Loco – Histology

• The physical properties of cartilage (like tensile strength and resilience) mainly depend on its extracellular matrix (ECM).

• Collagen type II fibrils are the dominant structural component in hyaline cartilage.

• They form a fine meshwork that provides tensile strength while allowing the ground substance (rich in proteoglycans and water) to resist compressive forces.

• This combination makes cartilage strong, flexible, and capable of withstanding mechanical stress.

🔴 Why the others are wrong

• Neurovascular bundles → Cartilage is avascular and aneural; nutrients diffuse through the matrix, so neurovascular bundles are absent.

• Fibronectin → A glycoprotein involved in cell adhesion and wound healing, but not the main determinant of cartilage’s physical strength.

• Chondronectin → Helps chondrocytes attach to collagen, but it does not determine the bulk physical properties of cartilage.

• Hydroxyapatite crystals → Found in bone, not cartilage. Cartilage matrix is not mineralized (except in calcified cartilage during endochondral ossification).

• Hyaline cartilage is the most common type of cartilage.

• It is found at articular ends of long bones, costal cartilages, nose, trachea, and bronchi.

• It provides a smooth, low-friction surface for joint movement and supports the respiratory tract.

• Its matrix is rich in type II collagen and proteoglycans, which make it resilient and glassy in appearance.

🔴 Why the others are wrong

• Vascular tissue → Cartilage is avascular; nutrients diffuse through the matrix from surrounding perichondrium or synovial fluid.

• It undergoes interstitial growth only → Hyaline cartilage shows both interstitial and appositional growth, not just interstitial.

• Contains type 1 collagen → That is characteristic of fibrocartilage, not hyaline. Hyaline has type II collagen.

• Chondrocytes arranged as parallel rows → Seen in fibrocartilage (between collagen bundles), not hyaline cartilage (where chondrocytes lie in isogenous groups inside lacunae).

• Osteoclasts are large, multinucleated, bone-resorbing cells.

• They originate from the monocyte–macrophage lineage of hematopoietic stem cells in the bone marrow.

• Monocytes fuse together to form multinucleated osteoclasts, which then attach to bone surfaces and secrete acid and enzymes for bone resorption.

🔴 Why the others are wrong

• Osteocytes → Mature bone cells derived from osteoblasts, involved in maintaining bone matrix, not resorption.

• Osteoblasts → Bone-forming cells, but they do not give rise to osteoclasts.

• Osteoprogenitor cells → Precursors for osteoblasts, not osteoclasts.

• Chondrocytes → Cartilage cells, unrelated to osteoclast lineage.

• Fibrocartilage is a hybrid between dense connective tissue and cartilage.

• Its hallmark under the light microscope is the presence of thick bundles of type I collagen fibers interspersed with chondrocytes in lacunae.

• These fibers give fibrocartilage great tensile strength (e.g., intervertebral discs, pubic symphysis, menisci).

🔴 Why the others are wrong

• Mineralized matrix → Seen in bone, not in cartilage.

• Elastic fibers → Present in elastic cartilage (e.g., external ear, epiglottis), not fibrocartilage.

• Acidophilic → General staining characteristic, but not a defining feature of fibrocartilage.

• Perichondrium → Absent in fibrocartilage (unlike hyaline or elastic cartilage, which usually have perichondrium).

• Compact bone:

  • Dense, strong, forms the outer shell of bones.

  • Contains Haversian systems (osteons) → concentric lamellae surrounding a central canal (with blood vessels & nerves).

  • Provides mechanical strength.

• Spongy (cancellous) bone:

  • Found at epiphyses of long bones, vertebrae, flat bones.

  • Lacks Haversian systems, instead has trabeculae with marrow in between.

  • Lighter, designed for shock absorption and hematopoiesis.

Why the other options are wrong:

• Lines medullary cavity → True for compact bone, but spongy bone also surrounds marrow in trabeculae. Not unique.

• Thoracic skeleton is made up from this → Thoracic skeleton (ribs, sternum, vertebrae) has both compact & spongy.

• Cranial bones are made up from this → Cranial bones are diploë (compact–spongy–compact). Not unique.

• It is also called cancellous bone → That’s spongy bone, not compact.

✅ Key Clinical Pearl:
When you see Haversian system, always think compact bone.
Spongy bone = trabeculae, no osteons.

• T-tubules (transverse tubules) are invaginations of the sarcolemma that penetrate deep into the muscle fiber, ensuring the action potential reaches the interior of the fiber quickly, which triggers calcium release from the sarcoplasmic reticulum and initiates contraction.

❌ Why the other options are incorrect:

• Sarcoplasmic reticulum provides ATP → Wrong; it stores Ca²⁺, not ATP. ATP is produced mainly by mitochondria and glycolysis.

• They do not have cross-striations → Wrong; skeletal muscle is striated due to the arrangement of actin and myosin filaments.

• Mitochondria are a store house of Ca²⁺ → Wrong; mitochondria produce ATP; calcium is stored in the sarcoplasmic reticulum.

• Ca²⁺-calmodulin complex binds to actin filament → Wrong; in skeletal muscle, troponin binds Ca²⁺, not calmodulin (calmodulin is more relevant in smooth muscle).

• Hyaline cartilage is the most prone to calcification with age (especially articular cartilage at the ends of long bones, costal cartilages, and tracheal rings).

• The calcification is part of the normal aging process and does not necessarily mean ossification, though in some cases it precedes endochondral bone formation.

❌ Why not the others?

• Elastic cartilage → never normally calcifies (e.g., ear, epiglottis).

