Bones and Bone Structure Chapter 6 Study Notes

Functions of the Skeletal System

• The skeletal system is composed of several components:
  • Bones of the skeleton.
  • Cartilages.
  • Ligaments.
  • Other connective tissues.
• Primary functions identified for the skeletal system include:
  • Support: Providing a structural framework for the body.
  • Storage of minerals and lipids: Acting as a reservoir for essential minerals especially calcium and phosphate, and storing energy-rich lipids in yellow marrow.
  • Blood cell production: Occurring in red bone marrow.
  • Protection: Enclosing and shielding soft organs (e.g., the ribs protecting the heart and lungs).
  • Leverage: Serving as levers that muscles pull against to create movement.

Classification of Bones by Shape and Structure

• Bones are primarily classified by their shape and internal structure. The six main shapes include:
  • Sutural Bones (Wormian bones): Small, flat, oddly shaped bones found between the flat bones of the skull. Their borders are irregular, resembling jigsaw puzzle pieces. They range in size from a grain of sand to the size of a quarter.
  • Irregular Bones: Characterized by complex shapes with short, flat, notched, or ridged surfaces. Examples include the vertebrae forming the spinal column, the pelvic bones, and several skull bones.
  • Short Bones: Boxlike in appearance. Key examples include the carpal bones located in the wrists and the tarsal bones in the ankles.
  • Flat Bones: Possess thin, parallel surfaces. These bones provide protection for underlying soft tissues and offer extensive surface area for muscle attachment. Examples include the roof of the skull (parietal bone), the sternum (breastbone), the ribs, and the scapulae (shoulder blades).
  • Long Bones: Relatively long and slender. They are found in the arm, forearm, thigh, leg, palms, soles, fingers, and toes. The femur (thigh bone) is the largest and heaviest bone in the body.
  • Sesamoid Bones: Usually small, round, and flat. They develop within tendons and are typically found near joints in the knees, hands, and feet. The patellae (kneecaps) are sesamoid bones that everyone possesses, though the count of other sesamoid bones varies between individuals.

Bone Markings and Surface Features

• Bone markings are surface features that serve specific functional roles:
  • Projections: These are areas where muscles, tendons, and ligaments attach, or where bones articulate with one another.
  • Openings and Depressions: These features allow for the passage of blood vessels and nerves through the bone.

Internal Structure of Bones

• Structure of a Long Bone (e.g., the femur):
  • Diaphysis (Shaft): The central, tubular part of the bone consisting of a wall of compact bone and a central space known as the medullary cavity (or marrow cavity).
  • Epiphysis: The wide part at each end of the bone. It consists mostly of spongy bone (also called trabecular bone).
  • Metaphysis: The narrow zone where the diaphysis and epiphysis meet.
• Structure of Flat Bones (e.g., the parietal bone):
  • Consists of a layer of spongy bone sandwiched between two layers of compact bone (the cortex).
  • Within the cranium, this central layer of spongy bone is specifically referred to as the diploë.

Bone Tissue and Matrix Composition

• Bone tissue is a dense, supportive connective tissue containing specialized cells and a solid extracellular matrix with collagen fibers.
• General Characteristics:
  • The matrix is dense due to the deposition of calcium salts.
  • Osteocytes (mature bone cells) reside in small pockets called lacunae, which are organized around blood vessels.
  • Canaliculi are narrow passageways through the matrix that facilitate the exchange of nutrients, wastes, and gases between osteocytes and the blood supply.
  • The Periosteum covers the outer surfaces of bones (except at joints) and consists of an outer fibrous layer and an inner cellular layer.
• Matrix Composition:
  • Approximately two-thirds of bone mass is calcium phosphate ($Ca_3(PO_4)_2$).
  • Calcium phosphate interacts with calcium hydroxide ($Ca(OH)_2$) to form crystals of hydroxyapatite ($Ca_{10}(PO_4)_6(OH)_2$).
  • The matrix also incorporates other salts like calcium carbonate ($CaCO_3$) and ions such as magnesium.
  • Approximately one-third of bone mass consists of collagen fibers.
  • Functional property: A bone lacking a calcified matrix would appear normal but be extremely flexible.

