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BDS 1st Year Anatomy - Board Exam Answers

Kathmandu University

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Q1. What is a joint? Classify joints. Give the characteristic features of synovial joint with labelled diagram. [3+1+1=5 → Full 10 marks version]

Definition of Joint (Arthrology)

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Classification of Joints

A. Structural Classification

B. Functional Classification

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Characteristics/Features of a Synovial Joint

1. Joint (articular) cavity - A potential space between articular surfaces containing synovial fluid
2. Articular cartilage - Hyaline cartilage covering articular surfaces of bones; avascular and aneural; reduces friction
3. Articular capsule - Surrounds the joint; consists of:
   • Outer fibrous layer - dense irregular connective tissue; provides strength
   • Inner synovial membrane - highly vascularized; secretes synovial fluid
4. Synovial fluid - Clear, viscous, egg-white-like fluid; acts as lubricant, nourishes cartilage, removes debris; contains hyaluronic acid and phagocytic cells
5. Ligaments - Capsular (thickened portions of capsule) or extracapsular/intracapsular; provide stability
6. Blood supply - From periarticular arterial plexus (branches of arteries around joint); most dense in synovial membrane
7. Nerve supply - According to Hilton's Law: nerves supplying muscles moving the joint also supply the joint and overlying skin

Accessory Features (not in all synovial joints):

• Articular discs / Menisci - fibrocartilaginous pads that improve congruence (e.g., TMJ disc, knee menisci)
• Labra - deepen the socket (e.g., acetabular labrum, glenoid labrum)
• Bursae - fluid-filled sacs that reduce friction
• Fat pads - intra-articular cushions (e.g., infrapatellar fat pad)
• Intracapsular tendons - e.g., long head of biceps, cruciate ligaments

Subtypes of Synovial Joints (by shape):

1. Plane - gliding; carpals, tarsals, facet joints
2. Hinge (Ginglymus) - uniaxial flexion/extension; elbow, ankle
3. Pivot (Trochoid) - uniaxial rotation; atlantoaxial, radioulnar
4. Condyloid (Ellipsoid) - biaxial; radiocarpal, metacarpophalangeal
5. Saddle (Sellar) - biaxial; 1st carpometacarpal joint (thumb)
6. Ball and socket (Spheroid) - multiaxial; hip, shoulder, TMJ is a modified condyloid joint

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Labelled Diagram of a Synovial Joint

        BONE
          |
    ======|======   ← Periosteum
    //////|\\\\\\   ← Articular (Hyaline) Cartilage
   |              |
   |  Joint       |  ← Fibrous Capsule (outer layer)
   |  Cavity      |  ← Synovial Membrane (inner layer)
   | (synovial    |
   |   fluid)     |  ← Ligament
    \\\\\\|//////
    ======|======   ← Articular Cartilage
          |
        BONE

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Q2. Describe the histological structure of elastic cartilage with a well labelled diagram. [3+2=5 → Full 10 marks]

Introduction

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Sites / Locations

• Pinna (auricle) of the external ear
• External auditory meatus
• Epiglottis
• Cuneiform cartilages of larynx
• Corniculate cartilages of larynx
• Auditory (Eustachian) tube

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Histological Structure

1. Perichondrium

• Dense fibrous connective tissue layer surrounding the cartilage
• Outer fibrous layer: dense collagen fibers, fibroblasts
• Inner chondrogenic (cellular) layer: chondroblasts that can differentiate into chondrocytes
• Provides nutrition (avascular tissue) and allows appositional growth

2. Chondrocytes

• Cells occupying lacunae in the matrix
• Rounded to oval in shape
• More numerous and closely packed than in hyaline cartilage
• In the central region: cells occur in isogenous groups (cell nests of 2-4 cells) - result of mitotic division
• At periphery: cells are smaller and more flattened (younger cells)
• Cytoplasm: abundant RER, Golgi complex, mitochondria, lipid droplets (active secretory cells)

3. Extracellular Matrix

• Ground substance:
  • Type II collagen (same as hyaline, but fibers are masked by ground substance)
  • Proteoglycans (aggrecan, versican) containing chondroitin sulfate and keratan sulfate
  • Hyaluronic acid
  • Glycoproteins (fibronectin, laminin)
• Elastic fibers:
  • Composed of elastin protein surrounded by fibrillin microfibrils
  • Form a dense network throughout the matrix
  • Stain with: Orcein stain (dark brown), Verhoeff's stain, Weigert's stain (appear dark/black)
  • Responsible for the yellow color seen macroscopically ("yellow elastic cartilage")
  • Provide resilience and elasticity
• Collagen fibers: Type II collagen, masked by ground substance in routine H&E
• Territorial matrix (capsular matrix): intensely basophilic zone immediately around each lacuna (rich in proteoglycans)
• Interterritorial matrix: lighter staining area between cell nests

4. Lacunae

• Small spaces within the matrix that house chondrocytes
• Each lacuna contains a chondrocyte (or isogenous group of 2-4 cells)

5. Vascular Supply

• Elastic cartilage is avascular - nutrients reach cells by diffusion through the matrix
• Blood vessels are found only in the perichondrium

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Staining Characteristics

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Labelled Diagram of Elastic Cartilage

     ┌──────────────────────────────────────┐
     │  PERICHONDRIUM                       │
     │  - Outer fibrous layer               │
     │  - Inner chondrogenic layer          │
     ├──────────────────────────────────────┤
     │                                      │
     │  ○○   Matrix with dense elastic     │
     │ ○[C]○  fiber network                │
     │  ○○                                 │
     │        [C] = Chondrocyte in lacuna  │
     │  ○[C][C]○   Isogenous group         │
     │             (2-4 cells)             │
     │  ○  = Elastic fibers throughout     │
     │                                      │
     └──────────────────────────────────────┘
Labels: Perichondrium | Lacuna | Chondrocyte | Isogenous cell nest | 
        Elastic fibers (network) | Territorial matrix | Interterritorial matrix

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Differences from Hyaline Cartilage (key points)

• Elastic cartilage has abundant elastic fibers (hyaline has none)
• More cellular (chondrocytes closer together)
• Does NOT calcify (hyaline may calcify with age)
• More flexible and resilient
• Yellow color macroscopically (hyaline is bluish-white)

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Q3. Explain the nerves and blood supply of a typical long bone with a labelled diagram. [3+2=5 → Full 10 marks]

Introduction

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Blood Supply of a Long Bone

A. Arteries supplying a long bone:

• The most important single artery
• Enters through the nutrient foramen in the diaphysis (shaft) at an oblique angle
• Passes through the compact bone to enter the medullary cavity
• Inside the medullary cavity it divides into:
  • Ascending branch (toward proximal epiphysis)
  • Descending branch (toward distal epiphysis)
• Each branch further divides into medullary sinusoids that nourish the inner 2/3 of cortical bone
• Blood flow direction: centrifugal (from medullary cavity outward)

• Arise from periosteal arteries that are branches of muscular and fascial vessels
• Penetrate via Volkmann's canals into the Haversian system
• Supply the outer 1/3 of the cortex (periosteal blood supply)
• Important for blood supply after fracture or when nutrient artery is disrupted

• Multiple small branches from arteries around the joint
• Supply the metaphysis (flared ends of diaphysis)
• During growth, these are important in the metaphysis

• Branch from articular arterial anastomoses
• Supply the epiphyses
• In children: the epiphyseal plate (growth plate) acts as a barrier separating epiphyseal and metaphyseal circulation

B. Veins

• Exit via nutrient vein, periosteal veins, and emissary veins
• Drain into regional venous plexuses

