Chapter 4: Tissues – The Living Fabric (Anatomy & Physiology I Study Notes)

Chapter 4: Tissues

The Living Fabric

Tissues are groups of cells similar in structure that perform a common or related function. The study of tissues is called histology. Understanding tissues is essential for comprehending how organs and organ systems function to maintain homeostasis in the body.

• Definition: A tissue is a group of cells that are similar in structure and perform a specific function.

• Four basic tissue types:

  • Epithelial tissue

  • Connective tissue

  • Muscle tissue

  • Nervous tissue

• Application: Each tissue type contributes to the structure and function of organs and organ systems.

Microscopy and Tissue Preparation

Preparing Tissues for Microscopy

To study tissues under a microscope, they must be properly prepared to preserve their structure and enhance visibility.

• Fixed: Tissue is preserved using a solvent to prevent decay.

• Sectioned: Tissue is cut into thin slices to allow light or electrons to pass through.

• Stained: Dyes or heavy metal salts are used to enhance contrast. Staining may introduce artifacts (distortions) not present in living tissue.

• Microscopy types:

  • Light microscopy: Uses colored dyes.

  • Electron microscopy: Uses heavy metal salts for contrast.

    • Transmission electron microscopy (TEM): Shows internal structures.

    • Scanning electron microscopy (SEM): Shows surface features.

Epithelial Tissue

Overview and Functions

Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines body cavities. It serves as a boundary between different environments and performs various functions such as protection, absorption, filtration, excretion, secretion, and sensory reception.

• Forms:

  • Covering and lining epithelium: Forms the outer layer of the skin, lines open cavities, and covers walls and organs of the ventral body cavity.

  • Glandular epithelium: Forms glands of the body (e.g., salivary glands).

• Locations: Lining of urogenital, digestive, and respiratory systems.

Special Characteristics of Epithelial Tissues

Epithelial tissues have five distinguishing characteristics that set them apart from other tissue types:

• Polarity: Cells have an apical (upper, free) surface and a basal (lower, attached) surface. The apical surface may have microvilli or cilia; the basal surface is attached to the underlying connective tissue via the basal lamina.

• Specialized contacts: Epithelial cells fit closely together to form continuous sheets, bound by tight junctions and desmosomes.

• Supported by connective tissue: All epithelial sheets rest upon and are supported by connective tissue (basement membrane, composed of basal and reticular lamina).

• Avascular but innervated: Epithelial tissues contain no blood vessels but are supplied by nerve fibers. They receive nutrients by diffusion from underlying connective tissues.

• Regeneration: Epithelial cells have a high regenerative capacity, especially in areas exposed to friction or hostile environments. Adequate nutrition and cell division are required for regeneration.

Classification of Epithelia

Epithelia are classified based on the number of cell layers and the shape of the cells.

• Number of layers:

  • Simple epithelia: Single cell layer; found where absorption, secretion, and filtration occur.

  • Stratified epithelia: Two or more layers; found in high-abrasion areas (e.g., skin, mouth lining).

• Cell shape:

  • Squamous: Flattened and scale-like.

  • Cuboidal: Box-like, as tall as they are wide.

  • Columnar: Tall and column-shaped.

  Note: In stratified epithelia, cell shape is named according to the shape of the apical layer.

Types of Epithelial Tissue

• Simple Squamous Epithelium: Single layer of flat cells; allows rapid diffusion and filtration. Locations: Air sacs of lungs, lining of heart, blood vessels, and lymphatic vessels.

• Simple Cuboidal Epithelium: Single layer of cube-like cells; functions in secretion and absorption. Locations: Kidney tubules, ducts, and secretory portions of small glands.

• Simple Columnar Epithelium: Single layer of tall cells; may have microvilli or cilia. Functions: Absorption, secretion of mucus and enzymes. Locations: Digestive tract, gallbladder, uterine tubes.

• Pseudostratified Columnar Epithelium: Appears stratified but is a single layer; often ciliated. Functions: Secretion, particularly of mucus; propulsion of mucus by cilia. Locations: Upper respiratory tract, ducts of large glands.

• Stratified Squamous Epithelium: Several layers; basal cells are cuboidal/columnar, surface cells are squamous. Function: Protection. Locations: Skin (keratinized), mouth, esophagus, vagina (non-keratinized).

• Stratified Cuboidal and Columnar Epithelium: Rare; found in some sweat and mammary glands (cuboidal), pharynx, male urethra (columnar).

• Transitional Epithelium: Resembles both stratified squamous and cuboidal; stretches readily. Locations: Urinary bladder, ureters, part of urethra.

Glandular Epithelia

Glands are one or more cells that make and secrete an aqueous fluid called a secretion.

• Endocrine glands: Ductless; secrete hormones into interstitial fluid, which is picked up by the circulatory system. Example: Thyroid gland.

• Exocrine glands: Secrete products into ducts that lead to body surfaces or cavities. Examples: Sweat, oil, and salivary glands.

