5.3 The Skeletal System & Bone Physiology

Overview of Skeletal Physiology

The skeletal system is a dynamic, living organ system comprising 206 distinct bones in the adult human body, along with cartilage, tendons, and ligaments. Beyond serving as a structural scaffold, bone tissue is continuously remodeled and acts as an endocrine regulator of mineral metabolism.

Primary Functions of the Skeletal System

1. Support: Provides a structural framework that supports soft body tissues and serves as attachment points for skeletal muscle tendons.
2. Protection: Encloses internal organs within hard bony walls (cranium protects brain, vertebral column protects spinal cord, rib cage protects heart and lungs).
3. Assistance in Movement: Bones act as rigid levers pulled by contracting skeletal muscles across joint fulcrums.
4. Mineral & Triglyceride Homeostasis: Stores 99% of the body's calcium and 85% of its phosphorus. Yellow bone marrow in medullary cavities stores triglycerides in adipocytes as energy reserves.
5. Hematopoiesis (Blood Cell Production): Red bone marrow contained within spongy bone spaces produces red blood cells, white blood cells, and blood platelets.

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Axial vs. Appendicular Skeleton

The 206 bones of the adult skeleton are divided into two anatomical divisions:

                        ┌── Axial Skeleton (80 bones) ────── Skull, Hyoid, Vertebrae, Ribs, Sternum
Human Adult Skeleton ───┤
                        └── Appendicular Skeleton (126) ── Pectoral/Pelvic Girdles, Limbs

1. Axial Skeleton (80 Bones)

Positioned along the central longitudinal axis of the body:

• Skull (22 bones): 8 cranial bones (frontal, parietal [2], temporal [2], occipital, sphenoid, ethmoid) that enclose the brain, and 14 facial bones (maxilla [2], zygomatic [2], nasal [2], lacrimal [2], palatine [2], inferior nasal conchae [2], vomer, mandible).
• Associated Bones (7 bones): 1 hyoid bone (located in anterior neck; unique because it does not articulate with any other bone) and 6 auditory ossicles (malleus [2], incus [2], stapes [2]).
• Vertebral Column (26 bones): 7 Cervical ($C1$ atlas, $C2$ axis), 12 Thoracic (articulate with ribs), 5 Lumbar (heavy, load-bearing), 1 Sacrum (5 fused sacral vertebrae), and 1 Coccyx (4 fused coccygeal vertebrae).
• Thoracic Cage (25 bones): 1 sternum (manubrium, body, xiphoid process) and 24 ribs (12 pairs: Ribs 1–7 True/Vertebrosternal; Ribs 8–10 False/Vertebrochondral; Ribs 11–12 Floating/Vertebral).

2. Appendicular Skeleton (126 Bones)

Bones of the upper and lower limbs and the girdles attaching limbs to the axial skeleton:

• Pectoral (Shoulder) Girdle (4 bones): Clavicle [2] and Scapula [2].
• Upper Limbs (60 bones): Humerus [2], Radius [2, lateral], Ulna [2, medial], Carpals [16 wrist bones], Metacarpals [10 palm bones], Phalanges [28 finger bones].
• Pelvic (Hip) Girdle (2 bones): Two coxal/hip bones (each formed by fusion of ilium, ischium, and pubis).
• Lower Limbs (60 bones): Femur [2, longest/strongest bone], Patella [2, kneecap sesamoid], Tibia [2, weight-bearing medial shin], Fibula [2, slender lateral leg], Tarsals [14 ankle bones], Metatarsals [10 foot bones], Phalanges [28 toe bones].

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Bone Classification by Shape

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Microscopic Anatomy of Bone Tissue

1. Compact (Cortical) Bone

Forms the dense outer protective shell of all bones. Organized into repeating structural units called Osteons (Haversian Systems):

• Haversian (Central) Canal: Runs longitudinally through the center of an osteon, containing blood vessels, lymphatic vessels, and nerve fibers.
• Volkmann's (Perforating) Canals: Transverse channels connecting Haversian canals to periosteum and endosteum.
• Concentric Lamellae: Rings of hard, calcified extracellular matrix consisting of mineral salts deposited around collagen fibers.
• Lacunae: Microscopic cavities located between adjacent lamellae housing mature osteocytes.
• Canaliculi: Microscopic fluid-filled canals radiating from lacunae containing osteocyte cytoplasmic processes connected via gap junctions. Canaliculi provide pathways for nutrients, oxygen, and metabolic waste diffusion between osteocytes and the central canal.

