Skeletal System: Bone Structure, Function, and Physiology Study Notes

Skeletal System Overview

Classification of Bones by Shape

The human skeletal system consists of 206 bones, which can be classified based on their shapes. Each type has distinct structural and functional characteristics.

• Sutural (Wormian) Bones: Small, flat, irregularly shaped bones found between flat bones of the skull. Number varies among individuals.

• Irregular Bones: Complex shapes, e.g., spinal vertebrae, pelvic bones.

• Short Bones: Boxy, e.g., carpal bones and tarsal bones.

• Flat Bones: Thin with parallel surfaces, e.g., bones of skull roof, sternum, ribs, scapulae.

• Long Bones: Long and slender, e.g., arms, legs, palms, soles, fingers, toes.

• Sesamoid Bones: Usually small, round, and flat; develop within tendons near joints of knees, hands, and feet. Location and number vary between individuals. Example: patellae.

Sinuses in Bones

Sinuses are air-filled chambers in the skull that serve several functions:

• Decrease weight of the skull

• Produce mucus to moisten and clean air

• Serve as resonating chambers in speech production

Bone Cells and Their Functions

Osteocytes

• Mature bone cells that do not divide

• Live in lacunae between layers of matrix

• Have cytoplasmic extensions that pass through canaliculi

• Functions:

  • Maintain protein and mineral content of matrix

  • Help repair damaged bone

Osteoblasts

• Absorb and remove bone matrix

• Large, multinucleate cells

• Secrete acids and protein-digesting enzymes to dissolve bone matrix and release stored minerals

• Derived from the same stem cells that produce monocytes and macrophages

• Most abundant bone cells

Osteoclasts

• Absorb and remove bone matrix

• Large, multinucleate cells

• Secrete acids and protein-digesting enzymes

• Dissolve bone matrix and release stored minerals

• Derived from stem cells that produce monocytes and macrophages

Bone Matrix Components

• Calcium phosphate makes up almost two-thirds of bone mass

• Bone contains collagen fibers for flexibility

• About one-third of bone mass is collagen fibers

• Matrix incorporates other calcium salts such as calcium carbonate and magnesium

Hydroxyapatite in Bone Matrix

• Hydroxyapatite is formed by calcium phosphate interacting with calcium hydroxide

• Purpose: To form crystals and incorporate other calcium salts, making bone matrix solid and not flexible

Bone Structure and Blood Supply

Blood Supply of Bone

• Nutrient artery and vein: Most bones have one of each (some have more)

• Metaphyseal vessels: Supply blood to epiphyseal cartilages

• Periosteal vessels: Supply blood to superficial osteons and secondary ossification centers

Regions of a Long Bone

• Diaphysis (shaft): Wall is made up of compact bone; central space contains medullary cavity (narrow cavity)

• Epiphysis (wide part at each end): Mostly spongy bone (trabecular bone)

• Metaphysis: Where diaphysis and epiphysis meet

Functional Unit of Compact Bone

• Osteon (Haversian system)

Bone Formation and Ossification

Endosteum

• Incomplete cellular layer lining medullary cavity

• Active during bone growth, repair, and remodeling

• Lines central canals of compact bone

• Consists of flattened layer of osteogenic cells

Steps of Endochondral and Intramembranous Ossification

• Most bones form by endochondral ossification

• Primary ossification center develops inside hyaline cartilage

• Cartilage is gradually replaced by bone

• Seven main steps:

  1. Chondrocytes near the center of the shaft increase greatly in size; matrix reduces to a series of small struts that soon begin to calcify; enlarged chondrocytes die and disintegrate, leaving cavities within the cartilage.

  2. Blood vessels grow around the edges of the cartilage, and cells of the perichondrium convert to osteoblasts. Shaft of the cartilage becomes ensheathed in a superficial layer of bone.

  3. Blood vessels penetrate the cartilage and invade the central region. Fibroblasts migrate with the blood vessels, differentiate into osteoblasts, and begin producing spongy bone at a primary ossification center. Bone formation spreads along the shaft towards both ends of the former cartilage model.

  4. Remodeling occurs as growth continues, creating a medullary cavity. The osseous tissue of the shaft becomes thicker, and cartilage near each epiphysis is replaced by shafts of bone. Further growth involves increases in length and diameter.

  5. Capillaries and osteoblasts migrate into the epiphyses, creating secondary ossification centers.

  6. The epiphyses eventually become filled with spongy bone. The metaphysis, a relatively narrow cartilaginous region, separates the epiphysis from the diaphysis.