• Unicellular exocrine glands: Mucous and goblet cells; secrete mucin, which forms mucus.

• Multicellular exocrine glands: Composed of a duct and a secretory unit, supported by connective tissue. Classified by duct structure (simple or compound) and secretory unit shape (tubular, alveolar, or tubuloalveolar).

• Modes of secretion:

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

  • Holocrine: Accumulate products until they rupture (e.g., sebaceous glands).

  • Apocrine: Accumulate products, then the apex of the cell pinches off (controversial in humans; mammary glands may use this method).

Connective Tissue

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue in the body. It supports, protects, insulates, stores fuel, and transports substances.

• Main classes:

  • Connective tissue proper (loose and dense)

  • Cartilage

  • Bone (osseous tissue)

  • Blood

• Functions: Binding and support, protection, insulation, energy storage, and transport.

Structural Elements of Connective Tissue

• Ground substance: Unstructured material that fills the space between cells; contains interstitial fluid, adhesion proteins, and proteoglycans (e.g., chondroitin sulfate, hyaluronic acid).

• Fibers:

  • Collagen fibers: Strongest and most abundant; provide high tensile strength.

  • Elastic fibers: Long, thin fibers that allow for stretch and recoil.

  • Reticular fibers: Short, fine, highly branched fibers that form networks for support.

• Cells:

  • "-blast" cells: Immature, actively secrete matrix (e.g., fibroblasts, chondroblasts, osteoblasts).

  • "-cyte" cells: Mature, maintain the matrix (e.g., fibrocytes, chondrocytes, osteocytes).

  • Other cells: Adipocytes (fat storage), leukocytes (immune response), mast cells (inflammation), macrophages (phagocytosis).

Types of Connective Tissue

• Connective Tissue Proper:

  • Loose connective tissue: Areolar, adipose, reticular.

  • Dense connective tissue: Dense regular, dense irregular, elastic.

• Cartilage: Hyaline, elastic, fibrocartilage.

• Bone (Osseous Tissue): Compact and spongy bone.

• Blood: Atypical connective tissue; consists of blood cells in plasma.

Comparison of Connective Tissue Classes

Type	Cells	Matrix	General Features
Loose connective tissue	Fibroblasts, defense cells, adipocytes	Gel-like ground substance, all three fiber types	Functions as a binding tissue; resists tension
Dense connective tissue	Fibroblasts	Primarily collagen fibers	Resists tension; found in tendons and ligaments
Cartilage	Chondroblasts, chondrocytes	Gel-like ground substance, collagen, elastic fibers	Resists compression; supports body structures
Bone	Osteoblasts, osteocytes	Hard, calcified matrix with collagen fibers	Supports and protects; stores calcium
Blood	Red and white blood cells, platelets	Plasma (fluid matrix)	Transports nutrients, wastes, and other substances

Muscle Tissue

Overview and Types

Muscle tissue is highly vascularized and responsible for most types of body movement. Muscle cells contain myofilaments (actin and myosin) that enable contraction.

• Skeletal muscle: Attached to bones; voluntary control; striated; multinucleate.

• Cardiac muscle: Found only in the heart; involuntary; striated; branched cells connected by intercalated discs.

• Smooth muscle: Found in walls of hollow organs (except heart); involuntary; non-striated; spindle-shaped cells with one nucleus.

Nervous Tissue

Structure and Function

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

• Neurons: Specialized cells that generate and conduct nerve impulses. They have dendrites (receive signals) and axons (transmit signals).

• Neuroglia (glial cells): Support, insulate, and protect neurons.

Body Membranes

Types of Membranes

• Cutaneous membrane: The skin; composed of keratinized stratified squamous epithelium (epidermis) attached to connective tissue (dermis); dry membrane.

• Mucous membranes (mucosae): Line body cavities open to the exterior (digestive, respiratory, urogenital tracts); moist membranes bathed by secretions; epithelial sheet over connective tissue (lamina propria).

• Serous membranes (serosae): Line closed ventral body cavities; composed of simple squamous epithelium (mesothelium) on areolar connective tissue; secrete serous fluid for lubrication. Examples: Pleura (lungs), pericardium (heart), peritoneum (abdominopelvic cavity).

Tissue Repair

Process of Tissue Repair

When body barriers are compromised, the inflammatory and immune responses are activated to repair tissue. Tissue repair occurs in two major ways:

• Regeneration: Replacement of destroyed tissue with the same kind of tissue, restoring original function.

• Fibrosis: Replacement with scar tissue (dense connective tissue), which may impair function.

The repair process involves inflammation, organization (restoration of blood supply), and regeneration/fibrosis.

• Tissues with high regenerative capacity: Epithelial tissues, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissue.

• Moderate capacity: Smooth muscle, dense regular connective tissue.

• Little or no capacity: Cardiac muscle, nervous tissue in brain and spinal cord.