Tissues

Introduction to Tissues

Tissues are groups of cells that work collectively to perform a specific function. The study of tissues is called histology.

• Definition: A tissue is a group of similar cells and their extracellular products that perform a common function.

• Types of Tissues: Four main types: epithelial, connective, muscle, and nervous.

Epithelial Tissue

General Characteristics

Epithelial tissue covers and lines all free body surfaces and forms glands. It serves as a protective barrier and is involved in absorption, filtration, and excretion.

• Locations: Skin, lining of cardiovascular system, vessels, blood, body cavities, digestive and respiratory tracts.

• Glandular Epithelium: Forms glands that secrete substances.

• Functions:

  • Establish boundaries between environments.

  • Protection: Protects underlying tissues.

  • Absorption: Allows selective passage of substances.

  • Filtration: Filters substances passing through.

  • Excretion: Eliminates waste products.

Structural Features

• Polarity: Has an apical (top) and basal (bottom) surface.

• Basement Membrane: Anchors epithelium to underlying connective tissue.

  • Basal Lamina: Glycoproteins, not cells.

  • Reticular Lamina: Deep to basal lamina, contains collagen fibers for strength.

  • Selective Membrane: Controls movement of substances.

Types of Epithelial Tissue

• Simple Epithelium: One cell layer; areas of absorption, filtration.

• Stratified Epithelium: More than one layer; high abrasion, protection.

• Pseudostratified Epithelium: Appears layered but is only one layer; often ciliated (e.g., PCCE).

Cell Shapes

• Squamous: Flat cells.

• Cuboidal: Cube-shaped cells.

• Columnar: Column-shaped cells.

• Transitional: Dome-shaped; found in organs like the urinary bladder, allows stretching.

Glandular Epithelium

Specialized for secretion. Glands can be classified by structure and function.

• Exocrine Glands: Secrete into ducts (e.g., sweat, gall bladder).

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

• Unicellular Glands: Goblet cells in PCCE produce mucus.

• Multicellular Glands: Have ducts and secretory units.

• Functional Classification:

  • Merocrine: Secrete by exocytosis (cell remains intact).

  • Holocrine: Cell ruptures to release secretion (cell dies).

Connective Tissue

General Characteristics

Connective tissue supports, binds, and protects other tissues and organs. All connective tissues are derived from the embryonic mesenchyme (middle layer of embryo).

• Types: Connective tissue proper, cartilage, bone, blood.

• Blood: Only liquid tissue in the body.

Structural Elements

• Ground Substance: Material in which cells and fibers are embedded.

• Fibers:

  • Collagen: Provides strength.

  • Elastic: Provides stretch and recoil.

  • Reticular: Provides support.

• Cells: Fibroblasts (produce fibers), chondroblasts (cartilage), osteoblasts (bone), etc.

Muscle Tissue

General Characteristics

Muscle tissue is specialized for contraction and movement. All muscle cells contain sarcoplasm (cytoplasm) and sarcoplasmic reticulum (specialized endoplasmic reticulum).

Types of Muscle Tissue

• Skeletal Muscle:

  • Attaches to bones.

  • Striated, voluntary, multi-nucleated.

• Cardiac Muscle:

  • Found in heart.

  • Striated, involuntary, uni-nucleated.

  • Has intercalated discs for communication (gap junctions).

• Smooth Muscle:

  • Found in walls of hollow organs.

  • Non-striated, involuntary, uni-nucleated.

Nervous Tissue

General Characteristics

Nervous tissue is specialized for communication, energy transfer, and control. It consists of neurons and supporting cells called neuroglia.

• Neurons: Transmit electrical signals.

• Neuroglia: Support, protect, and nourish neurons; many types exist.

Membranes

Definition and Types

Membranes are continuous multicellular sheets composed of at least two tissue types: epithelium bound to underlying connective tissue. Membranes can be classified as covering/lining or synovial (joint) membranes.

• Cutaneous Membrane: Skin; covers the body surface.

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

• Serous Membrane: Lines body cavities closed to the exterior; produces serous fluid to reduce friction.

Summary Table: Types of Tissues

Additional info:

• Basement Membrane Equation: The basement membrane is composed of the basal lamina and reticular lamina:

• Classification of Epithelia: Based on number of layers (simple vs. stratified) and cell shape (squamous, cuboidal, columnar, transitional).

• Functional Classification of Glands: Merocrine (exocytosis), holocrine (cell rupture).