Tissue Level of Organization

Chapter 4: The Tissue Level of Organization

Tissue Definition

  • Tissue: A group of similar cells that work together to perform a specific function.

  • Histology: The study of tissues.

Major Types of Tissues

  1. Connective Tissue

  2. Epithelial Tissue

  3. Muscle Tissue

  4. Nervous Tissue

Tissue Techniques and Sectioning

Procedure for preparing tissues for examination:

  1. Tissue is placed in a fixative to preserve structure.

  2. Tissue is embedded in paraffin wax for stability.

  3. Tissue is sectioned to a thickness of 1-2 cells.

  4. Slices are mounted on slides for viewing under a microscope.

  5. Slides are stained to enhance contrast and visibility of the cellular structures.

Cell Junctions

  • Tight Junctions:

    • Definition: Encircles the cell like a zipper.

    • Function: Creates a leakproof seal that prevents substances from passing between cells.

    • Example: Found in the urinary tract and gastrointestinal tract.

  • Adherens Junctions:

    • Definition: Encircles the cell like a belt.

    • Function: Keeps tissues from separating as they stretch or contract.

    • Mechanism: Transmembrane proteins called Cadherin join cells.

    • Example: Found in the urinary tract and gastrointestinal tract.

  • Desmosomes:

    • Definition: Holds cells together like a button.

    • Function: Allows substances to pass through while providing structural integrity.

    • Mechanism: Also utilizes cadherin proteins.

    • Example: Found in the skin (epidermis).

  • Hemidesmosomes:

    • Definition: Hold cells to the basement membrane.

    • Mechanism: Uses integrin proteins instead of cadherin proteins.

    • Example: Found in the base layer of the epidermis.

  • Gap Junctions:

    • Definition: Formed by a ring of trans-membrane proteins.

    • Function: Allows for cell-to-cell communication.

    • Example: Present in muscle cells of the heart.

Embryonic Tissues

  1. Ectoderm:

    • Description: Outermost layer of the developing fetus.

    • Derivatives: Forms the epidermis, skin glands, various cranial bones, pituitary and adrenal medulla, parts of the nervous system, mouth, and anus.

  2. Endoderm:

    • Description: Innermost layer of the developing fetus.

    • Derivatives: Forms the lining of airways and the digestive tract, and certain glands (digestive, endocrine, adrenal cortex), as well as the bladder.

  3. Mesoderm:

    • Description: Middle layer of the developing fetus.

    • Derivatives: Forms muscle, bone, cartilage, blood, blood vessels, lymph tissue, parts of the kidney, and parts of the gonads.

Epithelial Tissues

  • Definition: Epithelial tissues consist of closely adhering cells that line the surfaces of organs and structures.

  • Characteristics:

    • Arranged in layers (simple, stratified, etc.).

    • Basal (bottom) and apical (top) surfaces.

    • Anchored to the basement membrane by hemidesmosomes.

  • Arrangement of Layers:

    • Simple: Single layer of cells.

    • Types:

      • Simple Squamous: Flat, wide cells.

      • Simple Cuboidal: Height equals width.

      • Simple Columnar: Tall, thin cells.

    • Stratified: Multiple layers of cells.

    • Types:

      • Stratified Squamous: All layers touch the bottom, only the top layer may not.

      • Stratified Cuboidal and Columnar: Protects and secretes, with varying numbers of layers.

    • Pseudostratified: Appears to be stratified but is a single layer of varying cell heights.

Specific Types of Epithelial Tissues

  1. Simple Squamous Epithelium:

    • Function: Permits diffusion across the epithelium.

    • Found in: Lungs (alveoli), kidneys (glomeruli).

  2. Simple Cuboidal Epithelium:

    • Function: Aids in absorption (with microvilli), secretion, and mucus production.

    • Found in: Liver, thyroid, mammary glands, salivary glands, bronchioles, and kidney tubules.

  3. Simple Columnar Epithelium:

    • Function: Aids in absorption and secretion (often containing microvilli).

    • Found in: Lining of the gastrointestinal tract, uterus, kidneys, seminiferous tubules, and uterine tubes.

  4. Pseudostratified Columnar Epithelium:

    • Function: Important for secretion of mucus.

    • Found in: Lining of the trachea.

  5. Stratified Squamous Epithelium:

    • Function: Protects against friction.

    • Types:

      • Keratinized: Forms dry surfaces (e.g., skin).

      • Non-Keratinized: Forms moist tissues (e.g., tongue, esophagus, vagina).

  6. Stratified Columnar Epithelium:

    • Function: Protection and secretion.

    • Found in: Parts of the urethra, mammary ducts, and some glands.

  7. Stratified Cuboidal Epithelium:

    • Function: Secretes sweat and produces hormones.

