Chapter 6: The Skeletal System

Functions of Bone and the Skeletal System

Composition of Bone (Organ): Bone is not merely a hard structure but a complex organ composed of several different tissues including osseous (bone) tissue, dense connective tissue, epithelium, adipose tissue, and nervous tissue.
The Skeletal System: This refers to the entire framework of bones and their associated cartilages.
Primary Functions: * Support: Provides a structural framework for the body and attachment points for tendons of most skeletal muscles. * Protection: Shields internal organs from injury (e.g., cranial bones protect the brain, vertebrae protect the spinal cord, and the rib cage protects the heart and lungs). * Assistance in Movement: Most skeletal muscles attach to bones; when they contract, they pull on bones to produce movement. * Mineral Homeostasis: Bone tissue stores several minerals, most notably calcium ( $Ca$ ) and phosphorus ( $P$ ). On demand, bone releases minerals into the blood to maintain critical balances. * Blood Cell Production: Within certain bones, a connective tissue known as red bone marrow produces red blood cells, white blood cells, and platelets through a process called hemopoiesis. * Triglyceride Storage: Yellow bone marrow consists mainly of adipose cells, which store triglycerides. These stored lipids serve as a potential chemical energy reserve.

Types of Bones

Long Bones: Characterized by a length greater than their width. They consist of a shaft (diaphysis) and a variable number of ends (epiphyses). They are typically curved for strength to absorb the stress of body weight at several different points. Examples include: * Thigh ( $\text{femur}$ ) * Leg ( $\text{tibia}$ and $\text{fibula}$ ) * Arm ( $\text{humerus}$ ) * Forearm ( $\text{ulna}$ and $\text{radius}$ ) * Fingers and toes ( $\text{phalanges}$ )
Short Bones: Somewhat cube-shaped and nearly equal in length and width. Examples include most wrist ( $\text{carpals}$ ) and ankle ( $\text{tarsals}$ ) bones.
Flat Bones: Generally thin and composed of two nearly parallel plates of compact bone tissue enclosing a layer of spongy bone tissue. They afford considerable protection and provide extensive surfaces for muscle attachment. Examples include: * Cranial bones * Sternum (breastbone) and ribs * Scapulae (shoulder blades)
Irregular Bones: Possess complex shapes and cannot be grouped into the previous categories. Examples include: * Vertebrae * Certain facial bones

Structure of Bone

Parts of a Long Bone

Diaphysis: The bone’s shaft or body; the long, cylindrical, main portion of the bone.
Epiphyses: The distal and proximal ends of the bone.
Metaphyses: The regions where the diaphysis joins the epiphyses. In a growing bone, each metaphysis contains an epiphyseal (growth) plate.
Articular Cartilage: A thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation (joint) with another bone. It reduces friction and absorbs shock.
Periosteum: A tough sheath of dense irregular connective tissue and its associated blood vessels that surrounds the bone surface wherever it is not covered by articular cartilage. It contains bone-forming cells that enable bone to grow in thickness.
Medullary Cavity (Marrow Cavity): A hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow in adults.
Endosteum: A thin membrane that lines the medullary cavity.

Histology of Bone Tissue

Extracellular Matrix: Bone contains an abundant extracellular matrix surrounding widely separated cells. The composition is: * $25\%$ water * $25\%$ collagen fibers (providing flexibility and tensile strength) * $50\%$ crystallized mineral salts (primarily calcium phosphate)
Cell Types: 1. Osteoprogenitor Cells: Unspecialized stem cells derived from mesenchyme; they are the only bone cells to undergo cell division, and the resulting cells develop into osteoblasts. 2. Osteoblasts: Bone-building cells that synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix; they initiate calcification. As they become surrounded by their own secretions, they become osteocytes. 3. Osteocytes: Mature bone cells and the most numerous cells in bone tissue. They maintain the daily metabolism of bone tissue, such as the exchange of nutrients and wastes with the blood. 4. Osteoclasts: Huge cells derived from the fusion of as many as $50$ monocytes (a type of white blood cell). They release powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying bone matrix (resorption).

Bone Density Categories

Compact Bone: * Comprises $80\%$ of the skeleton. * Provides protection and support and resists the stresses produced by weight and movement. * Arranged in repeating structural units called osteons (Haversian systems). * Osteonic (Haversian) Canal: A central canal for blood vessels and nerves. * Concentric Lamellae: Rings of hard, calcified matrix. * Lacunae: Small spaces between lamellae which contain osteocytes. * Canaliculi: Tiny channels filled with extracellular fluid that connect lacunae, providing routes for nutrients and oxygen to reach osteocytes.
Spongy Bone: * Comprises $20\%$ of the skeleton. * Does not contain osteons. It consists of an irregular latticework of thin columns of bone called trabeculae. * The spaces between trabeculae are filled with red bone marrow. * Found in short, flat, and irregular bones, and the epiphyses of long bones.

