Skeletal System: Structure, Function, Bones, and Articulations

Vocabulary flashcards covering bone structure, function, cartilage, ossification, bone remodeling, axial and appendicular skeletal features, and joint classifications and movements.

Compact Bone

Bone connective tissue that appears white, smooth, and solid, making up about $80\%$ of total bone mass, and arranged in repeating functional units called osteons.

Spongy Bone

Porous bone connective tissue making up about $20\%$ of total bone mass, composed of an irregular pattern of thin columns called trabeculae, lacking osteons, and serving as the primary site of hemopoiesis.

Ligaments

Dense regular connective tissue structures that connect bone to bone.

Cartilage

Semirigid connective tissue that is more flexible than bone, with mature forms being avascular, existing as either hyaline cartilage or fibrocartilage.

Osteon

The repeating microscopic functional unit of compact bone structure.

Perforating Canal

Transverse tunnels in compact bone that carry blood vessels, lymphatic vessels, and nerves.

Central (Haversian) Canal

Longitudinal tunnels running through the center of osteons in compact bone.

Canaliculi

Microscopic canals radiating from lacunae that connect lacunae to the central canal, providing routes for nutrient and waste exchange.

Hyaline Cartilage

Resilient and flexible cartilage containing a high percentage of water that acts as a shock absorber; forms costal cartilage, articular cartilage, epiphyseal plates, and the embryonic fetal skeleton model.

Fibrocartilage

Weight-bearing cartilage designed to withstand compression; forms intervertebral discs, pubic symphysis, and cartilage pads of the menisci.

Periosteum

Tough sheath covering the outer surface of bone, consisting of an outer fibrous layer of dense irregular CT and an inner cellular layer of multipotent osteoprogenitor cells and osteoblasts.

Endosteum

Thin layer of connective tissue containing osteoprogenitor cells and osteoblasts that covers all internal surfaces of bone within the medullary cavity.

Diaphysis

The main shaft of a long bone that provides leverage and major weight support, composed externally of compact bone and internally of spongy bone spicules.

Epiphysis

The expanded proximal or distal end of a long bone composed of an outer thin layer of compact bone and an inner, thicker layer of spongy bone involved in hematopoiesis.

Medullary Cavity

Hollow cylindrical space within the diaphysis containing red bone marrow in children and yellow bone marrow in adults.

Nutrient Foramina

Small openings in bone through which nutrient blood vessels and sensory nerves enter and exit.

Diploe

The sandwich-like structural arrangement in flat bones of the skull, consisting of spongy bone situated between two layers of compact bone.

Red Bone Marrow

Soft connective tissue containing reticular connective tissue, developing blood cells, and adipocytes that functions in forming blood cells (hematopoiesis).

Yellow Bone Marrow

Fatty substance produced from red marrow disintegration, found in select portions of the adult axial skeleton and medullary cavities.

Osteoprogenitor Cells

Mitotically active, multipotent stem cells derived from mesenchyme that are located in the periosteum and endosteum.

Osteoblasts

Bone-building cells formed from osteoprogenitor cells that synthesize and secrete osteoid.

Osteocytes

Mature bone cells formed from trapped osteoblasts that maintain the bone matrix and detect mechanical stress.

Osteoclasts

Large phagocytic cells derived from fused bone marrow cells with a ruffled border that perform bone resorption.

Osteoid

The organic component of bone matrix secreted by osteoblasts, composed of collagen fibers and ground substance (proteoglycans and glycoproteins) providing tensile strength and flexibility.

Hydroxyapatite

The inorganic component of bone matrix formed from calcium phosphate and calcium hydroxide crystals that hardens the matrix and provides compressional strength.

Interstitial Growth (Cartilage)

Pattern of cartilage growth that increases length through mitosis of chondrocytes within lacunae and internal matrix production.

Appositional Growth

Pattern of growth occurring on the outer surface or periphery of cartilage or bone, increasing overall thickness or width.

Intramembranous Ossification

Bone formation process where bone develops directly from a fibrous membrane of mesenchyme, producing flat bones of the skull and the clavicle.

Endochondral Ossification

Bone formation process where bone replaces an initial hyaline cartilage model, forming most bones of the skeleton.

Epiphyseal Plate

Layer of hyaline cartilage between the epiphysis and diaphysis where interstitial growth in bone length occurs across five distinct histological zones.