• Fibrocartilage → found in intervertebral discs, menisci; it resists compression but rarely calcifies except in pathology.

• Articular cartilage → is a special type of hyaline cartilage; while it may undergo degenerative calcification, general hyaline cartilage is the most likely overall.

• Mixed cartilage → not a standard histological classification.

Osteocytes sit in lacunae and send long, thin processes through canaliculi. These processes need to communicate for:

• Nutrient exchange (since bone matrix is avascular)

• Signal transmission (mechanical stress → bone remodeling signals to osteoblasts/osteoclasts)

👉 This is achieved through gap junctions, which allow direct passage of ions and small molecules between adjacent osteocytic processes.

❌ Why not the others?

• Desmosomes / Macula adherens → provide strong mechanical adhesion (seen in epithelial cells, not osteocytes).

• Zonula adherens → actin anchoring junctions in epithelia, not for metabolic coupling.

• Tight junctions (zonula occludens) → seal intercellular spaces (common in blood-brain barrier, gut epithelium), not in bone.

• Osteoclasts are multinucleated, bone-dissolving cells derived from monocyte–macrophage lineage.

• Their main role: bone resorption → they secrete acids (to dissolve hydroxyapatite) and enzymes (to degrade collagen and matrix).

• This is the direct process by which bone is broken down, releasing calcium and phosphate into blood.

❌ Why the others are wrong

• Bone demineralization → refers specifically to loss of mineral content, not the complete breakdown of bone matrix (collagen + mineral) as done by osteoclasts.

• Bone remodeling → overall cycle of bone renewal, involving both osteoclasts (resorption) and osteoblasts (formation). Not just breaking.

• Bone mineralization → done by osteoblasts, where calcium and phosphate are deposited into osteoid.

• Bone modeling → shaping of bone during growth, involving both deposition and resorption, not simply breakdown.

Collagen type I constitutes about 90% of the organic matrix of bone. It forms a strong fibrillar framework that provides tensile strength to the bone and acts as a scaffold for the deposition of hydroxyapatite crystals, which give the bone its hardness.

Why the other options are incorrect:

• Elastin: Present in elastic tissues like arteries and ligaments; negligible in bone.

• Collagen type II: Found mainly in cartilage, not bone.

• Collagen type III: Present in reticular fibers of soft tissues, skin, and blood vessels; minor in bone.

• Fibronectin: Glycoprotein involved in cell adhesion and matrix organization; present but not the major structural protein of bone.

• The sarcomere is the basic structural and functional unit of a myofibril.

• It extends from one Z-line (Z-disc) to the next Z-line.

• Within a sarcomere:

  • A band = entire length of thick filament (myosin), includes overlap with thin filaments.

  • I band = only thin filaments (actin), on either side of Z-line.

  • H zone = central lighter part of A band, only thick filaments (no overlap).

  • M line = middle of the H zone, site where thick filaments are linked.

Thus, when asked “between two consecutive Z-lines” → it defines the sarcomere.

Why not the others?

• A band → thick filament region, does not span between Z-lines. ❌

• I band → thin filaments only, doesn’t cover the whole span. ❌

• H zone → central thick-only part, within A band. ❌

• M line → central axis of sarcomere, not the whole unit. ❌

• Cartilage is resilient and shock-absorbing because of its extracellular matrix, which is rich in proteoglycans and glycosaminoglycans (GAGs) such as chondroitin sulfate, keratan sulfate, and hyaluronic acid.

• These GAGs are highly negatively charged, attracting large amounts of water molecules → the matrix becomes hydrated, gel-like, and compressible.

• This hydration allows cartilage to act like a shock absorber, resisting compressive forces while remaining flexible.

• Why not others?

  • Non-covalent bonding between GAGs and proteins → helps in structure, but alone doesn’t explain the shock-absorbing property.

  • Regenerative ability of chondrocytes → is actually poor, so not the reason.

  • Ossification of cartilage → makes it rigid, not resilient.

  • All of these → incorrect, since only hydration of GAGs is the real mechanism.

• Howship’s lacunae = small resorption bays found on the surface of bone.

• They are created by osteoclast activity when bone is being resorbed.

• Osteoclasts are large, multinucleated cells derived from monocyte/macrophage lineage.

• They release acid + proteolytic enzymes → dissolve mineralized matrix → form these lacunae.

❌ Why the other options are wrong:

• Chondrocytes → found in cartilage lacunae, not bone.

• Osteoprogenitor cells → precursors of osteoblasts, found in periosteum and endosteum.

• Osteocytes → mature bone cells, live inside lacunae (but not Howship’s, rather osteocytic lacunae).

• Osteoblasts → bone-forming cells, line bone surfaces, do not occupy Howship’s lacunae.

Howship’s lacunae, also called resorption bays, are shallow depressions found on the surface of bone. These are active sites of bone resorption where osteoclasts are located.

• Osteoclasts are large, multinucleated cells derived from hematopoietic stem cells of the monocyte-macrophage lineage.

• They secrete acid and lysosomal enzymes (e.g., cathepsin K, H⁺ ions) that dissolve mineralized matrix and collagen, allowing remodeling and calcium homeostasis.

• These enzymes and protons are secreted into the Howship’s lacuna, forming a sealed acidic microenvironment for bone breakdown.

❌ Why the Other Options Are Incorrect:

• Osteocytes
  ➤ Mature bone cells trapped in lacunae within the bone matrix, not in Howship’s lacunae. They help maintain the matrix.

• Chondrocytes
  ➤ Found in cartilage, not bone. They reside in lacunae of hyaline or elastic cartilage, not Howship’s lacunae.

