Anatomy & Physiology Study Notes: Tissues

Embryonic Tissues

Germ Layers and Their Derivatives

During embryonic development, three primary germ layers form, each giving rise to specific tissues and organs in the body.

• Ectoderm: Forms the epidermis (skin), integumentary system, and nervous system.

• Mesoderm: Forms connective tissues, muscle, and the cardiovascular system.

• Endoderm: Forms the linings of internal organs such as the digestive and respiratory tracts.

These germ layers give rise to four adult tissue types: epithelial, connective, muscle, and nerve.

Epithelial Tissues

Characteristics

Epithelial tissues line body surfaces, cavities, and form glands. They are specialized for protection, secretion, and absorption.

• Apical Surface: The free surface facing the body cavity or external environment.

• Basement Membrane / Basal Surface: A non-living layer beneath the cells, anchoring them to underlying connective tissue.

• Cell Junctions: Includes desmosomes, tight junctions, and gap junctions for cell adhesion and communication.

• High Cell Turnover: Epithelial cells are frequently replaced due to wear and tear.

• Closely Packed Cells: Minimal intercellular space; cells are tightly joined.

Functions

• Protection: Covers all external and internal body surfaces.

• Secretion: Forms glands and produces substances like enzymes and hormones.

• Absorption: Allows uptake of nutrients and other substances.

• Excretion: Removes waste products.

• Sensation: Contains sensory nerve endings.

Types of Epithelial Tissue

• Simple Squamous Epithelium: Single layer of flat cells; highly permeable; lines blood vessels (endothelium) and body cavities (mesothelium).

• Simple Cuboidal Epithelium: Single layer of cube-shaped cells; found in kidney tubules and glands.

• Simple Columnar Epithelium: Single layer of tall, elongated cells; may have microvilli or cilia; lines digestive tract.

• Pseudostratified Columnar Epithelium: Appears multilayered due to uneven nuclei; commonly found in respiratory tract.

• Stratified Squamous Epithelium: Multiple layers; protects against abrasion; found in skin, mouth, esophagus.

• Keratinized Stratified Squamous Epithelium: Surface cells contain keratin; forms the epidermis.

• Transitional Epithelium: Stratified; stretches and recoils; lines urinary bladder.

Glandular Epithelium

Glands are groups of epithelial cells specialized for secretion.

• Exocrine Glands: Secrete products into ducts that lead to external or internal surfaces (e.g., sweat, salivary glands).

• Endocrine Glands: Secrete hormones directly into the bloodstream (e.g., thyroid, pituitary).

Exocrine Gland Types

• Unicellular: Single epithelial cell (e.g., goblet cell).

• Multicellular: Composed of many epithelial cells; may be simple or compound.

• Alveolar/Acinar: Sac-like secretory portion (e.g., mammary glands).

Classification of Glandular Secretions

• Merocrine: Secrete via exocytosis (e.g., salivary glands).

• Apocrine: Secrete by pinching off part of the cell (e.g., mammary glands).

• Holocrine: Secrete by cell rupture (e.g., sebaceous glands).

Connective Tissue (CT)

General Features

Connective tissue is the most abundant and diverse tissue type, providing support, protection, and storage.

• Extracellular Matrix (ECM): Consists of protein fibers and ground substance; may be liquid, gel-like, or solid.

• Hyaluronic Acid: Component of ground substance; provides viscosity.

• Glycosaminoglycans: Polysaccharides in ground substance.

• Mesenchyme: Embryonic CT from which all CT is derived.

Functions

• Bind structures and tissues together

• Support and protect organs

• Blood cell production

• Fat storage

• Repair tissue damage

CT Cell Types

• Fibroblasts: Produce fibers and secrete ECM.

• Macrophages: Phagocytic cells that engulf debris and pathogens.

• Mast Cells: Release histamine and heparin; involved in inflammation.

• White Blood Cells: Immune cells such as macrophages.

CT Fibers

• Collagenous (White) Fibers: Strong, flexible, resist stretching.

• Elastic (Yellow) Fibers: Stretch and recoil easily.

