Histology

Tissues

Introduction to Tissues

  • Definition of Tissues: A tissue is a group of structurally and functionally related cells and their external environment that together perform common functions.

  • Each tissue type differs in structure and function, yet all tissues share the same two basic components:

    • Discrete population of cells that are related in structure and function.

    • Extracellular matrix (ECM), the surrounding material.

Tissues Student Learning Outcomes

  • Objectives: To understand and describe the following:

    • Difference between epithelial and connective tissue.

    • Characterization of epithelial tissue by cell shape and layering, including identification from images of:

    • Simple squamous, cuboidal, and columnar epithelium.

    • Pseudostratified ciliated columnar epithelium.

    • Stratified squamous epithelium (both keratinized and non-keratinized).

    • Transitional epithelium.

    • Structure and function of and identification from images of:

    • Loose (areolar) connective tissue.

    • Dense regular connective tissue.

    • Adipose tissue.

    • Reticular tissue.

    • Hyaline cartilage.

Types of Tissues

Epithelial Tissue

  • Definition: Epithelial tissue consists of sheets of cells that are tightly packed together with no visible extracellular matrix (ECM).

  • Functions:

    • Covers body surfaces.

    • Lines hollow organs, body cavities, and ducts.

    • Forms glands.

    • Allows interaction with the internal and external environments.

Connective Tissue

  • Definition: Connective tissue protects and supports the body and its organs.

  • Functions:

    • Binds organs together.

    • Stores energy reserves as fat.

    • Provides immunity against disease-causing organisms.

Muscle Tissue

  • Definition: Composed of cells that can contract to generate physical force.

  • Functions:

    • Moves body structures.

    • Generates heat to warm the body.

Nervous Tissue

  • Definition: Detects changes in conditions inside and outside the body.

  • Functions:

    • Responds to changes by generating electrical signals called action potentials.

    • Activates muscular contractions and glandular secretions.

Extracellular Matrix (ECM)

  • Definition: The ECM is composed of substances surrounding the cells in a tissue.

  • Components:

    • Ground substance

    • Protein fibers

  • Functions of ECM:

    • Provides strength to resist tensile (stretching) and compressive forces.

    • Directs cells to their proper places within a tissue.

    • Regulates the development, mitotic activity, and survival of cells.

    • Holds cells in their proper positions.

Functions of Epithelial Tissue

  • Epithelial tissues are crucial for the following functions:

    • Protection: E.g., skin epithelium produces keratin, enhancing resistance to injury.

    • Immune Defense: Barriers against invading microorganisms.

    • Secretion: Form glands that produce substances (e.g., oil, hormones).

    • Transport: Selectively permeable barriers allow for passive/active transport of substances.

    • Sensation: Richly supplied with nerves for environmental detection (e.g., taste sensation in tongue).

Classification of Epithelia

  • Epithelia are classified based on:

    1. Number of cell layers:

    • Simple epithelia: Single layer of cells.

    • Stratified epithelia: More than one cell layer.

    1. Shape of cells:

    • Squamous: Flattened cells.

    • Cuboidal: Short, cube-shaped cells.

    • Columnar: Tall, elongated cells.

  • The number of layers and shapes are functionally significant.

Simple Epithelia

Characteristics

  • Composed of a single layer of cells:

    • Simple Squamous Epithelium:

    • Components: Single layer of flat cells with flattened, central nuclei.

    • Functions: Provides a barrier for gas and fluid exchange.

    • Locations: Air sacs of lungs, inner lining of body cavities, blood vessels.

    • Simple Cuboidal Epithelium:

    • Components: Single layer of cube-shaped cells with round, central nuclei.

    • Functions: Absorption and secretion.

    • Locations: Kidney tubules, certain glands.

    • Simple Columnar Epithelium:

    • Components: Single layer of rectangular cells with ovoid, basal nuclei.

    • Functions: Absorption and secretion of mucus and other substances.

    • Locations: Digestive tract and uterine tube.

    • Pseudostratified Columnar Epithelium:

    • Components: Single layer of columnar cells of different heights, giving a stratified appearance with cilia.

    • Functions: Secretion of mucus and propulsion via ciliary motion.

    • Locations: Upper respiratory passages.

Stratified Epithelia

Characteristics

  • Comprised of more than one layer of cells:

    • Stratified Squamous Epithelium:

    • Keratinized:

      • Components: Apical cells are dead, flaky, and filled with keratin.

      • Function: Provides protection and prevents water loss.

      • Location: Epidermis of skin.

    • Nonkeratinized:

      • Components: Apical cells are living and very flat.

      • Function: Offers protection against mechanical stresses.

      • Locations: Mouth, pharynx, esophagus, vagina, anal canal.

    • Stratified Cuboidal Epithelium:

    • Components: Two or more layers of cuboidal cells.

    • Functions: Provides some absorption and secretion; offers protection.

    • Locations: Ducts of sweat glands.

    • Stratified Columnar Epithelium:

    • Components: Two or more layers of columnar cells.

    • Functions: Some absorption and secretion; provides protection.

    • Locations: Ducts of certain glands, parts of male urethra.

    • Transitional Epithelium:

    • Components: Multiple layers that can change shape depending on stretch.

    • Functions: Provides protection and distensibility (ability to stretch).

    • Locations: Urinary bladder, ureters.

Connective Tissue Functions

  • General Functions:

    • Connecting and Binding: Binds other tissue layers and organs together.

    • Support: Structures like bone and cartilage provide support.

    • Protection: Various connective tissues provide protective functions against physical impacts.

    • Transport: Blood as a fluid connective tissue serves as the primary transport medium.

Types of Connective Tissue Proper

  1. Loose Connective Tissue:

    • Components: Fibroblast, ground substance, protein fibers.

    • Functions: Support, protection, housing blood vessels supplying epithelium.

    • Locations: Deep to epidermis, in membranes lining body cavities.

    • Properties: Strength and resistance to stress in all three planes.

  2. Dense Connective Tissue:

    • Components: Fibroblasts, collagen fibers, ground substance.

    • Types:

      • Irregular Dense CT: Resistance to stress in multiple directions (e.g., deep skin layer).

      • Regular Dense CT: Strength in one plane (e.g., tendons, ligaments).

  3. Reticular Tissue:

    • Components: Reticular fibers, leukocytes.

    • Functions: Forms the internal structure of organs (e.g., lymph nodes, spleen).

  4. Adipose Tissue:

    • Components: Adipocytes, protein fibers.

    • Functions: Energy reserve, warmth, shock absorption, protection.

    • Locations: Deep to skin in specific areas, surrounding heart and abdominal organs.

Specialized Connective Tissue

  • Definition: Specialized connective tissues have more defined functions than connective tissue proper.

  • Types:

    1. Cartilage:

    • Components: Chondrocytes and ECM.

    • Functions: Support, protection, resists compression.

    • Locations: Between bones in joints, between sternum and ribs, nose, and respiratory tract.

    1. Bone (Osseous Tissue):

    • Components: Osteocytes, osteoblasts, and osteoclasts, ECM.

    • Functions: Support, protection, lever mechanism for movement, stores calcium.

    • Locations: Skeletal system.

    1. Blood:

    • Components: Plasma, erythrocytes, leukocytes.

    • Functions: Transports nutrients, gases, wastes, and immune cells.

    • Locations: Within blood vessels and chambers of the heart.