C. Lymphatics

• Found only in the periosteum
• No lymphatics inside the bone substance

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Nerve Supply of a Long Bone

Periosteum

• Richly innervated by myelinated and unmyelinated fibers from adjacent nerves
• Contains nociceptors (pain receptors) - explains why periosteum is extremely sensitive to pain
• Periosteal pain: intense, well-localized (e.g., bone fracture, periostitis)

Cortical Bone and Haversian Canals

• Nerves accompany blood vessels through Haversian canals (central canals) and Volkmann's canals
• Vasomotor (sympathetic adrenergic) fibers: regulate blood vessel tone within the canals
• Sensory fibers: limited; provide some dull pain sensation from deep bone

Medullary Cavity and Endosteum

• Nerves enter with the nutrient artery
• Unmyelinated sympathetic fibers supply the marrow vasculature
• Sensory innervation is sparse (explains why deep bone marrow pain is dull and poorly localized)

Summary - Hilton's Law Applied to Bone

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Labelled Diagram of Blood Supply of a Long Bone

     EPIPHYSIS (proximal)
     ├── Epiphyseal arteries (from articular plexus)
     │       ↓ supply epiphyseal bone + marrow
     │
  ═══════════════════  ← Articular cartilage
  ───────────────────  ← Epiphyseal plate (growth plate)
     METAPHYSIS
     ├── Metaphyseal arteries
     │
     │         ← PERIOSTEUM
     DIAPHYSIS │ ← Periosteal arteries (outer cortex)
     │         │
     │     ----│----- ← Nutrient Foramen
     │         │  ↑ NUTRIENT ARTERY
     │         │   └→ Ascending branch
     │         │   └→ Descending branch → medullary sinusoids
     │
  ═══════════════════  ← Growth plate
     METAPHYSIS
     ├── Metaphyseal arteries
     EPIPHYSIS (distal)
     └── Epiphyseal arteries

Labels: Periosteum | Compact bone | Medullary cavity | Nutrient foramen | 
        Nutrient artery | Epiphyseal artery | Metaphyseal artery | 
        Periosteal arteries | Haversian canal | Volkmann's canal

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Q4. Describe the special features of sesamoid bone with two examples. [3+2=5 → Full 10 marks]

Definition

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Special Features of Sesamoid Bone

1. Location: Embedded within tendons or joint capsules, at points where tendons make sharp turns over joints or bony prominences
2. Structure:
   • Small, rounded, and nodular bones
   • Core of cancellous (spongy) bone covered by a thin shell of compact bone
   • Cartilage or fibrocartilage covers the smooth articular surface
   • Covered by fibrous tissue (not true periosteum) on the non-articular surfaces
   • Poor blood supply - receive nutrition from surrounding tissues and synovial fluid
3. Development: Form by ossification within tendon fibrocartilage (endochondral/intramembranous); develop in response to mechanical stress
4. Lack of Periosteum: On the surface embedded within a tendon; periosteum is absent or rudimentary; this makes healing very difficult after fracture (avascular necrosis is a complication)
5. Variable Presence: Some sesamoid bones are constant (always present) while others are inconstant/variable (present in only some individuals)
6. Function:
   • Protect tendons from friction and compression at bony angles
   • Improve mechanical advantage by increasing the angle (moment arm) of pull of a muscle
   • Act as pulleys to redirect tendon force
   • Reduce friction and wear on tendons
7. Development: They develop in response to mechanical forces (tension/compression) during fetal development or early life; they are sometimes called accessory ossicles when they form near joints
8. X-ray appearance: Dense opaque rounded bony shadows within soft tissue adjacent to a joint
9. No Nutrient Foramen: Because they are poorly vascularized; blood vessels enter only from periphery

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Examples (with clinical relevance)

Example 1: Patella (Kneecap)

• Location: Within the quadriceps femoris tendon, in front of the knee joint
• Size: Largest sesamoid bone in the body; triangular in shape
• Bone type: Contains cancellous bone centrally
• Function: Increases the angle of pull of the quadriceps, improving the mechanical advantage (moment arm) by ~30%; protects the knee joint anteriorly; acts as a pulley
• Clinical significance: Fractured patella is serious - fracture causes loss of extensor mechanism of the knee

Example 2: Two sesamoid bones at the 1st Metatarsophalangeal (MTP) joint of the hallux (big toe)

• Location: Embedded in the flexor hallucis brevis tendon, on the plantar aspect of the 1st metatarsal head
• They are constant sesamoid bones
• Function: Protect the tendon of flexor hallucis longus passing between them; distribute weight-bearing load; act as a pulley for flexor hallucis brevis; increase mechanical advantage during toe-off phase of walking
• Clinical significance: Sesamoiditis (inflammation), sesamoid fractures from impact loading

Other examples:

• Fabella: in lateral head of gastrocnemius tendon (inconstant)
• Pisiform: in flexor carpi ulnaris tendon at wrist (constant)
• Two sesamoids in flexor pollicis brevis tendon at thumb MCP joint (constant)

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Q5. Define epithelium and classify it with examples. [5 marks → 10 marks full answer]

Definition of Epithelium

• Cells are tightly packed with specialized junctions (tight junctions, desmosomes, gap junctions)
• Rests on a basement membrane (basal lamina + reticular lamina)
• Avascular - nutrition by diffusion from underlying connective tissue
• High regenerative capacity (rapid cell division)
• Performs functions: protection, absorption, secretion, filtration, sensation, reproduction

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Classification of Epithelium

1. Number of cell layers (simple vs stratified)
2. Shape of surface cells (squamous, cuboidal, columnar)

I. Simple Epithelium (Single layer of cells)

• Cells: flat, scale-like; width > height; nucleus flattened
• Function: filtration, diffusion, reduces friction
• Examples:
  • Bowman's capsule (glomerular epithelium) - filtration
  • Alveoli of lungs - gas exchange
  • Endothelium (lining blood vessels)
  • Mesothelium (lining pleura, peritoneum, pericardium)
  • Loop of Henle (thin segment)

• Cells: cube-shaped; height ≈ width; nucleus round and central
• Function: secretion, absorption
• Examples:
  • Thyroid follicles
  • Collecting tubules of kidney
  • Small excretory ducts of glands
  • Choroid plexus of brain

• Cells: taller than wide; nucleus oval and basally located
• Sub-types:
  • Non-ciliated: small intestine (with microvilli/brush border), stomach
  • Ciliated: bronchioles, uterine tube (fallopian tube)
  • With goblet cells: large intestine
• Function: absorption (with microvilli), secretion (mucus), transport by cilia
• Examples: Lining of GI tract, gallbladder, uterine tube

II. Pseudostratified Epithelium (appears stratified but is truly simple)

• All cells rest on the basement membrane
• Not all cells reach the surface
• Nuclei at different levels - gives false appearance of layers
• Sub-types:
  • Pseudostratified columnar ciliated with goblet cells: Upper respiratory tract (trachea, bronchi, nasal cavity) - respiratory epithelium
  • Pseudostratified columnar non-ciliated (stereocilia): Epididymis, vas deferens
• Function: Mucociliary clearance, secretion, absorption

III. Stratified Epithelium (Two or more layers of cells)

• Most common stratified epithelium
• Basal cells: columnar/cuboidal (germinal/proliferative layer)
• Surface cells: flat/squamous
• Two types:
  • Keratinized (cornified):
    • Surface cells are dead, filled with keratin (hard protein)
    • No nuclei in surface cells
    • Resistant to desiccation and mechanical abrasion
    • Location: Epidermis of skin, dorsum of tongue, hard palate (thick skin areas)
  • Non-keratinized:
    • Surface cells are living, nucleated
    • Moist surfaces
    • Location: Oral mucosa (cheek, lips), esophagus, vagina, cornea, conjunctiva
• Function: Protection against mechanical damage

