Topic 3: Basic Tissues & Integumentary System / บทที่ 3: เนื้อเยื่อพื้นฐานและระบบผิวหนัง

The human body is composed of four primary tissue classes:

• Epithelial Tissue: Avascular, tightly packed sheets covering surfaces and forming glands. Protects, secretes, absorbs, and filters.
• Connective Tissue: Widely separated cells within an abundant extracellular matrix (fibers + ground substance). Binds, supports, protects, and insulates.
• Muscular Tissue: Excitable and contractile cells specialized for mechanical force generation and movement.
• Nervous Tissue: Neurons (impulse generation/conduction) and neuroglia (supporting cells) for sensation, integration, and control.

Cell Junctions

Junction Type	Major Proteins	Key Function & Typical Location
Tight Junction (Zonula Occludens)	Occludin, Claudin	Seals adjacent plasma membranes; prevents paracellular fluid leakage (Blood-brain barrier, intestinal lining, stomach).
Adherens Junction (Zonula Adherens)	Cadherin + Actin microfilaments	Anchoring belt linking actin cytoskeleton; reinforces apical junctional complexes.
Desmosome (Macula Adherens)	Cadherin (Desmoglein/Desmocollin) + Keratin intermediate filaments	"Spot weld" resisting intense mechanical shear/pulling forces (Epidermis of skin, cardiac muscle).
Hemidesmosome	Integrin + Keratin	Anchors basal surface of epithelial cell to the underlying basal lamina/basement membrane.
Gap Junction (Nexus)	Connexin (forms 6-subunit connexon channel)	Direct cytoplasmic diffusion of small ions, cAMP, metabolites (<1 kDa); electrical coupling (Intercalated discs of cardiac muscle, single-unit smooth muscle).

Epithelia are classified by the number of cell layers (Simple, Stratified, Pseudostratified) and the shape of cells at the apical surface (Squamous, Cuboidal, Columnar, Transitional):

Epithelium Type	Histological Description	Key Locations & Functions
Simple Squamous	Single layer of flat, scale-like cells with disc-shaped nuclei	Lung alveoli (gas exchange), kidney glomeruli Bowman's capsule (filtration), endothelium (lining blood/lymph vessels), mesothelium (pericardium, pleura, peritoneum).
Simple Cuboidal	Single layer of cube-shaped cells with central round spherical nuclei	Kidney tubules (secretion/absorption), surface of ovary, thyroid gland follicles.
Simple Columnar	Single layer of tall rectangular cells with oval basal nuclei; often with microvilli or cilia, and goblet cells	Non-ciliated: GI tract lining (stomach, small & large intestine, gallbladder; absorption/secretion). Ciliated: Uterine (Fallopian) tubes (propels oocyte).
Pseudostratified Columnar	Single layer appearing multi-layered; all cells touch basement membrane but not all reach surface; ciliated with goblet cells	Lining of upper respiratory tract: nasal cavity, trachea, bronchi (mucus secretion and ciliary escalation of debris).
Stratified Squamous (Keratinized)	Multiple layers; superficial cells are dead, anucleate, filled with water-resistant keratin	Epidermis of skin (protection against abrasion, pathogens, desiccation).
Stratified Squamous (Non-keratinized)	Multiple layers; superficial cells living, nucleated, moist	Oral cavity, pharynx, esophagus, vagina, anal canal (protection against mechanical abrasion in moist cavities).
Transitional (Urothelium)	Stratified with dome-shaped/umbrella apical cells that flatten when stretched	Urinary bladder, ureters, renal calyces, proximal urethra (permits reversible distension while maintaining impermeable urine barrier).

Connective tissue consists of cells widely dispersed in an Extracellular Matrix (ECM):

• Matrix Components:
  • Ground substance: Gel-like medium of glycosaminoglycans (hyaluronic acid, chondroitin sulfate) and proteoglycans.
  • Fibers: Collagen fibers (thick, resists tensile pulling), Elastic fibers (elastin/fibrillin, stretch and recoil up to 150%), Reticular fibers (delicate Type III collagen meshwork supporting soft organ stroma).
• Cell Types: Fibroblasts (synthesize fibers/ground substance), Adipocytes (fat storage), Macrophages (phagocytosis), Plasma cells (antibodies), Mast cells (histamine, heparin in inflammation), White blood cells.