    • Found in: Sweat gland ducts, urethra, and ovarian follicles.

  8. Transitional Epithelium:

    • Function: Cells can compress and stretch.

    • Found in: Bladder.

Connective Tissues

  • Definition: Connective tissues are derived from mesenchymal cells originating from the mesoderm, crucial for filling internal spaces, supporting other tissues, transporting materials, storing energy, and protecting/insulating structures.

Anatomical Characteristics of Connective Tissues

  1. Fibrous Connective Tissues:

    • Fibrocytes, collagen, elastic, and gel-like ground substance with reticular fibers.

  2. Blood Cells:

    • Leukocytes (white blood cells) and Erythrocytes (red blood cells).

    • Liquid ground substance with fibrin fibers.

  3. Adipose Tissues:

    • Adipocytes store fats, with collagen and elastic fibers present.

    • Gel-like ground substance.

  4. Bone Tissues:

    • Osteocytes in a calcified ground substance.

  5. Cartilage:

    • Chondrocytes with collagen fibers in a gel-like ground substance.

Other Cells Found within Connective Tissue

  1. Permanent Cells (Matrix-Building):

    • Fibroblasts (fibrous CT).

    • Chondroblasts (cartilage CT).

    • Osteoblasts (bone CT).

  2. Transient Cells (Immune Function):

    • Macrophages, Neutrophils, Plasma Cells.

  3. Mast Cells secrete:

    • Heparin: Inhibits clotting.

    • Histamine: Causes dilation of blood vessels.

Connective Tissue Fibers

  1. Collagen Fibers:

    • Comprise 25% of your protein.

    • Tough, stretch-resistant, and flexible.

    • Found in: Tendons, ligaments, and deep skin.

  2. Reticular Fibers:

    • Thin, collagen-based fibers.

    • Found in spleen and lymph tissues.

  3. Elastic Fibers:

    • Thin, elastin protein-based fibers, important for stretch and recoil.

    • Found in: Skin, lungs, and arteries.

  4. Embryonic Connective Tissues:

    • Mesenchyme: Precursor to all connective tissues.

    • Mucous CT: Found in the umbilical cord and rich in stem cells.

Fibrous Connective Tissue

Fibrous Loose Connective Tissues

  1. Areolar Tissue:

    • Cell Type: Fibrocytes.

    • Function: Underlies epithelia and accommodates stretching.

    • Found: Throughout the body.

  2. Adipose Tissue:

    • Cell Type: Adipocytes.

    • Function: Lipid storage, protection, and energy storage.

    • Found: Throughout body, usually subcutaneous.

  3. Reticular Tissue:

    • Cell Type: Reticulocytes.

    • Function: Support framework for lymphatic system.

    • Found: Lymphatic organs (e.g., spleen, lymph nodes).

Fibrous Dense Connective Tissues

  1. Dense Regular Tissue:

    • Cell Type: Fibrocytes.

    • Function: Densely packed collagen suitable for stretching in one direction.

    • Found: Tendons and ligaments.

  2. Dense Irregular Tissue:

    • Cell Type: Fibrocytes.

    • Function: Densely packed collagen for stretching in multiple directions.

    • Found: Deep layers of the skin and around organ capsules.

  3. Elastic Tissue:

    • Cell Type: Fibrocytes.

    • Function: Densely packed elastin for stretching.

    • Found: Lungs, arteries, trachea, and penis.

Supportive Connective Tissues

Cartilaginous Tissues

  • Characteristics: Rubbery ground substance, no blood vessels, produced by chondroblasts.

  1. Hyaline Cartilage:

    • Cell Type: Chondrocytes.

    • Function: Supports airways and eases joint movements.

    • Found: Trachea, ends of bones, tip of nose.

  2. Fibro Cartilage:

    • Cell Type: Chondrocytes (with extensive collagen fibers).

    • Function: Resists compression and absorbs shock.

    • Found in: Meniscus, intervertebral disks, between hip bones.

  3. Elastic Tissue:

    • Cell Type: Chondrocytes (with elastin fibers).

    • Function: Provides flexibility and elastic support.

    • Found in: Ears, epiglottis.

Bone Tissues

  • Characteristics: Calcified matrix produced by osteoblasts, with osteocytes residing within it.

  • Function: Provides structural support, facilitates muscle leverage, and provides mineral storage.

  • Found in: Skeleton.

Liquid Connective Tissues

Blood Tissues

  • Components: Blood cells including:

    • Leukocytes (white blood cells).

    • Erythrocytes (red blood cells).

    • Function: Transports ions, gases, hormones, and immune cells.

    • Found in: Circulatory system.