• Two layers of cuboidal cells
• Rare in body
• Location: Large excretory ducts of sweat glands, salivary glands
• Function: secretion, protection

• Multiple layers; surface cells are columnar
• Very rare
• Location: Large ducts of mammary glands, parts of male urethra, conjunctiva
• Function: protection, secretion

• Unique stratified epithelium that can change shape (transitional)
• Relaxed (empty bladder): 4-6 cell layers; surface "umbrella cells" (dome-shaped, binucleate, large)
• Distended (full bladder): 2-3 layers; surface cells flatten out
• Location: Renal pelvis, ureter, urinary bladder, proximal urethra
• Function: Allows distension and recoil; acts as permeability barrier

IV. Special Types of Epithelium (Glandular Epithelium)

• Exocrine glands: secrete onto body surfaces or into ducts (salivary glands, sweat glands, sebaceous glands, mucous glands)
• Endocrine glands: secrete hormones directly into bloodstream; ductless (thyroid, adrenal, pituitary)
• Mixed glands: both exocrine and endocrine components (pancreas, liver)

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Summary Table

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Q6. Difference between hyaline, elastic, and white-fibrocartilage. [2+2+2=6 → Full 10 marks]

Comparative Table

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Detailed Descriptions

Hyaline Cartilage

• The most common type; the "model" cartilage
• Matrix appears ground-glass homogeneous (hence "hyaline" = glass-like)
• Type II collagen fibrils (20-200 nm diameter) masked by abundant proteoglycans (aggrecan-hyaluronic acid aggregates)
• Territorial matrix (intensely basophilic capsule around lacunae) due to high sulfated GAG concentration
• Chondrocytes in isogenous groups (2-8 cells) in interior; single cells at periphery
• Perichondrium present except at articular surfaces (replaced by synovial fluid nutrition) and growth plates

Elastic Cartilage

• Distinguishable feature: network of elastic fibers throughout matrix
• These fibers are composed of elastin core + fibrillin microfibrils
• Yellow color macroscopically due to elastin
• Cells more numerous and closely packed than hyaline
• Never calcifies - remains permanently cartilaginous

Fibrocartilage

• Transitional tissue between dense fibrous connective tissue and cartilage
• Type I collagen (same as tendons/ligaments) in thick parallel bundles
• Provides greatest tensile strength and resistance to compression among cartilages
• Chondrocytes arranged in rows between collagen bundles; no isogenous groups typically
• No distinct perichondrium - merges gradually with adjacent fibrous tissue

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Q7. Describe the stages of intracartilaginous (endochondral) ossification.

Introduction

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Stages of Endochondral Ossification

Stage 1: Formation of the Cartilage Model

• Mesenchymal stem cells at the site of future bone aggregate and differentiate into chondroblasts
• Chondroblasts secrete a hyaline cartilage matrix
• The cartilage model has the shape of the future bone, covered by a perichondrium
• The model grows by:
  • Interstitial growth: chondrocytes divide within the matrix
  • Appositional growth: chondroblasts from perichondrium add to the surface

Stage 2: Chondrocyte Hypertrophy and Matrix Calcification (Primary ossification center)

• Chondrocytes in the center of the diaphysis enlarge (hypertrophy) and accumulate glycogen
• Hypertrophied chondrocytes secrete alkaline phosphatase and matrix vesicles
• Matrix vesicles deposit calcium phosphate crystals - the cartilage matrix calcifies
• Calcified matrix prevents diffusion of nutrients - chondrocytes die (apoptosis)
• Lacunae are left empty (ghost lacunae)

Stage 3: Vascular Invasion (Periosteal bud / Primary ossification center)

• The perichondrium around the mid-diaphysis becomes a periosteum as osteoprogenitor cells appear
• A periosteal collar of woven bone is laid down around the mid-diaphysis (intramembranous ossification component)
• Osteoclasts (from perichondrium/periosteum) erode through the periosteal collar
• A periosteal bud (branch of periosteal artery + mesenchymal cells + osteoclasts + osteoblasts) invades the calcified cartilage
• This creates the primary center of ossification in the middle of the shaft (diaphysis)
• Osteoclasts resorb calcified cartilage partitions; osteoblasts deposit bone matrix (osteoid) on remaining calcified cartilage spicules → primary spongiosa (bone trabeculae)
• The central medullary cavity begins to form as osteoclasts continue to resorb primary spongiosa

Stage 4: Formation of Primary Ossification Center Expands

• The ossification front spreads toward both ends (epiphyses) of the cartilage model
• Cartilage persists only at:
  • The epiphyses (ends)
  • The epiphyseal (growth) plate region

Stage 5: Secondary Ossification Centers (Epiphyseal centers)

• Separate vascular buds invade the epiphyses, usually after birth (proximal femoral epiphysis is an exception - forms in fetal life)
• Secondary ossification centers form within each epiphysis
• Bone replaces cartilage in a radial pattern from the center outward
• Cartilage persists as:
  • Articular cartilage (permanent hyaline cartilage on joint surfaces - never ossifies)
  • Epiphyseal plate (growth plate / physis) - between diaphysis and epiphysis

Stage 6: Epiphyseal Plate (Growth Plate) - Mechanism of Longitudinal Growth

1. Zone of resting (reserve) cartilage: small, scattered chondrocytes; hyaline cartilage; anchors plate to epiphysis
2. Zone of proliferation: chondrocytes undergo rapid mitosis; arranged in longitudinal columns (like stacked coins); responsible for lengthening
3. Zone of maturation (hypertrophy): chondrocytes enlarge; accumulate glycogen; columns expand
4. Zone of calcification: matrix calcifies; chondrocytes die; matrix vesicles deposit calcium
5. Zone of ossification (zone of provisional calcification): osteoclasts resorb calcified cartilage; osteoblasts deposit bone on remaining spicules; bone trabeculae form

Stage 7: Closure of Epiphyseal Plate (Growth plate fusion)

• At the end of adolescence (females ~16-18 years; males ~18-20 years), under the influence of sex hormones
• Chondrocyte proliferation slows and stops
• Entire plate ossifies - epiphysis and diaphysis fuse → epiphyseal line (scar visible on X-ray)
• Growth in length ceases

Stage 8: Bone Remodeling

• Primary spongiosa is replaced by secondary spongiosa (more organized lamellar bone)
• Compact (cortical) bone replaces peripheral spongy bone
• Medullary cavity enlarges (osteoclasts resorb inner bone)
• Woven bone → replaced by lamellar bone (organized osteons/Haversian systems)
• Final bone has cortical diaphysis + spongy metaphysis/epiphysis

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Q8. Describe the histological structure of smooth muscle with a well labelled diagram. [5 marks → 10 marks]

Introduction

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Locations

• Walls of digestive tract (esophagus to rectum)
• Walls of blood vessels (tunica media of arteries and veins)
• Airways (bronchi, bronchioles)
• Urinary bladder, ureters, uterus
• Iris and ciliary body of eye
• Skin: arrector pili muscles
• Capsule and trabeculae of spleen

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Histological Features

1. Cell Shape and Size

• Spindle-shaped (fusiform) cells: elongated with tapered ends
• Size varies by location:
  • Small vessels: 20 μm long
  • Pregnant uterus: up to 500 μm long
  • Average: 100-200 μm long, 5-10 μm diameter
• Cells are arranged in sheets or bundles

