Functions and Divisions of the Skeletal System

Functions of the Skeletal System

• Support and protect the body
• Enable movement
• Store minerals (calcium and phosphate)
• Participate in blood cell production (red bone marrow)
• Store fat

Divisions of the Skeletal System

• Axial Skeleton: Forms the central axis of the body, including the skull, vertebral column, and rib cage
• Appendicular Skeleton: Consists of the bones of the limbs and their girdles (shoulder and pelvic girdles)

Bone Shapes

• Sutural bones: Small, irregular bones found within sutures (joints) of the skull
• Long bones: Longer than they are wide (e.g., femur, humerus)
• Flat bones: Thin and flattened, often curved (e.g., skull bones, ribs, sternum)
• Short bones: Roughly cube-shaped (e.g., carpals, tarsals)
• Irregular bones: Bones with complex shapes (e.g., vertebrae, facial bones)
• Sesamoid bones: Small round bones embedded in tendons (e.g., patella)

Parts of a Long Bone

• Epiphyses: The expanded ends of a long bone, covered with articular cartilage
• Metaphyses: The regions between the diaphysis and epiphyses, containing the epiphyseal plate/line
• Diaphysis: The shaft or central region of a long bone
  • Periosteum: A tough fibrous membrane that surrounds the diaphysis, with collagen fibers that attach it to the bone
  • Compact bone: Dense, hard outer layer of the diaphysis
  • Endosteum: A thin membranous layer that lines the medullary cavity
  • Medullary cavity: The hollow space within the diaphysis, containing bone marrow

Gross Anatomy: Flat/Short Bones

• Compact bone: Outer layer
• Spongy (Cancellous) bone: Inner layer, also known as diploe
• Red bone marrow: Found within spongy bone, responsible for hematopoiesis (blood cell production)

Bone Matrix and Chemistry

• Organic component (33-35%): Primarily collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and osteocalcin
• Inorganic component (65-67%): Mainly hydroxyapatite crystals (calcium salts and potassium), along with bicarbonate, magnesium, phosphate, and sodium salts

Blood and Nerves to Bone

• Periosteum: Contains a network of blood vessels and nerves, especially for short, flat, irregular, and sesamoid bones
• Diaphysis: Received blood supply from nutrient arteries entering through the nutrient foramen
• Epiphyses: Supplied by small blood vessels entering through small holes and nutrient arteries

Bone Cells

• Osteoprogenitor cells: Undifferentiated cells give rise to osteoblasts
• Osteoblasts: Bone-building cells, responsible for depositing new bone matrix
• Osteocytes: Mature bone cells, maintain bone matrix
• Osteoclasts: Bone-destroying cells, responsible for bone resorption and remodeling

Bone Production and Resorption

• Osteoblasts: Deposit new bone matrix.
• Osteocytes: Maintain bone.
• Osteoclasts: Breakdown bone matrix to release calcium, contributing to bone remodeling.

Compact Bone Histology

• Consists of osteons (Haversian systems): concentric rings of lamellae (layers) containing osteocytes, surrounding a central canal containing blood vessels and nerves.
• Canaliculi: Tiny channels that connect lacunae (spaces housing osteocytes) and allow for communication and nutrient exchange.

Spongy (Cancellous) Bone Histology

• Composed of trabeculae: interconnected bony plates that form a lattice-like structure.
• Bone marrow: fills spaces within the trabeculae.

Bone Development: 2 Routes

• Intramembranous ossification: Direct formation of bones from mesenchyme, occurs in flat bones (skull, clavicle)
• Endochondral ossification: Replacement of hyaline cartilage by bone, responsible for the formation of long bones.

Intramembranous Ossification

• Primary Ossification: Begins in the center of the bone and moves outwards
• Secondary Ossification: Begins in the epiphyses of long bones, after birth.

Endochondral Ossification

• Hyaline cartilage acts as a template for bone formation.
• Chondrocytes enlarge, calcify, die, and degenerate.
• Perichondrium becomes the periosteum.

