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• The axial skeleton supports the head, neck, back, and chest, forming the body's vertical axis.
• The axial skeleton includes the skull, vertebral column (sacrum and coccyx), and thoracic cage (ribs and sternum).
• The appendicular skeleton consists of all bones of the upper and lower limbs.
• The bodies of adjacent vertebrae are separated and united by an intervertebral disc.
• Intervertebral discs provide padding and allow for movements between adjacent vertebrae.
• Discs consist of the anulus fibrosus and the nucleus pulposus.
• The anulus fibrosus is fibrous outer layer of the disc.
• The nucleus pulposus is a gel-like center.
• The intervertebral foramen is the opening between adjacent vertebrae for spinal nerve exit.
• Weakening of the anulus fibrosus leads to herniation of nucleus pulposus, compressing spinal nerves.
• Herniated Nucleus Pulposus (HNP) results in pain and muscle weakness in areas served by the nerve.

Abnormal Curvatures of the Vertebral Column

• Scoliosis is an abnormal lateral bending of the vertebral column.
• Excessive kyphosis is an excessive curvature of the upper thoracic vertebral column.
• Excessive lordosis occurs in the lumbar region of the vertebral column.

Bone Markings

• Bone surface features depend on function, location, and attachment to ligaments, tendons, blood vessels, and nerves.
• Head is a bony expansion carried on a narrow neck.
• Facet is a smooth, nearly flat articular surface.
• Condyle is a rounded articular projection.
• Ramus is an arm-like bar of bone.
• Groove is a furrow.
• Fissure is a narrow, slit-like opening.
• Foramen is a round or oval opening through a bone.
• Notch is an indentation at the edge of a structure.
• Meatus is a canal-like passageway.
• Sinus is a cavity within a bone, filled with air and lined with a mucous membrane.
• Fossa is a shallow, basin-like depression in a bone, often serving as an articular surface.
• Tuberosity is a large, rounded projection that may be roughened.
• Crest is a narrow ridge of bone, usually prominent.
• Trochanter is a very large, blunt, irregularly shaped process (only on the femur).
• Line is a narrow ridge of bone that is less prominent than a crest.
• Tubercle is a small rounded projection or process.
• Epicondyle is a raised area on or above a condyle.
• Spine is a sharp, slender, often pointed projection.
• Process is any bony prominence.

Bone Tissue

• Compact bone is the dense outer layer that appears smooth and solid.
• Spongy bone is the internal honeycomb of flat pieces called trabeculae.
• Articular cartilage covers the external bone surface.
• Periosteum forms the outer surface of bone.
• Endosteum lines the medullary cavity.
• Osteocytes maintain bone tissue.
• Osteoblasts form bone matrix.
• Osteogenic cells are stem cells.
• Osteoclasts resorb bone.

Anatomy of a Long Bone

• Typical bones have a diaphysis, epiphyses, and membranes.
• The diaphysis is the shaft.
• Epiphyses are the bone ends.
• Compact bone is the dense outer layer.
• Spongy bone is the internal network.
• The medullary cavity contains marrow.
• The periosteum is the outer fibrous layer.
• Endosteum lines the marrow cavity.

Functions of Bones

• Bones support the body and soft organs.
• Bones protect the brain, spinal cord, and vital organs.
• Bones facilitate movement via muscle attachment.
• Bones store minerals and growth factors like calcium and phosphorus.
• Bones enable blood cell formation (hematopoiesis) in red marrow.
• Bones store triglyceride (fat) in bone cavities.
• Bones produce hormones.
• Osteocalcin regulates bone formation.

Importance of Calcium

• Functions include nerve impulse transmission, muscle contraction, and blood coagulation.
• Calcium aids in secretion by glands and nerve cells as well as cell division.
• The body contains 1200-1400 grams of calcium, 99% stored as bone minerals.
• Intestinal absorption requires Vitamin D.