2. Nucleus

• Single, centrally placed, elongated/oval nucleus
• Nucleus is rod-shaped in relaxed state
• Nucleus appears corkscrew-shaped or wavy in contracted state (due to shortening of cell)
• Nuclei are staggered in adjacent cells (alternating positions) - gives the tissue a characteristic appearance

3. Cytoplasm (Sarcoplasm)

• Homogeneous, eosinophilic (pink on H&E)
• No striations (hence "non-striated") - this distinguishes it from skeletal and cardiac muscle
• Thick (myosin) and thin (actin) filaments are present but NOT arranged in regular sarcomeres
• Contains dense bodies (analogous to Z-discs) attached to the cell membrane and scattered in cytoplasm - serve as anchoring points for actin filaments
• Intermediate filaments (desmin, vimentin) connect dense bodies

4. Cell Membrane (Sarcolemma)

• Numerous caveolae (pinocytotic vesicles) on the surface = analogous to T-tubules of skeletal muscle
• Caveolae are involved in calcium regulation and signal transduction

5. Connective Tissue Framework

• Individual cells are surrounded by a basal lamina (external lamina)
• Reticular fibers (type III collagen) connect individual cells to each other and to the basal lamina
• Small amounts of collagen and elastin in extracellular matrix

6. Intercellular Junctions

• Gap junctions (nexus): low-resistance electrical connections between adjacent smooth muscle cells; allow coordinated contraction (cells act as functional syncytium)
• Particularly numerous in GI tract and uterus
• NOT all smooth muscle has them (multi-unit smooth muscle - e.g., iris, ciliary muscle, large vessels - lacks gap junctions)

7. Contractile Apparatus

• Thin filaments: actin + tropomyosin (no troponin in smooth muscle!)
• Thick filaments: myosin (arranged differently than skeletal muscle; side-polar arrangement)
• Dense bodies: anchor actin filaments; contain alpha-actinin
• Contraction is regulated by calmodulin (not troponin) through phosphorylation of myosin light chain kinase (MLCK)

8. Sarcoplasmic Reticulum (SR)

• Less developed than in skeletal muscle
• Does not form a triad
• Works with caveolae for calcium storage and release

9. Arrangement

• Unitary (single-unit) smooth muscle: cells connected by gap junctions; contracts as a unit; found in GI tract, uterus, urinary bladder
• Multi-unit smooth muscle: cells act independently; no gap junctions; found in iris, ciliary body, large arteries, hair follicles (arrector pili)

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Labelled Diagram

  ┌──────────────────────────────────────────────┐
  │                                              │
  │   ~~~~○~~~~  ~~~~○~~~~  ~~~~○~~~~           │
  │   Cell 1    Cell 2     Cell 3               │
  │                                              │
  │  Each cell:                                 │
  │  ┌─────────────────────────────────┐        │
  │  │  Tapered end                    │        │
  │  │    ↑                            │        │
  │  │  Cytoplasm (sarcoplasm)         │        │
  │  │  [Central oval nucleus]         │        │
  │  │  Dense bodies • • •             │        │
  │  │  Actin + myosin filaments       │        │
  │  │  Caveolae on membrane           │        │
  │  │    ↓                            │        │
  │  │  Tapered end                    │        │
  │  └─────────────────────────────────┘        │
  │  Basal lamina surrounds each cell           │
  │  Reticular fibers between cells             │
  │  Gap junctions connecting adjacent cells    │
  └──────────────────────────────────────────────┘

Labels: Nucleus (central, oval) | Cytoplasm/Sarcoplasm | Tapered cell ends | 
        Dense bodies | Caveolae | Basal lamina | Reticular fibers | Gap junctions

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Q9. Draw a labelled histological diagram of trachea. [5 marks → 10 marks]

Introduction

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Layers of the Tracheal Wall (from lumen outward)

1. Mucosa

• This is the diagnostic feature of the trachea
• Cell types present:
  • Ciliated columnar cells (most numerous): possess numerous cilia that beat in coordinated metachronal waves to move mucus toward the pharynx (mucociliary escalator)
  • Goblet cells: unicellular mucous glands; secrete mucus (mucin); abundant in trachea
  • Basal cells: low cells resting on basement membrane; serve as stem cells for renewal
  • Brush cells (rare): columnar cells with microvilli; may have sensory function
  • Small granule cells (Kulchitsky/Feyrter cells): neuroendocrine cells containing dense-core vesicles; APUD cells
  • Serous cells: produce watery secretion
• All cells rest on the prominent basement membrane (thick, clearly visible on H&E)
• Cilia beat in a layer of watery periciliary fluid (sol layer) beneath the mucus blanket

• Loose connective tissue with elastic fibers, blood vessels, lymphatics
• Contains mucous and serous glands (submucous glands)
• Small lymphocytes and occasional lymphoid aggregates (MALT)

2. Submucosa

• Loose connective tissue
• Contains mixed (seromucous) glands - the submucosal glands (tracheal glands)
  • Produce mucus that reaches the lumen via ducts
  • Mixed type: both mucous acini and serous demilunes
• Blood vessels, lymphatics, nerves
• Adipose tissue may be present

3. Cartilaginous Layer - C-shaped Hyaline Cartilage Rings

• 16-20 incomplete (C-shaped) rings of hyaline cartilage
• Open (deficient) portion is posteriorly located (toward esophagus)
• Function: maintain patency of tracheal lumen; prevents collapse during inspiration
• Cartilage rings are connected to each other by annular ligaments (fibroelastic tissue)
• The cartilage rings are embedded in a dense fibroelastic connective tissue

4. Trachealis Muscle (Posterior wall)

• Located at the posterior opening (gap) of the C-shaped cartilage ring
• Composed of smooth muscle (transversely arranged)
• Connects the free ends of the C-shaped cartilage posteriorly
• Allows some constriction/dilation of the lumen
• Allows the trachea to accommodate esophageal distension during swallowing

5. Adventitia

• Outermost layer
• Loose connective tissue
• Connects trachea to surrounding structures (esophagus posteriorly, thyroid anteriorly)
• Contains blood vessels, lymphatics, nerves

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Labelled Diagram of Tracheal Wall (Cross-section)

                    ╔══ LUMEN ══╗
                    ║           ║
  ─────────────────────────────────────── ← Basement membrane (thick)
  Pseudostratified ciliated columnar      ← MUCOSA: Epithelium
  epithelium + goblet cells
  ─────────────────────────────────────── 
  Lamina propria (loose CT, elastic fibers)
  ───────────────────────────────────────
  SUBMUCOSA: mixed seromucous glands,     ← SUBMUCOSA
  loose CT, vessels
  ───────────────────────────────────────
  Hyaline cartilage (C-ring)              ← CARTILAGE LAYER
  Perichondrium | Chondrocytes in lacunae
  ─────────────────────────────────────── 
  Adventitia (loose CT)                   ← ADVENTITIA

POSTERIOR: Trachealis smooth muscle bridges the gap of C-cartilage

Labels: Ciliated cells | Goblet cells | Basal cells | Basement membrane | 
        Lamina propria | Submucosal gland (mixed) | C-shaped hyaline cartilage ring | 
        Perichondrium | Trachealis smooth muscle | Adventitia

---

Q10. Histology of Thin Skin

Introduction

---

Layers of Thin Skin (from surface to deep)

