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Epithelial Tissue Overview

• Epithelial Tissue Characteristics:

  • Polarity: Epithelial cells have an apical surface and a basal surface.

  • Avascular but Innervated: Epithelial tissue lacks blood vessels but contains nerve endings.

  • High Regeneration: Exhibits high regenerative capacity, especially in places with frequent wear and tear.

Example of Regeneration

• Paper Cuts: Damage to the epithelium leads to pain without bleeding. Bleeding occurs only when deeper tissues are affected.

• The gastrointestinal (GI) tract epithelium is renewed every 3-4 days due to the harsh conditions it endures (acid, enzymes).

Importance of Polarity

• Polarity Definition: Epithelial cells are polarized, having distinct apical (top) and basal (bottom) surfaces.

  • Apical Surface: Faces the external environment or lumen (e.g., wants to absorb nutrients in the GI tract).

  • Basal Surface: Attaches to basement membrane connected to underlying connective tissue.

Structure Reference

• Apical surface = free surface facing the cavity (lumen).

• Basal surface = secure side attached via the basement membrane to connective tissues.

Structures of Epithelia

• Microvilli: Found in cells of the small intestine, increasing surface area for absorption.

• Cilia: Found in respiratory tract cells, helping in the movement of mucus and debris.

Support by Connective Tissue

• Epithelial cells are supported by connective tissue through the basement membrane, which consists of:

  • Basal Lamina: Acts as a filter.

  • Reticular Lamina: Provides strength and anchoring to underlying connective tissue, contributing to structural integrity.

  • Collagen Fibers: Offer tensile strength, resisting stretching and tearing.

Specialized Contacts

• Cell Junctions: Include desmosomes (anchoring cells) and tight junctions (prevent flow between cells), allowing epithelial tissue to maintain continuity and protection.

Classification of Epithelial Tissue

• Epithelial tissues have two names based on:

  • Cell Layers:

    • Simple Epithelium: One layer of cells.

    • Stratified Epithelium: Multiple layers of cells.

  • Cell Shape:

    • Squamous: Flat cells.

    • Cuboidal: Cube-shaped cells.

    • Columnar: Taller than wide.

Functions of Epithelial Layers

• Simple Epithelia: Primarily involved in absorption, secretion, and filtration due to their thin structure.

• Stratified Epithelia: Primarily for protection due to their multiple layers.

Simple Squamous Epithelium

• Found in locations like lungs (air sacs) and capillaries; facilitates diffusion and filtration due to its thin structure.

• Special terms:

  • Endothelium: Specific name for simple squamous epithelium lining blood vessels.

  • Mesothelium: Lines serous membranes in body cavities.

Simple Cuboidal Epithelium

• Found in kidney tubules, aids in absorption and secretion, particularly in the filtration and processing of urine.

Simple Columnar Epithelium

• Types:

  • Non-ciliated: Found in GI tract; involved in absorption and secreation (goblet cells for mucus).

  • Ciliated: Found in bronchi and female reproductive tract; aids in moving substances (e.g., egg towards uterus).

Pseudostratified Columnar Epithelium

• Appears layered but is a single layer; found in the trachea and upper respiratory tract; functions in secretion and propulsion of mucus.

Stratified Epithelia Types

• Stratified Squamous: Can be keratinized (epidermis) for skin protection or non-keratinized (moist linings).

• Transitional Epithelium: Specialized for stretching; found in the urinary bladder.

Glandular Epithelium

• Definition of Gland: Epithelial origin, producing secretions.

• Classification:

  • Exocrine Glands: Have ducts (e.g., sweat, salivary glands).

  • Endocrine Glands: No ducts; release hormones directly into the blood.

Exocrine vs. Endocrine Glands

• Exocrine: Secretes products on surfaces or cavities.

• Endocrine: Product enters bloodstream, systemic effects.

Unicellular and Multicellular Exocrine Glands

• Unicellular: Goblet and mucus cells;

• Multicellular: Classified by duct structure (simple vs. compound) and secretory unit shape (tubular vs. alveolar).

Connective Tissue Overview

• Main types of connective tissue classified as CCBV:

  • Connective tissue proper

  • Cartilage

  • Bone

  • Blood

Connective Tissue Types

1. Connective Tissue Proper

• Subtypes: Loose and dense connective tissues.

  • Loose Connective Tissue:

    • Areolar: Contains open spaces, wraps and cushions organs, and provides reservoir for salt and water.

    • Adipose: Stores energy in the form of fat, insulates and cushions organs. Contains adipocytes (fat cells) filled with triglycerides.

    • Reticular: Comprised of reticular cells and fibers, supports lymphoid organs (spleen, lymph nodes).

  • Dense Connective Tissue:

    • Regular: Collagen fibers aligned in one direction. Found in tendons and ligaments, withstands tensile forces.

    • Irregular: Thicker collagen fibers arranged in various directions. Provides strength and elasticity; found in the dermis and surrounding organs.

    • Elastic: High density of elastic fibers allowing for recoil, found in lungs and elastic arteries (e.g., aorta).

2. Cartilage

• Types:

  • Hyaline Cartilage: Provides support and flexibility; found at ends of long bones, respiratory structures (trachea, larynx).

  • Elastic Cartilage: Similar to hyaline but with more elastic fibers; provides strength with stretchability (found in ears and epiglottis).

  • Fibrocartilage: Contains thick collagen fibers, resists heavy pressure and tension; found in intervertebral discs, menisci of knees, pubic symphysis.

3. Bone (Osseous Tissue)

• Two types:

  • Compact Bone: Hard tissue structure, composed of osteons with a central canal for blood vessels and nerves; strong support and protection.

  • Spongy Bone: Lighter, less dense, contains trabeculae that support marrow.

4. Blood

• Liquid connective tissue, matrix is plasma.

• Components:

  • Erythrocytes (Red Blood Cells): Transport oxygen and carbon dioxide.

  • Leukocytes (White Blood Cells): Immune defense.

  • Platelets: Function in clotting.

Muscle Tissue

• Skeletal Muscle: Voluntary control, striated, multi-nucleated fibers.

• Cardiac Muscle: Involuntary, found only in the heart; branched with intercalated discs for synchronized contraction.

• Smooth Muscle: Involuntary, found in walls of hollow organs (blood vessels, GI tract), no striations; responsible for involuntary movements.

Nervous Tissue

• Composed of neurons (transmit nerve impulses) and glial cells (support and protect neurons).

• Neurons consist of cell body, dendrites (receive signals), and axon (transmits impulses).

Tissue Repair

1. Inflammation

• Cardinal signs: Redness, heat, swelling, and pain. Mast cells release histamine causing blood vessels to dilate and leak fluid into the area.

2. Organization

• Replacement of blood clot with granulation tissue; restores blood supply, contains fibroblasts that create collagen to bridge the wound gap.

3. Regeneration or Fibrosis

• Regeneration restores the original tissue type, maintains function.

• Fibrosis replaces damaged tissue with scar tissue that often lacks function.

Regeneration Capacity of Tissues

• High Capacity for Regeneration: Epithelial, bone, and loose connective tissues.

• Moderate Capacity: Smooth muscle and dense regular connective tissue.

• Low Capacity: Skeletal muscle and cartilage.

• Very Low Capacity: Cardiac muscle and nervous tissues (CNS).

Epithelial Membranes

• Comprised of epithelium and underlying connective tissue. Includes cutaneous (skin), mucous (moist linings of cavities such as the digestive tract) and serous membranes (line body cavities, like pleurae around lungs).