Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells similar in structure that perform common or related functions, maintaining homeostasis in the body. The study of tissues is called histology. There are four basic tissue types: epithelial, connective, muscle, and nervous tissue.

• Epithelial tissue: Covers body surfaces and lines cavities.

• Connective tissue: Supports, protects, and binds other tissues together.

• Muscle tissue: Contracts to produce movement.

• Nervous tissue: Initiates and transmits electrical impulses for communication.

4.1 Tissue Preparation for Microscopy

Steps in Tissue Preparation

To study tissues under a microscope, samples must be:

• Fixed: Preserved with a solvent to prevent decay.

• Sectioned: Cut into thin slices to allow light or electrons to pass through.

• Stained: Treated with dyes to enhance contrast. Light microscopy uses colored dyes, while electron microscopy uses heavy metal salts.

Two main types of electron microscopy:

• Transmission Electron Microscopy (TEM): Shows internal sections.

• Scanning Electron Microscopy (SEM): Shows surface details.

4.2 Epithelial Tissue

Definition and Functions

Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines body cavities. It has two main forms:

• Covering and lining epithelium: Forms the outer layer of skin and lines open cavities and organs.

• Glandular epithelium: Forms glands that secrete substances (e.g., salivary glands).

Main functions include protection, absorption, filtration, excretion, secretion, and sensory reception.

Special Characteristics of Epithelial Tissues

• Polarity: Cells have an apical (top) surface and a basal (bottom) surface. The apical surface may have microvilli for absorption, while the basal surface attaches to the basal lamina.

• Specialized contacts: Cells are tightly joined by junctions such as tight junctions and desmosomes.

• Supported by connective tissue: The basement membrane (basal and reticular lamina) reinforces the epithelium.

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

• Regeneration: High capacity for renewal, especially in areas exposed to friction or damage.

Classification of Epithelial Tissue

Epithelia are classified by the number of cell layers and cell shape:

• Number of layers:

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

  • Stratified: Two or more layers (for protection in high-abrasion areas).

• Cell shape:

  • Squamous: Flattened, scale-like.

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

  • Columnar: Tall, column-shaped.

Types of Epithelial Tissue

• Simple Squamous Epithelium: Single layer of flat cells; allows diffusion and filtration. Found in air sacs of lungs, kidney glomeruli, lining of heart and blood vessels.

• Simple Cuboidal Epithelium: Single layer of cube-shaped cells; secretion and absorption. Found in kidney tubules, ducts, and small glands.

• Simple Columnar Epithelium: Single layer of tall cells; absorption and secretion of mucus and enzymes. May have microvilli or cilia. Found in digestive tract, gallbladder, and some ducts.

• Pseudostratified Columnar Epithelium: Appears multilayered but is a single layer; often ciliated. Secretes and moves mucus. Found in upper respiratory tract and large gland ducts.

• Stratified Squamous Epithelium: Multiple layers; protects underlying tissues. Keratinized type forms skin; nonkeratinized type lines moist cavities (mouth, esophagus).

• Transitional Epithelium: Resembles both stratified squamous and cuboidal; stretches to allow filling of urinary organs (bladder, ureters).

Glandular Epithelia

A gland consists of one or more cells that secrete an aqueous fluid (secretion). Glands are classified by:

• Site of product release: Endocrine (ductless, hormones) or exocrine (ducts, non-hormonal products).

• Number of cells: Unicellular (e.g., goblet cells) or multicellular (e.g., salivary glands).

• Unicellular Exocrine Glands: Mainly goblet and mucous cells; secrete mucin, which forms mucus for protection and lubrication.

• Multicellular Exocrine Glands: Composed of a duct and secretory unit; classified by structure (simple/compound, tubular/alveolar/tubuloalveolar) and mode of secretion (merocrine, holocrine, apocrine).

• Modes of Secretion:

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

  • Holocrine: Accumulate products until cell ruptures (e.g., sebaceous glands).

  • Apocrine: Apex of cell pinches off (controversial in humans; mammary glands are closest example).