Bone Formation (Ossification)

Initial Bone Formation

Ossification occurs in four situations: initial formation in an embryo/fetus, growth until adulthood, remodeling, and repair of fractures.

Intramembranous Ossification: Bone forms directly within mesenchyme arranged in sheetlike layers. * Development of Ossification Center: Mesenchymal cells cluster and differentiate into osteoprogenitor cells, then osteoblasts, which secrete the organic matrix. * Calcification: Secretion stops, and the cells (now osteocytes) lie in lacunae. Calcium and other mineral salts are deposited, and the matrix hardens. * Formation of Trabeculae: The bone matrix develops into trabeculae that fuse to form spongy bone around the network of blood vessels. * Development of the Periosteum: Mesenchyme at the periphery condenses and develops into the periosteum. A thin layer of compact bone replaces the surface layers of the spongy bone.
Endochondral Ossification: Bone forms within hyaline cartilage that develops from mesenchyme. * Development of Cartilage Model: Mesenchymal cells crowd together in the shape of the future bone and develop into chondroblasts. * Growth of Cartilage Model: Chondrocytes divide, and the model grows. Chondrocytes in the mid-region hypertrophy (increase in size) and the surrounding matrix begins to calcify. Chondrocytes die, leaving behind small cavities (lacunae). * Development of Primary Ossification Center: Nutrient artery penetrates the perichondrium, stimulating osteoprogenitor cells to differentiate into osteoblasts. Bone tissue replaces most of the cartilage in the diaphysis. * Development of Medullary Cavity: Osteoclasts break down newly formed spongy bone to create the cavity. * Development of Secondary Ossification Centers: These develop in the epiphyses around the time of birth. * Formation of Articular Cartilage and Epiphyseal Plate: Hyaline cartilage remains covering the epiphyses (articular cartilage) and between the diaphysis and epiphysis (epiphyseal plate).

Bone Growth

Growth in Length: Occurs at the epiphyseal plate. Chondrocytes on the epiphyseal side of the plate divide, while chondrocytes on the diaphyseal side are replaced by bone. At the end of adolescence (ages $18-21$ ), the plate closes, leaving the epiphyseal line.
Growth in Thickness: Osteoblasts in the periosteum secrete bone matrix to form new osteons at the periphery, while osteoclasts in the endosteum enlarge the medullary cavity by resorbing bone on the inner surface.

Bone Remodeling and Factors Affecting Homeostasis

Remodeling Process

Bone Resorption: Removal of minerals and collagen fibers from bone by osteoclasts.
Bone Deposition: Addition of minerals and collagen fibers to bone by osteoblasts.

Factors Influencing Bone Metabolism

Minerals: * Calcium and Phosphorus: Make bone matrix hard. * Magnesium: Helps form bone matrix. * Fluoride: Strengthens bone matrix. * Manganese: Activates enzymes for matrix synthesis.
Vitamins: * Vitamin A: Needed for osteoblast activity during remodeling. * Vitamin C: Needed for collagen synthesis; deficiency leads to scurvy and slowed growth. * Vitamin D (Calcitriol): Increases calcium absorption from the GI tract. Deficiency causes faulty calcification. * Vitamins K and B12: Needed for bone protein synthesis.
Hormones: * hGH (Human Growth Hormone) & IGFs (Insulinlike Growth Factors): Stimulate osteoblasts, promote cell division at the epiphyseal plate, and enhance protein synthesis. * Thyroid Hormones (Thyroxine & Triiodothyronine): Promote normal bone growth by stimulating osteoblasts. * Insulin: Increases bone protein synthesis. * Sex Hormones (Estrogens & Testosterone): Responsible for the teenage growth spurt and eventually cause the closure of the epiphyseal plates. They also slow bone resorption in adults. * Parathyroid Hormone (PTH): Increases blood calcium levels by stimulating osteoclasts to resorb bone. * Calcitonin (CT): Decreases blood calcium levels by inhibiting osteoclasts.

Exercise and Aging

Mechanical Stress: Bone tissue becomes stronger under stress through increased mineral deposition and collagen production. Absence of stress leads to demineralization and bone loss.
Aging: As sex hormone levels diminish (especially after menopause in women), bone resorption outpaces deposition, leading to decreased bone mass and an increased risk of osteoporosis.