Specialized Bone Cells

• Cells make up only approximately $2\%$ of total bone mass. There are four types:
  • Osteogenic Cells (Osteoprogenitor cells): Mesenchymal stem cells that divide to produce osteoblasts. They are located in the inner cellular layer of the periosteum and in the endosteum. They play a vital role in fracture repair.
  • Osteoblasts: Immature bone cells that produce new bone matrix through a process called osteogenesis (or ossification). They secrete osteoid, the organic matrix that has not yet been calcified. Once surrounded by matrix, osteoblasts differentiate into osteocytes.
  • Osteocytes: Mature bone cells that no longer divide. They live in lacunae between matrix layers and maintain the protein and mineral content of the surrounding matrix. They also participate in the repair of damaged bone. They have cytoplasmic extensions that reach through the canaliculi.
  • Osteoclasts: Large, multinucleate cells that absorb and remove bone matrix. They secrete acids and protein-digesting enzymes to dissolve the matrix and release stored minerals (a process called osteolysis), which is essential for mineral homeostasis. They are derived from the same stem cells that produce monocytes and macrophages.

Compact Bone and Spongy Bone Structure

• Compact Bone Structure:
  • The functional unit is the Osteon (Haversian system).
  • Central Canal: Contains one or more blood vessels.
  • Perforating Canals: Run perpendicular to the bone surface, carrying blood vessels into deep bone and the marrow cavity.
  • Lamellae: Layers of bone matrix.
    • Concentric lamellae: Surround the central canal.
    • Interstitial lamellae: Fill the spaces between various osteons.
    • Circumferential lamellae: Found at the outer and inner surfaces of the bone.
• Spongy Bone Structure:
  • Lacks osteons.
  • The matrix forms an open network of struts called trabeculae.
  • Lacks capillaries and venules; nutrients reach osteocytes by diffusion along canaliculi.
  • Spaces between trabeculae are filled with bone marrow:
    • Red Bone Marrow: Responsible for blood cell formation.
    • Yellow Bone Marrow: Stores fat for energy.

Bone Membranes: Periosteum and Endosteum

• Periosteum:
  • Covers the exterior of bones except within joint cavities.
  • Consists of an outer fibrous layer and an inner cellular layer.
  • Contains perforating fibers (collagen fibers) that incorporate into the bone tissue to strengthen the attachment of tendons and ligaments.
  • Functions: Isolates bone from surrounding tissue, provides a route for blood vessels and nerves, and participates in bone growth and repair.
• Endosteum:
  • An incomplete cellular layer that lines the medullary cavity.
  • Covers the trabeculae of spongy bone and lines the central canals of compact bone.
  • Active during bone growth, repair, and remodeling.
  • Consists of a flattened layer of osteogenic cells, osteoblasts, and osteoclasts.

Bone Formation and Growth (Ossification)

• Ossification (Osteogenesis): The process of bone formation.
• Calcification: The deposition of calcium salts, which occurs during ossification.
• Human bones generally grow until approximately age $25$.
• Endochondral Ossification (Bone replaces hyaline cartilage):
  1. Cartilage model enlarges; chondrocytes near the center of the shaft die and disintegrate as matrix calcifies.
  2. Blood vessels grow around the cartilage edges; perichondrium cells convert to osteoblasts, forming a superficial layer of bone (bone collar).
  3. Blood vessels penetrate the cartilage; fibroblasts differentiate into osteoblasts to produce spongy bone at a primary ossification center.
  4. Remodeling creates a medullary cavity; the shaft becomes thicker and replaced by bone.
  5. Capillaries and osteoblasts migrate into the epiphyses to create secondary ossification centers.
  6. Epiphyses fill with spongy bone; the epiphyseal cartilage (epiphyseal plate) at the metaphysis separates the epiphysis from the diaphysis.
  7. At puberty, osteoblast activity accelerates and cartilage production slows until the epiphyseal cartilage disappears (epiphyseal closure), leaving a visible epiphyseal line.
• Intramembranous Ossification (Dermal ossification):
  • Occurs in the deeper layers of the dermis.
  • Produces dermal bones like the mandible and clavicles.
  1. Mesenchymal cells differentiate into osteoblasts and secrete osteoid, which mineralizes.
  2. Developing bone grows outward from the ossification center in small struts called spicules.
  3. Blood vessels branch between spicules; as spicules interconnect, they trap blood vessels.
  4. Osteoblasts continue deposition, resulting in a plate of spongy bone.
  5. Remodeling creates the diploë and a thin covering of compact bone.
• Types of Growth:
  • Interstitial Growth: Growth in length (via the epiphyseal cartilage).
  • Appositional Growth: Growth in width. Circumferential lamellae are added at the outer surface while osteoclasts remove bone at the inner surface to enlarge the medullary cavity.