I. Epidermis (thinner than in thick skin - 75-150 μm)

1. Stratum basale (germinativum)
   • Deepest layer; single row of columnar/cuboidal cells
   • Mitotically active - stem cell layer (keratinocyte renewal every 28-56 days)
   • Attached to basement membrane by hemidesmosomes
   • Contains melanocytes (neural crest origin; produce melanin in melanosomes)
   • Contains Merkel cells (sensory mechanoreceptors for touch)
   • Keratin 5 and 14 intermediate filaments
2. Stratum spinosum (prickle cell layer)
   • Several layers of polygonal cells
   • Connected by desmosomes (appear as "prickles" on EM)
   • Langerhans cells (dendritic antigen-presenting cells; CD1a+) are found here
   • Keratohyalin granules begin to appear
   • Keratin 1 and 10
3. Stratum granulosum
   • 3-5 layers of flattened cells
   • Cells contain keratohyalin granules (basophilic; contain profilaggrin and loricrin)
   • Lamellar bodies (Odland bodies) at cell periphery: release lipid bilayers into intercellular space → forms the water permeability barrier
   • Cells begin to lose nuclei and organelles → transition to dead cells
   • ⚠ In THIN skin: stratum granulosum is present but thinner than thick skin
4. Stratum corneum
   • Multiple layers of dead, flattened, anucleate cells filled with keratin (hard keratin)
   • Cells called corneocytes (cornified cells)
   • Thinner than in thick skin (fewer layers)
   • Constantly shed (desquamation) from surface
   • "Brick and mortar" model: corneocytes = bricks; lipid bilayers = mortar
   • Functions: barrier to water loss, protection from abrasion, microbial defense

Note: Stratum lucidum (a clear layer present in thick skin between stratum granulosum and corneum) is ABSENT in thin skin

II. Dermis

• Loose (areolar) connective tissue
• Projects dermal papillae up into the epidermis (more numerous and prominent in thick skin)
• Contains: thin collagen fibers (type I and III), elastic fibers, capillary loops (nourish avascular epidermis), Meissner's corpuscles (touch receptors in dermal papillae)
• Contains free nerve endings (pain, temperature)

• Dense irregular connective tissue
• Thick collagen bundles (type I) in interlacing network
• Elastic fibers (give skin elasticity and recoil)
• Contains Pacinian corpuscles (deep pressure/vibration receptors)
• Sebaceous glands, sweat glands, hair follicles embedded here

III. Epidermal Appendages (present in thin skin, absent in thick skin)

1. Hair follicles:
   • Invaginations of epidermis into dermis
   • Produce hair shaft composed of hard keratin
   • Inner root sheath, outer root sheath, hair matrix (germinal cells at base)
   • Hair bulb: contains dermal papilla with capillaries
2. Sebaceous glands:
   • Holocene secretory glands (entire cell disintegrates to release product)
   • Almost always associated with hair follicles (pilosebaceous unit)
   • Secrete sebum (mixture of lipids, wax esters, squalene)
   • Sebum: lubricates hair and skin; has some antimicrobial properties
   • Acinar structure; large pale lipid-filled cells (sebocytes); flattened basal cells at periphery
3. Eccrine sweat glands:
   • Present in thin skin (also thick skin)
   • Long coiled secretory portion in deep dermis/hypodermis
   • Straight duct opens directly onto skin surface (pore)
   • Secretory coil: clear (water-secreting) cells and dark (mucus-secreting) cells + myoepithelial cells
   • Function: thermoregulation (evaporative cooling)
4. Arrector pili muscle:
   • Small bundle of smooth muscle
   • Connects hair follicle to papillary dermis
   • Contraction (in cold/fear/emotion): erects hair ("goosebumps"/cutis anserina) and compresses sebaceous gland

IV. Hypodermis (Subcutaneous tissue)

• Not part of skin proper but underlies it
• Loose connective tissue with large fat cells (adipocytes)
• Lobules of adipose tissue held in connective tissue septa
• Contains larger blood vessels, lymphatics, Pacinian corpuscles, and nerve trunks

---

Q11. Describe briefly about splenic circulation.

Introduction

---

Arterial Supply

1. Splenic artery (largest branch of celiac trunk)
   • Tortuous course along upper border of pancreas
   • Enters spleen at the hilum
2. Trabecular arteries
   • Branches run within fibrous trabeculae from hilum
   • Give off arteries that leave the trabeculae to enter the spleen parenchyma (pulp)
3. Central arteries (Follicular arteries)
   • Leave trabeculae; immediately acquire a periarteriolar lymphoid sheath (PALS)
   • PALS = T-cell zone (white pulp); central artery is eccentrically placed within it
   • Give off branches to lymphoid follicles (B-cell areas)
4. Penicillar arteries (Pencillar arteries)
   • Terminal branches of central arteries
   • Leave white pulp; divide into 3 segments: a) Pulp arteries: short, straight; run in red pulp b) Sheathed arteries (ellipsoidal arteries): have a thick macrophage sheath (ellipsoid/Schweigger-Seidel sheath) that filters blood c) Terminal capillaries: open into the red pulp

---

The Open vs. Closed Circulation Debate

• Open circulation: Blood flows from capillaries directly into the splenic sinusoids or cords of Billroth (red pulp); this allows macrophages to filter the blood (phagocytose old RBCs, foreign material). This is the predominant pathway in humans.
• Closed circulation: Direct capillary-to-sinusoid connection (less prominent in humans)

---

Red Pulp Circulation

• Blood entering cords of Billroth (splenic cords, red pulp cords):
  • Macrophages screen blood for old, deformed, or parasitized RBCs
  • Old RBCs must squeeze through the narrow slits (2-3 μm gaps) between lining cells of venous sinusoids - rigid or aged RBCs cannot deform and are phagocytosed (culling and pitting functions)
  • Normal, flexible RBCs squeeze through the slit pores into the sinuses
  • Venous sinusoids are lined by elongated endothelial cells arranged parallel to long axis, bound by hoop-like reticular fibers - "barrel stave" arrangement

---

Venous Drainage

1. Venous sinusoids → Collect filtered blood
2. Pulp veins → run in red pulp
3. Trabecular veins → within trabeculae
4. Splenic vein → exits at hilum → joins superior mesenteric vein → portal vein → liver

---

Special Features of Splenic Circulation

• Blood reservoir: spleen can hold 200-300 mL of blood; sympathetic stimulation contracts capsule and trabeculae → expels blood into circulation
• Slow transit: blood dwells in red pulp cords for minutes → allows thorough screening by macrophages
• Platelet pooling: about 30% of the total platelet mass is stored in the spleen
• Culling: phagocytosis of entire old/abnormal RBCs
• Pitting: removal of inclusions (Howell-Jolly bodies, Heinz bodies, ring forms) from RBCs while returning the "cleaned" RBC to circulation

---

Q12. Histological Difference Between Thick and Thin Skin

---

Q13. Mention the microscopic features of lymph node with the help of a labelled diagram. [7+3]

Introduction

---

Gross Features (brief)

• Enclosed by a fibrous capsule with trabeculae
• Has a convex surface where afferent lymphatic vessels enter
• Has a concave hilum where blood vessels and efferent lymphatic vessels emerge

---

Microscopic Structure

1. Capsule

• Dense fibrous connective tissue (collagen)
• Subcapsular (marginal) sinus lies immediately deep to the capsule
• Lymph flows from afferent vessels → subcapsular sinus → cortex

2. Trabeculae

• Fibrous partitions extending from capsule inward
• Carry blood vessels into the node
• Peritrabecular sinuses run alongside trabeculae

3. Cortex (Outer zone)

• Dense, spherical aggregates of mature, unstimulated B lymphocytes (naïve B cells)
• Present in antigen-naive or non-stimulated nodes

