Skeletal System: Bone Structure and Function

Vocabulary terms and definitions regarding skeletal system growth, remodeling, hormonal regulation, calcium homeostasis, and fracture repair based on the Chapter 07C lecture notes.

Appositional Bone Growth

Bone growth that occurs within the periosteum where bone matrix is deposited in layers parallel to the surface.

Achondroplasia

A condition characterized by the abnormal conversion of hyaline cartilage to bone; it is the most common cause of achondroplastic dwarfism where long bones stop growing in childhood.

Bone Remodeling

A process that continues throughout adulthood where approximately $20\%$ of the skeleton is replaced yearly at periosteal and endosteal surfaces.

Distal femur replacement rate

The distal part of the femur is typically replaced every $4$ to $6$ months during bone remodeling.

Growth hormone (somatotropin)

Produced by the anterior pituitary gland, it stimulates the liver to produce insulin-like growth factors (IGFs) to stimulate cartilage growth in the epiphyseal plate.

Thyroid hormone

Secreted by the thyroid gland, this hormone influences the basal metabolic rate of bone cells and regulates activity at epiphyseal plates.

Sex hormones (Estrogen and Testosterone)

Hormones secreted in large amounts at puberty that dramatically accelerate bone growth and eventually cause cartilage to be replaced by bone.

Glucocorticoids

A group of steroid hormones from the adrenal cortex that regulate blood glucose; high levels increase bone loss and impair growth at the epiphyseal plate.

Serotonin

A neurotransmitter and hormone that, if levels are too high, prevents osteoprogenitor cells from differentiating into osteoblasts, potentially leading to low bone density.

Calcitriol

The active form of vitamin D that stimulates the absorption of calcium from the small intestine into the blood.

7-dehydrocholesterol

The precursor molecule in the skin that is converted to vitamin D3 (cholecalciferol) when exposed to ultraviolet (UV) light.

Calcidiol

A molecule produced in the liver by converting Vitamin D3; it is later converted to calcitriol in the kidneys.

Parathyroid hormone (PTH)

A hormone released in response to decreased blood calcium that works with calcitriol to increase osteoclast activity and decrease calcium excretion in urine.

Rickets

A disease caused by vitamin D deficiency in childhood characterized by deficient calcification of osteoid tissue and a bowlegged appearance.

Calcitonin

A hormone released from the thyroid gland in response to high blood calcium levels that inhibits osteoclast activity and increases calcium loss in urine.

Osteopenia

A decrease in bone mineral density (BMD) below reference values that occurs slightly in all people with age, typically beginning between ages $35$ and $40$.

Osteoporosis

A condition of reduced bone mass sufficient to compromise normal function, often linked to age, menopause, and decreased hormone levels.

DEXA scan

A dual-energy x-ray absorptiometry scan used to determine a decrease in bone mineral density.

Stress fracture

A thin break in the bone caused by increased physical activity or repeated, stressful impact.

Pathologic fracture

A fracture that occurs in a bone already weakened by a disease process, such as cancer.

Compound (open) fracture

A fracture where one or both broken ends of the bone protrude through the skin.

Fracture hematoma

The first step of fracture repair where blood vessels torn within the periosteum form a blood clot.

Fibrocartilaginous (soft) callus

The second step of fracture repair where a procallus is formed and fibroblasts produce collagen fibers while chondroblasts form dense regular CT.

Hard (bony) callus

The third step of fracture repair where osteoblasts produce trabeculae that replace the soft callus.

Colles fracture

A fracture of the distal end of the radius (lateral forearm bone) that produces a "dinner fork" deformity.

Comminuted fracture

A fracture where the bone is splintered into several small pieces between the main parts.

Greenstick fracture

A partial fracture where one side of the bone breaks and the other side is bent; most commonly seen in children.

Pott fracture

A fracture occurring at the distal ends of both the tibia and the fibula.