• Chondroblasts
  ➤ Precursor cells of cartilage that form the extracellular matrix of cartilage, unrelated to bone resorption.

• Osteoblasts
  ➤ Bone-forming cells found on bone surfaces, responsible for laying down new matrix, not in resorption bays.

Proteoglycans are heavily glycosylated proteins composed of a core protein with one or more glycosaminoglycan (GAG) chains attached. These GAGs are long, negatively charged polysaccharide chains that attract water and cations (like Na⁺), causing the matrix to swell and resist compressive forces. This property is especially important in cartilage, intervertebral discs, and synovial joints, where tissues must withstand mechanical stress and compression.

The high water content due to the hydrophilic nature of proteoglycans provides both lubrication and shock absorption in connective tissues.

Why the other options are incorrect:

• Mucins ❌
  Mucins are glycoproteins found in mucus secretions, contributing to lubrication and protection of epithelial surfaces but not primarily responsible for resisting compressional forces.

• Fibrin ❌
  Fibrin is a fibrous protein involved in blood clotting, forming a mesh that stabilizes a blood clot, not related to hydration or resistance to compression.

• Collagen ❌
  Collagen is a fibrous structural protein that provides tensile strength to tissues (like skin, tendons, bones), not compressive resistance or hydration.

• Laminin ❌
  Laminin is a component of the basement membrane, playing roles in cell adhesion and differentiation, not primarily in hydration or swelling pressure.

• Long bones increase in length through the epiphyseal (growth) plate, which is made of hyaline cartilage.
• The cartilage here undergoes interstitial growth (growth from within) by proliferation of chondrocytes.
• This provides a scaffold that is then replaced by bone via endochondral ossification.
• Important distinction: bone itself cannot grow interstitially because of its rigid mineralized matrix. Growth in length therefore depends on cartilage.

❌ Removal of calcified cartilage

• This is part of endochondral ossification, but removal alone does not account for bone lengthening.
• It is a step in remodeling, not the primary growth mechanism.

❌ Endochondral deposition of bone tissue

• Endochondral ossification is indeed how cartilage is replaced by bone, but the increase in length comes first from cartilage proliferation (interstitial growth), not from bone deposition itself.

❌ Appositional deposition of bone tissue

• Appositional growth = addition of new bone layers to the outer surface (beneath periosteum).
• This increases the thickness (diameter) of bone, not its length.

❌ Interstitial growth of bone tissue

• Bone cannot grow interstitially because its matrix is calcified and rigid.
• Only cartilage is capable of interstitial growth.

✅ Correct Answer:

Osteoblasts

• Osteoblasts are the bone-forming cells derived from osteoprogenitor cells.
• They are found lining the bone surfaces, especially in areas of active bone formation.
• Their main roles are:
  • Secreting osteoid (unmineralized bone matrix).
  • Initiating mineralization.
• Once surrounded by matrix, osteoblasts differentiate into osteocytes.

❌ Osteoclast

• Large multinucleated cells involved in bone resorption.
• Found in Howship’s lacunae (resorption pits), not simply along intact bone surfaces.

❌ Osteon

• Not a cell, but a structural unit of compact bone (Haversian system).
• Composed of concentric lamellae around a central canal.
• Therefore, this is a histological structure, not a cell.

❌ Lymphocytes

• White blood cells involved in immune responses.
• Not part of normal bone histology unless there’s pathology (e.g., inflammation, infection, marrow involvement).

❌ Chondrocytes

• Cells of cartilage, found in lacunae within cartilage matrix.
• Not present in bone tissue.

• T-tubules (transverse tubules) are invaginations of the sarcolemma (the muscle cell membrane).
• They extend deep into the muscle fiber, allowing the action potential to travel quickly into the interior of the cell.
• This ensures synchronous contraction of all myofibrils by triggering the sarcoplasmic reticulum to release calcium.
• So, while the T-tubules are closely associated with the sarcoplasmic reticulum (forming the triad in skeletal muscle: T-tubule + 2 terminal cisternae), structurally they are derived from the sarcolemma, not the SR.

❌ Nucleus

• The nuclei of skeletal muscle are peripheral, multinucleated, but they do not form tubular extensions.

❌ Rough Endoplasmic Reticulum

• RER is mainly involved in protein synthesis.
• In muscle, the specialized ER is the sarcoplasmic reticulum (SR), not the T-tubules.

❌ Sarcoplasmic Reticulum

• The SR stores and releases calcium for contraction.
• It lies adjacent to the T-tubules, but T-tubules are not extensions of the SR—they are separate but functionally linked.

❌ Sarcoplasm

• The sarcoplasm is simply the cytoplasm of the muscle cell (contains mitochondria, glycogen, myoglobin).
• It is not structurally continuous with T-tubules.

Compact bone is organized into osteons (Haversian systems), each consisting of a central canal surrounded by concentric lamellae.
The bone matrix is rich in type I collagen, which provides tensile strength and resists pulling forces.
This collagen, along with hydroxyapatite crystals, makes bone both strong and rigid.

❌ Why the other options are incorrect:

• Type VII:
  Found in anchoring fibrils of the basement membrane, not in bone.

• Type IV:
  Found in the basement membrane (forms a meshwork), not bone matrix.

• Type II:
  Found in cartilage, not compact bone.

• Type III:
  Found in reticular fibers (e.g., lymphoid organs, bone marrow), not in compact bone.

Compact bone and spongy bone are two structural types of bone tissue:

• Compact bone:

  • Dense, strong, and forms the outer layer of bones.