2. Spongy (Cancellous/Trabecular) Bone

Located in the interior of short, flat, irregular bones and epiphyses of long bones. Lacks true osteons; consists of an open lattice of thin bony spicules called trabeculae, aligned precisely along lines of mechanical stress. Spaces between trabeculae house red bone marrow.

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Bone Cell Types & Dynamic Remodeling

Bone is a dynamic tissue continuously broken down and rebuilt through bone remodeling (~10% replaced per year), governed by four cell types:

1. Osteogenic (Osteoprogenitor) Cells: Unspecialized mesenchymal stem cells found in the periosteum and endosteum. Undergo mitosis to differentiate into osteoblasts.
2. Osteoblasts: Bone-forming cells that synthesize and secrete organic unmineralized bone matrix (osteoid, rich in Type I collagen). They initiate calcification by depositing calcium phosphate as hydroxyapatite crystals $[Ca_{10}(PO_4)_6(OH)_2]$. Once trapped in matrix, osteoblasts become osteocytes.
3. Osteocytes: Mature, star-shaped bone cells trapped in lacunae. Maintain daily bone matrix homeostasis, act as mechanosensors (detecting physical stress and microcracks), and signal osteoblasts or osteoclasts.
4. Osteoclasts: Giant multinucleated cells derived from hematopoietic monocyte/macrophage stem cell lines. Positioned in resorption bays (Howship's lacunae). They secrete hydrochloric acid (HCl) (dissolving inorganic hydroxyapatite minerals) and lysosomal enzymes like cathepsin K (digesting collagen matrix).

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Endocrine Regulation of Calcium Homeostasis

Normal blood calcium concentration is strictly maintained between 9.0 and 11.0 mg/dL through antagonistic hormonal feedback mechanisms:

\text{Hypocalcemia }(Ca^{2+} < 9.0\text{ mg/dL}) &\rightarrow \text{Parathyroid Glands Release PTH} \\ &\rightarrow \text{Osteoclast Resorption (Bone }Ca^{2+}\text{ to Blood)} \\ &\rightarrow \text{Renal }Ca^{2+}\text{ Reabsorption} \\ &\rightarrow \text{Calcitriol Synthesis and Intestinal }Ca^{2+}\text{ Absorption} \end{aligned}$$ $$\begin{aligned} \text{Hypercalcemia }(Ca^{2+} > 11.0\text{ mg/dL}) &\rightarrow \text{Thyroid C-Cells Release Calcitonin} \\ &\rightarrow \text{Decreased Osteoclast Resorption} \\ &\rightarrow \text{Increased Osteoblast Matrix Deposition} \\ &\rightarrow \text{Increased Renal }Ca^{2+}\text{ Excretion} \end{aligned}$$ --- ## Joint Classification (Arthrology) Joints (articulations) are sites where two or more bones meet. They are classified structurally (by binding tissue and cavity presence) and functionally (by degree of movement): ### 1. Structural Classification - **Fibrous Joints**: Bones joined by dense fibrous connective tissue; no joint cavity. Immovable or slightly movable. - *Sutures*: Between skull bones (e.g., coronal suture). - *Syndesmoses*: Bones connected by ligament/interosseous membrane (e.g., distal tibiofibular joint). - *Gomphoses*: Peg-in-socket tooth joint in alveolar socket. - **Cartilaginous Joints**: Bones joined by cartilage; no joint cavity. - *Synchondroses*: Connected by hyaline cartilage (e.g., epiphyseal growth plates, 1st rib to sternum). - *Symphyses*: Connected by fibrocartilage pad (e.g., pubic symphysis, intervertebral discs). - **Synovial Joints**: Bones separated by a fluid-filled **synovial cavity**. Freely movable diarthroses. Feature articular hyaline cartilage, fibrous capsule, synovial membrane secreting **synovial fluid** (lubricates, absorbs shock, nourishes chondrocytes), and reinforcing ligaments. ### 2. Types of Synovial Joints - **Hinge (Uniaxial)**: Flexion/extension (elbow, knee, interphalangeal joints). - **Ball-and-Socket (Multiaxial)**: Flexion/extension, abduction/adduction, rotation, circumduction (hip and shoulder joints). - **Pivot (Uniaxial)**: Rotation around axis ($C1 {--}C2$ atlantoaxial joint, proximal radioulnar joint). - **Plane / Gliding (Nonaxial)**: Back-and-forth sliding (intercarpal and intertarsal joints). - **Condyloid / Ellipsoid (Biaxial)**: Flexion/extension, abduction/adduction (radiocarpal wrist joint). - **Saddle (Biaxial)**: Greater movement freedom (first carpometacarpal joint of thumb).