Skeletal System Anatomy, Development, and Bone Tissue

Vocabulary practice flashcards detailing skeletal histology, developmental ossification, bone markings, calcium homeostasis, and the comprehensive anatomy of the axial and appendicular skeleton.

Articulation

A bone marking where two bones meet, such as the knee joint.

Condyle

A rounded surface on a bone that typically forms a joint, such as the occipital condyles.

Tuberosity

A rough surface marking on a bone, such as the deltoid tuberosity of the humerus.

Fossa

An elongated basin or shallow depression in a bone, such as the mandibular fossa.

Foramen

A hole through bone through which blood vessels and nerves can pass, such as the foramen magnum.

Canaliculi

Channels within the bone matrix that house cytoplasmic extensions of osteocytes, used to communicate and receive nutrients.

Central canal

A longitudinal channel located in the center of each osteon that houses blood vessels, nerves, and lymphatic vessels; also known as the Haversian canal.

Osteon

The basic structural unit of compact bone composed of concentric layers of calcified matrix; also known as the Haversian system.

Endosteum

A delicate membranous lining covering the inside of a bone's medullary cavity.

Periosteum

A fibrous membrane covering the outer surface of bone that is continuous with ligaments, except where articular cartilage is present.

Diploë

The central layer of spongy bone sandwiched between two layers of compact bone within flat bones.

Osteoid

The uncalcified organic bone matrix secreted by osteoblasts.

Osteoclast

A specialized bone cell responsible for the resorption and breakdown of bone tissue.

Osteoblast

A bone cell responsible for forming and secreting new bone matrix.

Osteogenic cell

An undifferentiated cell with high mitotic activity; the only bone cell that divides, eventually developing into an osteoblast.

Sesamoid bone

A small, round bone embedded within a tendon that protects the tendon from compressive forces, such as the patella.

Reserve zone

The region of the epiphyseal plate that anchors the plate to the osseous tissue of the epiphysis.

Proliferative zone

The region of the epiphyseal plate that actively produces new chondrocytes to replace those that die at the diaphyseal end, contributing to longitudinal bone growth.

Zone of maturation and hypertrophy

The region of the epiphyseal plate where chondrocytes from the proliferative zone grow, mature, and enlarge, contributing to longitudinal bone growth.

Zone of calcified matrix

The region of the epiphyseal plate closest to the diaphyseal end that connects the epiphyseal plate to the diaphysis.

Intramembranous ossification

The process by which bone tissue develops directly from mesenchymal connective tissue, forming the flat bones of the skull and clavicle.

Endochondral ossification

The process of bone development in which hyaline cartilage models are gradually replaced by osseous tissue, forming the base of the skull, facial bones, and long bones.

Sclerotome

The medial part of an embryonic somite composed of mesenchyme that gives rise to the vertebrae, ribs, and associated fibrous connective tissues.

Fontanelle

A broad, fibrous connective tissue space separating the incompletely ossified cranial bones of a newborn skull, the largest of which closes by age $2$.

Calcitriol

The active form of vitamin D produced by the kidneys that stimulates the absorption of calcium and phosphate from the digestive tract.

Parathyroid hormone (PTH)

A hormone that stimulates osteoclast proliferation and bone resorption, promotes calcium reabsorption in kidney tubules, and indirectly increases calcium uptake in the small intestine.

Calcitonin

A hormone secreted by the thyroid gland that inhibits osteoclast activity and stimulates calcium uptake into bones, reducing blood calcium levels.

Hypocalcemia

A clinical condition characterized by abnormally low blood calcium levels (below the normal level of about $10\,\text{mg/dL}$), impairing blood clotting, cardiac rhythm, nerve impulses, and muscle contractions.

Hypercalcemia

A condition characterized by abnormally high levels of calcium in the blood, resulting in an underactive nervous system, lethargy, sluggish reflexes, constipation, and confusion.

Paget's disease

A skeletal disorder characterized by overactive osteoclasts causing rapid resorption followed by haphazard, disorganized bone formation by osteoblasts, accompanied by elevated blood levels of alkaline phosphatase.

Osteoporosis

An age-related skeletal disease characterized by a decrease in bone mass when the rate of resorption exceeds formation, without the elevated alkaline phosphatase levels found in Paget's disease.

Axial skeleton

The vertical, central axis of the body consisting of $80$ bones in adults, including the skull ($22$ bones plus $7$ associated bones), vertebral column ($24$ vertebrae, sacrum, and coccyx), and thoracic cage ($12$ pairs of ribs and sternum).