• Have a pale-staining germinal center surrounded by a dark mantle zone (corona)
• Germinal center components:
  • Centroblasts (large, rapidly dividing B cells; undergo somatic hypermutation)
  • Centrocytes (smaller B cells; undergo selection based on antigen affinity)
  • Follicular dendritic cells (FDC): present antigen to B cells; not a true DC (derived from stromal cells)
  • Tingible body macrophages (macrophages engulfing apoptotic B cells - "starry sky" appearance)
• Dark zone (bottom): proliferating centroblasts
• Light zone (top): centrocytes + FDCs + selection
• Function: clonal expansion, affinity maturation, class switching, memory B cell formation, plasma cell formation

4. Paracortex (Deep cortex / Thymus-dependent zone)

• Region between cortex and medulla
• Predominantly T lymphocytes (CD4+ helper T cells)
• Contains High Endothelial Venules (HEV): specialized post-capillary venules lined by plump (tall) cuboidal/columnar endothelial cells
  • Express addressins (MAdCAM-1, PNAd) that bind L-selectin and CCR7 on lymphocytes
  • Allow lymphocyte homing from blood into lymph node
• Interdigitating dendritic cells (IDC): present antigen (MHC II) to T cells
• Expands dramatically during T cell-mediated immune responses (cell-mediated immunity)

5. Medulla

• Irregular cords of lymphoid tissue
• Contain: plasma cells (antibody-secreting; clock-face nucleus), B lymphocytes, macrophages, and occasional mast cells
• Active plasma cells are the end-product of germinal center reactions

• Channels between medullary cords
• Lined by littoral cells (flattened endothelial-like cells + macrophages)
• Macrophages in sinuses phagocytose debris, pathogens, and aged cells
• Lymph flows through: cortical sinuses → peritrabecular sinuses → medullary sinuses → hilum → efferent lymphatic vessel

6. Blood Supply

• Arteries enter at hilum
• Branch into trabecular arteries → HEV in paracortex (lymphocyte entry) → capillaries → venous drainage → hilum
• Lymphocyte recirculation: blood → HEV → paracortex → cortex/germinal center → medullary sinus → efferent lymphatic → thoracic duct → subclavian vein → blood

7. Lymphatic Sinuses

• Subcapsular (marginal) sinus: just below capsule; drains from afferent lymphatics
• Cortical (peritrabecular) sinuses: alongside trabeculae
• Medullary sinuses: between medullary cords
• Sinuses lined by littoral cells (sinus-lining cells with phagocytic activity)

---

Labelled Diagram

            Afferent lymphatics (multiple)
                    ↓ ↓ ↓
       ┌──────────────────────────────────┐
       │ CAPSULE (fibrous)                │
       │  └─ Subcapsular sinus            │
       │                                  │
       │  CORTEX:                         │
       │  ○ Primary follicle (dense)      │
       │  ⊙ Secondary follicle with      │
       │    germinal center (pale center) │
       │                                  │
       │  PARACORTEX (T-cell zone):       │
       │  [HEV] ← lymphocyte homing       │
       │  Interdigitating dendritic cells │
       │                                  │
       │  MEDULLA:                        │
       │  ═══ Medullary cords (plasma     │
       │       cells, B cells, Mφ)        │
       │  --- Medullary sinuses           │
       │                                  │
       │    HILUM                         │
       └──────────────────────────────────┘
              ↓                  ↓
       Efferent lymphatic    Blood vessels
              ↓
         (to blood)

Labels: Capsule | Trabecula | Subcapsular sinus | Primary follicle | 
        Secondary follicle | Germinal center | Mantle zone | Paracortex | 
        HEV | Medullary cord | Medullary sinus | Hilum | Afferent lymphatics | 
        Efferent lymphatics

---

Q14. Describe the structure of a neuron with a labelled diagram. [5 marks → 10 marks]

Definition

---

General Features of a Neuron

1. Cell Body (Soma/Perikaryon)

• Main metabolic center of the neuron
• Contains the nucleus: large, round, euchromatic (light-staining), with prominent nucleolus (active protein synthesis)
• Cytoplasm contains:
  • Nissl bodies: clusters of rough ER and free ribosomes; represent the site of protein synthesis (motor proteins, neurotransmitters, enzymes); stain with basic dyes (thionine, cresyl violet); absent in axon hillock and axon
  • Mitochondria: numerous; produce ATP for active transport and signal transmission
  • Golgi apparatus: modifies and packages proteins for axonal transport
  • Neurofilaments and microtubules: cytoskeletal elements; support cell shape; form basis of neurofibrils seen in silver-stained sections
  • Lysosomes: recycling
  • Lipofuscin granules: "wear and tear" pigment; accumulates with age
  • Melanin: in substantia nigra and locus coeruleus neurons
• Axon hillock: Cone-shaped region of soma where axon originates; lacks Nissl bodies; initial segment of axon (site of action potential generation)

2. Dendrites

• Multiple, branching processes extending from the soma
• Short and tapering (generally)
• Receive synaptic input from other neurons (afferent stimulation)
• Contain Nissl bodies (unlike axon)
• Surface has dendritic spines (in some neurons) = postsynaptic specializations; increase surface area for synaptic contacts
• Conduct impulses toward the cell body (centripetal direction)
• Internal structure: similar to soma (organelles present)

3. Axon

• Single, long process arising from the axon hillock
• May be very short (interneurons) or up to 1 meter long (motor neurons to toe muscles)
• Uniform diameter along entire length (unlike tapering dendrites)
• No Nissl bodies (no RER); no protein synthesis
• Cytoplasm = axoplasm; membrane = axolemma
• Contains: mitochondria, smooth ER, microtubules, neurofilaments, actin
• Collateral branches: side branches along axon length
• Terminal branches (telodendria): at the end; each terminates as a synaptic terminal (bouton/knob)
• Synaptic terminals contain: synaptic vesicles (with neurotransmitters), mitochondria, presynaptic membrane specializations
• Conducts impulses away from cell body (centrifugal direction)
• Myelin sheath (in myelinated neurons): formed by Schwann cells (PNS) or oligodendrocytes (CNS); acts as electrical insulator; speeds up conduction by saltatory conduction
• Nodes of Ranvier: gaps in myelin sheath where axolemma is exposed; sites of action potential regeneration
• Axonal transport:
  • Anterograde (soma → terminal): fast (motor proteins kinesin; organelles, vesicles) and slow (cytoskeletal elements)
  • Retrograde (terminal → soma): fast (dynein; used by neurotrophins, viruses, toxins)

---

Classification of Neurons

1. Unipolar: one process (rare; invertebrates)
2. Pseudounipolar: single process divides into central and peripheral branches (dorsal root ganglion neurons, sensory neurons of cranial nerve ganglia)
3. Bipolar: two processes - one dendrite, one axon (retinal neurons, vestibular ganglion, olfactory neurons)
4. Multipolar: many dendrites + one axon (most common; motor neurons, interneurons)

• Afferent (sensory): transmit to CNS
• Efferent (motor): transmit from CNS
• Interneurons (association): within CNS

