Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells with similar structure and function that form the foundation of organs and systems in the human body. Understanding tissue types is essential for monitoring health and diagnosing conditions such as bedsores and cancer.

• Histology: The study of tissues.

• Four Basic Tissue Types: Epithelial, Connective, Muscle, and Nervous tissue.

Preparation of Tissue Samples for Microscopy

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

• Fixation: Preserves tissue using solvents.

• Sectioning: Slices tissue thinly for light or electron transmission.

• Staining: Uses dyes or heavy metals to highlight structures; may introduce artifacts.

• Microscopy Types:

  • Light microscopy: colored dyes.

  • Transmission electron microscopy (TEM): shows internal sections.

  • Scanning electron microscopy (SEM): shows surface details.

Epithelial Tissue

Definition and Functions

Epithelial tissue forms protective sheets that cover surfaces and line cavities. It is specialized for various functions essential to homeostasis.

• Forms:

  • Covering and lining epithelium: skin, linings of body cavities.

  • Glandular epithelium: forms glands (e.g., salivary glands).

• Main Functions: Protection, absorption, filtration, excretion, secretion, sensory reception.

Special Characteristics of Epithelial Tissue

Epithelial tissues possess unique features that distinguish them from other tissue types.

• Polarity: Cells have distinct apical (top) and basal (bottom) surfaces. The apical surface may have microvilli for absorption; the basal surface attaches to the basal lamina.

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

• Supported by Connective Tissue: The Basement membrane (basal and reticular laminae) reinforces and defines boundaries.

• Avascular but Innervated: No blood vessels; nutrients diffuse from underlying tissues. Supplied by nerve fibers.

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

Classification of Epithelial Tissue

Epithelial tissues are classified by the number of cell layers and cell shape.

• Number of Layers:

  • Simple: single layer (for absorption, filtration).

  • Stratified: multiple layers (for protection).

• Cell Shape:

  • Squamous: flat, scale-like.

  • Cuboidal: cube-shaped.

  • Columnar: tall, column-like.

• In stratified epithelia, cell shape is named according to the apical layer.

Types of Simple Epithelia

Simple Squamous Epithelium

Single layer of flat cells, ideal for rapid diffusion and filtration.

• Locations: Kidney glomeruli, air sacs of lungs, lining of heart, blood vessels, lymphatic vessels.

• Special Types:

  • Endothelium: lines vessels and heart.

  • Mesothelium: lines serous membranes.

Simple Cuboidal Epithelium

Single layer of cube-shaped cells, specialized for secretion and absorption.

• Locations: Kidney tubules, ducts of glands.

Simple Columnar Epithelium

Single layer of tall cells, often with microvilli or cilia, and goblet cells for mucus secretion.

• Functions: Absorption, secretion of mucus and enzymes.

• Locations: Digestive tract, gallbladder, bronchi, uterine tubes.

Connective Tissue

Definition and Functions

Connective tissue is the most abundant tissue type, providing support, protection, insulation, storage, and transport.

• Main Classes: Connective tissue proper, cartilage, bone, blood.

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

Common Characteristics

• Extracellular Matrix: Nonliving material (ground substance and fibers) in which cells are suspended.

• Common Origin: All connective tissues arise from mesenchyme (embryonic tissue).

Structural Elements

• Ground Substance: Gel-like material with interstitial fluid, cell adhesion proteins, and proteoglycans.

• Fibers:

  • Collagen: strong, abundant.

  • Elastic: stretch and recoil.

  • Reticular: branched, supportive.

• Cells:

  • "-blast" cells: immature, secrete matrix.

  • "-cyte" cells: mature, maintain matrix.

  • Other cells: adipocytes, leukocytes, mast cells, macrophages.

Muscle Tissue

Definition and Types

Muscle tissue is specialized for contraction and movement. It is highly vascularized and contains myofilaments (actin and myosin).

• Skeletal Muscle: Voluntary, striated, multinucleated, attached to bones.

• Cardiac Muscle: Involuntary, striated, single nucleus, found in the heart, connected by intercalated discs.

• Smooth Muscle: Involuntary, non-striated, spindle-shaped, single nucleus, found in walls of hollow organs.

Nervous Tissue

Definition and Components

Nervous tissue forms the main component of the nervous system, regulating and controlling body functions.

• Neurons: Specialized cells that respond to stimuli and transmit electrical signals via dendrites and axons.

• Glial Cells (Neuroglia): Support, insulate, and protect neurons.

Membranes

Types of Membranes

Membranes are organs composed of more than one tissue type that cover and line body surfaces.

• Cutaneous Membrane: Skin; dry, keratinized stratified squamous epithelium attached to connective tissue.

• Mucous Membrane: Lines cavities open to the exterior; moist, often secretes mucus.

• Serous Membrane: Lines closed ventral cavities; moist, simple squamous epithelium (mesothelium) on areolar tissue.

Tissue Repair

Steps in Tissue Repair

Tissue repair is activated by inflammation and occurs in three main steps:

1. Inflammation: Release of chemicals, dilation of vessels, clotting.

2. Organization: Blood clot replaced by granulation tissue, regeneration begins, fibroblasts bridge gap.

3. Regeneration and Fibrosis: Scab detaches, tissue matures, scar tissue may form.

Regenerative Capacity

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

• Moderate: Smooth muscle, dense regular connective tissue.

• Low: Cardiac muscle, nervous tissue.

Developmental Aspects of Tissues

Primary Germ Layers

Embryonic development forms three germ layers that specialize into primary tissues:

• Ectoderm: Nerve tissue.

• Mesoderm: Muscle and connective tissue.

• Endoderm: Epithelial tissue arises from all three layers.

Aging and Tissue Function

• Epithelia are thin; repair is less efficient; tissues atrophy; DNA mutations increase cancer risk.