Control of Remodeling

• Bone remodeling occurs continuously and is regulated by genetic factors and two control loops.
• A negative feedback hormonal loop maintains Ca2+ homeostasis in blood.
• Bone responds to mechanical and gravitational forces (Wolff's law).
• Bone in a healthy animal adapts to the loads under which it is placed.
• Loading increases cause a bone to remodel and become stronger to resist that sort of loading.
• Serotonin neurotransmitter regulates mood and sleep, secreted into the blood after eating.
• Serotonin interferes with osteoblast activity.
• Serotonin reuptake inhibitors (Prozac) can cause lower bone density.

Calcium Homeostasis

• Dangerous changes in blood calcium levels can cause severe neuromuscular problems.
• Hyperexcitability occurs if calcium levels are too low.
• Nonresponsiveness occurs if calcium levels are too high.
• Sustained high blood calcium levels lead to hypercalcemia and deposits of calcium salts.
• Deposits of calcium salts in blood vessels and kidneys can interfere with function.

Bone Homeostasis: Bone Remodeling

• Both bone deposit and bone resorption occur at the periosteum and endosteum surfaces.
• Remodeling units are osteoblasts and osteoclasts.

Hormonal Regulation of Bone Growth

• Growth hormone stimulates epiphyseal plate activity, especially in infancy and childhood.
• Thyroid hormone modulates growth hormone activity.
• Testosterone and estrogens promote adolescent growth spurts.
• End growth is achieved through the induction of epiphyseal plate closure.
• The epiphyseal plate is responsible for longitudinal bone growth.

Growth of Cartilage

• Appositional growth occurs when cells secrete matrix against the external face of existing cartilage.

Endochondral Ossification

• Endochondral ossification follows five steps:
  • Mesenchymal cells differentiate into chondrocytes.
  • The cartilage model of the future bony skeleton and perichondrium form.
  • Capillaries penetrate cartilage.
  • Perichondrium transforms into periosteum, leading to the development of a periosteal collar and a primary ossification center.
  • Cartilage and chondrocytes continue to grow at the ends of the bone, and secondary ossification centers develop.
• Cartilage remains at the epiphyseal (growth) plate and the joint surface as articular cartilage.

Fractures

• Fracture repair involves a series of steps.
• A fracture hematoma forms.
• Internal and external calli form.
• Cartilage of the calli is replaced by trabecular bone.
• Remodeling occurs.
• Timelines range from 4-20 weeks.
• A nondisplaced fracture retains normal bone alignment.
• A displaced fracture ends out of normal alignment.
• An open (compound) fracture penetrates the skin.
• A closed (simple) fracture does not penetrate the skin.
• A complete fracture fractures all the way through the bone.
• An incomplete fracture does not fracture all the way through the bone.

Types of Fractures

• Include closed, open, transverse, spiral, comminuted, impacted, greenstick, and oblique.

Red Marrow

• Red marrow occupies trabecular cavities in spongy bone and flat bones.
• In newborns, red marrow is found in medullary cavities and spongy bone.
• Adult long bones have little red marrow; it is mainly in the heads of the femur and humerus.

Homeostatic Imbalances

• Osteoporosis occurs when bone resorption outpaces bone deposit.
• The spongy bone of the spine and femur neck is most susceptible to osteoporosis.
• Vertebral and hip fractures are common.
• Males with prostate cancer taking androgen-suppressing drugs are at higher risk.
• The female athlete triad, involving menstrual dysfunction, low energy availability, and decreased bone mineral density, increases risk.
• Risk factors include; insufficient exercise, poor diet (calcium & protein), and smoking.

Bone Diseases and Infections

• Osteomalacia (Rickets) in children.
• Osteomyelitis is a bacterial bone infection that causes pain, tenderness, lump, redness, and pus.
• Osteoarthritis treatments include antibiotics and surgery.
• Osteosarcoma is the most common type of bone cancer, often affecting teenage boys.
• Paget's Disease is a bone disease with an unknown cause (genetic predisposition).
• Normal leg bones affected by Paget's disease are porous and curved.
• A bone infection causes pain, tenderness, lump, and redness. Treatments include antibiotics and surgery.