The Tissue Level of Organization

The Tissue Level of Organizationsd

Chapter Objectives

After completing this chapter, you will be able to:

• Identify the main tissue types and discuss their functions in the human body.

• Identify the four types of tissue membranes and explain their functional characteristics.

• Explain the functions of various epithelial tissues and relate their forms to their functions.

• Explain the functions of various connective tissues and relate their forms to their functions.

• Describe the characteristics of muscle tissue and how they facilitate movement.

• Discuss the characteristics of nervous tissue and how they enable information processing and control of muscular and glandular activities.

Types of Tissues

Four Primary Tissue Types

There are four fundamental types of tissues that make up the human body:

1. Epithelial Tissue: Covers exterior body surfaces, lines internal cavities and passageways, and forms certain glands.

2. Connective Tissue: Binds cells and organs together, providing protection, support, and integration of all body parts.

3. Muscle Tissue: Excitable, responds to stimulation by contracting to produce movement. It exists in three major types:

   • Skeletal (voluntary) muscle.

   • Smooth muscle.

   • Cardiac muscle (in the heart).

4. Nervous Tissue: Also excitable, it propagates electrochemical signals (nerve impulses) to facilitate communication between different regions of the body.

Embryonic Origin of Tissues

Totipotent Cells

• The very first embryonic cells are totipotent, meaning each has the capacity to divide, differentiate, and develop into any type of cell in the body.

Three Germ Layers

As development progresses, these totipotent cells give rise to three main germ layers:

1. Ectoderm ($ecto- = \text{"outer"}$):

   • Gives rise to: epidermis, glands on skin, some cranial bones, pituitary and adrenal medulla, the nervous system, the mouth between the cheek and gums, and the anus.

   • Examples of tissues/cells derived: skin cells, neurons, pigment cells.

2. Mesoderm ($meso- = \text{"middle"}$):

   • Gives rise to: connective tissues proper, bone, cartilage, blood, endothelium of blood vessels, muscle, synovial membranes, serous membranes lining body cavities, kidneys, and lining of gonads.

   • During embryonic development, it forms a gelatinous tissue called mesenchyme, characterized by wispy collagen fibers and fibroblasts in a gel matrix.

   • Examples of tissues/cells derived: cardiac muscle, skeletal muscle, tubule cell of kidney, red blood cells, smooth muscle.

3. Endoderm ($endo- = \text{"inner"}$):

   • Gives rise to: lining of airways and the digestive system (except the mouth and distal rectum/anal canal), and glands (digestive glands, endocrine glands, adrenal cortex).

   • Examples of tissues/cells derived: lung cell, thyroid cell, pancreatic cell.

Tissue Membranes

Categories of Tissue Membranes

There are two broad categories of tissue membranes in the body:

1. Connective Tissue Membranes:

   • Synovial membranes: Found in joints.

2. Epithelial Membranes:

   • Mucous membranes: Line cavities open to the exterior.

   • Serous membranes: Line body cavities not open to the exterior (e.g., pleura, peritoneum, pericardium).

   • Cutaneous membrane: The skin.

Epithelial Tissue

Characteristics of Epithelial Tissue

• Polarity: Epithelial cells exhibit differences in structure and function between their exposed (apical) surface and their basal surface (close to underlying structures).

• Basal Lamina: A mixture of glycoproteins and collagen, providing an attachment site for the epithelium and separating it from underlying connective tissue.

• Reticular Lamina: Secreted by the underlying connective tissue, it attaches to the basal lamina, forming a basement membrane that helps hold the epithelium together.

• Avascularity: Epithelial tissues are nearly completely avascular, meaning they do not have blood vessels. They are usually nourished by diffusion from the underlying connective tissue.

• Closely Adhering Cells: Epithelia are sheets of cells that are very close together.

• Covers & Lines: Covers body surfaces and lines body cavities.

• Glands: Constitutes most glands.

• High Rate of Mitosis: Epithelial cells have a high rate of cell division, allowing for regeneration and replacement.

• Surfaces:

  • Basal surface: Faces the basement membrane.

  • Apical surface: Faces away from the basement membrane, often exposed to the environment or an internal space.

  • Lateral surface: Where cells are very close together.

Lining Epithelial Tissues

• Endothelium: A type of simple squamous epithelium that lines the interior surface of blood vessels and lymphatic vessels. Its thinness facilitates material passage.