  7. Osteoblasts continue to produce bone matrix, replacing cartilage with bone. Epiphyseal closure occurs when the epiphyseal cartilage is replaced by bone, leaving an epiphyseal line.

Intramembranous Ossification

• Mesenchymal cells cluster together, differentiate into osteoblasts, and start to secrete organic components of the matrix. The resulting osteoid then becomes mineralized with calcium salts forming bone matrix.

• Some osteoblasts are trapped inside bony pockets where they differentiate into osteocytes.

• Blood vessels begin to branch within the region and grow between the spicules.

• Subsequent remodeling produces typical compact bone.

• Osteoblasts on the bone surface along with connective tissue around the bone become the periosteum.

Location of Ossification Centers

• Primary ossification center in the long bone is in the diaphysis

• Secondary ossification center in the long bone is in the epiphysis

Starting Tissue for Ossification

• Endochondral ossification: hyaline cartilage

• Intramembranous ossification: mesenchymal tissue

Examples of Bones Formed by Each Type

• Endochondral ossification: long bones (tibia, humerus, and phalanges)

• Intramembranous ossification: flat bones (frontal, mandible, clavicles, skull)

Bone Growth: Appositional vs. Interstitial

• Appositional growth: Growth in width; layers of circumferential lamellae are added at outer surface

• Interstitial growth: Growth in length; epiphyseal closure—completion of epiphyseal growth

Bone Remodeling and Homeostasis

Bone Remodeling

• Occurs throughout life

• Functions in bone maintenance

• Involves osteocytes, osteoblasts, and osteoclasts

• If removal is faster than replacement, bones weaken

• If deposition predominates, bones strengthen

Hormonal Regulation of Bone

• Calcitonin: Decreases blood calcium levels by inhibiting osteoclast activity, increasing calcium excretion, and reducing calcitriol secretion by kidneys.

• Parathyroid hormone (PTH): Increases blood calcium levels by stimulating osteoclast activity, increasing intestinal absorption of calcium, and decreasing calcium excretion by kidneys.

Organ Systems Involved in Calcium Homeostasis

• Skeletal organ systems

• Urinary organ systems

• Digestive organ systems

Calcium Absorption and Storage

• Blood calcium levels are maintained by the 3 organ systems storing, absorbing, and excreting calcium as the body needs it.

Bone Fractures and Repair

Types of Fractures

• Open/compound fracture: Bone breaks and also punctures the skin

• Closed/simple fracture: Bone breaks but does not damage the skin

• Major types: Transverse, spiral, epiphyseal, comminuted, greenstick, colles, and pott's fractures

Steps of Bone Repair

• Bone repair occurs in four steps:

  1. Fracture hematoma formation

  2. Callus formation (cells of endosteum and periosteum divide and migrate into fracture area)

  3. Spongy bone formation (osteoblasts replace central cartilage of external callus with spongy bone)

  4. Compact bone formation (repaired bone may be slightly thicker and stronger than normal)

Osteoporosis and Aging

Osteoporosis

• Osteoporosis affects women more than men because women lose estrogen with age, which helps maintain bone mass.

• Osteoporosis—severe loss of bone mass, over age 45, occurs in 29% of women and 18% of men.

• Compromises normal function; vertebrae, epiphyses, and jaws are most affected.

• Hormones and bone loss: Estrogen helps women maintain bone mass; in women, osteoporosis accelerates after menopause.

• Cancer and bone loss: Cancerous tissue releases osteoclast-activating factor, which stimulates osteoclasts and produces severe osteoporosis.

Effects of Aging on the Skeletal System

• Bone becomes thinner and weaker with age

• Osteopenia—beginning of ossification reduction, begins between ages 30 and 40

• Epiphyses, vertebrae, and jaws most affected

• Results in fragile limbs, reduced height, and tooth loss

Prevention of Osteoporosis

• Exercise

• Nutrition (adequate calcium and vitamin D)

Vertebrae Identification

• Know how to identify the different parts of a vertebra from a picture

Summary Table: Bone Cell Types and Functions

Cell Type	Location	Function
Osteocyte	Lacunae in bone matrix	Maintain bone matrix, repair damaged bone
Osteoblast	Bone surface	Produce new bone matrix (osteogenesis)
Osteoclast	Bone surface, endosteum	Remove bone matrix (osteolysis)

Key Equations

• Hydroxyapatite formation: $Ca_3(PO_4)_2 + Ca(OH)_2 \rightarrow Ca_5(PO_4)_3OH$

Additional info: Some explanations and examples have been expanded for clarity and completeness.