Lymph Tissues

  • Components: Lymphocytes (type of leukocytes).

  • Function: Provides immunity and aids in movement of immune cells.

  • Found in: Lymphatic organs.

Nervous and Muscular Tissues

Muscular Tissues

  1. Skeletal Muscle Tissue:

    • Cell Type: Myocytes.

    • Function: Moves limbs and produces heat.

    • Found: Attached to bones.

  2. Cardiac Muscle Tissue:

    • Cell Type: Myocytes.

    • Function: Pushes blood through the heart.

    • Found: Heart.

  3. Smooth Muscle Tissue:

    • Cell Type: Myocytes.

    • Function: Expels or moves substances along tubes.

    • Found: Lining tubes such as the GI tract.

  • Control: Skeletal muscle can be controlled voluntarily; cardiac and smooth muscle are involuntary.

Nervous Tissues

  • Definition: Comprised of neurons and supporting cells (glial cells).

  • Function: Communication throughout the body.

  • Found in: Brain, spinal cord, and ganglia.

Membranes

Definition: Membranes are the simplest organs in the body that line cavities and cover viscera, composed of both epithelial and connective tissues.

Types of Membranes

  1. Serous Membrane (Serosa):

    • Function: Internal lining that produces serous fluid.

    • Serous Fluid: Thin, watery fluid that keeps epithelia moist.

    • Found: Cavities not exposed to outside, such as the lungs.

  2. Mucous Membrane (Mucosa):

    • Function: Lines external passageways and produces sticky mucus.

    • Found: Digestive tract.

  3. Cutaneous Membrane (Skin):

    • Function: Protects the body.

    • Found: Skin.

  4. Synovial Membrane:

    • Function: Lines joint cavities and secretes synovial fluid.

    • Synovial Fluid: Lubricates joints for movement.

    • Found in: Knuckles, shoulders, etc.

Glands

Definition: Glands are composed of epithelial tissues and function in secretion.

Characterization of Glands

  1. Where the Secretion Goes:

    • Exocrine Glands: Secrete substances outside the body.

      • Examples: Sweat glands, digestive glands.

    • Endocrine Glands: Secrete substances inside the body (hormones).

      • Examples: Hormonal glands.

  2. The Shape of the Gland:

    • Simple: No branches.

    • Compound: One or more branches.

    • Tubular: Tube-shaped.

    • Acinar: Spherical-shaped.

  3. How the Gland Secretes:

    • Merocrine: By exocytosis.

      • Examples: Regular sweat, tears, gastric glands.

    • Apocrine: Budding of cytoplasm and plasma membrane.

      • Examples: Mammary glands, post-pubertal sweat glands.

    • Holocrine: By apoptosis (programmed cell death).

      • Example: Sebaceous glands by hair follicles.

Tissue Change

Types of Tissue Change

  1. Metaplasia: A mature tissue changes into another type of mature tissue.

    • Example: Vaginal tissue changes from simple cuboidal/columnar before puberty to stratified squamous epithelium after puberty.

  2. Differentiation: Unspecialized tissue becomes specialized mature tissue.

    • Example: Mesenchyme to areolar tissue.

Stem Cells

  1. Totipotent: Early embryo can become anything (includes replication of itself or development into pluripotent).

  2. Pluripotent: After totipotent, can become some types (e.g., blood stem cells, muscle stem cells, nerve stem cells, bone stem cells).

  3. Unipotent: A single cell type becomes one specific type (e.g., skin).

Types of Tissue Growth

  1. Hypertrophy: Increase in cell size, leads to muscle growth.

    • Example: Bigger cells, not necessarily more cells.

  2. Hyperplasia: Increase in cell number, leads to tissue growth.

    • Example: More cells, not larger cells.

  3. Neoplasia: Abnormal growth, can result in tumors which may be benign or malignant.

Tissue Shrinkage and Death

Types of Tissue Reduction

  1. Atrophy: Loss of cell size or number, opposite of hypertrophy and hyperplasia.

    • Example: Muscle atrophy due to lack of use.

  2. Necrosis: Permanent pathological death of tissue, caused by:

    • Trauma, toxins, infections, lack of oxygen, lack of blood supply.

    • Infarction: Sudden necrosis (e.g., a heart attack).

    • Gangrene: Necrosis due to inadequate blood supply (e.g., frostbite).

  3. Apoptosis: Programmed cell death that occurs intentionally.

    • Example: Cells shrink and are phagocytized.

Tissue Repair

  • Regeneration: Function of the tissue is restored (e.g., surface skin injury healing).

  • Fibrosis: Function is not restored; damaged cells are replaced with scar tissue (e.g., muscle injuries, lung tissue affected by tuberculosis, severe cuts, or burns).