Supraorbital foramen

A small opening located near the midpoint of the supraorbital margin of the frontal bone that provides passage for a sensory nerve to the forehead skin.

Nasal septum

The internal vertical wall that divides the nasal cavity into halves, formed superiorly by the perpendicular plate of the ethmoid bone and inferiorly by the vomer bone.

Inferior nasal concha

An independent, curved bone forming the lowest bony shelf projecting from the lateral wall of the nasal cavity.

Zygomatic arch

The bony bridge spanning from the cheek to above the ear canal, formed by the junction of the temporal process of the zygomatic bone and the zygomatic process of the temporal bone.

Pterion

An H-shaped suture junction uniting the frontal, parietal, temporal, and sphenoid (greater wing) bones, representing the structurally weakest point of the skull.

Sella turcica

A saddle-shaped bony region of the sphenoid bone within the middle cranial fossa that contains the hypophyseal fossa housing the pituitary gland.

Occipital condyle

An oval-shaped projection on the inferior base of the occipital bone flanking the foramen magnum that articulates with the C1 vertebra (atlas).

Petrous ridge

A prominent diagonal ridge of the temporal bone in the floor of the cranial cavity that separates the middle and posterior cranial fossae and houses middle and inner ear cavities.

Mandibular fossa

An oval depression on the external base of the temporal bone anterior to the acoustic meatus that articulates with the mandible to form the temporomandibular joint.

Styloid process

A downward, needle-like projection on the external base of the temporal bone that anchors several small muscles and a ligament supporting the hyoid bone.

Stylomastoid foramen

A small opening located between the styloid and mastoid processes of the temporal bone that allows the exit of the cranial nerve supplying facial muscles.

Carotid canal

A zig-zag shaped passageway through the temporal bone that conducts a major brain-supplying artery into the middle cranial cavity.

Crista galli

A small, upward midline bony projection of the ethmoid bone that serves as an anterior attachment point for one of the covering layers of the brain.

Cribriform plate

A flattened, sieve-like area of the ethmoid bone with olfactory foramina that transmit sensory nerve branches from the nasal cavity into the brain.

Primary curvatures

The spinal curvatures retained from original fetal posture, consisting of the thoracic and sacrococcygeal curves.

Secondary curvatures

The spinal curvatures that develop after birth as a child learns to hold up the head and walk, consisting of the cervical and lumbar curves.

Vertebral arch

The posterior portion of a vertebra formed by paired pedicles and paired laminae that surrounds and protects the spinal cord.

Intervertebral foramen

An opening formed between notches in the pedicles of adjacent vertebrae through which a spinal nerve exits the vertebral column.

Atlas

The first cervical vertebra ($C1$) that supports the skull; it lacks a vertebral body and spinous process, consisting instead of an anterior and posterior arch.

Axis

The second cervical vertebra ($C2$) characterized by the upward-projecting dens, serving as the pivot axis for rotation when turning the head side to side.

Dens

A tooth-like bony projection extending upward from the vertebral body of the axis ($C2$), held against the anterior arch of the atlas by the transverse ligament; also known as the odontoid process.

Transverse foramen

An opening found exclusively within the transverse processes of cervical vertebrae through which an essential artery ascends to supply the brain.

Costal facet

An articulation site located on the body or transverse process of a thoracic vertebra for forming joints with the head or tubercle of a rib.

Anulus fibrosus

The tough, fibrous outer ring of an intervertebral disc that anchors to the outer margins of adjacent vertebral bodies.

Nucleus pulposus

The soft, gel-like center of an intervertebral disc with a high water content that resists compression and aids in weight bearing.

Median sacral crest

A bumpy ridge running down the posterior midline of the sacrum formed by the fusion of the sacral spinous processes.

Sacral promontory

The prominent anterior lip of the superior weight-bearing base of the sacrum.

Sternal angle

The junction between the manubrium and body of the sternum that forms a palpable slight bend and serves as the attachment site for the second rib.

Jugular notch

A shallow, U-shaped border on the superior margin of the manubrium palpable between the medial clavicular ends; also known as the suprasternal notch.

Xiphoid process

The inferior cartilaginous tip of the sternum that ossifies into bone during middle age.

True ribs

Ribs $1$ through $7$ (vertebrosternal ribs) whose costal cartilages attach directly to the sternum.

False ribs

Ribs $8$ through $12$ (vertebrochondral ribs) whose costal cartilages do not attach directly to the sternum, including those that attach to higher costal cartilage or end in muscle.

