The Skeletal System: Bones and Cartilage

Vocabulary practice flashcards covering bone anatomy, microscopic histology, ossification, postnatal growth, remodeling, fracture repair, and hormonal calcium homeostasis.

Compact Bone Tissue

Dense bone tissue composed of repeating structural units called osteons (Haversian systems).

Spongy Bone Tissue

A honeycomb-like network of bone tissue composed of needle-like structural units called trabeculae.

Trabeculae

The needle-like structural units of spongy bone that align along lines of greatest tensile stress and compression load.

Epiphyses

The expanded proximal and distal ends of a long bone, consisting internally of spongy bone surrounded by a thin exterior layer of compact bone.

Diaphysis

The shaft forming the long axis of a long bone, composed of a thick collar of compact bone surrounding a central medullary cavity.

Medullary Cavity

The central internal cavity within the diaphysis of a long bone that contains red bone marrow in childhood and yellow bone marrow in adulthood.

Hematopoiesis

The physiological process by which blood cells and platelets are formed, occurring exclusively within red bone marrow.

Periosteum

A double-layered external membrane covering bone, consisting of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer containing osteoblasts and osteoclasts.

Sharpey's Fibers

Tough collagenous perforating fibers that securely anchor the periosteum to the underlying compact bone.

Endosteum

A delicate internal connective tissue membrane containing osteoblasts and osteoclasts that covers internal bone surfaces such as bony canals and trabeculae.

Osteogenic Cells

Stem cells of osseous tissue that give rise to osteoblasts.

Osteoblasts

Bone-forming cells that synthesize and secrete the unmineralized organic bone matrix known as osteoid.

Osteocytes

Mature bone cells derived from trapped osteoblasts that occupy lacunae and maintain contact with one another through long cytoplasmic processes.

Osteoclasts

Multinucleated giant bone-resorbing cells possessing a brush border rich in tartrate-resistant acid phosphatase that actively degrade bone tissue.

Osteon

The cylindrical structural unit of compact bone, also known as a Haversian system, composed of concentric rings called lamellae around a central canal.

Lamellae

Concentric cylindrical tubes of mineralized bone matrix within an osteon, in which collagen fibers in adjacent layers run in opposing directions to resist torsional and twisting forces.

Central Canal (Haversian Canal)

A longitudinal channel running through the core of an osteon that houses blood vessels and nerve fibers.

Perforating Canals (Volkmann's Canals)

Canals oriented at right angles to the central canals that connect blood vessels and nerves from the periosteum to the central canals and medullary cavity.

Lacunae

Small cavities within the calcified bone matrix that accommodate mature osteocytes.

Canaliculi

Hairlike microscopic canals connecting adjacent lacunae to each other and to the central canal, enabling the exchange of nutrients and wastes.

Intramembranous Ossification

The prenatal bone formation pathway that develops directly from a fibrous connective tissue membrane derived from mesenchyme, producing membrane bones such as the cranial bones and clavicles.

Endochondral Ossification

The prenatal bone formation process wherein hyaline cartilage models produced by chondroblasts are broken down and replaced by bone tissue to form all skeletal bones except cranial bones and clavicles.

Longitudinal Bone Growth

Postnatal linear bone growth occurring at the epiphyseal plates that increases the length of long bones and body height.

Appositional Bone Growth

Bone growth in thickness and diameter where osteoblasts in the periosteum deposit new bone on the external surface while osteoclasts in the endosteum slightly resorb bone internally.

Growth (Proliferation) Zone

The zone of the epiphyseal plate located at the epiphyseal face where chondroblasts rapidly undergo mitosis and secrete new hyaline cartilage matrix.

Hypertrophic Zone

The zone of the epiphyseal plate where older chondrocytes enlarge significantly in size.

Calcification Zone

The zone of the epiphyseal plate where hydroxyapatite crystals calcify the surrounding matrix, cutting off nutrients and causing chondrocytes to die.

Ossification (Osteogenic) Zone

The zone on the diaphyseal face of the epiphyseal plate where osteoblasts invade deteriorating cartilage and deposit new osteoid matrix.

Epiphyseal Plate Closure

The complete ossification of the epiphyseal plates stimulated by sex steroid hormones near the end of adolescence, ending further longitudinal bone growth.

Gigantism

An endocrine disorder caused by hypersecretion of growth hormone during childhood prior to the closure of the epiphyseal plates, resulting in abnormally tall stature.

Acromegaly

An endocrine disorder resulting from the excess secretion of growth hormone in adults after epiphyseal plate closure, producing excessive appositional bone growth.

Bone Remodeling

A lifelong coordinated process involving the resorption of existing bone by osteoclasts and the deposition of new bone by osteoblasts to maintain calcium homeostasis and repair microdamage.

Osteoporosis

A metabolic bone disease characterized by bone depletion in which bone resorption outpaces bone formation, reducing both organic and inorganic matrix density.

Dowager's Hump

An abnormal kyphotic curvature of the thoracic spine resulting from osteoporotic vertebral wedge compression fractures, which can lead to height loss of up to $18\,\text{cm}$.

Paget's Disease

A chronic bone condition characterized by excessive bone resorption paired with disorganized, haphazard bone reformation, leading to bowed weight-bearing bones and cortical thickening.

Comminuted Fracture

A bone fracture in which the bone shatters into three or more fragments, commonly occurring in elderly individuals with brittle bones.

Greenstick Fracture

An incomplete fracture in which one side of the bone breaks while the opposite side bends, typical in children whose bones are more flexible.

Spiral Fracture

A ragged fracture occurring when excessive twisting forces are applied across a bone, common in sports injuries.

Stages of Fracture Healing

The four sequential stages of bone repair: 1) Hematoma formation ($1\text{--}2\,\text{weeks}$), 2) Fibrocartilaginous callus formation ($2\text{--}4\,\text{weeks}$), 3) Bony callus formation ($4\text{--}8\,\text{weeks}$), and 4) Bone remodeling ($8\text{--}12+\,\text{weeks}$).

Wolff's Law

The physiological law stating that bones adapt, grow, and remodel in direct response to the mechanical demands and stresses placed upon them.

Parathyroid Hormone (PTH)

An 84-amino acid peptide hormone released by chief cells of the parathyroid glands in response to hypocalcemia (serum calcium $<9\,\text{mg}/100\,\text{cc}$) and hyperphosphatemia.

Osteoclast Activating Factors (OAFs)

Signaling factors released by osteoblasts following PTH receptor binding that indirectly activate osteoclasts to promote bone resorption.

Calcitriol (1,25-dihydroxyvitamin D)

The biologically active form of vitamin D synthesized via 1-alpha-hydroxylase in the kidneys that directly increases calcium and phosphate absorption from the gastrointestinal tract.

Calcitonin

A peptide hormone produced by parafollicular cells (C cells) of the thyroid gland in response to hypercalcemia that stimulates osteoblast mineral deposition and inhibits osteoclast resorption.

Haglund's Deformity

A bony enlargement on the posterior calcaneus (heel), also termed 'pump bump,' caused by chronic irritation from rigid shoe counters or high heels.