Connective Tissue Classification

• Loose Connective Tissue:
  • Areolar: Loose meshwork; lamina propria under epithelia, wraps organs and nerves.
  • Adipose: Dominated by adipocytes; thermal insulation, energy depot, organ cushion.
  • Reticular: Stroma of lymphoid organs (spleen, lymph nodes, bone marrow).
• Dense Connective Tissue:
  • Dense Regular: Parallel, dense collagen bundles; withstands high unidirectional tension (tendons, ligaments, aponeuroses).
  • Dense Irregular: Randomly interwoven collagen bundles; withstands multidirectional pull (reticular layer of dermis, periosteum, organ capsules).
  • Elastic: Abundant parallel elastic fibers; permits expansion and recoil (aorta wall, vocal cords, ligamentum flavum).
• Specialized Connective Tissues: Cartilage (Hyaline, Elastic, Fibrocartilage), Bone (Compact, Spongy), and Fluid (Blood and Lymph).

The skin (integument) consists of two main anatomical layers resting on a subcutaneous base:

1. Epidermis (Avascular keratinized stratified squamous)

5 Strata from deep to superficial (mnemonic: "Before Signing, Get Legal Counsel"):

1. Stratum Basale (Germinativum): Single row of stem cells on basal lamina; highly mitotic; contains melanocytes (produce UV-protective melanin) and Merkel cells (touch).
2. Stratum Spinosum: 8–10 layers of cells connected by prominent desmosomes; contains Langerhans cells (dendritic antigen-presenting immune cells).
3. Stratum Granulosum: 3–5 layers; cells undergoing apoptosis; contain keratohyalin and lamellar lipid granules (waterproofing).
4. Stratum Lucidum: Clear layer of dead flat cells; present ONLY in thick skin (palms and soles).
5. Stratum Corneum: 20–30 layers of dead, anucleate, keratin-packed scales continually shed (desquamation).

2. Dermis (Vascular, innervated dense irregular CT)

• Papillary Layer (superficial 20%): Areolar CT; dermal papillae protruding into epidermal ridges (fingerprints); houses capillary loops, Meissner's corpuscles (light touch), and free nerve endings.
• Reticular Layer (deep 80%): Dense irregular CT with coarse collagen; contains hair roots, sebaceous glands, sweat glands, and deep Pacinian corpuscles (vibration and deep pressure).

3. Hypodermis (Subcutis / Superficial Fascia)

Adipose and loose connective tissue; anchors skin to underlying deep fascia; provides thermal insulation and mechanical shock absorption.

Cutaneous Sensory Receptors

• Free nerve endings: Pain (nociceptors), temperature (cold/warmth), itch.
• Meissner's (tactile) corpuscles: Light discriminative touch, low-frequency vibration (in dermal papillae).
• Pacinian (lamellar) corpuscles: Deep pressure, high-frequency vibration (deep dermis/hypodermis).
• Ruffini corpuscles: Skin stretch, continuous pressure.

Classification of Burns

• First-degree burn: Damage confined to epidermis (erythema, pain, edema, no blisters; e.g. mild sunburn). Heals in 3–6 days.
• Second-degree burn: Destroys epidermis and upper papillary/reticular dermis (blisters, severe pain, edema; partial-thickness). Heals in 2–4 weeks.
• Third-degree burn: Destroys entire thickness of skin into hypodermis (charred/pearly white, painless due to sensory nerve ending destruction; full-thickness). Requires skin grafting.

4 Phases of Deep Wound Healing

1. Hemostasis (Clotting): Immediate vasoconstriction, platelet plug aggregation, and fibrin mesh clot formation to arrest bleeding.
2. Inflammatory Phase: Vasodilation and increased vascular permeability; infiltration of neutrophils followed by monocytes (macrophages) to phagocytose pathogens, debris, and necrotic tissue. Cardinal signs: rubor (redness), calor (heat), tumor (swelling), dolor (pain).
3. Proliferative Phase: Re-epithelialization beneath scab; fibroblasts synthesize abundant collagen and ground substance; angiogenesis (new capillary buds) forms vascularized granulation tissue; wound contraction by myofibroblasts.
4. Maturation (Remodeling) Phase: Scab sloughs off; collagen fibers reorganized and cross-linked along lines of tension; capillaries recede; scar tissue formation (fibrosis). Complications: Hypertrophic scar (confined within wound borders) vs. Keloid (collagen overgrowth extending beyond original boundaries).