• Mesothelium: A membrane composed of simple squamous cells that forms the pleura (lungs), peritoneum (abdominal cavity), mesentery (supports intestines), and pericardium (heart). It provides a smooth, protective surface.

Classification of Epithelia: Simple vs. Stratified

• Simple Epithelia:

  • One layer of cells.

  • Named by the shape of the cells.

  • All cells touch the basement membrane.

  • Types:

    • Simple Squamous: Thin, scaly cells. Nuclei are flat, horizontal, elliptical. Found in air sacs of lungs, lining of blood and lymphatic vessels (endothelium), serous membranes (mesothelium). Allows rapid diffusion and filtration.

    • Simple Cuboidal: Square or round (box-like) cells. Nucleus is round. Found in kidney tubules, ducts, and secretory portions of small glands. Functions in secretion and absorption.

    • Simple Columnar: Tall, narrow (column-like) cells. Nucleus located in the basal end. Found in the digestive system and female reproductive tract. Functions in absorption and secretion. Some are ciliated.

    • Pseudostratified Columnar: Appears stratified due to irregular shaped cells and nuclei at different levels, but every cell reaches the basement membrane (though not all reach the free surface). Often ciliated and contains goblet cells. Found in the respiratory tract. Secretes mucus and propels it with cilia.

• Stratified Epithelia:

  • More than one layer of cells (ranging from $2$ to $20$ or more).

  • Named by the shape of the apical cells.

  • Only the deepest layer attaches to the basement membrane; some cells rest on top of others.

  • Deepest layers undergo continuous mitosis, with daughter cells pushing upwards, flattening, dying, and flaking off (exfoliation or desquamation).

  • Types:

    • Stratified Squamous: Most widespread epithelium in the body. Apical cells are squamous; basal layer contains columnar or cuboidal cells. Functions in protection against abrasion.

      Keratinized: Found on skin surface (epidermis); abrasion-resistant due to a surface layer of dead cells packed with keratin.

      • Nonkeratinized: Lacks surface layer of dead cells. Found in the lining of the esophagus, mouth, and vagina.

    • Stratified Cuboidal: Apical cells are cuboidal. Generally involved in glands and ducts (e.g., sweat glands, salivary glands, mammary glands). Protective tissue. Uncommon.

    • Stratified Columnar: Apical cells are columnar. Less common; found in male and female urethra and ducts of some glands. Functions in secretion and protection.

    • Transitional (Urothelium): Specializes in changing shape. Found in the urinary system (bladder, ureters, urethra). When the bladder is empty, it has a convoluted shape with cuboidal apical cells; when full, apical cells stretch and become squamous-shaped. Allows for expansion and stretching.

Goblet Cells

• Wineglass-shaped, mucus-secreting cells commonly found in simple columnar and pseudostratified epithelia.

• Interspersed with other epithelial cells, such as in the lining of the small intestine or respiratory tract.

Exocrine Glands

Classification by Structure

Exocrine glands are classified by the structure of their ducts and secretory portions:

• Simple Glands (unbranched duct):

  • Simple Tubular: Intestinal glands.

  • Simple Branched Tubular: Glands in the stomach (gastric), mucous glands of the esophagus, tongue, duodenum.

  • Simple Coiled Tubular: Merocrine sweat glands.

  • Simple Alveolar (Acinar): Not common in adult humans; found in development of simple branched glands.

  • Simple Branched Alveolar: Sebaceous glands.

• Compound Glands (branched duct):

  • Compound Tubular: Mucous glands in the mouth, bulbourethral glands in the male reproductive system, and testes (seminiferous tubules).

  • Compound Alveolar (Acinar): Mammary glands.

  • Compound Tubuloalveolar: Salivary glands, glands of the respiratory passages, and pancreas.

Modes of Glandular Secretion

1. Merocrine Secretion: The most common type (e.g., eccrine/cooling sweat glands).

   • The cell remains intact.

   • Products are released by exocytosis.

2. Apocrine Secretion: Not as common.

   • The apical portion of the cell is released along with the product.

   • The cell repairs itself and continues to secrete.

3. Holocrine Secretion: (e.g., sebaceous glands which secrete oils to lubricate and protect the skin).

   • The cell is destroyed as it releases its product.

   • The entire cell itself becomes part of the secretion.

   • New glandular cells must form to replace the destroyed cells.

Connective Tissue

Overview and Functions

Connective tissue is the most abundant and widely distributed of the primary tissues, with highly variable structure and function.