---

Labelled Diagram

                    DENDRITES
                   /    |    \
                  /     |     \
                 /      |      \
                ↓       ↓       ↓  (Impulse flow: toward soma)
          ┌─────────────────────────┐
          │   CELL BODY (SOMA)      │
          │  ┌───┐ Nucleus          │
          │  │ N │ (large, pale,    │
          │  │ u │  prominent       │
          │  └───┘  nucleolus)      │
          │  ≡ Nissl bodies (RER)   │
          │  ⊕ Mitochondria         │
          │  Golgi complex          │
          └────────────┬────────────┘
                       │ ← AXON HILLOCK
                       │   (no Nissl bodies)
                       │ AXON
                       │─────────── Collateral branch
                       │
                  ╔════╧════╗
                  ║ Myelin   ║
                  ║ sheath   ║
                  ╚════╤════╝
                       │ ← Node of Ranvier
                  ╔════╧════╗
                  ║ Myelin   ║
                  ╚════╤════╝
                       │
                   /   │   \
              Synaptic terminals (boutons)
              (contain synaptic vesicles
               + mitochondria)
                       ↓
               NEXT NEURON or EFFECTOR
               (impulse flow: away from soma)

Labels: Dendrite | Dendritic spines | Cell body (soma) | Nucleus | Nucleolus | 
        Nissl bodies | Axon hillock | Axon | Myelin sheath | Node of Ranvier | 
        Schwann cell | Collateral branch | Synaptic terminal/bouton | 
        Synaptic vesicles

---

Q15. Describe the histological structure of spleen with a well labelled diagram. [6 marks → 10 marks]

Introduction

---

Histological Structure

1. Capsule

• Outermost layer
• Dense fibrous connective tissue (collagen + elastic fibers)
• Contains smooth muscle (allows contraction → expels stored blood)
• Covered by visceral peritoneum (mesothelium) on the outer surface
• Thicker at the hilum

2. Trabeculae

• Fibrous partitions projecting inward from the capsule
• Contain: collagen, elastic fibers, smooth muscle
• Carry trabecular arteries and veins
• Divide the spleen into compartments (imperfect lobules)

3. Splenic Pulp (Parenchyma)

---

A. WHITE PULP (Lymphoid tissue)

• Sleeve of lymphoid tissue surrounding the central artery (which is eccentrically placed)
• Predominantly T lymphocytes (CD4+ and CD8+)
• Contains dendritic cells (antigen presentation to T cells)
• Equivalent to thymus-dependent zone of lymph node

• Spherical aggregates of B lymphocytes on one side of the PALS
• Primary follicles: dense, unstimulated B cells
• Secondary follicles: have germinal center (after antigen stimulation)
  • Germinal center: pale-staining; centroblasts, centrocytes, FDC, tingible body macrophages
  • Mantle zone: dark ring of naïve B cells
• Blood-borne antigens arriving via central artery contact B cells here

---

B. MARGINAL ZONE

• Interface between white and red pulp
• Contains unique marginal zone B cells (respond rapidly to blood-borne polysaccharide antigens; produce IgM; T-independent responses)
• Marginal zone macrophages: phagocytose particulate antigens
• Contains marginal sinus (terminal capillaries open here)
• The marginal zone is the first site where blood-borne antigens are trapped and presented

---

C. RED PULP (Blood-filtering tissue)

• Irregular, anastomosing cords of loose connective tissue
• Contain: macrophages (responsible for blood filtration), plasma cells, lymphocytes, red blood cells (in transit)
• Macrophages engulf old, rigid, or damaged RBCs (culling), parasitized RBCs (malaria), iron recycling
• Also perform "pitting" - removing inclusions from RBCs without destroying the cell

• Wide, irregular, blood-filled channels
• Lined by unique elongated stave cells (endothelial cells arranged parallel to sinus long axis, like barrel staves)
• Stave cells are supported by hoop-like reticular fibers (ring fibers perpendicular to long axis)
• Gaps (slit pores) between stave cells (2-3 μm wide)
• Normal deformable RBCs squeeze through these slits to re-enter circulation
• Old/rigid/abnormal RBCs cannot pass → remain in cords → phagocytosed by macrophages

---

4. Reticular Framework

• Both white and red pulp are supported by a reticular fiber network (type III collagen)
• Produced by reticular cells (fibroblast-like stromal cells)

---

Labelled Diagram

     ╔═══════════════════════════════════════════╗
     ║ CAPSULE (fibrous + smooth muscle)          ║
     ╠═══════════════════════════════════════════╣
     ║ TRABECULA ─── Trabecular artery/vein       ║
     ╠═══════════════════════════════════════════╣
     ║                                           ║
     ║  WHITE PULP:                              ║
     ║  ┌────────────────────────┐               ║
     ║  │ PALS (T cells) ─ around│               ║
     ║  │ Central artery          │               ║
     ║  │  ⊙ Lymphoid follicle   │               ║
     ║  │  (B cells + germinal   │               ║
     ║  │   center)              │               ║
     ║  └────────────────────────┘               ║
     ║                                           ║
     ║  MARGINAL ZONE (marginal B cells,         ║
     ║  marginal Mφ, marginal sinus)             ║
     ║                                           ║
     ║  RED PULP:                                ║
     ║  ═══ Splenic (Billroth's) cords           ║
     ║  --- Venous sinusoids (stave cells)        ║
     ║      Macrophages | Plasma cells           ║
     ║      RBCs squeezing through slit pores    ║
     ╚═══════════════════════════════════════════╝

Labels: Capsule | Trabecula | Trabecular artery | Central artery | PALS (T zone) | 
        Lymphoid follicle | Germinal center | Mantle zone | Marginal zone | 
        Marginal zone B cells | Splenic cords (Billroth's cords) | 
        Venous sinusoids | Stave cells | Slit pores | Macrophages | 
        Reticular fibers

---

Q16. Describe the histological structure of skeletal muscle with a well labelled diagram. [3+2=5 → 10 marks]

Introduction

---

Organization (Connective Tissue Framework)

• Blood vessels travel from epimysium → perimysium → endomysium → capillary network
• Nerves travel same route to reach motor end plates

---

Histological Features of a Muscle Fiber (Cell)

1. Shape and Size

• Cylindrical/elongated fibers, running parallel to muscle long axis
• Length: 1 mm to 30 cm
• Diameter: 10-100 μm

2. Multinucleated

• Each fiber has multiple nuclei (hundreds per cell)
• Nuclei are peripherally located (just beneath sarcolemma)
• This is the key difference from cardiac muscle (central nuclei) and smooth muscle (single central nucleus)

3. Sarcolemma

• Cell membrane of muscle fiber
• Has T-tubules (transverse tubules): invaginations of sarcolemma at A-I junction
• T-tubules transmit electrical signal from surface deep into the fiber

4. Sarcoplasm (cytoplasm)

• Filled with myofibrils (longitudinal cylinders of contractile proteins)
• Mitochondria: numerous, between myofibrils (aerobic energy production)
• Sarcoplasmic reticulum (SR): smooth ER equivalent; surrounds myofibrils; stores and releases Ca²⁺
• Triad: one T-tubule flanked by two terminal cisternae of SR (at A-I junction)

5. Striations (Cross-striations)

• A band (dark, anisotropic): overlapping actin and myosin; does NOT shorten during contraction
  • Contains M line: bisects H zone; composed of proteins linking adjacent thick filaments
  • Contains H zone: lighter central area of A band; contains only thick filaments (no actin overlap)
• I band (light, isotropic): contains only thin (actin) filaments; anchored to Z discs; shortens during contraction
  • Z disc (Z line): bisects I band; anchors actin filaments of adjacent sarcomeres; contains alpha-actinin
• Sarcomere: functional unit of contraction; from one Z disc to the next Z disc
  • Length at rest: ~2.2 μm
  • During contraction: I bands shorten, H zone shortens/disappears, A band remains constant (sliding filament theory)

6. Myofibrils

• Each myofibril consists of repeating sarcomeres
• Sarcomeres of adjacent myofibrils are in register → explains the cross-striations