Blood Supply and Innervation

• Nutrient Artery and Vein: Most bones have one of each passing through nutrient foramina in the diaphysis.
• Metaphyseal Vessels: Supply blood to the epiphyseal cartilages where bone growth occurs.
• Periosteal Vessels: Supply blood to superficial osteons and secondary ossification centers.
• The periosteum also contains a network of lymphatic vessels and sensory nerves.

Bone Remodeling, Exercise, and Nutrition

• Remodeling: The continuous recycling and renewal of the bone matrix involving osteocytes, osteoblasts, and osteoclasts.
• Exercise Effects:
  • Mineral recycling allows bones to adapt to stress.
  • Heavily stressed bones become thicker and stronger; weight-bearing exercise stimulates osteoblasts.
  • Inactivity causes rapid degeneration; up to one-third ($\frac{1}{3}$) of bone mass can be lost in a few weeks of inactivity.
• Nutrition and Hormones:
  • Dietary requirements: Calcium, phosphorus, and small amounts of magnesium, fluoride, iron, and manganese.
  • Calcitriol: Synthesized in the kidneys from Vitamin $D_{3}$ (cholecalciferol); required for the absorption of calcium and phosphate in the digestive tract.
  • Vitamin C: Required for collagen synthesis and stimulates osteoblast differentiation.
  • Vitamin A: Stimulates osteoblast activity.
  • Vitamins K and $B_{12}$: Required for the synthesis of bone proteins.
  • Growth Hormone and Thyroxine: Stimulate bone growth.
  • Sex Hormones (Estrogen and Testosterone): Stimulate osteoblasts to grow bone.

Calcium Homeostasis

• Bones store $99\%$ of the body's calcium, $99\%$ of phosphate, $80\%$ of carbonate, $50\%$ of magnesium, $35\%$ of sodium, and $4\%$ of potassium.
• Calcium is the most abundant mineral in the body and is vital for many physiological processes.
• Parathyroid Hormone (PTH):
  • Produced by parathyroid glands.
  • Increases blood calcium levels by indirectly stimulating osteoclast activity, increasing intestinal calcium absorption (via calcitriol), and decreasing calcium excretion by kidneys.
• Calcitonin:
  • Secreted by C cells in the thyroid gland.
  • Decreases blood calcium levels by inhibiting osteoclast activity, increasing renal calcium excretion, and decreasing intestinal absorption.

Fractures and Repair

• Fractures are cracks or breaks in bones caused by physical stress.
• Classification: Open (compound) or closed (simple).
• Major Types: Transverse, displaced, compression, spiral, epiphyseal, comminuted, greenstick, Colles, Pott’s.
• The Repair Process (Four Steps):
  1. Fracture Hematoma Formation: A large blood clot forms to establish a fibrous network; nearby bone cells die.
  2. Callus Formation: Cells from the endosteum and periosteum migrate to the zone. An internal callus develops in the medullary cavity, and an external callus of cartilage and bone surrounds the break.
  3. Spongy Bone Formation: Osteoblasts replace the central cartilage of the external callus with spongy bone.
  4. Compact Bone Formation: Spongy bone is replaced; the area may be slightly thicker/stronger than before.

Effects of Aging on the Skeletal System

• Osteopenia: Inadequate ossification leading to thinner and weaker bones. This begins between ages $30$ and $40$. Women lose approximately $8\%$ of bone mass per decade, while men lose approximately $3\%$. Most affected areas include the epiphyses, vertebrae, and jaws.
• Osteoporosis: Severe loss of bone mass that compromises function. In people over age $45$, it affects $29\%$ of women and $18\%$ of men. In women, the process accelerates after menopause due to declining sex hormones.
• Cancer also contributes to bone loss; cancerous tissues release "osteoclast-activating factor," which stimulates osteoclasts and can produce severe osteoporosis.