• Functions:

  • Binding of Organs: Tendons (muscle to bone) and ligaments (bone to bone).

  • Support: Bones and cartilage provide structural support.

  • Physical Protection: Cranium, ribs, and sternum protect vital organs.

  • Immune Protection: White blood cells attack foreign invaders.

  • Movement: Bones provide a lever system for muscle action.

  • Storage: Stores fat (adipose tissue), calcium, and phosphorus.

  • Heat Production: Metabolism of brown fat in infants.

  • Transport: Blood carries nutrients, gases, wastes, and hormones.

Five Main Types of Connective Tissue

1. Fibrous Tissue

2. Adipose Tissue

3. Cartilage

4. Bone

5. Blood (Note: Bone and blood are considered connective tissues)

Fibrous Connective Tissue

• Fibers:

  • Collagenous fibers: Most abundant body protein ($25\%$ of all protein).

    • Tough, flexible, and stretch-resistant.

    • Major component of tendons, ligaments, and the deep layer of the skin.

    • Less visible in the matrix of cartilage and bone.

  • Reticular fibers: Thin collagen fibers coated with glycoprotein.

    • Form a delicate framework for organs like the spleen and lymph nodes.

  • Elastic fibers: Thinner than collagenous fibers, branch and rejoin each other.

    • Made of the protein elastin.

    • Allows for stretch and recoil in tissues.

• Types of Fibrous Connective Tissue:

  • Loose Connective Tissue: Characterized by much gel-like ground substance between cells.

    • Areolar Tissue: Loosely organized fibers, abundant blood vessels, and much empty space. Possesses all six cell types found in connective tissue proper. Fibers run in random directions (mostly collagenous, also elastic and reticular). Found in almost every part of the body, surrounding blood vessels and nerves. Nearly every epithelium rests on a layer of areolar tissue, which provides nutrition, waste removal, and infection-fighting leukocytes.

    • Reticular Tissue: Loose connective tissue made up of a network of reticular fibers providing a supportive framework for soft organs (e.g., lymph nodes, spleen, bone marrow).

  • Dense Connective Tissue: Fibers fill spaces between cells, with less ground substance.

    • Dense Regular Connective Tissue: Densely packed, parallel collagen fibers, with compressed fibroblast nuclei. Forms tendons (attach muscles to bones) and ligaments (hold bones together).

    • Dense Irregular Connective Tissue: Densely packed, randomly arranged collagen fibers and few visible cells. Withstands unpredictable stresses. Found in the deeper layer of the skin (dermis) and capsules around organs.

Adipose Tissue

• A loose connective tissue consisting of fat cells (adipocytes) with little extracellular matrix.

• Functions: Stores fat for energy, provides insulation, and cushions organs.

Cartilage (Supportive Connective Tissue)

• Stiff connective tissue with a flexible matrix.

• Chondroblasts: Cartilage cells that actively produce the matrix.

• Chondrocytes: Mature cartilage cells that become trapped in small cavities called lacunae within the matrix.

• Contains a reserve population of chondroblasts that contribute to cartilage growth throughout life.

• Avascular: Cartilage has no (or rarely any) blood vessels. Nutrients and wastes are exchanged via diffusion, leading to slow healing.

• Types of Cartilage (vary with fiber composition):

  • Hyaline Cartilage: Provides support with some flexibility (e.g., articular cartilage, costal cartilage, tracheal rings).

  • Fibrocartilage: Provides compressibility and can absorb pressure due to dense collagen fibers (e.g., intervertebral discs, menisci of knee).

  • Elastic Cartilage: Provides firm but elastic support due to abundant elastic fibers (e.g., external ear, epiglottis).

Blood (Fluid Connective Tissue)

• A fluid connective tissue containing erythrocytes (red blood cells), various types of leukocytes (white blood cells), and platelets, which circulate in a liquid extracellular matrix called plasma.

Muscle Tissue and Motion

Characteristics of Muscle Tissue

• Excitable: Responds to stimulation.

• Contractile: Ability to shorten, providing movement.

Types of Muscle Tissue

1. Skeletal Muscle:

   • Cells have prominent striations (alternating light and dark bands) and multiple nuclei located on their periphery.

   • Voluntary control (conscious control).

   • Responsible for movement of the skeleton.

2. Smooth Muscle:

   • Cells have a single, centrally located nucleus and no visible striations.

   • Involuntary control.

   • Found in the walls of internal organs (e.g., digestive tract, blood vessels, bladder).

