integumentary system includes the skin and its accessory structures. It serves as the body's first line of defense and plays roles in protection, sensation, thermoregulation, excretion, vitamin D synthesis, and water balance.

• Organs: Skin + accessory structures (hair, nails, glands).

• Functions: Protection, sensation, thermoregulation, excretion, vitamin D synthesis, water balance.

5.2 Epidermis

The epidermis is a keratinized stratified squamous epithelium, providing a barrier against environmental damage.

• Cell Types:

  • Keratinocytes: Main, produce keratin.

  • Melanocytes: Produce melanin.

  • Dendritic cells: Immune.

  • Merkel cells: Touch receptors.

• Layers (deep → superficial):

  • Stratum basale: Mitosis, melanocytes, Merkel cells.

  • Stratum spinosum: Keratinocytes, desmosomes.

  • Stratum granulosum: Keratohyalin granules, waterproofing.

  • Stratum corneum: Dead keratinized cells.

  • Stratum lucidum: Only in thick skin (palms, soles).

• Thick vs. Thin Skin:

  • Thick: Palms, soles (5 layers, no hair).

  • Thin: Most of body (4 layers, hair present).

5.3 Dermis

The dermis is a connective tissue layer beneath the epidermis, providing strength and elasticity.

• Papillary layer: Areolar CT, dermal papillae, fingerprints, capillaries.

• Reticular layer: Dense irregular CT, strength, houses glands, hair follicles.

5.4 Pigmentation

Skin color is determined by several pigments, each with specific functions.

• Melanin: Main pigment, protects DNA from UV.

• Carotene: Yellow-orange.

• Hemoglobin: Red/pink.

• Clinical notes: Cyanosis (blue), jaundice (yellow), pallor (pale).

5.5 Accessory Structures

Accessory structures include hair, glands, and nails, each with specialized functions.

• Hair:

  • Root (in skin), shaft (outside).

  • Arrector pili muscle causes goosebumps.

  • Protects, senses touch.

• Glands:

  • Eccrine sweat glands: Watery sweat, cooling.

  • Apocrine sweat glands: Scent, axilla/groin.

  • Sebaceous glands: Sebum (oil), lubricates skin/hair.

5.6 Pathology

Common skin pathologies include burns and cancers, each classified by severity and risk.

• Burns:

  • 1st degree: Epidermis.

  • 2nd degree: Epidermis + part dermis.

  • 3rd degree: Full thickness.

• Skin Cancer:

  • Basal cell carcinoma: Most common, least dangerous.

  • Squamous cell carcinoma: Intermediate.

  • Melanoma: Least common, most deadly (ABCDE rule).

Chapter 6 – Bones & Bone Tissue

6.1 Introduction

Bones provide support, protection, movement, mineral storage, blood cell formation, and fat storage. They are classified by shape and structure.

• Functions: Support, protection, movement, mineral storage (Ca, phosphate), blood cell formation (hematopoiesis), fat storage.

• Bone Classifications:

  • Long (femur, humerus)

  • Short (carpals)

  • Flat (skull, sternum)

  • Irregular (vertebrae)

  • Sesamoid (patella)

• Bone Structure:

  • Diaphysis: Shaft, compact bone.

  • Epiphysis: Ends, spongy bone.

  • Medullary cavity: Yellow marrow (fat storage).

  • Red marrow: Hematopoiesis.

  • Periosteum: Outer covering.

6.2 Microscopic Structure

Bone tissue is organized into compact and spongy bone, each with specialized cells and matrix components.

• Compact bone: Osteons, central canal, concentric lamellae, lacunae, canaliculi.

• Spongy bone: Trabeculae, lighter, filled with marrow.

• Bone cells:

  • Osteoblasts: Build bone.

  • Osteocytes: Mature, maintain bone.

  • Osteoclasts: Break down bone.

• Matrix:

  • Inorganic: Hydroxyapatite (calcium phosphate crystals).

  • Organic: Collagen fibers.

6.3 Bone Formation

Bone formation (ossification) occurs through two main processes and involves the transition from immature to mature bone.

• Primary bone: Immature, woven.

• Secondary bone: Mature, stronger.

• Ossification Types:

  • Intramembranous: Flat bones (skull, clavicle).

  • Endochondral: Cartilage model replaced (most bones).

6.4 Bone Growth

Bones grow in length and width through specific processes regulated by hormones.

• Longitudinal growth: Epiphyseal plate, zones of cartilage.

• Appositional growth: Width, addition under periosteum.

• Hormonal control: GH, thyroid hormone, testosterone, estrogen.

6.5 Bone Remodeling & Repair

Bone remodeling is a continuous process balancing deposition and resorption, regulated by hormones and essential for bone health and repair.

• Remodeling: Deposition (osteoblasts) vs. resorption (osteoclasts).

• Regulators:

  1. PTH (parathyroid hormone): Increases in blood (stimulates osteoclasts).

  2. Calcitonin: Decreases in blood (stimulates deposition).

  3. Vitamin D: Helps absorb from gut.

• Fracture Repair:

  1. Hematoma (blood clot).

  2. Fibrocartilaginous callus.

  3. Bony callus (spongy bone).

  4. Remodeling (compact bone).

Quick Memory Triggers

• Epithelial layers: "Come, Let's Get Sun Burned" (Corneum, Lucidum, Granulosum, Spinosum, Basale).

• Bone cells: Blasts Build, Clasts Crush.

• Skin cancers: "Basal is Basic, Squamous is Somewhat bad, Melanoma is Most deadly."