Chapter 4: Tissue: The Living Fabric - BIOL 2740

4.1 Tissue Samples are Fixed, Sliced, and Stained for Microscopy

• Tissue samples are prepared for microscopy through three main steps: fixation, slicing, and staining.

• Fixation: preserves tissue and halts decay/biological activity to maintain structure for observation.

• Slicing: thin sections are cut so structures can be viewed under light or electron microscopy.

• Staining: dyes are used to enhance contrast and visualize different tissue components.

• These prepared samples form the basis for histological study of tissues and their organization.

4.2 Epithelial Tissue Covers Body Surfaces, Lines Cavities, and Forms Glands

• Epithelial tissue originates from the word Epithelium, meaning "laid on" or "covering"; epithelia are sheets of cells that cover body surfaces or line cavities.

• Epithelial tissues line or cover surfaces and cavities; epithelia (plural) are the tissues themselves.

• Basement membrane: the thin layer that anchors epithelium to underlying connective tissue.

• Goblet cells: mucus-secreting cells frequently found in epithelium; mucus helps protect and lubricate surfaces.

• Epithelia are avascular (no blood vessels) and obtain nutrients via diffusion from underlying connective tissue.

• Forms of epithelial tissue (two main forms):

  • Covering and lining epithelium: covers external surfaces and lines internal cavities (e.g., skin, lumen linings).

  • Glandular epithelium: consists of secreting cells that form glands in the body.

• Classification of epithelia by cell shape (nucleus shape follows cell shape):

  • Squamous: flattened, scale-like

  • Cuboidal: box-like, cube-shaped

  • Columnar: tall, column-like

• Classification by layers based on cell organization is also used (e.g., simple, stratified, pseudostratified).

• Key epithelial tissue types (types, functions, locations):

  • Simple squamous epithelium

  • Description: single layer of flattened cells with disc-shaped nuclei.

  • Function: allows diffusion and filtration; secretes lubricating substances in serosae where protection is not essential.

  • Location: lines kidney glomeruli; air sacs of lungs; lining of heart, blood vessels, lymphatic vessels; lining of ventral body cavity (serosae).

  • Stratified squamous epithelium

  • Description: multiple cell layers; outermost cells are flattened.

  • Function: protects underlying tissues from abrasion, drying, infection.

  • Location: moist linings of mouth, esophagus, vagina; skin (keratinized forms).

  • Simple cuboidal epithelium

  • Description: a single layer of cube-shaped cells with large, spherical nuclei.

  • Function: secretion and absorption.

  • Location: kidney tubules; ducts and secretory portions of small glands; ovarian surface.

  • Stratified cuboidal epithelium

  • Description: multiple cell layers; surface cells are cuboidal.

  • Function: secretion of water and ions.

  • Location: ducts of sweat glands.

  • Simple columnar epithelium

  • Description: a single layer of tall cells; some bear microvilli; some contain goblet cells.

  • Function: absorption; secretion of mucus, enzymes, and other substances; ciliated type propels mucus or reproductive cells by ciliary action.

  • Location: nonciliated lines most digestive organs (stomach to rectum); gallbladder; excretory ducts of some glands; ciliated variety lines small bronchi, uterine tubes, and parts of the uterus.

  • Stratified columnar epithelium

  • Description: several layers with at least the apical cells columnar.

  • Function: protection and secretion.

  • Location: large ducts (e.g., some glandular ducts), portions of the male urethra, and conjunctiva of the eye (less common in humans).

  • Pseudostratified columnar epithelium

  • Description: single layer of cells of differing heights; nuclei at different levels; may contain mucus-secreting goblet cells and bear cilia.

  • Function: secretion of mucus; propulsion of mucus by ciliary action.

  • Location: nonciliated type in male ducts and ducts of large glands; ciliated variety lines the trachea and most of the upper respiratory tract.

  • Transitional epithelium

  • Description: resembles both stratified squamous and stratified cuboidal; basal cells are cuboidal or columnar; surface cells dome-shaped or squamous-like, depending on stretch.

  • Function: stretches readily, permits stored urine to distend urinary organ.

  • Location: lines ureters, bladder, and part of the urethra.

• Special notes:

  • Many epithelial tissues exhibit goblet cells and may be ciliated (as in pseudostratified columnar epithelium).

  • Transitional epithelium demonstrates stretch: relaxed vs distended states.

  • Some epithelia are keratinized (e.g., epidermis) and provide extra protection.

4.3 Connective Tissues are the Most Abundant and Widely Distributed Tissue in the Body

• Connective tissue is the most abundant and widely distributed primary tissue; the amount in an organ varies.

• Major functions: binding and support; protecting; insulating; storing reserve fuel; transporting substances.