  • Organized into Haversian systems (osteons) → concentric lamellae around a central Haversian canal containing blood vessels and nerves.

  • Provides mechanical strength.

• Spongy bone (cancellous bone):

  • Consists of trabeculae with bone marrow in between.

  • No Haversian systems.

  • Found mainly in epiphyses of long bones and in flat bones (like cranial bones).

❌ Why the other options are incorrect:

• Thoracic skeleton is made up from this:
  Thoracic skeleton (e.g., sternum, ribs, vertebrae) has both compact and spongy bone. Not unique to compact bone.

• Lines medullary cavity:
  The medullary cavity is lined by spongy bone internally, with compact bone forming the outer wall. Not distinguishing.

• It is also called cancellous bone:
  Cancellous bone is another name for spongy bone, not compact bone.

• Cranial bones are made up from this:
  Cranial bones are primarily spongy (diploë) sandwiched between compact bone layers. Not a unique feature of compact bone.

Cell adhesion relies on specialized proteins that mediate interactions between cells or between cells and the extracellular matrix. These proteins maintain tissue architecture, enable signaling, and regulate cell movement. Cadherins, integrins, selectins, and CAMs are all families of adhesion molecules essential for these processes. In contrast, calmodulin is not a cell adhesion protein; it is a calcium-binding regulatory protein inside the cell, important in activating enzymes and signaling pathways, not in adhesion.

❌ Why the other options are incorrect:

• Cadherin:
  A calcium-dependent adhesion molecule responsible for cell-to-cell adhesion in adherens junctions and desmosomes.

• Integrins:
  Mediate cell-to-extracellular matrix adhesion, linking the cytoskeleton to extracellular proteins like fibronectin and laminin.

• Selectins:
  Play a role in transient cell-cell interactions, especially in the immune system (e.g., leukocyte rolling on endothelium).

• Cell adhesion molecules (CAMs):
  A general term for proteins (like NCAM, ICAM, VCAM) that facilitate stable cell-to-cell adhesion and communication.

Osteoclasts are large, multinucleated cells derived from the monocyte–macrophage lineage. Their main function is bone resorption, which requires secretion of enzymes (like collagenases and cathepsins) and acids to dissolve bone matrix. To perform this, osteoclasts have abundant rough endoplasmic reticulum (RER) for protein synthesis and numerous mitochondria for energy. They attach to the bone surface, creating Howship’s lacunae during resorption.

❌ Why the other options are incorrect:

• Housed as isogenous cells in lacunae:
  This describes chondrocytes in cartilage, not osteoclasts. Osteoclasts sit in resorption bays (Howship’s lacunae).

• They communicate through gap junctions:
  This is true for osteocytes, which extend processes through canaliculi and connect via gap junctions. Osteoclasts do not use this mechanism.

• They give rise to osteophytes:
  Osteophytes (bony outgrowths in osteoarthritis) are formed by osteoblast activity, not osteoclasts. Osteoclasts only break down bone.

• They are immature cells:
  Osteoclasts are fully differentiated giant cells. Immature precursors are osteoprogenitor cells (for osteoblasts), not osteoclasts.

Cartilage is primarily composed of Type II collagen fibers, which are thin fibrils designed to resist compressive stress and provide tensile strength in structures like hyaline and elastic cartilage. They are associated with proteoglycans (e.g., aggrecan) that allow cartilage to withstand pressure.

❌ Why the other options are incorrect:

• Type 4: Found in the basement membrane (forms a network, not fibrils).

• Type 1: Major collagen of bone, tendons, ligaments, skin, providing tensile strength, not typical of cartilage.

• Type 6: Found in the intervertebral disc and connective tissue interfaces, not the primary collagen in cartilage.

• Type 3: Known as reticular fibers, present in lymphoid tissues and bone marrow, not in cartilage.

Howship’s lacunae are small depressions or cavities on the bone surface where bone resorption occurs. These lacunae are formed by the osteoclasts, which are large, multinucleated cells that break down bone matrix by secreting acids and proteolytic enzymes. This process is essential for bone remodeling and calcium homeostasis.

❌ Why the other options are incorrect:

• Osteoblasts: Bone-forming cells; they synthesize new bone rather than resorb it, and are not found in Howship’s lacunae.

• Osteocytes: Mature bone cells embedded in the lacunae of the bone matrix, not on the surface resorption pits.

• Chondrocytes: Cartilage cells; unrelated to bone resorption or Howship’s lacunae.

• Osteoprogenitor cells: Precursors of osteoblasts; located in the periosteum and endosteum, not in resorption lacunae.

The sarcolemma is the thin, specialized cell membrane that surrounds each skeletal muscle fiber. It plays a crucial role in the initiation and conduction of action potentials, as well as in maintaining the integrity of the muscle cell. The sarcolemma is located just beneath the endomysium and surrounds the sarcoplasm (cytoplasm of muscle cell), enclosing the myofibrils within.

Let’s break down each option:

• Sarcolemma ✅
  This is the correct answer. It is the plasma membrane of a muscle fiber and is essential for electrical signal conduction and muscle fiber integrity.

• Actin ❌
  Actin is a thin filament protein involved in muscle contraction. It is found within the myofibrils but does not enclose the muscle fiber.

• Myofibril ❌
  These are bundles of contractile proteins (actin and myosin) inside the muscle fiber. Each muscle fiber contains many myofibrils — they are internal components, not membranes.

• Sarcomere ❌
  The sarcomere is the structural and functional unit of contraction, composed of actin and myosin arranged in a repeating pattern within myofibrils. It is inside the myofibril, not a covering membrane.