Bone Remodeling

Ongoing replacement of old bone tissue by new bone tissue through balanced osteoclastic resorption and osteoblastic deposition, replacing about $20\%$ of the skeleton yearly.

Parathyroid Hormone (PTH)

Hormone secreted in response to low blood calcium levels that inhibits osteoblasts and stimulates osteoclasts to increase bone resorption and elevate blood calcium levels.

Calcitonin

Hormone secreted in response to elevated blood calcium levels that stimulates osteoblasts for bone formation and inhibits osteoclasts to lower blood calcium levels.

Compound Fracture

A bone fracture in which the broken ends of the bone protrude through the skin.

Simple Fracture

A bone fracture in which the broken bone does not break or pierce through the skin.

Axial Skeleton

Skeletal division consisting of the skull, vertebral column, and thoracic cage, functioning primarily in protection, support, and hematopoiesis.

Appendicular Skeleton

Skeletal division consisting of the pectoral girdle, pelvic girdle, and upper and lower limbs, functioning in movement and manipulation.

Sella Turcica

Bony enclosure on the sphenoid bone surrounding the hypophyseal fossa that houses the pituitary gland.

Cribriform Plate

Perforated structure of the ethmoid bone that forms the roof of the nasal cavities and floor of the anterior cranial fossa, allowing passageway for olfactory nerves.

Paranasal Sinuses

Mucous membrane-lined cavities within cranial and facial bones surrounding the nasal cavity that serve as resonating chambers to intensify sound.

Hyoid Bone

Slender, U-shaped bone located inferior to the skull between the mandible and larynx that does not articulate with any other bone.

Fontanelles

Unossified fibrous membrane regions (soft spots) between cranial bones in infants that allow head flexibility during childbirth.

Atlas (C1)

First cervical vertebra lacking a body and spinous process, articulating with the occipital condyles to allow nodding of the head.

Axis (C2)

Second cervical vertebra characterized by the dens (odontoid process) that acts as a pivot allowing rotational head movement to shake 'no'.

Synarthrosis

Functional joint classification denoting an immovable joint.

Amphiarthrosis

Functional joint classification denoting a slightly movable joint.

Diarthrosis

Functional joint classification denoting a freely movable joint.

Gomphosis

Fibrous, synarthrotic peg-in-socket joint holding teeth within the alveolar processes of the mandible and maxillae via periodontal ligaments.

Syndesmosis

Fibrous, amphiarthrotic joint bound by an interosseous membrane between parallel bones such as the radius/ulna or tibia/fibula.

Synchondrosis

Cartilaginous, synarthrotic joint where bones are joined together by hyaline cartilage, such as the epiphyseal plate.

Symphysis

Cartilaginous, amphiarthrotic joint containing a pad of fibrocartilage to resist compression and absorb shock, such as the pubic symphysis or intervertebral joints.

Synovial Joint

Freely movable (diarthrotic) joint characterized by a fluid-filled joint cavity separated by an articular capsule and lined with synovial membrane.

Synovial Fluid

Viscous secretion produced by the inner synovial membrane that lubricates articular cartilage, nourishes chondrocytes, and absorbs joint shock.

Bursae

Sac-like structures containing synovial fluid located between tendons, ligaments, and bones to cushion movement and reduce friction.

Flexion

Angular motion at a synovial joint that decreases the angle between two articulating bones.

Extension

Angular motion at a synovial joint that increases the angle between two articulating bones.

Abduction

Angular motion that moves a limb laterally away from the midline of the body.

Adduction

Angular motion that moves a limb toward the midline of the body.

Pronation

Medial rotation of the forearm causing the palm to face posteriorly, crossing the radius over the ulna.

Supination

Lateral rotation of the forearm returning the palm to an anterior anatomical position with parallel radius and ulna.

Dorsiflexion

Special movement of the talocrural joint where the toes and dorsum of the foot are lifted superiorly toward the leg.

Plantar Flexion

Special movement of the talocrural joint where the foot bends downward so the toes point inferiorly.

Rotator Cuff

Group of four muscle tendons (subscapularis, supraspinatus, infraspinatus, teres minor) that encircle and stabilize the glenohumeral joint.

Osteoarthritis

Degenerative 'wear-and-tear' form of arthritis involving progressive breakdown of articular cartilage within joints.