Tissues: Structure, Function, and Classification in Anatomy & Physiology

Tissues

Overview of Tissues

Tissues are groups of cells with specific functions, forming the structural and functional foundation of organs. Each organ contains unique tissue types, which differ histologically and functionally.

• Definition: A tissue is a group of similar cells and their extracellular products that perform a common function.

• Four Major Types:

  • Epithelial

  • Connective

  • Muscle

  • Nervous

Microscopy in Tissue Study

Microscopy is essential for studying tissue structure. Tissues are preserved, cut, and stained to visualize cellular details.

• Stains:

  • Acidic stains

  • Basic stains

Epithelial Tissue

Types and Classification

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is classified by cell shape and number of layers.

• Types by Shape:

  • Simple squamous

  • Simple cuboidal

  • Simple columnar

  • Pseudostratified columnar

  • Stratified squamous

  • Transitional

• Classification by Layers:

  • Simple Epithelium: Single layer

  • Stratified Epithelium: Multiple layers

Functions of Epithelial Tissue

• Protection

• Absorption

• Filtration

• Excretion

• Secretion

• Sensory Reception

Characteristics of Epithelial Tissue

• Polarity: Apical (top) and basal (bottom) surfaces differ in function.

  • Apical Surface: May have microvilli or cilia.

  • Basal Surface: Rests on basal lamina (selective filter).

  • Lateral Surface: Bound by tight junctions and desmosomes (maintain polarity).

• Basement Membrane: Composed of basal lamina and reticular lamina; provides support and boundary.

• Avascular but Innervated: Nourished by diffusion from connective tissue below.

• High Regeneration: Rapid cell division when nutrients are available.

Classification Table: Epithelial Tissue

Type	Description	Function	Location
Simple Squamous	Single layer, flat cells	Diffusion, filtration	Alveoli, kidney glomeruli, endothelium, mesothelium
Simple Cuboidal	Single layer, cube-shaped cells	Secretion, absorption	Secretory glands
Simple Columnar	Single layer, tall cells	Absorption, secretion	Digestive tract, fallopian tubes (ciliated)
Pseudostratified Columnar	Appears layered, but all cells touch basement membrane	Secretion, movement of mucus	Respiratory tract (ciliated), male reproductive tract (non-ciliated)
Stratified Squamous	Multiple layers, flat cells at surface	Protection	Epidermis (keratinized), esophagus/vagina (non-keratinized)
Transitional	Multiple layers, cells change shape	Stretching	Urinary bladder

Glandular Epithelium

• Glands: One or more cells that make and secrete a product.

• Products: Aqueous fluids, lipid or steroid-rich substances.

• Types:

  • Endocrine: Ductless glands, secrete hormones into blood.

  • Exocrine: Ducted glands, secrete onto surfaces or into cavities.

• Modes of Secretion:

  • Merocrine: Secrete via exocytosis.

  • Holocrine: Accumulate product then rupture.

  • Apocrine: Controversial, may involve partial cell loss.

Connective Tissue (CT)

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue type, providing support, protection, insulation, and transportation.

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

• Functions:

  • Binding and support

  • Protection

  • Insulation

  • Transportation

Characteristics of Connective Tissue

• Origin: Mesenchyme (embryonic tissue)

• Degrees of Vascularity: Varies from avascular (cartilage) to highly vascular (bone)

• Extracellular Matrix (ECM): Nonliving matrix that gives CT its strength and weight-bearing properties

Elements of Connective Tissue

• Ground Substance: Fills space between fibers and cells, absorbs compressive forces.

  • Composed of interstitial fluid, cell adhesion proteins, proteoglycans

  • Proteoglycans attract water, making fluid viscous

• Fibers:

  • Collagen: Thick, high tensile strength

  • Elastic: Stretch and recoil

  • Reticular: Form networks

• Cells:

  • Blast (to bud or sprout)

  • Fibroblasts, chondroblasts, osteoblasts

  • Other: adipocytes, WBCs, mast cells, macrophages

Classification Table: Connective Tissue Types

Type	Description	Function	Location
Areolar CT	Loose, supports and binds other tissues	Support, defense	Lamina propria (all over)
Adipose CT	Fat storage	Insulation, energy storage	Under skin
Reticular CT	Network of reticular fibers	Support	Lymphoid organs
Dense Regular CT	Parallel collagen fibers	Strength	Tendons, ligaments
Dense Irregular CT	Irregularly arranged collagen fibers	Strength	Dermis
Elastic CT	Elastic fibers	Stretch and recoil	Walls of large arteries (aorta)
Hyaline Cartilage	Most common cartilage	Support, flexibility	Fetal skeleton, ends of long bones
Elastic Cartilage	Elastic fibers	Flexibility	Ear, epiglottis
Fibrocartilage	Thick collagen fibers	Strength	Intervertebral discs, pubic symphysis, menisci
Bone	Calcified matrix	Support, protection	Bones
Blood	Fluid matrix	Transport	Blood vessels

Nervous Tissue

Overview

Nervous tissue is specialized for communication and control, consisting of neurons and neuroglia.

• Neurons: Transmit electrical signals

• Neuroglia: Support and protect neurons

Functions and Locations

• Function: Communication, integration, control

• Location: Brain, spinal cord, nerves

Muscle Tissue

Overview

Muscle tissue is responsible for movement and force generation. It is classified into three types based on structure and function.

• Skeletal Muscle: Voluntary movement, attached to bones

• Cardiac Muscle: Involuntary, found in heart; contains intercalated discs

• Smooth Muscle: Involuntary, found in walls of hollow organs

Membranes: Covering and Lining

Types of Membranes

• Cutaneous Membrane: Skin; keratinized stratified squamous epithelium

• Mucous Membrane: Lines body cavities open to exterior; wet membrane

• Serous Membrane: Lines closed cavities; produces serous fluid

Examples

• Mucous: Digestive, respiratory tracts

• Serous: Pleura, peritoneum, pericardium

Tissue Repair

Process of Repair

Tissue repair involves cell division and migration, with two main steps: inflammation and organization.

• Inflammation: Increased capillary permeability, WBCs, clotting proteins, antibodies; forms a scab

• Organization: Restores blood supply, fibroblasts produce collagen fibers, macrophages clear debris

• Regeneration and Fibrosis: Epithelium thickens, scar tissue forms

Regenerative Capacity

• Good: Epithelial, bone, areolar CT, dense irregular CT, blood

• Moderate: Smooth muscle, dense regular CT

• Poor: Skeletal muscle, cartilage

• None: Cardiac muscle, nervous tissue

Primary Germ Layers

Overview

Primary germ layers are embryonic tissues that give rise to all body tissues.

• Ectoderm: Forms nervous tissue

• Mesoderm: Forms muscle and connective tissue

• Endoderm: Forms epithelial linings of digestive and respiratory tracts