Chapter 4: The Tissue Level of Organization

Learning Objectives

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

  • Epitheal, connective, muscular, nervous

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

• Explain the functions of epithelial tissues and how their forms enable these functions.

• Explain the functions of connective tissues and how their forms enable these functions.

• Describe the characteristics of muscle tissue and how these enable function.

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

Types of Tissues

• The four basic types of tissues are: epithelial, connective, muscular, and nervous

what is a tissue

• A tissue is a group of cells with a common embryonic origin that function together to carry out specialized activities.

• There are four basic types of tissues in the human body, categorized by structure and function.

Cell Junctions

• Cell junctions are points of contact between cells that hold them together.

  Intercellular Junction Interactions

• Types of intercellular junctions include:

• tight junctions,

• adhering junctions,

• desmosomes,

• hemidesmosomes

• gap junctions.

Embryonic Origin of Tissues and Major Organs

• Epithelial and connective tissues are compared based on their embryonic origin.

Epithelial Tissue

• Cells are arranged in sheets and are densely packed.

• Many cell junctions are present.

• Epithelial cells attach to a basement membrane.

• Epithelial tissue is avascular but has a nerve supply.

• Mitosis occurs frequently.

Surfaces of Epithelial Cells and the Basement Membrane

• Covering and lining epithelia are classified according to the shape of the cells and the number of layers.

• The name of stratified epithelial tissue depends on the shape of the apical cells.

Epithelial Tissue: Surface Epithelium

Simple Squamous Epithelium

• Description: Single layer of flat cells resembling a tiled floor with a centrally located nucleus.

• Location: Lines the cardiovascular and lymphatic system (endothelium), forms the epithelial layer of serous membranes in abdominal and thoracic cavities (mesothelium), air sacs of lungs, kidneys, cornea, and tympanic membrane.

• Function: Filtration (kidneys), diffusion (lungs), and secretion in serous membranes.

Simple Cuboidal Epithelium

• Description: Single layer of cube-shaped cells with a round, centrally located nucleus. Easily viewed when sectioned and viewed from the side.

• Location: Covers the surface of the ovary, lines the anterior surface of the capsule of the eye lens, forms pigmented epithelium at the posterior surface of the retina, lines kidney tubules and small ducts in glands.

• Function: Secretion and absorption.

Nonciliated Simple Columnar Epithelium

• Description: Single layer of nonciliated column-like cells with oval nuclei near the base of the cells; contains columnar epithelial cells with microvilli at the apical surface and goblet cells.

• Location: Lines the gastrointestinal tract, ducts of many glands, and gallbladder.

• Function: Secretion and absorption; mucus lubricates the lining of the digestive, respiratory, and reproductive tracts, and most of the urinary tract; helps prevent destruction of the stomach lining by acidic stomach juices.

Ciliated Simple Columnar Epithelium

• Description: Single layer of ciliated column-like cells with oval nuclei near the base of the cells. Goblet cells are usually interspersed.

• Location: Lines some bronchioles, uterine tubes, uterus, paranasal sinuses, central canal of the spinal cord, and ventricles of the brain.

• Function: In the respiratory system, cilia beat in unison to move mucus and foreign particles toward the throat to be coughed up or swallowed. Cilia also move oocytes from the ovaries through the fallopian tubes into the uterus.

Nonciliated Pseudostratified Columnar Epithelium

• Description: Appears to have several layers due to nuclei at various levels. All cells attach to the basement membrane in a single layer, but some cells do not extend to the apical surface, giving a false impression of a multi-layered tissue. Contains cells without cilia and lacks goblet cells.

• Location: Lines the epididymis, larger ducts of many glands, and parts of the male urethra.

• Function: Absorption and secretion.

Ciliated Pseudostratified Columnar Epithelium

• Description: Appears to have several layers because cell nuclei are at various levels. All cells are attached to the basement membrane in a single layer, but some cells do not extend to the apical surface. Contains cells that extend to the surface and secrete mucus (goblet cells) or bear cilia.