Floating ribs

The last two pairs of false ribs (ribs $11$ and $12$, or vertebral ribs) that do not connect to the sternum at all and terminate in the lateral abdominal musculature.

Costal groove

A shallow depression located along the inferior margin of a rib shaft that accommodates blood vessels and an intercostal nerve.

Sternoclavicular joint

The only bony joint between the pectoral girdle of the upper limb and the axial skeleton, formed between the sternal end of the clavicle and the manubrium.

Coracoclavicular ligament

A strong band of connective tissue anchoring the scapula's coracoid process to the inferior acromial end of the clavicle, providing primary support against shoulder separation.

Glenoid cavity

A shallow depression on the lateral corner of the scapula that articulates with the head of the humerus to form the glenohumeral (shoulder) joint.

Coracoid process

A hook-like projection on the superior border of the scapula that projects anteriorly and curves laterally, anchoring chest and arm muscles and a ligament to the clavicle.

Acromion

The flattened lateral expansion of the scapular spine that forms the bony tip of the superior shoulder and articulates with the acromial end of the clavicle.

Intertubercular groove

A narrow channel between the greater and lesser tubercles of the humerus that provides passage for a tendon of the biceps brachii muscle; also known as the bicipital groove.

Trochlea

A spindle- or pulley-shaped condylar region on the medial distal humerus that articulates with the trochlear notch of the ulna.

Capitulum

A smooth, rounded knob on the anterior lateral aspect of the distal humerus that articulates with the head of the radius.

Olecranon process

The posterior and superior bony prominence of the proximal ulna that forms the bony tip of the elbow and slots into the olecranon fossa of the humerus during forearm extension.

Radial notch of the ulna

A small, smooth depression on the lateral side of the coronoid process of the ulna that articulates with the head of the radius at the proximal radioulnar joint.

Synarthrosis

A functional joint category characterized by an immobile or nearly immobile union that provides strong protection for internal organs.

Amphiarthrosis

A functional joint category characterized by limited mobility, exemplified by an intervertebral disc or the pubic symphysis.

Diarthrosis

A functional joint category encompassing all freely movable synovial joints of the body.

Syndesmosis

A type of fibrous joint in which two parallel bones are united by a ligament or a wide connective tissue band called an interosseous membrane.

Gomphosis

A specialized peg-and-socket fibrous joint that anchors the root of a tooth into its bony socket within the jaw.

Periodontal ligament

Short bands of dense connective tissue that firmly anchor the root of a tooth to the alveolar bone socket.

Synchondrosis

A cartilaginous joint in which the articulating bones are joined exclusively by hyaline cartilage.

Symphysis

A cartilaginous joint in which the connecting material between articulating bones is a pad of fibrocartilage.

Synostosis

The complete bony fusion formed when connective tissue or cartilage between adjacent bones ossifies.

Articular capsule

A fibrous connective tissue structure attached to adjacent bones that surrounds and encloses the synovial joint cavity.

Synovial membrane

The delicate cellular inner lining of an articular capsule that secretes lubricating fluid into the joint cavity.

Bursa

A fluid-filled sac lined with synovial membrane that reduces friction between skin, muscle, tendon, and underlying bone around a joint.

Meniscus

A C-shaped fibrocartilaginous pad positioned between the condyles of the femur and tibia to provide cushioning, shock absorption, and joint stabilization.

Uniaxial joint

A joint classification that permits movement in only a single anatomical plane around one axis.

Biaxial joint

A joint classification that allows movement in two anatomical planes around two perpendicular axes.

Multiaxial joint

A joint classification permitting movement in three anatomical planes and around three axes, including rotational movements.

Pivot joint

A synovial joint in which a rounded bone projection rotates within a ring formed partly by another bone and partly by a ligament.

Hinge joint

A uniaxial synovial joint where the convex surface of one bone articulates with the concave surface of another to permit only bending and straightening.

Condyloid joint

A biaxial synovial joint where an oval depression receives an egg-shaped condyle, permitting angular motions in two planes; also known as an ellipsoidal joint.

Saddle joint

A biaxial synovial joint characterized by complementary saddle-shaped articulating surfaces that fit together like a rider on horseback.

Plane joint

A synovial joint featuring flat or slightly curved surfaces that permit bones to slide past one another; also termed a gliding joint.

Ball-and-socket joint

A multiaxial synovial joint featuring a spherical head fitting into a cup-shaped depression, offering the greatest range of motion in the skeleton.

Flexion

An angular movement typically in the sagittal plane that decreases the angle between articulating bones.

Extension

An angular movement typically in the sagittal plane that straightens a joint or increases the angle between articulating bones.