Chapter 4: Tissue—The Living Fabric (A&P Study Notes)

Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells similar in structure that perform common or related functions, maintaining the body's homeostasis. The study of tissues is called histology. There are four primary tissue types in the human body, each with specialized roles.

• Epithelial tissue: Covers surfaces, lines cavities, forms glands.

• Connective tissue: Supports, protects, stores energy, and provides immunity.

• Muscle tissue: Produces movement.

• Nervous tissue: Generates signals to coordinate body activities.

Epithelial Tissue

Overview and Functions

Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines body cavities. It is involved in protection, absorption, filtration, excretion, secretion, and sensory reception.

• Covering and lining epithelium: Outer layer of skin, lines open cavities (urogenital, digestive, respiratory), covers organs in closed ventral body cavities.

• Glandular epithelium: Secretory tissue in glands (e.g., salivary glands).

Five Characteristics of Epithelial Tissues

1. Polarity: Cells have an apical (top) and basal (bottom) surface, each with distinct structures and functions.

2. Specialized contacts: Cells are tightly joined by junctions (tight junctions and desmosomes).

3. Supported by connective tissues: All epithelial sheets rest on a basement membrane (basal lamina + reticular lamina).

4. Avascular but innervated: No blood vessels, but supplied by nerves; nutrients diffuse from underlying tissues.

5. Regeneration: High capacity for renewal if nutrients are available.

Surface Specializations

• Microvilli: Increase surface area for absorption/secretion (e.g., intestinal lining).

• Cilia: Move substances along the surface (e.g., trachea).

Classification of Epithelia

• By layers:

  • Simple: Single layer (absorption, secretion, filtration).

  • Stratified: Multiple layers (protection).

• By cell shape:

  • Squamous: Flattened, scale-like.

  • Cuboidal: Cube-shaped.

  • Columnar: Tall, column-like.

Types of Epithelia

• Simple squamous: Single layer, rapid diffusion (kidney, lungs). Special types:

  • Endothelium: Lining of blood vessels, heart, lymphatics.

  • Mesothelium: Serous membranes in ventral body cavity.

• Simple cuboidal: Secretion and absorption (kidney tubules, ducts).

• Simple columnar: Absorption, secretion; may have microvilli, cilia, goblet cells (digestive tract, bronchi, uterine tubes).

• Pseudostratified columnar: Appears stratified but is single-layered; often ciliated, with goblet cells (respiratory tract).

• Stratified squamous: Multiple layers; protection (skin, mouth, esophagus). Keratinized (skin) or nonkeratinized (moist linings).

• Stratified cuboidal: Rare; sweat and mammary glands.

• Stratified columnar: Rare; pharynx, male urethra, glandular ducts.

• Transitional: Stretches; found in urinary bladder, ureters, urethra.

Glandular Epithelia

• Gland: One or more cells that secrete an aqueous fluid (secretion).

• Endocrine glands: Ductless, secrete hormones into interstitial fluid/blood.

• Exocrine glands: Secrete onto body surfaces or into cavities via ducts (sweat, oil, salivary glands).

• Unicellular exocrine glands: Goblet and mucous cells (produce mucin).

• Multicellular exocrine glands: Duct + secretory unit; classified by structure (simple/compound, tubular/alveolar/tubuloalveolar) and mode of secretion (merocrine, holocrine, apocrine).

Connective Tissue

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue type. It binds, supports, protects, insulates, stores energy, and transports substances.

• Four main classes: Connective tissue proper, cartilage, bone, blood.

Characteristics of Connective Tissue

• Common embryonic origin: All arise from mesenchyme.

• Degrees of vascularity: From avascular (cartilage) to highly vascular (bone).

• Extracellular matrix: Nonliving matrix separates living cells, allowing tissue to bear weight and withstand tension.

Structural Components

• Ground substance: Unstructured material (interstitial fluid, cell adhesion proteins, proteoglycans with glycosaminoglycans).

• Fibers:

  • Collagen: Strongest, high tensile strength.

  • Elastic: Stretch and recoil.

  • Reticular: Short, fine, branched; support soft tissues.