• Location: Lines airways of most of the upper respiratory tract.

• Function: Secretes mucus that traps foreign particles, and cilia sweep away mucus for elimination from the body.

Stratified Squamous Epithelium

• Description: Contains two or more layers of cells. Cells in the apical layer and several layers deep to it are squamous; cells in deeper layers are cuboidal and columnar. As basal cells divide, daughter cells push upward toward the apical layer, becoming dehydrated and less metabolically active. Tough proteins predominate as the cytoplasm is reduced, and cells become tough, hard structures that eventually die. At the apical layer, dead cells lose cell junctions and are sloughed off but are replaced continuously as new cells emerge from basal cells. There are two types: keratinized (contain keratin) and nonkeratinized (does not contain large amounts of keratin).

• Location: Keratinized variety forms the superficial layer of skin; nonkeratinized variety lines wet surfaces (lining of the mouth, esophagus, part of the epiglottis, part of the pharynx, and vagina) and covers the tongue.

• Function: Protection against abrasion, water loss, UV radiation, and foreign invasion. First line of defense against microbes.

Stratified Cuboidal Epithelium

• Description: Has two or more layers of cells; cells in the apical layer are cube-shaped; fairly rare type.

• Location: Ducts of adult sweat glands and esophageal glands, part of the male urethra.

• Function: Protection; limited secretion and absorption.

Stratified Columnar Epithelium

• Description: Basal layers consist of shortened, irregularly shaped cells; only the apical layer has columnar cells; uncommon.

• Location: Lines part of the urethra; large excretory ducts of some glands, such as esophageal glands; small areas in the anal mucous membrane; part of the conjunctiva of the eye.

• Function: Protection and secretion.

Urothelium (Transitional Epithelium)

• Description: Has a variable appearance (transitional). In a relaxed or unstretched state, looks like stratified cuboidal epithelium, except apical layer cells tend to be large and rounded. As tissue is stretched, cells become flatter, giving the appearance of stratified squamous epithelium. Multiple layers and elasticity make it ideal for lining hollow structures (urinary bladder) subject to expansion from within.

• Location: Lines the urinary bladder, ureters, and portions of the urethra.

• Function: Allows urinary organs to stretch and maintain a protective lining while holding variable amounts of fluid without rupturing.

Glandular Epithelium

• A gland is a single cell or a mass of epithelial cells adapted for secretion.

• The secretions of endocrine glands enter the interstitial fluid and diffuse into the bloodstream.

• The secretions of exocrine glands enter ducts that empty onto the surface of a covering/lining epithelium such as the skin surface or the lumen of the stomach.

Endocrine Glands

• Description: Endocrine gland secretions (hormones) enter interstitial fluid and then diffuse into the bloodstream without flowing through a duct.

• Location: Examples include the pituitary gland at the base of the brain, the pineal gland in the brain, thyroid and parathyroid glands near the larynx (voice box), adrenal glands superior to the kidneys, pancreas near the stomach, ovaries in the pelvic cavity, testes in the scrotum, thymus in the thoracic cavity.

• Function: Hormones regulate many metabolic and physiological activities to maintain homeostasis.

Exocrine Glands

• Description: Exocrine gland secretory products are released into ducts that empty onto the surface of a covering and lining epithelium, such as the skin surface or the lumen of a hollow organ.

• Location: Sweat, oil, and earwax glands of the skin; digestive glands such as salivary glands (secrete into the mouth cavity) and the pancreas (secretes into the small intestine).

• Function: Produce substances such as sweat to help lower body temperature, oil, earwax, saliva, or digestive enzymes.

Structural Classification of Glandular Epithelium

• Unicellular - single cells: Goblet cells.

• Multicellular - composed of many cells that form a distinctive microscopic structure or macroscopic organ:

  • Sweat glands

  • Oil glands

  • Salivary glands

Functional Classification of Glandular Epithelium

• Modes of Glandular Secretion:

  • Merocrine secretion: the cell remains intact.

  • Apocrine secretion: the apical portion of the cell is released, as well.