• Myosin ❌
  Myosin is the thick filament protein involved in contraction and found inside the sarcomere. Like actin, it is a contractile protein, not a membrane.

Bone is a composite tissue composed of inorganic minerals (mainly hydroxyapatite) and organic components, primarily proteins. The organic matrix, also known as osteoid, contributes to the tensile strength and flexibility of bone.

Among the organic matrix:

• Type I collagen is the dominant protein, making up more than 90% of the organic content of bone.

  • It forms strong triple-helical fibers, which serve as the scaffolding for mineral deposition.

  • Its structure is crucial for mechanical strength and resistance to stretching.

Why the Other Options Are Incorrect:

• Osteocalcin:
  ❌ A non-collagenous protein produced by osteoblasts, involved in calcium binding and mineralization, but constitutes only a small fraction of the organic matrix.

• Bone Morphogenic Protein (BMP):
  ❌ A group of growth factors that regulate bone formation and healing, not a structural component of the matrix.

• Type V Collagen:
  ❌ Present in small amounts in bone and other connective tissues but does not contribute significantly to the organic content of bone.

• Osteopontin:
  ❌ Another non-collagenous protein, important for cell adhesion and signaling during bone remodeling, but not a major structural protein.

• Osteocytes are mature bone cells derived from osteoblasts. When an osteoblast becomes embedded in the matrix it secreted, it transforms into an osteocyte.

• Osteocytes reside in lacunae and maintain communication with other osteocytes through canaliculi.

• Their role is to maintain the bone matrix and regulate mineral homeostasis.

❌ Why the Other Options Are Incorrect:

• Osteoblasts → Responsible for synthesizing new bone matrix (bone formation), not for long-term maintenance.

• Osteoclasts → Multinucleated giant cells responsible for bone resorption, not matrix maintenance.

• Both osteoblasts and osteoclasts → They work in bone remodeling, but neither maintains the already-formed matrix.

• Fibroblasts → Found in connective tissue; they secrete collagen and ECM, but they are not bone cells.

• Cartilage is primarily composed of type II collagen, which forms fine fibrils that are well suited for resisting compressive forces.

• Type II collagen is the major fibril-forming collagen in hyaline cartilage (the most common type of cartilage).

• It provides the framework that traps proteoglycans and water, giving cartilage its resilience and ability to withstand pressure.

❌ Why the Other Options Are Incorrect:

• Collagen type I → Found mainly in bone, tendon, ligaments, and fibrocartilage, not in hyaline cartilage.

• Collagen type III → Known as reticular collagen; found in skin, blood vessels, and lymphoid tissues.

• Collagen type IV → Does not form fibrils; instead forms a meshwork in basement membranes.

• Collagen type V → Involved in the regulation of fibril assembly, but not the main structural collagen in cartilage.

The Z disk (or Z line) is the structure where actin (thin) filaments attach and extend in both directions into adjacent sarcomeres.

• Each sarcomere extends from one Z disk to the next.

• Thin filaments (actin) are firmly anchored in the Z disk by a protein complex (α-actinin and other proteins).

• During muscle contraction, thin filaments slide past the thick filaments (myosin), shortening the sarcomere.

❌ Why the Other Options Are Incorrect:

• I band → This is the region containing only thin filaments (actin), but it is not where they attach. It is the light band between Z disks.

• H zone → Central lighter area of the A band containing only thick filaments (myosin) when relaxed. No actin filaments attach here.

• M line → Middle of the sarcomere; where thick filaments (myosin) are anchored, not actin.

• A band → The dark band where thick and thin filaments overlap. It is a region, not a point of attachment.

• Longitudinal growth of bones occurs at the epiphyseal (growth) plate, which is made of hyaline cartilage.
• The cartilage cells (chondrocytes) undergo interstitial growth—that is, the cells divide and expand within the cartilage matrix.
• This interstitial expansion pushes the epiphysis and diaphysis apart, increasing bone length.
• Later, the cartilage is replaced by bone through endochondral ossification, but the actual “lengthening” mechanism is the interstitial growth of cartilage.

Why the other options are wrong

• Appositional deposition of bone growth
  → This increases bone thickness (diameter), not length.
• Endochondral deposition of bone tissue
  → Important for replacing cartilage with bone, but not the actual driver of elongation. It maintains the strength of the bone after cartilage expansion.
• Removal of calcified cartilage
  → Happens during ossification, but again, this is remodeling—not the cause of lengthening.
• Interstitial growth of bone tissue
  → Bone tissue itself cannot grow interstitially, only appositionally. Cartilage, unlike bone, can grow interstitially.

• Osteoblast → Correct
  These are bone-forming cells. They line the surface of bone and secrete osteoid (organic bone matrix). Once entrapped, they mature into osteocytes.
• Osteoclast → Also surface cells but resorptive
  These are large multinucleated cells that resorb bone. They too are found on surfaces but specifically in Howship’s lacunae (resorption pits).
  However, in a general slide of bone, the most characteristic surface cells you’ll spot are osteoblasts.
• Lymphocytes → Incorrect
  Not normal bone cells. Present only in bone marrow or pathological infiltration.
• Osteon → Incorrect
  Osteon = structural unit of compact bone, not a cell.
• Chondrocytes → Incorrect
  These are cartilage cells, not bone cells.

To understand the principal protein in bone, we need to look at the organic matrix of bone tissue.

Bone is composed of:

• Organic part (mainly collagen and some ground substance)
• Inorganic part (mainly hydroxyapatite crystals)

🔬 Organic Matrix of Bone:

• Collagen type I makes up ~90% of the organic matrix of bone.
• It provides tensile strength, allowing bone to resist pulling forces.
• These collagen fibers act as a scaffold for mineral deposition, which provides rigidity and compressive strength.