• Cells:

  • "Blast" cells: Immature, secrete matrix (fibroblasts, chondroblasts, osteoblasts).

  • "Cyte" cells: Mature, maintain matrix (fibrocytes, chondrocytes, osteocytes).

  • Other cells: Fat cells (store nutrients), white blood cells (immunity), mast cells (inflammation), macrophages (phagocytosis).

Types of Connective Tissue Proper

• Loose connective tissue:

  • Areolar: Supports, binds, holds fluids, defends against infection; widely distributed.

  • Adipose: Stores energy, insulates, shock absorption; white and brown fat.

  • Reticular: Supports blood cells in lymph nodes, spleen, bone marrow.

• Dense connective tissue:

  • Dense regular: Parallel collagen fibers; tendons, ligaments.

  • Dense irregular: Irregularly arranged collagen; dermis, joint capsules.

  • Elastic: High proportion of elastic fibers; walls of large arteries, some ligaments.

Cartilage

• Chondroblasts: Produce matrix in growing cartilage.

• Chondrocytes: Mature cells in lacunae.

• Features: 80% water, avascular, tough yet flexible, surrounded by perichondrium.

• Types:

  • Hyaline: Most abundant; articular surfaces, nose, trachea, embryonic skeleton.

  • Elastic: More elastic fibers; ear, epiglottis.

  • Fibrocartilage: Thick collagen; intervertebral discs, knee menisci.

Bone (Osseous Tissue)

• Functions: Supports, protects, stores fat, synthesizes blood cells.

• Structure: Osteoblasts (produce matrix), osteocytes (maintain matrix), osteons (structural units), richly vascularized.

• Matrix: Collagen fibers, inorganic calcium salts.

Blood

• Most atypical connective tissue: Fluid matrix (plasma).

• Cells: Red blood cells (erythrocytes), white blood cells (leukocytes), platelets.

• Functions: Transport of nutrients, wastes, gases, and other substances.

Muscle Tissue

Overview and Types

Muscle tissue is highly vascularized and responsible for movement. Muscle cells contain myofilaments (actin and myosin) for contraction.

• Skeletal muscle: Attached to bones, voluntary, striated, multinucleate.

• Cardiac muscle: Heart walls, involuntary, striated, one nucleus, intercalated discs.

• Smooth muscle: Walls of hollow organs, involuntary, non-striated, spindle-shaped, 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.

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

Covering and Lining Membranes

Types of Membranes

• Cutaneous membrane: Skin; keratinized stratified squamous epithelium + dense irregular connective tissue; dry membrane.

• Mucous membranes (mucosae): Line body cavities open to exterior (digestive, respiratory, urogenital); moist, may secrete mucus; epithelium + lamina propria.

• Serous membranes (serosae): Line closed ventral body cavities; simple squamous epithelium (mesothelium) + areolar connective tissue; secrete serous fluid. Special names: pleurae (lungs), pericardium (heart), peritoneum (abdomen).

Tissue Repair

Overview

When tissues are damaged, repair occurs via regeneration (replacement with same tissue) or fibrosis (replacement with connective tissue, forming scar tissue). The process involves inflammation, organization, and permanent repair.

1. Inflammation: Release of chemicals, dilation and increased permeability of blood vessels, clot formation.

2. Organization: Blood clot replaced by granulation tissue, epithelium regenerates, fibroblasts produce collagen, debris removed by macrophages.

3. Regeneration/Fibrosis: Scab detaches, tissue matures, epithelium thickens, scar tissue may remain.

Regenerative Capacity of Tissues

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

• Moderate: Smooth muscle, dense regular CT.

• Weak: Skeletal muscle, cartilage.

• None (functional): Cardiac muscle, nervous tissue in brain/spinal cord (replaced by scar tissue).

Clinical Relevance

• Scar tissue can impair organ function, block substances, interfere with muscle contraction or nerve transmission, and cause adhesions.

• Excessive scarring may lead to organ failure, especially in the heart.

Aging and Tissues

• Tissue function declines with age; epithelia thin, repair is less efficient, and tissues atrophy.

• Increased risk of DNA mutations and cancer.