  • Holocrine secretion: the cell is destroyed as it releases its product, and the cell itself becomes part of the secretion.

Connective Tissue

General Features of Connective Tissue

• Consists of two basic elements:

  • Cells

  • Extracellular matrix

• Connective tissue cells do not have any free surfaces.

• Connective tissue is highly vascularized and has a nerve supply, with exceptions for tendons and cartilage.

Connective Tissue Cells

Connective Tissue Extracellular Matrix

• The extracellular matrix is located in the spaces between connective tissue cells.

• The extracellular matrix is composed of fibers and ground substance.

• Fibers in the extracellular matrix provide strength and support to a tissue:

  • Collagen fibers

  • Elastic fibers

  • Reticular fibers

Classification of Connective Tissue

Embryonic

• Mesenchyme

• Mucous

Mature

• Loose

• Dense

• Cartilage

• Bone

• Blood

Connective Tissue Anatomy Overview: Connective Tissues

Cartilage

• Cartilage is a connective tissue consisting of collagenous fibers embedded in a firm matrix of chondroitin sulfates.

• Hyaline cartilage provides support with some flexibility.

• Fibrocartilage provides some compressibility and can absorb pressure.

• Elastic cartilage provides firm but elastic support.

Mature Connective Tissue: Supporting Connective Tissue: Cartilage

Hyaline Cartilage

• Description: Contains resilient gel as ground substance and appears as a bluish-white, shiny substance; prominent chondrocytes are found in cartilage lacunae surrounded by perichondrium (exceptions: articular cartilage in joints and cartilage of growth plates, where bones lengthen during growth).

• Location: Most abundant cartilage in the body; at ends of long bones, anterior ends of ribs, nose, parts of the larynx, trachea, bronchi, bronchial tubes, embryonic and fetal skeleton.

• Function: Provides smooth surfaces for movement at joints, flexibility, and support; weakest type of cartilage and can be fractured.

Fibrous Cartilage

• Description: Has chondrocytes among clearly visible thick bundles of collagen fibers within the extracellular matrix; lacks perichondrium.

• Location: Public symphysis, intervertebral discs, menisci of the knee, portions of tendons that insert into cartilage.

• Function: Support and joining structures together. Strength and rigidity make it the strongest type of cartilage.

Elastic Cartilage

• Description: Has chondrocytes in a threadlike network of elastic fibers within the extracellular matrix; perichondrium present.

• Location: Lid on top of the larynx (epiglottis), part of the external ear (auricle), auditory (Eustachian) tubes.

• Function: Provides strength and elasticity; maintains the shape of certain structures.

Mature Connective Tissue: Supporting Connective Tissue: Bone Tissue

• Description: Compact bone tissue consists of osteons (Haversian systems) that contain bone lamellae, bone lacunae, osteocytes, bone canaliculi, and central (Haversian) canals. Spongy bone tissue consists of thin columns called trabeculae; spaces between trabeculae are filled with red bone marrow.

• Location: Both compact and spongy bone tissue make up the various parts of bones of the body.

• Function: Support, protection, storage; houses blood-forming tissue; serves as levers that act with muscle tissue to enable movement.

Mature Connective Tissue: Liquid Connective Tissue: Blood

• Description: Consists of blood plasma and formed elements: red blood cells (erythrocytes), white blood cells (leukocytes), platelets (thrombocytes).

• Location: Within blood vessels (arteries, arterioles, capillaries, venules, veins), within chambers of the heart.

• Function: Red blood cells: transport oxygen and some carbon dioxide; White blood cells: carry on phagocytosis and mediate allergic reactions and immune system responses; Platelets: essential for clotting.

Membranes

• Membranes are flat sheets of pliable tissue that cover or line a part of the body.

• Two types of membranes:

  • Epithelial membranes:

    • Mucous membranes

    • Serous membranes

    • Cutaneous membranes

  • Synovial membranes

Muscular Tissue

General Characteristics of Muscular Tissue

• Muscular tissue consists of fibers that provide motion, maintain posture, and produce heat.