Fractures and Repair

Types of Fractures: * Partial: An incomplete break across the bone (e.g., a crack). * Complete: The bone is broken into two or more pieces. * Closed (Simple): The fracture does not break the skin. * Open (Compound): Broken ends of the bone protrude through the skin.
Repair Steps: 1. Phagocyte Cleanup: Phagocytes begin to remove dead bone tissue. 2. Fibrocartilage Formation: Chondroblasts form a fibrocartilage bridge at the fracture site. 3. Spongy Bone Formation: Osteoblasts convert fibrocartilage into spongy bone. 4. Remodeling: Osteoclasts absorb dead portions of bone; spongy bone is converted to compact bone.

Divisions of the Skeletal System

Total Bones in Adult: $206$ .
Axial Skeleton ( $80$ bones): Bones arranged along the longitudinal axis. * Skull (cranial and facial bones) * Auditory ossicles (ear bones) * Hyoid bone * Ribs and Sternum * Vertebrae
Appendicular Skeleton ( $126$ bones): Bones of the limbs and the girdles that attach limbs to the axial skeleton. * Pectoral (shoulder) girdle ( $\text{clavicle}$ and $\text{scapula}$ ) * Upper limbs ( $\text{humerus, ulna, radius, carpals, metacarpals, phalanges}$ ) * Pelvic (hip) girdle ( $\text{hip bone}$ ) * Lower limbs ( $\text{femur, patella, tibia, fibula, tarsals, metatarsals, phalanges}$ )

Anatomy of the Skull and Fontanels

The Skull (Cranial and Facial)

Cranial Bones: Frontal, Parietal ( $2$ ), Temporal ( $2$ ), Occipital, Sphenoid, Ethmoid.
Facial Bones: Nasal ( $2$ ), Maxilla ( $2$ ), Zygomatic ( $2$ ), Mandible, Lacrimal ( $2$ ), Palatine ( $2$ ), Inferior nasal conchae ( $2$ ), Vomer.
Sutures: Coronal, Sagittal, Lambdoid, Squamous.
Special Features: * Paranasal Sinuses: Found in the frontal, sphenoid, ethmoid, and maxillary bones. * Foramen Magnum: Large hole in the occipital bone for the spinal cord.

Fontanels (Soft Spots)

Anterior Fontanel: Largest; diamond-shaped; between parietal and frontal bones. Closes $18-24$ months after birth.
Posterior Fontanel: Diamond-shaped; between parietal and occipital bones. Closes at approximately $2$ months.
Anterolateral Fontanel: Small/irregular; between frontal, parietal, temporal, and sphenoid bones. Closes at $3$ months.
Posterolateral Fontanel: Irregular; between parietal, occipital, and temporal bones. Begins closing at $1-2$ months; complete by $12$ months.

Vertebral Column and Thorax

Vertebral Regions

Cervical ( $7$ ): Include the Atlas ( $C1$ ) and Axis ( $C2$ ). Characterized by bifid spinous processes and transverse foramina.
Thoracic ( $12$ ): Articulate with the ribs.
Lumbar ( $5$ ): Largest and strongest vertebrae.
Sacrum: $5$ fused sacral vertebrae ( $S1-S5$ ).
Coccyx: $4$ fused coccygeal vertebrae ( $Co1-Co4$ ).
Curves: Cervical and Lumbar (anteriorly convex); Thoracic and Sacral (anteriorly concave).

The Thorax

Sternum: Consists of the manubrium, body, and xiphoid process.
Ribs: $12$ pairs that provide structural support to the thoracic cavity. They connect to the sternum via costal cartilage.

The Girdles and Limbs

Pectoral Girdle and Upper Limb

Girdle: Clavicle (collarbone) and Scapula (shoulder blade). Features include the acromion, coracoid process, and glenoid cavity.
Humerus: Largest bone of the upper limb; articulates with the scapula.
Radius and Ulna: Forearm bones. The ulna is on the medial (little finger) side; the radius is on the lateral (thumb) side.
Carpals (Wrist): $8$ bones. Mnemonic: "Stop Letting Those People Touch The Cadaver's Hand" (Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate).
Metacarpals & Phalanges: Hand and finger bones.

Pelvic Girdle and Lower Limb

Girdle: Formed by the hip bones ( $\text{os coxa}$ ), which consist of the ilium, ischium, and pubis. Features include the acetabulum (socket for femur) and obturator foramen.
Femur: Thigh bone; the longest, heaviest, and strongest bone in the body.
Patella: Kneecap; a sesamoid bone.
Tibia and Fibula: Leg bones. Tibia is the larger medial weight-bearing bone. Fibula is the smaller lateral bone.
Tarsals (Ankle): $7$ bones. Mnemonic: "Never Take Shots From Tall Centers" (Navicular, Talus, Second cuneiform, First cuneiform, Third cuneiform, Calcaneus, Cuboid).
Metatarsals & Phalanges: Foot and toe bones.