3. Cardiac Muscle:

   • Cells appear striated and typically have a single, centrally located nucleus.

   • Unique features include intercalated discs, which are specialized junctions between cells.

   • Involuntary control.

   • Found only in the heart.

Nervous Tissue Mediates Perception and Response

Characteristics of Nervous Tissue

• Excitable: Specialized to transmit and receive electrochemical impulses.

• Allows the propagation of nerve impulses that communicate between different regions of the body.

• Enables information processing and control of muscular and glandular activities.

Components of Nervous Tissue

• Neurons: The primary functional cells of the nervous system, specialized for transmitting electrical signals.

  • Cell body (soma): Contains the nucleus and mitochondria; the main metabolic and synthetic center.

  • Dendrites: Branching extensions that receive nerve impulses and transfer them to the soma.

  • Axon: A single, long extension that carries action potentials away from the soma to another excitable cell.

• Neuroglia (Glial Cells): Support cells that protect, insulate, and nourish neurons, and aid in nervous communication.

Cell Junctions

Types of Cell-to-Cell Junctions

Epithelial cells often have specialized connections that hold them together and allow for communication:

1. Tight Junctions:

   • Linkage between two adjacent cells formed by transmembrane cell-adhesion proteins.

   • In epithelia, they form a zone that completely encircles each cell near its apical pole.

   • Function: Seals off intercellular space, making it difficult for substances to pass between cells (e.g., ensures proper absorption in the intestines, preventing leakage).

2. Desmosomes (Anchoring Junctions):

   • Patch-like structures that hold cells together, similar to a clothing snap.

   • Function: Keeps cells from pulling apart and resists mechanical stress. Hook-like, J-shaped proteins arise from the cytoskeleton and anchor to a membrane plaque, while transmembrane proteins from each cell are joined by cell adhesion proteins.

   • Location: Abundant in tissues subject to mechanical stress, such as the epidermis of the skin and heart muscle.

3. Gap Junctions:

   • Formed by ring-like structures called connexons.

   • Each connexon consists of six transmembrane proteins arranged like segments of an orange around a water-filled pore.

   • Function: Allows ions, nutrients, and other small solutes to pass directly between cells, facilitating rapid intercellular communication.

   • Location: Found in cardiac and smooth muscle, and embryonic tissue, where rapid communication and coordinated activity are crucial.

Tissue Repair: Wound Healing

Mechanisms of Tissue Repair

Damaged tissues can be repaired in two primary ways:

1. Regeneration:

   • Replacement of dead or damaged cells by the same type of cell as before.

   • Result: Restores normal function.

   • Examples: Skin, liver, and skeletal muscle have good regenerative capacity.

2. Fibrosis:

   • Replacement of damaged cells with scar tissue (collagen fibers).

   • Result: Holds organs together but does not restore normal function.

   • Examples: Healing of severe cuts and burns, and healing of cardiac muscle (which has limited regenerative capacity).

• Wound healing typically involves a combination of both regeneration and fibrosis.

Four Major Steps of Wound Healing

1. Step $1$: Initial Injury & Hemostasis:

   • Blood plasma seeps into the wound, carrying antibodies, clotting proteins (e.g., fibrinogen), and blood cells.

   • A blood clot forms.

2. Step $2$: Inflammation & Scab Formation:

   • A scab forms over the wound as the clot dries.

   • Immune cells (macrophages, leukocytes) move in to fight infection and clean up debris.

3. Step $3$: Proliferative (Fibroblastic) Phase:

   • Begins approximately $3-4$ days after injury and can last up to $2$ weeks.

   • Macrophages remove the blood clot.

   • New capillaries grow into the wound bed.

   • Granulation tissue forms, characterized by fibroblasts depositing new collagen fibers and extracellular matrix.

4. Step $4$: Remodeling Phase:

   • Begins several weeks after injury and may last up to $2$ years.

   • Surface epithelial cells multiply, and the epithelium regenerates, covering the wound.

   • The connective tissue undergoes fibrosis, forming scar tissue as collagen fibers are laid down randomly by fibroblasts.

   • The scar tissue remodels and contracts over time.

Tissue Injury and Aging

• During wound repair, collagen fibers are laid down randomly by fibroblasts that migrate into the damaged area.

• Aging can impact tissue healing and the overall health of tissues.

• Changes in cell size, nucleus size, and organization within tissues can be indicators of conditions like cancer development.