Tissue: The Living Fabric

Introduction to Tissues

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

Epithelial Tissue

Overview and Functions

Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines body cavities. It exists in two main forms: covering and lining epithelia (e.g., skin, lining of organs) and glandular epithelia (secretory tissue in glands). The main functions of epithelial tissue include:

• Protection

• Absorption

• Filtration

• Excretion

• Secretion

• Sensory reception

Special Characteristics of Epithelial Tissues

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

• Specialized contacts: Cells are closely packed and bound by tight junctions and desmosomes.

• Supported by connective tissues: 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 hostile environments.

Classification of Epithelia

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

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

• Stratified epithelia: Two or more layers (for protection)

Types of Simple Epithelia

• Simple squamous epithelium: Single layer of flat cells; allows rapid diffusion (e.g., alveoli in lungs, kidney glomeruli, endothelium, mesothelium).

• Simple cuboidal epithelium: Single layer of cube-shaped cells; functions in secretion and absorption (e.g., kidney tubules, ducts of glands).

• Simple columnar epithelium: Single layer of tall cells; may have microvilli or cilia; involved in absorption and secretion (e.g., digestive tract, gallbladder, bronchi).

• Pseudostratified columnar epithelium: Appears multilayered but is a single layer; often ciliated; secretes and moves mucus (e.g., upper respiratory tract).

Types of Stratified Epithelia

• Stratified squamous epithelium: Multiple layers; protects against abrasion; keratinized (skin) or nonkeratinized (esophagus, mouth, vagina).

• Stratified cuboidal epithelium: Rare; found in sweat, mammary, and salivary glands; usually two layers thick.

• Stratified columnar epithelium: Rare; found in pharynx, male urethra, and some glandular ducts; only apical layer is columnar.

• Transitional epithelium: Lines urinary organs; cells change shape to allow stretching (e.g., bladder, ureters, urethra).

Glandular Epithelia

Glands are one or more cells that secrete an aqueous fluid (secretion). They are classified by:

• Site of product release: Endocrine (hormones, ductless) or exocrine (onto surfaces, via ducts)

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

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

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

• Modes of secretion: Merocrine (exocytosis), holocrine (cell rupture), apocrine (apex ruptures; controversial in humans).

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 transport of substances (e.g., blood).

• Four main classes: connective tissue proper, cartilage, bone, blood

Common Characteristics of Connective Tissue

• All arise from mesenchyme (embryonic tissue)

• Varying degrees of vascularity (cartilage is avascular, bone is highly vascularized)

• Composed of cells and extracellular matrix (ECM) (protein-sugar mesh)

Structural Elements of Connective Tissue

• Ground substance: Unstructured material filling space between cells; contains interstitial fluid, cell adhesion proteins, proteoglycans, and water.

• Fibers: Collagen (strongest), elastic (stretch and recoil), reticular (short, fine, branched).

• Cells: "Blast" cells (immature, secrete matrix), "cyte" cells (mature, maintain matrix), fat cells, white blood cells, mast cells, macrophages.

Types of Connective Tissue Proper

• Loose connective tissues: Areolar, adipose, reticular

• Dense connective tissues: Dense regular, dense irregular, elastic

Loose Connective Tissues

• Areolar: Supports and binds tissues; holds body fluids; defends against infection; stores nutrients.

• Adipose: Stores energy, insulates, supports, and protects organs.

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

Dense Connective Tissues

• Dense regular: Parallel collagen fibers; withstands tension in one direction (e.g., tendons, ligaments).

• Dense irregular: Irregularly arranged collagen fibers; withstands tension in many directions (e.g., dermis, joint capsules).

• Elastic: Contains high proportion of elastic fibers; allows tissue to recoil (e.g., walls of large arteries).

Types of Cartilage

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

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

• Fibrocartilage: Strong, absorbs compressive shock; found in intervertebral discs, pubic symphysis, knee menisci.

Bone (Osseous Tissue)

Bones support and protect body structures, store fat, and synthesize blood cells. They have a hard, calcified matrix with collagen fibers and are richly vascularized. The main cell types are osteoblasts (produce matrix) and osteocytes (maintain matrix).

Blood

Blood is a fluid connective tissue consisting of cells (red and white blood cells, platelets) suspended in plasma. It functions in the transport of gases, nutrients, wastes, and other substances.

Muscle Tissue

Overview

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, multinucleate; attached to bones.

• Cardiac muscle: Involuntary, striated, usually one nucleus, intercalated discs; found in heart walls.

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

Nervous Tissue

Overview

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

• Neurons: Generate and conduct nerve impulses.

• Glial cells: Support, insulate, and protect neurons.

Covering and Lining Membranes

Types of Membranes

• Cutaneous membrane: The skin; keratinized stratified squamous epithelium attached to connective tissue; dry membrane.

• Mucous membranes (mucosae): Line body cavities open to the exterior; moist; may secrete mucus; epithelium over lamina propria.

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

Tissue Repair

Overview

When tissues are injured, the body initiates inflammatory and immune responses. Repair occurs by:

• Regeneration: Replacement with the same tissue type; restores function.

• Fibrosis: Replacement with connective tissue (scar); function may be lost.

Steps in Tissue Repair

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 removed.

3. Regeneration and fibrosis: Scab detaches; epithelium thickens; scar tissue forms as needed.

Regenerative Capacity of Tissues

• High: 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

Tissues function well through youth and middle age with proper nutrition and minimal injury. With aging, epithelia thin, repair is less efficient, and tissues such as bone, muscle, and nervous tissue atrophy. DNA mutations increase the risk of cancer.