Tissues: An Overview

Definition and General Features

Tissues are groups of similar cells that work together to perform a common function. They are supported by an extracellular matrix (ECM), which includes interstitial fluid and various proteins that provide structural and biochemical support.

• Four basic tissue types: Epithelial, Connective, Muscle, and Nervous tissue

• Extracellular matrix: Non-cellular material found between cells, crucial for tissue integrity and function

Epithelial Tissue

General Characteristics

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is specialized for protection, absorption, filtration, excretion, secretion, and sensory reception.

• Polarity: Has an apical (free) surface and a basal surface attached to a basement membrane

• Specialized contacts: Cells are tightly joined by junctions (tight junctions, desmosomes, gap junctions)

• Supported by connective tissue: The basement membrane anchors epithelium to underlying tissues

• Avascular but innervated: Contains no blood vessels but is supplied by nerve fibers

• Regenerative: High capacity for renewal via mitosis

Cell Junctions in Epithelial Tissue

Cell junctions are specialized structures that connect epithelial cells, providing mechanical strength and communication.

• Tight junctions: Seal adjacent cells to prevent passage of molecules

• Adherens junctions: Connect actin filaments between cells

• Desmosomes: Anchor intermediate filaments, providing strong adhesion

• Hemidesmosomes: Attach cells to the basement membrane

• Gap junctions: Allow passage of ions and small molecules between cells

Classification of Epithelial Tissue

Epithelial tissues are classified based on the number of cell layers and the shape of the cells.

• Number of layers:

  • Simple: One layer of cells

  • Stratified: Multiple layers of cells

  • Pseudostratified: Appears layered but all cells touch the basement membrane

• Cell shape:

  • Squamous: Flat and scale-like

  • Cuboidal: Cube-shaped

  • Columnar: Tall and column-like

  • Transitional: Changes shape (found in urinary tract)

Types of Epithelial Tissue

• Simple Squamous Epithelium: Allows rapid exchange of substances; found in alveoli and blood vessels (endothelium, mesothelium)

• Simple Cuboidal Epithelium: Specialized for absorption and secretion; found in kidney tubules and ducts of exocrine glands

• Simple Columnar Epithelium: Absorption and secretion; may be ciliated or nonciliated (e.g., digestive tract)

• Pseudostratified Columnar Epithelium: Appears layered, often ciliated, secretes and moves mucus; lines respiratory tract

• Stratified Squamous Epithelium: Multiple layers for protection; keratinized (skin) or nonkeratinized (esophagus, vagina)

• Stratified Cuboidal Epithelium: Protection and secretion; found in ducts of sweat glands

• Stratified Columnar Epithelium: Protection and secretion; found in male urethra and salivary ducts

• Transitional Epithelium: Stretches to accommodate fluctuation in urinary tract volume

Glandular Epithelium

Glands are specialized epithelial structures that produce and secrete substances. They are classified as endocrine (ductless, secrete hormones into blood) or exocrine (secrete via ducts to surfaces).

• Unicellular glands: Goblet and mucus cells

• Multicellular glands: Classified by structure (simple/compound, tubular/acinar) and mode of secretion (merocrine, apocrine, holocrine)

Connective Tissue

General Characteristics

Connective tissue (CT) supports, binds, and protects other tissues and organs. It is characterized by abundant ECM, which consists of fibers and ground substance, and various cell types.

• Varying vascularity: From avascular (cartilage) to highly vascular (bone)

• Three main components: Cells, fibers, ground substance

Cells of Connective Tissue

• Fibroblasts, chondroblasts, osteoblasts: Build ECM

• Fibroclasts, osteoclasts: Break down ECM

• Cytes: Mature, less active cells (e.g., osteocytes)

• Other cells: Immune cells (macrophages, lymphocytes, mast cells, neutrophils), adipocytes

Fibers and Ground Substance

• Collagen fibers: Strong, resist tension

• Elastic fibers: Stretch and recoil

• Reticular fibers: Form supportive networks

Classification of Connective Tissue

Connective tissue is classified into four main types: connective tissue proper, cartilage, bone, and blood.

• Connective tissue proper: Loose (areolar, adipose, reticular) and dense (regular, irregular, elastic)

• Cartilage: Hyaline, elastic, fibrocartilage (avascular, poor regeneration)

• Bone (osseous tissue): Highly vascular, supports and protects, site of hematopoiesis

• Blood: Fluid matrix (plasma), transports substances

Muscle Tissue

Types and Functions

Muscle tissue is specialized for contraction and movement. There are three types:

• Skeletal muscle: Striated, multinucleate, voluntary; attached to bones, responsible for body movement

• Cardiac muscle: Striated, uninucleate, involuntary; found in heart, contains intercalated discs and gap junctions

• Smooth muscle: Non-striated, uninucleate, involuntary; found in walls of hollow organs (except heart)

Nervous Tissue

Structure and Function

Nervous tissue is specialized for communication via electrical signals. It consists of neurons and supporting cells (neuroglia).

• Neurons: Transmit electrical impulses

• Neuroglia: Support, protect, and nourish neurons

Body Membranes

Types of Membranes

• Mucous membranes (mucosae): Line body cavities open to the exterior (e.g., digestive, respiratory tracts)

• Serous membranes: Line closed body cavities; secrete serous fluid to reduce friction (e.g., pericardium, pleura, peritoneum)

• Cutaneous membrane: The skin; covers the body surface

Tissue Repair

Steps in Tissue Repair

Tissue repair involves a series of steps to restore structure and function after injury:

1. Inflammation: Sets the stage for repair by bringing immune cells to the site

2. Organization: Restores blood supply and forms granulation tissue

3. Regeneration and fibrosis: Permanent repair by replacing damaged tissue with the same tissue (regeneration) or scar tissue (fibrosis)

Additional info: The regenerative capacity of tissues varies: epithelial and connective tissues (except cartilage) regenerate well, muscle tissue regenerates poorly or moderately, and nervous tissue has very limited regenerative ability.