W4 MHS1101 TISSUES

W4 Tissues of the Body

Four Types of Tissue

• Epithelial:
  • Covers surfaces, lines passages, forms glands.
• Connective:
  • Fills spaces, supports, transports, stores energy.
• Muscle:
  • Specialised for contraction (skeletal, cardiac, smooth).
• Nervous:
  • Conducts electrical signals.

Epithelial Tissue

• Includes:
  • Epithelia (coverings) & glands (secretion).
• Functions:
  • Physical protection
  • Control permeability
  • Provide sensation
  • Produce secretions
• Characteristics:
  • Polarity (apical/basal)
  • Cellularity (junctions)
  • Attachment (basement membrane)
  • Avascularity
  • High regeneration
• Specialisations:
  • Move fluids (cilia, microvilli)
  • Produce secretions
  • Polarity: apical (microvilli/cilia), basolateral surfaces
• Integrity Maintained by:
  • Intercellular connections (CAMs, proteoglycans)
  • Attachment to basement membrane (basal/reticular lamina)
  • Ongoing repair (stem cells)

Cell Junctions

• Gap junctions: rapid communication, coordination (e.g., heart)
• Tight junctions: block passage (digestive tract)
• Desmosomes: strong connections (skin)
• Hemidesmosomes: attach to basement membrane

Classification of Epithelia

• By Shape:
  • Squamous (flat), cuboidal (square), columnar (tall).
• By Layers:
  • Simple (one layer), stratified (multiple).

Epithelia Types

• Simple squamous: Absorption/diffusion (mesothelium, endothelium)
• Stratified squamous: Protection (skin, oral cavity)
• Simple cuboidal: Secretion/absorption (glands, kidneys)
• Stratified cuboidal: Rare (sweat, mammary glands)
• Transitional: Stretches (bladder)
• Pseudostratified columnar: Typically, ciliated (respiratory tract)
• Simple columnar: Absorption/secretion (GI tract)
• Stratified columnar: Rare (pharynx, anus, urethra)

Glandular Epithelium

• Glands: Clusters of epithelial cells producing secretions.
  • Endocrine: Release hormones to blood (ductless).
  • Exocrine: Secrete via ducts (surface).
• Methods of Secretion:
  • Merocrine: Exocytosis (salivary/sweat)
  • Apocrine: Shedding cytoplasm (mammary)
  • Holocrine: Cell bursts (sebaceous glands)
• Structure:
  • Unicellular (goblet cells – mucin)
  • Multicellular (simple/compound ducts, tubular/acinar shape, branched)
• Types of Exocrine Secretions:
  • Serous (watery)
  • Mucous (mucin)
  • Mixed

Connective Tissue

• Components
  • Specialised cells
  • Extracellular protein fibres
  • Ground substance (fluid)
  • Matrix: fibres + ground substance, main tissue volume
• Functions
  • Structural support
  • Transport
  • Protection
  • Connects/supports other tissues
  • Energy storage (triglycerides)
  • Defence (immune)
• Categories
  1. Connective tissue proper (connects/protects)
  2. Fluid connective tissue (blood, lymph – transport)
  3. Supporting connective tissue (cartilage, bone – strength)

Connective Tissue Proper

Loose Connective Tissue

• Areolar: Least specialised, supports, elastic, holds capillaries.
• Adipose: Fat storage, insulation, energy.
• Reticular: Support (stroma), found in liver, spleen, lymph nodes, bone marrow.

Dense Connective Tissue

• Dense regular: Parallel collagen (tendons, ligaments, aponeuroses).
• Dense irregular: Interwoven collagen (dermis, organ capsules, sheaths).
• Elastic tissue: Elastic fibres (vertebral ligaments).

Cells

• Fibroblasts: Most abundant, produce fibres/ground substance.
• Fibrocytes: Maintain fibres.
• Adipocytes: Store fat.
• Mesenchymal cells: Stem cells (repair).
• Melanocytes: Pigment (melanin).
• Macrophages: Engulf pathogens/debris (fixed/free).
• Mast cells: Inflammation (histamine, heparin).
• Lymphocytes: Immune response (can become plasma cells).
• Microphages: Phagocytic WBCs (neutrophils, eosinophils).