• Components:

  • Ground substance: unstructured gel-like material filling space between cells; serves as diffusion medium for solutes between blood capillaries and cells.

  • Extracellular matrix: composed of ground substance plus fibers.

  • Fibers: collagen, elastic, and reticular fibers.

• Cell types commonly found include: fibroblasts, fibrocytes, macrophages, mast cells, adipocytes, lymphocytes, neutrophils, and fat cells.

• Embryonic connective tissue: mesenchyme

  • Description: gel-like ground substance with fibers; star-shaped mesenchymal cells.

  • Function: gives rise to all other connective tissue types.

  • Location: primarily in the embryo.

• Connective tissue proper: six types total, organized into loose vs dense subclasses.

  • Loose connective tissue (areolar, adipose, reticular)

  • Dense connective tissue (regular, irregular, elastic)

  • Cells: Fibroblasts, fibrocytes, defense cells, adipocytes.

  • Matrix: gel-like ground substance; all three fiber types (collagen, reticular, elastic).

  • General features: binds tissues together; resists mechanical stress (tension); reservoir for water and salts; energy storage (fat).

• Areolar connective tissue (loose)

  • Description: gel-like matrix with all three fiber types; cells include fibroblasts, macrophages, mast cells, white blood cells.

  • Function: wraps and cushions organs; macrophages phagocytize bacteria; important in inflammation; contains and transports tissue fluid.

  • Location: widely distributed under epithelia (lamina propria of mucous membranes); surrounds organs; around capillaries.

• Adipose connective tissue (loose)

  • Description: matrix similar to areolar but with very sparse matrix; adipocytes with nuclei pushed to the side by large fat droplets.

  • Function: energy storage (fat), insulation, protection of organs.

  • Location: under skin; around kidneys and eyes; within abdomen; in breasts.

• Reticular connective tissue (loose)

  • Description: loose network of reticular fibers with reticular cells on the network.

  • Function: forms internal skeleton (stroma) to support white blood cells, mast cells, macrophages.

  • Location: lymphoid organs (lymph nodes, bone marrow, spleen).

• Dense regular connective tissue

  • Description: primarily parallel collagen fibers with a few elastic fibers; major cell type is the fibroblast.

  • Function: attaches muscles to bones or other muscles; attaches bones to bones; withstands great tensile stress in one direction.

  • Location: tendons, most ligaments, aponeuroses.

• Dense irregular connective tissue

  • Description: primarily irregularly arranged collagen fibers with some elastic fibers; fibroblasts predominant.

  • Function: resists tension in multiple directions; provides structural strength.

  • Location: fibrous capsules of organs and joints; dermis of the skin; submucosa of digestive tract.

• Elastic connective tissue

  • Description: dense regular connective tissue with a high proportion of elastic fibers.

  • Function: allows tissue recoil after stretching; maintains pulsatile flow of blood through arteries; aids passive recoil of lungs after inspiration.

  • Location: walls of large arteries (e.g., aorta); some ligaments; walls of bronchial tubes.

• Cartilage (hyaline, elastic, fibrocartilage)

  • General features: gel-like ground substance; fibers (collagen, elastic in some) in matrix; chondroblasts produce matrix; chondrocytes reside in lacunae; avascular and lacks nerves.

  • Hyaline cartilage

  • Description: amorphous but firm matrix; collagen fibers form an imperceptible network; chondroblasts produce matrix; mature chondrocytes sit in lacunae.

  • Function: cushions and supports; resists compressive stress.

  • Location: forms most of the embryonic skeleton; covers ends of long bones in joints; forms costal cartilages of ribs; cartilages of nose, trachea, and larynx.

  • Notable fact: up to 80 ext{\%} water in the matrix.

  • Elastic cartilage

  • Description: hyaline-like but with more elastic fibers in the matrix.

  • Function: maintains shape while allowing great flexibility.

  • Location: external ear (pinna); epiglottis.

  • Fibrocartilage

  • Description: matrix similar to hyaline but with thick collagen fibers.

  • Function: tensile strength; absorbs compressive shock.

  • Location: intervertebral discs; pubic symphysis; discs of knee joint.

• Bone tissue (osseous tissue)

  • Subclasses: compact bone and spongy bone

  • Cells: osteoblasts (build), osteocytes (in lacunae within bone matrix).

  • Matrix: gel-like ground substance mineralized with inorganic salts; fibers are collagen.

  • General features: hard tissue that resists compression and tension; supports and protects; stores minerals and fat; marrow inside bones is a site for hematopoiesis.

  • Location: bones throughout the skeleton.

• Blood (a fluid connective tissue)

  • Components: plasma (fluid matrix), erythrocytes (RBCs), leukocytes (WBCs), platelets.