✅ Why Collagen Type I is Correct:

• It is the most abundant and strongest collagen type in the body.
• It is specifically responsible for the structural integrity of bones, tendons, and ligaments.

❌ Why the Other Options Are Incorrect:

• Collagen type III: Found in reticular fibers, especially in soft tissues like liver, spleen, and bone marrow.
• Collagen type IV: Found in basement membranes, not in bone.
• Fibronectin: A glycoprotein involved in cell adhesion, not the main structural component of bone.
• Elastin: Provides elasticity in tissues like lungs, skin, and large arteries — not found significantly in bone.

Let’s clarify a small confusion in the question:

• A muscle fiber is a long, multinucleated cell, often several centimeters to tens of centimeters long.
• A sarcomere, on the other hand, is the smallest functional and structural unit of a myofibril — the part of the muscle fiber that actually contracts.

The question seems to ask:

What is the typical length of a sarcomere, the repeating unit within a muscle fiber?

✅ Why 2 μm Is Correct:

• In a resting skeletal muscle, the average length of a sarcomere is about 2.0 to 2.2 micrometers (μm).
• This length allows optimal overlap of the thick (myosin) and thin (actin) filaments for maximal force generation.
• During contraction, it can shorten to ~1.5 μm, and when stretched, it can lengthen to ~3.5 μm, but 2 μm is the standard physiological resting length.

❌ Why the Other Options Are Incorrect:

• 1 μm:
  Too short for a resting sarcomere; may occur only during extreme contraction.
• 3 μm, 4 μm, 5 μm:
  These values are closer to the maximum stretched length of a sarcomere but are not representative of its typical state.
  A sarcomere at 5 μm would be so stretched it wouldn’t function properly.

✅ It contains predominantly type I collagen fibers – Correct. Fibrocartilage is rich in type I collagen, giving it great tensile strength. It also contains some type II collagen, but type I dominates, which is unlike hyaline cartilage (type II).

❌ Nasal cartilage is of fibrocartilaginous variety – Incorrect. Nasal cartilage is made of hyaline cartilage, not fibrocartilage.

❌ It is abundant in proteoglycans – Incorrect. While fibrocartilage contains some proteoglycans, it is less abundant in them compared to hyaline cartilage, and the extracellular matrix is more fibrous than hydrated.

❌ It is covered by perichondrium – Incorrect. Fibrocartilage lacks a perichondrium, which differentiates it from hyaline and elastic cartilage that do have this outer covering.

❌ Regions surrounding lacunae are relatively free of fibers – Incorrect. In fibrocartilage, collagen fibers extend throughout the matrix, including around lacunae.

🔹 Components of the Haversian System:

1. Haversian canal:

   • Central channel that runs longitudinally through each osteon

   • Contains blood vessels, lymphatics, and nerves

2. Blood capillaries:

   • Located within the Haversian canal

   • Supply nutrients and remove waste products

3. Osteocytes:

   • Mature bone cells

   • Found within lacunae between the concentric lamellae

   • Maintain the bone matrix

4. Interstitial lamellae:

   • Remnants of old osteons that have been partially resorbed

   • Fill gaps between newer, complete osteons

❌ Why Osteoprogenitor cells Are Not Part of the Haversian System:

• Osteoprogenitor cells are stem-like cells found primarily in:

  • Inner layer of periosteum

  • Endosteum

  • Lining of Haversian and Volkmann’s canals (but not within the osteon structure itself)

• They are involved in bone growth and repair, not structural maintenance of compact bone.

• They differentiate into osteoblasts, which later become osteocytes.

Fibrocartilage is a specialized connective tissue that combines features of dense regular connective tissue and cartilage. It is found in areas where tensile strength and resistance to compression are both required — such as:

• Intervertebral discs

• Pubic symphysis

• Menisci of the knee

• Temporomandibular joint

Let’s evaluate the options one by one:

❌ Nasal cartilage is of fibrocartilaginous variety — Incorrect. Nasal cartilage is hyaline cartilage, not fibrocartilage.

❌ It is abundant in proteoglycans — Incorrect. Compared to hyaline cartilage, fibrocartilage contains fewer proteoglycans and more collagen, making its matrix less basophilic.

❌ It is composed of type 2 collagen only — Incorrect. Fibrocartilage contains both type I and type II collagen, but is predominantly type I, which gives it its dense, fibrous texture.

❌ It is covered by perichondrium — Incorrect. Unlike hyaline or elastic cartilage, fibrocartilage lacks a perichondrium.

✅ Regions surrounding lacunae are relatively free of fibers — Correct. In fibrocartilage, the chondrocytes are found in rows within lacunae, and the matrix surrounding these lacunae appears more translucent and less fibrous, due to a relative lack of collagen fibers immediately around the cells.

Type II collagen is the principal collagen in hyaline cartilage and elastic cartilage.

It provides tensile strength and resists compressive forces while maintaining flexibility.

Found in:

• Articular cartilage (joints)

• Costal cartilage

• Tracheal rings

• Nasal septum

• Embryonic skeleton

❌ Why the Other Options Are Incorrect:

• Type I collagen
  → Found in bone, tendon, dermis, and ligaments — not predominant in cartilage.

• Type III collagen
  → Known as reticular fibers, found in lymphoid organs and wound healing, not typical of cartilage.

• Type IV collagen
  → Forms the basement membrane, not found in cartilage.

• Type VI collagen
  → Present in pericellular matrix around chondrocytes, but in very small amounts, not the primary type.