Fibres

• Collagen: Strong, unbranched (tendons/ligaments).
• Reticular: Network (support, sheaths).
• Elastic: Stretch, recoil (vertebrae).

Fluid Connective Tissue: Blood

Functions

• Transport gases, nutrients, wastes, hormones
• Regulate pH, ions
• Restrict fluid loss
• Defend against toxins/pathogens
• Stabilise temperature

Components

• Plasma: 55% of blood, mostly water, proteins, solutes
• Formed elements: Red cells (erythrocytes), white cells (leukocytes), platelets (thrombocytes)

Red Blood Cells

• 99.9% of formed elements
• Biconcave, no nucleus/mitochondria/ribosomes
• Filled with haemoglobin (binds O₂/CO₂)
• Live ~120 days

Blood Types

• Surface antigens: A, B, Rh (D)
• Type A: A antigen, anti-B antibody
• Type B: B antigen, anti-A antibody
• Type AB: A/B antigens, no antibodies
• Type O: No antigens, both antibodies
• Rh+: D antigen present; Rh-: D antigen absent
• Agglutinogens: RBC antigens
• Agglutinins: Plasma antibodies (attack foreign antigens)
• Transfusion reaction: Incompatibility causes agglutination/haemolysis
• Universal donor: O- (still risk due to other antigens)

White Blood Cells

• Defence (pathogens, toxins, abnormal cells)
• Types: Neutrophils, eosinophils, basophils, monocytes, lymphocytes
• Can migrate from bloodstream, amoeboid movement, chemotaxis, some phagocytic

Platelets

• Cell fragments, clotting, patch damaged vessels, reduce break size
• Circulate 9–12 days, stored in spleen

Supporting Connective Tissues

Cartilage

• Shock absorption, protection
• Matrix: firm gel with chondroitin sulphates
• Cells: chondrocytes in lacunae, avascular, antiangiogenesis factor (no blood vessels)
• Perichondrium: Outer fibrous/inner cellular
• Types:
  • Hyaline: Most common, tough, flexible, reduces friction (joints, ribs, sternum, trachea)
  • Elastic: Flexible, supports (ear, epiglottis)
  • Fibrocartilage: Durable, prevents bone contact (joints, pubic bones, vertebrae)

Bone (Osseous Tissue)

• Weight support, rigid (calcium salts), resists shattering (collagen)
• Cells: osteocytes in lacunae, organised around central canals (blood supply)
• Periosteum: Covers bone (fibrous/cellular layers)
• Bone structure: 1/3 organic (collagen/proteins), 2/3 inorganic (hydroxyapatite, minerals)
• Spongy bone: Lattice structure, red marrow, strength with less weight

Bone Cells

• Osteogenic cells: Stem cells, create other bone cells
• Osteoblasts: Build bone, form matrix, produce osteocalcin
• Osteocytes: Former osteoblasts, maintain matrix, act as strain sensors, in lacunae connected by canaliculi
• Osteoclasts: Dissolve bone (resorption), large, multinucleate, ruffled border

Bone Development

• Ossification/Osteogenesis: Bone formation in foetus/infant
  • Intramembranous: Flat bones (skull, clavicle)
  • Endochondral: Most other bones (cartilage model replaced by bone)
• Bone Remodeling: Continuous absorption/deposition (10%/year), repairs, reshapes, releases minerals
  • Wolff’s law: Bone structure adapts to mechanical stress

Bone Physiology & Mineral Homeostasis

• Active tissue: Exchanges minerals with fluids, impacts body systems.
• Mineral deposition (mineralisation): Ca^{2+}, phosphate deposited in bone by osteoblasts.
• Resorption: Osteoclasts dissolve bone, release minerals.
• Calcium homeostasis: Balance of intake, loss, bone storage, regulated by hormones:
  • Calcitriol: Increases blood Ca^{2+} (vitamin D)
  • Calcitonin: Lowers blood Ca^{2+} (thyroid, inhibits osteoclasts, stimulates osteoblasts; important in children)
  • Parathyroid hormone (PTH): Raises blood Ca^{2+} (increases osteoclasts, kidney reabsorption, activates calcitriol, inhibits osteoblasts)
• Hypocalcaemia: Low Ca^{2+}, excitable nerves/muscles, risk in pregnancy/lactation.
• Hypercalcaemia: High Ca^{2+}, weak muscles, cardiac issues.