• Three types of muscle tissue:

  • Skeletal muscle

  • Cardiac muscle

  • Smooth muscle

Muscle Tissue Anatomy Overview: Muscular Tissue

Skeletal Muscle Tissue

• Description: Consists of long, cylindrical, striated fibers. Vary greatly in length, from a few centimeters in short muscles to 30-40 cm in the longest muscles. A muscle fiber is a roughly cylindrical, multinucleated cell with nuclei at the periphery. It is voluntary because it can be made to contract or relax by conscious control.

• Location: Usually attached to bones by tendons.

• Function: Motion, posture, heat production, protection.

Cardiac Muscle Tissue

• Description: Consists of branched, striated fibers with usually only one centrally located nucleus. Attach end to end by transverse thickenings of plasma membrane called intercalated discs, which contain desmosomes and gap junctions. Desmosomes strengthen tissue and hold fibers together during vigorous contractions. Gap junctions provide a route for quick conduction of electrical signals throughout the heart. Involuntary control.

• Location: Heart wall.

• Function: Pumps blood to all parts of the body.

Smooth Muscle Tissue

• Description: Consists of nonstriated fibers (lacking striations, hence the term smooth). A smooth muscle fiber is a small spindle-shaped cell thickest in the middle, tapering at each end, and containing a single, centrally located nucleus. Gap junctions connect many individual fibers in some smooth muscle tissue. Usually involuntary; can produce powerful contractions as many muscle fibers contract in unison. Where gap junctions are absent, such as in the iris of the eye, smooth muscle fibers contract individually, like skeletal muscle fibers.

• Location: Iris of eyes; walls of hollow internal structures such as blood vessels, airways to lungs, stomach, intestines, gallbladder, urinary bladder, and uterus.

• Function: Motion (constriction of blood vessels and airways, propulsion of foods through the gastrointestinal tract, contraction of the urinary bladder and gallbladder).

Nervous Tissue

General Characteristics of Nervous Tissue

• Two kinds of cells:

  • Neurons

  • Neuroglia

• Most neurons have a cell body, dendrites, and axons.

• Neurons can carry sensory or motor information, and they can perform integrative functions.

• Neuroglia protect and support neurons.

Nervous Tissue Anatomy Overview: Nervous Tissue

Nervous Tissue

• Description: Consists of (1) neurons (nerve cells), which consist of a cell body and processes extending from the cell body (one to multiple dendrites and a single axon); and (2) neuroglia, which do not generate or conduct nerve impulses but have other essential supporting functions.

• Location: Nervous system.

• Function: Exhibits sensitivity to various types of stimuli; converts stimuli into nerve impulses (action potentials); conducts nerve impulses to other neurons, muscle fibers, or glands.

Excitable Cells

• Neurons and muscle fibers are considered excitable cells because they exhibit electrical excitability.

• Electrical excitability is the ability to respond to certain stimuli by producing electrical signals, such as action potentials, which travel along the plasma membrane of a neuron or muscle fiber due to the presence of specific voltage-gated channels.

Tissue Repair: Restoring Homeostasis

Tissue Repair

• Tissue repair is the process that replaces worn-out, damaged, or dead cells.

• Epithelial cells are replaced by the division of stem cells or undifferentiated cells.

• Not all connective tissue cells have the ability to repair.

• Muscle cells can perform limited repair.

• Some nervous cells can perform limited repair, others cannot.

• Fibrosis is the formation of scar tissue.

Aging and Tissues

• Younger bodies generally experience:

  • A better nutritional state

  • A better blood supply to tissues

  • A faster metabolic rate

• Aging slows the process of tissue repair.

• Aging also results in stiffening and loss of elasticity in tissues.

Homeostatic Imbalances

• Disorders of epithelial tissues tend to be specific to individual organs, such as ulcers in the stomach.

• Disorders of connective tissues tend to be autoimmune in nature, such as lupus.

• Disorders of muscular and nervous tissues will be discussed in later chapters.