Four Basic Types of Tissue – Detailed Study Notes

A tissue is a collection of cells that carry out a specific function.- May contain >1 cell type within the same tissue.
- Tissues are organisational units that combine to form organs and, ultimately, body systems.

1) Epithelial tissue
2) Connective tissue
3) Muscular tissue
4) Nervous tissue
Each category has distinctive structural and functional profiles yet integrates with the others to maintain homeostasis.

Epithelial (adjective) / Epithelium (singular noun) / Epithelia (plural noun)

Avascular: contains no blood vessels; relies on diffusion from connective tissue for nutrition.
Cells tightly packed with minimal extracellular matrix.
Polarity (three distinct surfaces): - Apical surface – faces outward/external space or internal lumen; may bear cilia or microvilli to enhance movement or absorption.
- Lateral surfaces – interface with neighbouring epithelial cells; contain multiple junctional complexes.
- Basal surface – attaches to the basement membrane, which in turn anchors epithelium to underlying connective tissue and houses the blood supply.

Tight junctions – form near the apical region; seal intercellular space to regulate paracellular transport.
Gap junctions – protein channels allowing passage of ions, nutrients, and signalling molecules; coordinate activity.
Desmosomes – “spot welds”; provide strong cell–cell adhesion to withstand mechanical stress.
Hemidesmosomes – anchor basal plasma membrane to the basement membrane using integrins.

Arrangements (layers) - Simple – single cell layer.
- Pseudostratified – single layer that appears multilayered due to nuclei at differing heights.
- Stratified – ">=2 layers, offering protection.
Shapes (apical cell morphology) - Squamous – flat, thin.
- Cuboidal – cube-like, central nucleus.
- Columnar – tall, rectangular, basal nuclei.
Naming convention: arrangement + shape (e.g., simple squamous epithelium).

Protection – resists abrasion, pathogens, chemical insults.
Selective permeability – regulates entry/exit of water, solutes, gases.
Secretion – forms glandular epithelia releasing mucus, hormones, enzymes, sweat, etc.

External surface: skin (epidermis).
Internal linings: digestive tract, kidney tubules, blood vessels (endothelium), alveoli of lungs, reproductive tract, exocrine & endocrine glands.

Vascular (rich blood supply) except cartilage.
Cells dispersed in abundant extracellular matrix (ECM)—ECM dominates tissue volume.- ECM = protein fibres + ground substance.
Provides support, binding, protection, energy storage, immune surveillance.

Fibroblasts – secrete collagen, elastin, reticular fibres + ground substance.
Leukocytes – neutrophils, eosinophils migrate during infection.
Mast cells – release histamine (vasodilation) + antimicrobial factors.
Macrophages – phagocytose debris & pathogens.
Plasma cells – derived from B-lymphocytes; secrete antibodies.
Adipocytes – store triglycerides (energy, insulation).
Collagen fibres – strongest, resist tension.
Elastic fibres – contain elastin; provide stretch & recoil.
Ground substance – viscous gel of water, GAGs (e.g., chondroitin sulfate), proteoglycans.

Loose Connective Tissue- Areolar: most ubiquitous; surrounds almost every structure → offers strength, elasticity, support.
- Adipose: clusters of adipocytes among areolar tissue; reduces heat loss, cushions organs, energy reservoir.
Dense Connective Tissue- Higher fibre:cell ratio → greater tensile strength.
- Dense regular: parallel collagen → tendons (muscle→bone) & ligaments (bone→bone).
- Dense elastic: elastic fibres + fibroblasts → walls of elastic arteries (aorta), trachea, vocal cords; allows stretch/recoil.
Cartilage- ECM: firm gel (chondroitin sulfate); avascular & aneural.
- Cells = chondrocytes within lacunae.
- Types (e.g., hyaline on articular surfaces, nose, larynx).
Bone (Osseous Tissue)- ECM mineralised: Ca3(PO4)2 predominates.
- Cells = osteocytes in lacunae; confers rigidity, lever action, protection, Ca2+ reservoir.
Liquid Connective Tissues- Blood: plasma (~55%) + formed elements (RBCs, WBCs, platelets).
• RBCs transport O2/CO2.
• WBCs mediate immunity.
• Platelets enable clotting.
- Lymph: filtrate resembling plasma; resides in lymphatic vessels, returns excess interstitial fluid to circulation.

Presence of contractile filaments (actin & myosin) → shorten cells, generating force.

Skeletal Muscle - Voluntary, striated, multinucleate.
- Produces body movement, posture, stabilises joints, generates heat via shivering.
Cardiac Muscle - Involuntary, striated, branched, single nucleus, intercalated discs.
- Generates rhythmic contractions → propels blood throughout the circulatory system.
Smooth Muscle - Involuntary, non-striated, spindle-shaped.
- Regulates diameter of blood vessels, propels food (peristalsis) in GI tract, contracts bladder & reproductive organs.

Neurons (excitable) – generate & conduct electrical impulses. - Anatomy: dendrites (input) → cell body (integration) → axon (output).
Neuroglia – supportive “glue”; maintain homeostasis, form myelin, supply nutrients, remove debris.

Communication & Control - Rapid transmission of information (electrical + chemical).
- Detects stimuli (light, sound, chemicals, touch).
- Integrates sensory input (language, vision, music perception).
- Orchestrates responses (muscle contraction, gland secretion, emotion).
Locations: brain, spinal cord, peripheral nerves, autonomic ganglia.

Epithelial Connective interface: basement membrane separates yet tightly joins the two, ensuring nutrient diffusion to avascular epithelium.
ECM content increases from epithelium (minimal) → muscle (moderate endomysium) → connective tissue (maximal).
Excitability shared by muscle & neurons, enabling coordinated movement and signalling.
Repair capacity: epithelia & many connective tissues regenerate well; cartilage & neurons exhibit limited intrinsic repair.