7. Contractile Proteins

• Thick filaments (myosin): in A band; myosin has a head (ATPase) and tail
• Thin filaments (actin): in I and A bands; double helix of G-actin
  • Tropomyosin: wraps around actin; blocks myosin-binding sites
  • Troponin complex: TnT (binds tropomyosin), TnC (binds Ca²⁺), TnI (inhibitory)
• Titin: elastic protein from Z disc to M line; spring-like; maintains sarcomere integrity
• Nebulin: inextensible; runs length of thin filament; template for thin filament assembly
• Dystrophin: links actin cytoskeleton to extracellular matrix via sarcolemma

8. Muscle Fiber Types

---

Labelled Diagram

     ← Epimysium (outer sheath) ─────────────────────
     ← Perimysium (around fascicle) ─────────────────
     ← Endomysium (around individual fiber) ──────────
     
     Muscle Fiber:
     ┌──────────────────────────────────────────────┐
     │ Peripheral nuclei  □□□□□□□□□□□               │
     │                                              │
     │ ╠═╬═╬═╬═╬═╬═╬═╬═╬═╬═╬═╬═╬═╬═╬═╬═╬═╬═╬═╣   │ ← Myofibrils
     │                                              │
     │   |Z|..I..|Z|=====A====|Z|..I..|Z|          │
     │      ←sarcomere→                             │
     │                                              │
     │   I band  │  A band  │  I band               │
     │     (light)│  (dark)  │  (light)             │
     │           H zone                             │
     │           M line                             │
     │   T-tubule ↕ (at A-I junction)               │
     │   SR terminal cisterna on each side          │
     └──────────────────────────────────────────────┘

Labels: Epimysium | Perimysium | Endomysium | Peripheral nucleus | 
        Sarcolemma | T-tubule | Sarcoplasmic reticulum | Triad | 
        Sarcomere | Z disc | I band | A band | H zone | M line | 
        Thick (myosin) filaments | Thin (actin) filaments | Myofibril

---

Q17. Name the components / Histological structure of thymus with a well labelled diagram.

Introduction

---

Gross Organization

• Two asymmetrical lobes
• Each lobe divided into lobules by connective tissue septa (from the capsule)
• Each lobule has a cortex (dark) and a medulla (light)
• Medullary regions of adjacent lobules are continuous

---

Histological Components

1. Capsule

• Thin connective tissue capsule (less dense than lymph node capsule)
• From capsule, fibrous trabeculae (septa) extend inward, dividing the gland into incomplete lobules
• No afferent lymphatics (unlike lymph nodes)

2. Thymic Lobules

• Cortex: densely packed lymphocytes (thymocytes) → appears dark on H&E
• Medulla: fewer lymphocytes → appears lighter; contains unique structures

---

A. CORTEX

• Cortex is densely packed with immature thymocytes (T cell precursors from bone marrow)
• These are the most numerous cells: immature, rapidly dividing, undergoing selection
• Stages: pro-T → pre-T → double-negative (CD4-/CD8-) → double-positive (CD4+/CD8+) → single-positive (CD4+ or CD8+) → mature naïve T cell
• ~95% of thymocytes die by apoptosis (negative selection = self-reactive T cells eliminated)
• 5% pass positive + negative selection → enter medulla → exit as mature T cells

• Form a reticular epithelial framework (not true reticular fibers - these are epithelial cells)
• Type I TEC: subcapsular; form blood-thymus barrier
• Type II and III TEC: cortical; present self-antigens (MHC I and II) to thymocytes for positive selection
• Joined by desmosomes; stellate shape
• Do NOT contain Hassall's corpuscles (only in medulla)

• Prevents blood-borne antigens from reaching cortical thymocytes
• Composed of:
  • Capillary endothelial cells (continuous, with tight junctions)
  • Basement membrane
  • Perivascular space (macrophages here)
  • Basement membrane of epithelial cells
  • Type I thymic epithelial cells (pericapillary sheath)
• Ensures thymocytes are educated in an antigen-free environment during positive selection

---

B. MEDULLA

• Fewer in number (only mature T cells that have passed selection)
• Single-positive (CD4+ or CD8+) mature T cells
• About to exit the thymus

• More numerous relative to thymocytes than in cortex
• Express AIRE (Autoimmune Regulator) gene → enables them to present peripheral self-antigens (e.g., insulin, thyroglobulin) → negative selection (deletion of self-reactive T cells)
• Do not form blood-thymus barrier (no barrier equivalent in medulla)

• Diagnostic/pathognomonic feature of thymus
• Concentrically arranged whorls of keratinized and degenerating medullary epithelial cells
• Central cells: large, eosinophilic, may be keratinized, calcified, or necrotic
• Peripheral cells: more flattened, arranged in concentric layers (onion-skin appearance)
• Appear as pale, round/oval concentric structures in medulla
• Function: may secrete cytokines (TSLP) that promote regulatory T cell (Treg) differentiation; may be end-stage epithelial cells
• Stain: eosinophilic on H&E; number increases with age

• Numerous throughout thymus
• Remove apoptotic thymocytes (negative selection results in ~95% cell death in cortex)
• "Starry sky" appearance from macrophages engulfing apoptotic cells

• Located in medulla and corticomedullary junction
• Present self-antigens for negative selection of T cells

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C. Corticomedullary Junction

• Region between cortex and medulla
• Post-capillary venules (HEV-like) through which mature T cells exit
• Entry point of T-cell precursors from bone marrow (via blood)

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Summary of Functions

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Labelled Diagram

       ┌──────────────────────────────────────────────┐
       │ CAPSULE (thin fibrous CT)                     │
       │      └── TRABECULA/SEPTUM                     │
       │                                              │
       │  ┌──────────────────────────────────┐        │
       │  │ LOBULE:                          │        │
       │  │                                  │        │
       │  │  CORTEX (DARK):                  │        │
       │  │  ●●●●●●● Immature thymocytes     │        │
       │  │  ╲  ╱  Cortical TEC (reticular)  │        │
       │  │  Blood-Thymus Barrier             │        │
       │  │  (capillary → TEC sheath)         │        │
       │  │                                  │        │
       │  │........CORTICOMEDULLARY JUNCTION  │        │
       │  │                                  │        │
       │  │  MEDULLA (PALE):                 │        │
       │  │  ○ ○ ○ Mature T cells (fewer)    │        │
       │  │  Medullary TEC (AIRE+)           │        │
       │  │  ⊕ HASSALL'S CORPUSCLE           │        │
       │  │  (concentric whorls, eosinophilic)│        │
       │  │  Macrophages | Dendritic cells    │        │
       │  └──────────────────────────────────┘        │
       └──────────────────────────────────────────────┘

Labels: Capsule | Fibrous septa/trabeculae | Cortex | Medulla | 
        Immature thymocytes (cortex) | Mature thymocytes (medulla) | 
        Cortical thymic epithelial cells | Medullary thymic epithelial cells | 
        Blood-thymus barrier | Capillary | Hassall's corpuscle | 
        Macrophage | Dendritic cell | Corticomedullary junction

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Study Tips for KU BDS Board Exam:

• Every 10-mark question expects: definition (2 marks), classification/features (5-6 marks), well-labeled diagram (2-3 marks)
• Diagrams do not need to be artistic - clear labels and correct relationships score full marks
• Always mention staining characteristics for histology questions
• For cartilage: Orcein stain = elastic; H&E = hyaline; PAS = ground substance
• Memorize locations for each tissue type - examiners frequently ask "where is it found?"