  

4.3 Connective Tissue

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue type. Its major functions include binding and support, protection, insulation, energy storage, and transportation of substances (e.g., blood).

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

Common Characteristics of Connective Tissue

• Extracellular matrix: Nonliving material (ground substance and fibers) separates cells, allowing tissue to bear weight and withstand tension.

• Common origin: All connective tissues arise from embryonic mesenchyme.

Structural Elements of Connective Tissue

• Ground substance: Unstructured material filling space between cells; composed of interstitial fluid, cell adhesion proteins, and proteoglycans (e.g., chondroitin sulfate, hyaluronic acid).

• Fibers: Collagen (strength), elastic (stretch and recoil), reticular (supportive networks).

• Cells: Immature "-blast" cells (secrete matrix) and mature "-cyte" cells (maintain matrix). Other cells include adipocytes, leukocytes, mast cells, and macrophages.

Types of Connective Tissue Proper

• Loose Connective Tissue:

  • Areolar: Supports and binds tissues, holds body fluids, defends against infection.

  • Adipose: Stores energy, insulates, cushions organs.

  • Reticular: Forms a soft internal skeleton (stroma) supporting other cells.

• Dense Connective Tissue:

  • Dense Regular: Parallel collagen fibers; found in tendons and ligaments.

  • Dense Irregular: Irregularly arranged collagen fibers; found in dermis, joint capsules.

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

Cartilage

• Hyaline cartilage: Most abundant; supports and reinforces; found in nose, trachea, larynx, ends of long bones.

• Elastic cartilage: Maintains shape with flexibility; found in ear, epiglottis.

• Fibrocartilage: Strong, resists compression; found in intervertebral discs, knee.

Bone (Osseous Tissue)

• Supports and protects, stores fat, synthesizes blood cells.

• Matrix is hard, with collagen fibers and calcium salts.

• Osteoblasts produce matrix; osteocytes maintain it.

• Highly vascularized.

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Blood

• Most atypical connective tissue; consists of cells in a fluid matrix (plasma).

• Transports gases, nutrients, wastes, and other substances.

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4.4 Muscle Tissue

Types and Functions

Muscle tissue is highly vascularized and responsible for movement. Muscle cells contain myofilaments (actin and myosin) for contraction. There are three types:

• Skeletal muscle: Voluntary, striated, multinucleated; attached to bones for movement.

• Cardiac muscle: Involuntary, striated, one nucleus per cell, intercalated discs; found only in the heart.

• Smooth muscle: Involuntary, non-striated, spindle-shaped cells; found in walls of hollow organs (except heart).

4.5 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. Two main cell types:

• Neurons: Respond to stimuli and transmit electrical signals via dendrites and axons.

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

4.6 Membranes

Types of Membranes

Membranes are organs composed of at least two tissue types. Three main types:

• Cutaneous membrane: Skin; keratinized stratified squamous epithelium attached to connective tissue (dermis); dry membrane.

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

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

4.7 Tissue Repair

Steps in Tissue Repair

When tissues are damaged, repair occurs via two main processes:

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

• Fibrosis: Replacement with scar tissue (dense connective tissue), resulting in loss of function.

1. Inflammation: Release of chemicals, dilation of blood vessels, increased permeability, clotting.

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

3. Regeneration and Fibrosis: Scab detaches, epithelium thickens, scar tissue forms.

Regenerative Capacity of Tissues

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

• Moderate: Smooth muscle, dense regular connective tissue.

• Low/none: Cardiac muscle, nervous tissue in brain and spinal cord.

Developmental Aspects of Tissues

Embryonic Germ Layers

All tissues derive from three primary germ layers formed early in embryonic development:

• Ectoderm: Forms nerve tissue.

• Mesoderm: Forms muscle and connective tissues.

• Endoderm: Contributes to epithelial tissues.

Tissues function well through youth and middle age, but with aging, epithelia thin, repair is less efficient, and risk of cancer increases due to DNA mutations.