Chapter 4: Tissue – The Living Fabric

4.1: Tissue Samples are Fixed, Sliced, and Stained

Before microscopic examination, tissue samples must be preserved (fixed), cut into thin sections (sliced), and treated with dyes (stained) to highlight cellular structures. This process allows for detailed study of tissue architecture and cell types.

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

Functions of Epithelial Tissue

• Protection: Shields underlying tissues from mechanical and chemical injury.

• Absorption: Uptake of substances, such as nutrients in the digestive tract.

• Filtration: Selective movement of substances, e.g., in kidney tubules.

• Secretion: Production and release of substances like sweat, mucus, and enzymes.

A. Specialized Characteristics of Epithelium

• Polarity: Cells have apical (top) and basal (bottom) surfaces/layers with different structures and functions.

• Specialized Contacts: Cells are tightly joined by junctions (tight junctions, desmosomes).

• Supported by Connective Tissues: The basal surface is attached to a basement membrane derived from connective tissue.

• Avascular but Innervated: No blood vessels, but supplied by nerve fibers.

• Regeneration: High capacity to replace lost or damaged cells by cell division.

B. Classification of Epithelial Tissue (ET) (Covering and Lining)

Epithelia are classified by cell shape and number of layers:

1. Simple Epithelium: Single layer of cells; functions in absorption, secretion, and filtration.

   • Simple squamous: Flat cells; e.g., alveoli of lungs.

   • Simple cuboidal: Cube-shaped; e.g., kidney tubules.

   • Simple columnar: Tall, column-like; e.g., lining of digestive tract.

2. Stratified Epithelium: Multiple layers; protection is the major function.

   • Stratified squamous: Skin, mouth lining.

   • Stratified cuboidal: Rare, found in some glands.

   • Stratified columnar: Rare, found in male urethra.

   • Transitional epithelium: Stretches; found in urinary bladder.

3. Pseudostratified columnar epithelium: Appears layered but all cells touch the basement membrane; found in respiratory tract.

C. Glandular Epithelium (figure 4.5)

• Endocrine glands: Ductless; secrete hormones directly into blood.

• Exocrine glands: Secrete products into ducts (e.g., sweat, saliva).

• Modes of Secretion:

  • Merocrine: Secrete by exocytosis (e.g., sweat glands).

  • Apocrine: Apex of cell pinches off (e.g., mammary glands).

  • Holocrine: Entire cell ruptures (e.g., sebaceous glands).

4.3: Connective Tissue is the Most Abundant

A. Common Characteristics of Connective Tissue (CT)

• Extracellular matrix: Non-living material between cells; provides support.

• Common origin: All CT arises from mesenchyme (embryonic tissue).

B. Structural Components of CT

• Ground substance: Unstructured material filling space between cells; contains interstitial fluid, cell adhesion proteins (e.g., proteoglycans).

• CT fibers:

  • Collagen fibers: Strong, most abundant; made of collagen protein.

  • Reticular fibers: Short, fine, highly branched; continuous with collagen fibers.

  • Elastic fibers: Contain elastin; allow stretch and recoil.

• CT cells:

  • Fibroblasts/cytes: In CT proper.

  • Chondroblasts/cytes: In cartilage.

  • Osteoblasts/cytes: In bone.

  • Blood cells: Different types for immune response and transport.

  • Other cells: Adipocytes (fat storage), white blood cells (WBC), mast cells (inflammation), macrophages (phagocytosis).

C. Types of Connective Tissue (see Table 4.1; figure 4.10)

4.4: Muscle Tissue is Responsible for Body Movement

• Excitability: Responds to stimuli by generating electrical signals.

• Contractility: Able to shorten and generate force.

• Highly cellular and well vascularized.

• Responsible for most types of body movement.

• Myofilaments: Actin and myosin proteins enable contraction.

• Three types: Skeletal (voluntary, striated), cardiac (involuntary, striated, heart), smooth (involuntary, non-striated, walls of hollow organs).

A. Skeletal Muscle

• Long, cylindrical, multinucleate cells; obvious striations.

• Voluntary movement; locomotion.

B. Cardiac Muscle

• Branching, striated, generally uninucleate cells; intercalated discs.

• Involuntary; propels blood in the heart.

C. Smooth Muscle

• Spindle-shaped cells with central nuclei; no striations.

• Involuntary; moves substances along internal passageways.

4.5: Nervous Tissue is Specialized Tissue of the Nervous System (NS)

• 98% in brain and spinal cord.

• Two cell types:

  • Neurons: Generate and conduct nerve impulses.

  • Supporting cells (neuroglia): Support, insulate, and protect neurons.

4.6: The Cutaneous Membrane is Dry; Mucous & Serous Membranes are Wet (figure 4.14)

• Cutaneous membrane: Skin; composed of epithelial and connective tissue layers; dry membrane.

• Mucous membranes: Line body cavities open to the exterior (e.g., digestive, respiratory tracts); moist; epithelial sheet over connective tissue (lamina propria).

• Serous membranes: Line closed ventral body cavities; consist of parietal and visceral layers with serous fluid between.

4.7: Tissue Repair Involves Inflammation, Organization, and Regeneration

• Cell damage and death: Changes the composition of interstitial fluid, leading to necrosis, pus, and abscesses.

A. Steps in Tissue Repair (figure 4.15)

• Regeneration: Replacement of destroyed tissue with the same kind of tissue.

• Fibrosis: Replacement with scar tissue (dense connective tissue).

B. Regenerative Capacity of Different Tissues

• Regenerate well: Epithelial tissue, areolar CT, dense irregular CT, bone, blood-forming tissue.

• Regenerate moderately: Smooth muscle, dense regular CT.

• Regenerate weakly: Skeletal muscle, cartilage.

• Virtually no regeneration: Cardiac muscle, nervous tissue of brain and spinal cord.