• Reticular Fibers: Thin, branched fibers; support soft tissues.

Types of Connective Tissue

• Loose CT (Areolar): Loosely arranged fibers; supports and binds tissues.

• Adipose Tissue: Specialized for fat storage; insulates and cushions.

• Reticular CT: Network of reticular fibers; supports lymphoid organs.

• Dense Regular CT: Parallel collagen fibers; forms tendons and ligaments.

• Dense Irregular CT: Irregularly arranged fibers; found in dermis.

• Elastic CT: Mainly elastic fibers; found in walls of large arteries.

Cartilage

General Features

Cartilage is a rigid, supportive CT with a gel-like matrix, providing frameworks and attachments.

• Chondrocytes: Cartilage cells located in lacunae.

• Perichondrium: Dense CT surrounding cartilage.

Types of Cartilage

Type	Main Features	Locations
Hyaline	Dense collagen, glassy appearance	Ends of long bones, nose, trachea, embryonic skeleton
Elastic	Flexible, elastic fibers	External ear, epiglottis
Fibrocartilage	Tough, many collagen fibers	Intervertebral discs, pubic symphysis, menisci

Bone (Osseous Tissue)

General Features

Bones are the most rigid connective tissue, providing support and protection. Hardness is due to calcium phosphate crystals.

• Cancellous (Spongy) Bone: Lattice network of trabeculae.

• Bone Marrow: Red marrow (hematopoietic) produces blood cells; yellow marrow stores fat.

Blood

Composition and Function

Blood is a connective tissue with a liquid ECM (plasma) and formed elements.

• Plasma: Fluid portion with proteins, ions, nutrients, and wastes.

• Red Blood Cells (Erythrocytes): Carry oxygen via hemoglobin.

• White Blood Cells (Leukocytes): Immune functions.

• Platelets: Cell fragments involved in clotting.

Physiology of Inflammation & Tissue Repair

Inflammatory Response

Inflammation is a protective response to injury, mediated by connective tissue and immune cells.

• Histamine: Released by mast cells; increases vascular permeability.

• Vasodilation: Blood vessels widen, increasing blood flow to the area.

• Edema: Fluid accumulation in tissues.

• Granulation Tissue: New tissue formed during healing.

• Fibrosis: Scar formation due to excess collagen deposition.

Anti-Inflammatory Agents

• Antihistamines: Block histamine effects; used for allergies.

• NSAIDs: Non-steroidal anti-inflammatory drugs (e.g., aspirin, ibuprofen).

• Corticosteroids: Block prostaglandin synthesis; potent anti-inflammatory effects.

Muscle Tissue

Characteristics

Muscle tissue is specialized for contraction, enabling movement and force generation.

• Cells are excitable and can contract in response to stimuli.

• Muscle cells possess excitable membranes and can be stimulated by nerve impulses.

Types of Muscle Tissue

Type	Main Features	Location
Skeletal	Striated, voluntary, multinucleated	Attached to bones
Cardiac	Striated, involuntary, single nucleus, intercalated discs	Heart
Smooth	Non-striated, involuntary, single nucleus	Walls of hollow organs

Skeletal Muscle

• Usually connected to bones via tendons; responsible for voluntary movement.

• Cells are long, cylindrical, and multinucleated.

• Exhibits striations due to arrangement of actin and myosin filaments.

• Functions in locomotion, posture, and heat production.

Cardiac Muscle

• Found only in the heart; responsible for pumping blood.

• Cells are branched, interconnected, and have a single nucleus.

• Intercalated discs allow rapid electrical conduction.

Smooth Muscle

• Found in walls of hollow organs (e.g., intestines, blood vessels).

• Cells are spindle-shaped, non-striated, and have a single nucleus.

• Functions in involuntary movements such as peristalsis and vasoconstriction.

Key Equations

• Resting Membrane Potential: $V_m = V_{inside} - V_{outside}$

Additional info: Some details, such as the full classification of gland types and the specific mechanisms of inflammation, were expanded for academic completeness.