Appositional Growth

• Increase in bone diameter
• New bone deposited by osteoblasts in the periosteum and endosteum.

Periosteum and Endosteum

• Periosteum: Outer dense fibrous membrane covering the bone, contains osteoprogenitor cells.
• Endosteum: Thin, inner membranous layer lining bone marrow cavities, contains osteoprogenitor cells.

Bone Remodeling

• Continuous process of bone deposition (by osteoblasts) and resorption (by osteoclasts).
• Ensures bone strength and adaptation to mechanical stress.
• Spongy bone: Replaced by compact bone.

Hormonal Regulation of Bone Growth

• Growth Hormone (GH): Stimulates bone growth.
• Thyroxine (TH): Influences bone growth, working with GH.
• Sex hormones (testosterone and estrogen): Promote bone growth and contribute to the closure of epiphyseal plates.
• Progesterone: Influences bone growth, working with estrogen.
• Calcitonin: Inhibits bone resorption.
• Parathyroid Hormone (PTH): Stimulates bone resorption.

Metabolic Role of Bones

• Calcium: Involved in many bodily functions, including muscle contraction, nerve function, and enzyme activity.
• Hormonal control of blood calcium:
  • PTH: Raises blood calcium levels by stimulating bone resorption.
  • Calcitonin: Lowers blood calcium levels by inhibiting bone resorption.

Epiphyseal Plate and Line

• Epiphyseal plate: Layer of hyaline cartilage responsible for lengthwise bone growth.
• Epiphyseal line: Forms when the epiphyseal plate closes, marking the end of bone growth.

Vitamins, etc. for Bone Growth

• Calcium: Essential for bone mineralization.
• Vitamin C: Required for collagen synthesis.
• Vitamin D: Aids in calcium absorption and prevents calcium loss through the kidneys.
• Vitamin K: Aids in the production of calcium-binding glycoproteins.
• Protein: Required organic matrix synthesis.

Osteoporosis

• Risk factors:
  • Age, genetics (runs in families), gender (female), race, nutrition (low calcium intake, low vitamin D intake, high soda consumption), lack of exercise, certain medications.
• Causes:
  • Decreased bone density and weakening of bone tissue, increasing the risk of fractures.

Other Bone Weakening Disorders

• Rickets: A condition caused by vitamin D deficiency in children, resulting in soft and weakened bones.
• Osteomalacia: A condition caused by vitamin D deficiency in adults, resulting in soft and weakened bones.
• Paget's Disease: A condition characterized by abnormal bone remodeling and disorganized bone structure.

Bone Fracture Types

• Incomplete fracture: A crack in the bone that does not completely break it.
• Complete fracture: A bone break that completely severs the bone.
• Displaced fracture: A bone break where the bone fragments are out of alignment.
• Non-displaced fracture: A bone break where the bone fragments remain in alignment.
• Closed (simple) fracture: A bone break that does not break the skin.
• Open (compound) fracture: A bone break that breaks the skin.

Fracture Repair

• Hematoma Formation: Blood clots form at the fracture site.
• Fibrocartilage Callus Formation: Soft callus forms, composed of cartilage and fibrous tissue.
• Bony Callus Formation: Hard callus forms, composed of bone tissue.
• Remodeling: The bony callus is remodeled to resemble the original bone.

Abnormal Growth

• Gigantism: Excessive growth hormone (GH) production, resulting in excessive bone growth.
• Pituitary Dwarfism: Deficiency in GH production, resulting in abnormally short stature.
• Acromegaly: Excessive GH production after epiphyseal plates have closed, causing bone growth in face, hands, and feet.
• Achondroplasia: A genetic disorder characterized by abnormal cartilage growth, resulting in dwarfism.
• Marfan Syndrome: A genetic disorder caused by excessive cartilage growth, leading to long limbs, a tall stature, and other skeletal abnormalities.