  • General features: atypical connective tissue; does not connect tissues or provide a typical structural support.

  • Function: transports O2, CO2, nutrients, wastes, and other substances (e.g., hormones).

  • Location: contained within blood vessels.

4.4 Muscle Tissue is Responsible for Body Movement

• Muscle tissue is responsible for most body movements and includes three types:
  1) Skeletal muscle tissue
  2) Cardiac muscle tissue
  3) Smooth muscle tissue

• Skeletal muscle tissue

  • Description: long, cylindrical, multinucleate cells with obvious striations.

  • Function: voluntary movement; locomotion; manipulation of the environment; facial expression; under voluntary control.

  • Location: in skeletal muscles attached to bones or sometimes to skin.

• Cardiac muscle tissue

  • Description: branched, striated, generally uninucleate cells that interdigitate at specialized junctions called intercalated discs.

  • Function: contraction pumps blood through the heart and into the circulation; involuntary control.

  • Location: walls of the heart.

• Smooth muscle tissue

  • Description: spindle-shaped (elongated) cells with central nuclei; no striations; cells arranged in sheets.

  • Function: propels substances or objects (foodstuffs, urine, a baby) along internal passageways; involuntary control.

  • Location: walls of hollow organs (e.g., digestive, urinary, vascular systems).

4.5 Nervous Tissue is a Specialized Tissue of the Nervous System

• Nervous tissue is the main component of the nervous system (brain, spinal cord, nerves) and regulates and controls body functions.

• Two cell types:

  • Neurons: specialized nerve cells that generate and conduct nerve impulses.

  • Supporting (glial) cells: support, insulate, and protect neurons.

• Neurons: description and arrangement

  • Description: neurons are branching cells with processes extending from the cell body; the cell body contains the nucleus; processes include axons and dendrites.

  • Function: transmit electrical signals from sensory receptors to effectors (muscles and glands).

  • Location: brain, spinal cord, and nerves.

• Supporting cells: provide structural and metabolic support to neurons.

• Visual cues in the slides: nervous tissue is shown with neuron cell bodies, axons, dendrites, and supporting cell nuclei (photomicrographs).

Connections to foundational principles and real-world relevance

• Tissue preparation (4.1) underpins accurate microscopic analysis used in research and medical diagnosis. Understanding fixation, sectioning, and staining helps interpret tissue structure and pathology.

• Epithelial tissues (4.2) form barriers and interfaces essential for protection, absorption, and secretion. Their avascular nature explains the reliance on underlying connective tissue for nutrients and highlights why damage to the basement membrane can disrupt tissue integrity.

• The epithelial classification system (shape and layers) provides a framework for predicting function based on structure (e.g., simple squamous for diffusion, stratified squamous for protection).

• Connective tissues (4.3) show great diversity and specialization, from soft areolar tissue that cushions organs to rigid bone that supports the body and marrow for blood formation. The matrix components (ground substance and fibers) determine tissue mechanics (titting, cushioning, movement, and resilience).

• Cartilage’s avascularity and high water content (e.g., up to 80 ext{\%} water in hyaline cartilage) explain its durability and limited capacity for repair, highlighting clinical considerations in joint and growth plate injuries.

• Muscle tissues (4.4) illustrate how tissue specialization aligns with function: skeletal for voluntary movement, cardiac for pumping blood, and smooth for slow, involuntary propulsion in hollow organs.

• Nervous tissue (4.5) integrates signaling networks, enabling rapid communication across the body; neurons transmit impulses while glial cells provide essential support, protection, and metabolic maintenance.

Practical and ethical considerations

• Proper tissue handling and staining are critical for accurate histological assessment; poor preparation can obscure diagnostic features.

• Understanding tissue types aids in interpreting pathology (e.g., epithelial degeneration, connective tissue fibrosis, or muscle atrophy) and informs treatment strategies.

• Knowledge of tissue vascularity and nerve supply helps in planning surgical approaches and predicting healing potential, particularly in cartilage and dense connective tissues.

Key terms and quick reference

• Epithelium, epithelia, basement membrane, goblet cell, avascular, glandular epithelium

• Simple vs stratified, squamous vs cuboidal vs columnar, pseudostratified, transitional

• Connective tissue proper, cartilage, bone, blood

• Ground substance, extracellular matrix, collagen fibers, elastic fibers, reticular fibers

• Areolar, adipose, reticular, dense regular, dense irregular, elastic

• Hyaline cartilage (up to 80 ext{\%} water), elastic cartilage, fibrocartilage

• Bone: compact vs spongy; osteoblasts, osteocytes

• Blood: plasma, erythrocytes, leukocytes, platelets

• Muscle: skeletal, cardiac, smooth

• Nervous tissue: neurons, glial cells