The resilience and shock-absorbing properties of cartilage are primarily due to the hydrated nature of glycosaminoglycans (GAGs) in the extracellular matrix. GAGs such as chondroitin sulfate and keratan sulfate are highly negatively charged and attract large amounts of water, forming a gel-like matrix that resists compression. Additionally, non-covalent bonding between GAGs and core proteins forms proteoglycans, which contribute to the structural integrity and elasticity of cartilage.

✅ Hydration of glycosaminoglycans (GAGs) — Correct. This is the key factor responsible for cartilage’s shock-absorbing capacity.

❌ Non-covalent bonding between glycosaminoglycans (GAGs) and proteins — Important for forming proteoglycans but not the main reason for shock absorption.

❌ Regenerative ability of chondrocytes — Cartilage actually has limited regenerative capacity, especially in adults.

❌ Ossification of cartilage — Ossification leads to bone formation, not elasticity or shock absorption.

❌ All of these — Incorrect; only hydration of GAGs directly explains the resilience and compressive strength of cartilage.

Skeletal muscle tissue is organized in three concentric layers of connective tissue, each surrounding a different level of muscle structure:

🔹 1. Endomysium

• A delicate connective tissue layer

• Surrounds each individual muscle fiber

• Contains capillaries, nerves, and extracellular matrix

• Made primarily of reticular fibers (type III collagen)

🔹 2. Perimysium

• Thicker connective tissue

• Surrounds a fascicle (a bundle of muscle fibers)

• Provides pathways for larger blood vessels and nerves

🔹 3. Epimysium

• Outermost layer

• Dense irregular connective tissue

• Encloses the entire muscle

• Continuous with the tendon

❌ Why the Other Options Are Incorrect:

• Perimysium
  → Wraps fascicles, not individual muscle fibers

• Sarcoplasmic reticulum
  → A specialized organelle inside the muscle fiber; stores calcium, not connective tissue

• Terminal cisternae
  → Enlarged ends of the sarcoplasmic reticulum; not part of any connective wrapping

• Epimysium
  → Surrounds the entire muscle, not individual fibers

🔹 T-tubules (Transverse Tubules):

• Invaginations of the sarcolemma
• Penetrate into the muscle fiber at regular intervals
• Run perpendicular to the muscle fiber’s length
• Located at the junction of the A and I bands
• Carry action potentials from the surface deep into the fiber, ensuring synchronous contraction of the myofibrils

❌ Why the Other Options Are Incorrect:

• Pili
  → Not an anatomical term in muscle histology

• Terminal cisternae
  → Enlarged sacs of the sarcoplasmic reticulum, not part of the sarcolemma; store and release calcium

• Sarcoplasmic reticulum
  → Specialized endoplasmic reticulum in muscle fibers, stores Ca²⁺, but is not an indentation of the sarcolemma

• Lamellae
  → Refers to layers of bone matrix, not muscle structure

Howship’s lacunae (also called resorption bays) are shallow depressions or pits found on the surface of bones undergoing resorption. These are created by the activity of osteoclasts, which are the bone-resorbing cells of the skeletal system.

❌ Why the Other Options Are Incorrect:

Osteoblasts

• Bone-forming cells

• Located on the surface of newly formed bone, not within resorption pits

Osteocytes

• Mature bone cells

• Reside in lacunae within bone matrix (not Howship’s lacunae)

Osteoprogenitor cells

• Precursor cells that differentiate into osteoblasts

• Found in the periosteum and endosteum, not in resorption bays

Chondrocytes

• Found in cartilage, not bone

• Reside in cartilage lacunae, not Howship’s lacunae

Lamellae are concentric layers of bone matrix found in mature lamellar bone. Osteoblasts are bone-forming cells that line the outer and inner surfaces of bone but are not embedded within the lamellae. Osteocytes, not osteoblasts, are the cells that reside within lacunae between the lamellae.

✅ Osteoblasts lie within the lamellae — Correct. This statement is incorrect. Osteoblasts do not lie within the lamellae; rather, they are found on bone surfaces where they deposit new matrix. Once trapped, they become osteocytes.

❌ Osteoblasts lie in between the lamellae — True. Osteocytes (which were osteoblasts) reside in lacunae between lamellae.

❌ Osteoclasts lie within lacunae — True in the context of Howship’s lacunae, which are surface resorption bays, not the lacunae within the lamellae proper.

❌ Lamellae of woven bone are irregular — True. Woven bone is immature and its collagen fibers and lamellae are irregularly arranged.

❌ Lamellae are parallel to each other — True for compact bone, where lamellae are arranged in a parallel or concentric pattern.

Howship’s lacunae are shallow depressions or resorption bays found on the surface of bone. These are created as a result of the activity of osteoclasts, which are large, multinucleated cells specialized in bone resorption. As osteoclasts dissolve the mineral matrix and degrade collagen, they form and sit in these lacunae.

✅ Osteoclasts — Correct. Osteoclasts are the cells actively present in Howship’s lacunae, responsible for bone resorption.

❌ Osteoblasts — Bone-forming cells that synthesize new bone matrix; they line bone surfaces but do not occupy Howship’s lacunae.

❌ Osteoprogenitor cells — Stem cells that differentiate into osteoblasts; they are located in the periosteum and endosteum, not in Howship’s lacunae.

❌ Osteocytes — Mature bone cells embedded within the bone matrix inside lacunae (different from Howship’s lacunae); they maintain the bone but do not resorb it.

❌ Chondrocytes — Cartilage cells found in lacunae within cartilage, not in bone or Howship’s lacunae.

