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: Covers body surfaces and lines cavities.

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

• Muscle tissue: Responsible for movement.

• Nervous tissue: Enables internal communication by transmitting electrical impulses.

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). Its 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, each with distinct structures and functions.

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

• Number of layers: Simple (one layer) or stratified (multiple layers).

• Cell shape: Squamous (flat), cuboidal (cube-like), columnar (tall).

Types of Simple Epithelia

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

• 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, uterine tubes).

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

Types of Stratified Epithelia

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

• 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.

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

Glandular Epithelia

Glands are one or more cells that secrete an aqueous fluid called a secretion. They are classified by the site of product release (endocrine or exocrine) and the number of cells (unicellular or multicellular).

• Endocrine glands: Ductless; secrete hormones into interstitial fluid, which enters the blood.

• Exocrine glands: Secrete products into ducts or onto surfaces (e.g., sweat, oil, salivary glands).

Unicellular exocrine glands (e.g., goblet cells) produce mucin, which forms mucus.

Multicellular exocrine glands are classified by duct structure (simple or compound) and secretory unit shape (tubular, alveolar, or tubuloalveolar).

Modes of secretion include merocrine (by exocytosis), holocrine (cell ruptures), and 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). The four main classes are connective tissue proper, cartilage, bone, and blood.

Comparison of Classes of Connective Tissues

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 embedded in an extracellular matrix (ECM) of ground substance and fibers.

Structural Elements of Connective Tissue

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

• Fibers: Collagen (strong), elastic (stretchy), and reticular (supportive networks).

• Cells: "Blast" cells (immature, matrix-secreting) and "cyte" cells (mature, maintain matrix); also includes fat cells, white blood cells, mast cells, and macrophages.

Types of Connective Tissue Proper

Loose Connective Tissues

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

• Adipose: Stores energy, insulates, cushions organs; cells are adipocytes.

• Reticular: Forms a soft internal skeleton (stroma) supporting other cell types (e.g., lymphoid organs).

Dense Connective Tissues

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

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

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

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 and epiglottis.

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

Bone (Osseous Tissue)

Bone supports and protects body structures, stores fat, and synthesizes blood cells. It is richly vascularized and composed of osteoblasts (produce matrix) and osteocytes (maintain matrix) in lacunae. The structural unit is the osteon.

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, cardiac, and smooth muscle.

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

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

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

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 impulses) and glial cells (support, insulate, 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

Tissue repair occurs when barriers are compromised, activating inflammatory and immune responses. Repair can occur by regeneration (same tissue replaces lost tissue) or fibrosis (scar tissue replaces lost tissue).

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, fibrous tissue matures, epithelium thickens, scar tissue may remain.

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.

• Poor/None: Cardiac muscle, nervous tissue in brain and spinal cord.

Developmental Aspects and Clinical Considerations

• Tissues function well through youth and middle age with proper nutrition and minimal injury.

• Aging leads to thinning epithelia, less efficient repair, atrophy of bone, muscle, and nervous tissue, and increased cancer risk due to DNA mutations.

• Scar tissue can impair organ function, cause adhesions, and potentially lead to organ failure, especially in the heart.