 The bone matrix has two main components:

1. Organic part (osteoid): ~90% of which is collagen type I

2. Inorganic part: mostly hydroxyapatite crystals (calcium phosphate)

🦴 Why Collagen Type I is the correct answer:

• Collagen type I is the most abundant collagen in the human body.

• It is synthesized by osteoblasts, and its proper formation is essential for bone integrity and resistance to fractures.

• Mutations in type I collagen lead to diseases like osteogenesis imperfecta (brittle bone disease).

❌ Why the Other Options Are Incorrect:

Collagen type IX

• Found in cartilage, especially hyaline cartilage

• It binds to type II collagen and helps stabilize the cartilage matrix

• Not found in significant amounts in bone

Collagen type XI

• Also involved in cartilage structure, especially in fetal cartilage and intervertebral discs

• Associates with type II collagen

• Not a major component of bone

Collagen type II

• The main collagen in cartilage, not bone

• Provides tensile strength to articular cartilage, vitreous humor, and nucleus pulposus

• Not involved in bone matrix

Collagen type V

• Found in small amounts in bone, but its role is modulatory

• Helps regulate type I collagen fibril assembly

• It is not the primary collagen in bone

Bone is a connective tissue that serves several vital roles, including support, protection, movement, mineral storage, and blood cell formation. From a molecular standpoint, its extracellular matrix (ECM) is primarily composed of two major components:

1. Organic component – mostly collagen, giving bone its tensile strength.
2. Inorganic component – primarily hydroxyapatite crystals (calcium phosphate), giving bone its hardness and compressive strength.

Among collagens, Type I collagen is the most abundant collagen in bone, accounting for about 90% of the organic matrix. Osteoblasts synthesize this protein, and it forms the scaffold upon which mineralization occurs.

❌ Explanation of Incorrect Options:

Type III Collagen

This is found in reticular fibers, which support organs like the liver, spleen, and lymph nodes. While it plays a role in some connective tissues and wound healing, it is not a major component of bone.

Laminin

Laminin is a glycoprotein that is a key component of the basement membrane, involved in cell adhesion, differentiation, and migration. It is not associated with bone ECM, which is more about mineralized collagen and less about basement membranes.

All of these

Tempting—but incorrect. While fibronectin and Type I collagen are found in bone, not all the listed proteins are major components. For instance, laminin and Type III collagen are not significantly present in bone tissue.

Fibronectin

Fibronectin is a multifunctional glycoprotein involved in cell adhesion and matrix organization, and yes, it is present in bone, especially during development and repair. However, it is not the predominant protein—that role belongs to Type I collagen.

The correct answer is: Interstitial growth of cartilage tissue

Why is This the Correct Answer?

Long bones grow in length through endochondral ossification, a process where cartilage is replaced by bone. This occurs at the epiphyseal (growth) plate, which is composed of hyaline cartilage.

• The cartilage within the epiphyseal plate grows by interstitial growth (expansion from within), increasing the bone’s length.

• Eventually, cartilage is replaced by bone through ossification at the metaphysis.

• Once the growth plate closes (typically after puberty), longitudinal growth ceases.

Thus, interstitial growth of cartilage drives the increase in bone length, not the direct growth of bone itself.

Why Are the Other Options Incorrect?

1. Appositional deposition of bone growth (Incorrect)

   • Appositional growth refers to bone width/thickness growth, not length.

   • It occurs by new bone deposition on the outer surface of existing bone via osteoblasts in the periosteum.

2. Endochondral deposition of bone tissue (Incorrect)

   • While long bones do form through endochondral ossification, this refers to bone replacing cartilage, not the initial growth of the cartilage itself.

   • The increase in length happens before ossification, via interstitial cartilage growth.

3. Removal of calcified cartilage (Incorrect)

   • Calcified cartilage is removed during endochondral ossification, but this does not directly cause the bone to lengthen.

   • The increase in length happens first through cartilage growth, and then calcified cartilage is replaced with bone.

4. Interstitial growth of bone tissue (Incorrect)

   • Bone tissue does not grow interstitially because its rigid matrix prevents expansion from within.

   • Instead, bone grows through appositional growth, where new layers are added to the surface.

The correct answer is: Osteoblast

Why is This the Correct Answer?

Osteoblasts are bone-forming cells found along the surface of bones, where they play a crucial role in bone deposition and remodeling.

• They synthesize and secrete osteoid, which later mineralizes to form bone.

• Osteoblasts are typically aligned in a single layer along bone surfaces, especially in areas of active bone formation.

• As they mature, some osteoblasts become osteocytes, which are embedded within the bone matrix.

Why Are the Other Options Incorrect?

1. Osteoclast (Incorrect)

   • Osteoclasts are bone-resorbing cells that break down bone tissue.

   • While they are also found on bone surfaces, they are less common than osteoblasts under normal conditions.

   • They function in bone remodeling and calcium homeostasis, but they appear in different regions compared to osteoblasts.

2. Lymphocytes (Incorrect)

   • Lymphocytes are immune cells, primarily found in blood, lymphoid organs, and connective tissues.

   • They do not play a direct role in bone formation or resorption.

3. Osteon (Incorrect)

   • An osteon (Haversian system) is a structural unit of compact bone, consisting of concentric lamellae surrounding a central canal.

   • Osteons are microscopic structures within the bone matrix, not individual cells found on the bone surface.

4. Chondrocytes (Incorrect)

   • Chondrocytes are cartilage cells, found in structures like articular cartilage, growth plates, and intervertebral discs.

   • They do not reside in bone tissue.