The Skeletal System: Bone Tissue and the Axial Skeleton

Essential vocabulary flashcards covering skeletal functions, bone histology, osteogenesis, fractures, and the detailed anatomy of the axial skeleton.

Diaphysis

The extended tubular shaft of a long bone that surrounds a central marrow-filled space termed the medullary cavity.

Epiphysis

The expanded articular region located at each end of a long bone, composed primarily of spongy (trabecular) bone.

Metaphysis

The narrow transitional zone connecting the diaphysis and epiphysis of a long bone, containing the epiphyseal plate during growth.

Periosteum

A double-layered connective tissue covering wrapping the exterior of the diaphysis, featuring an outer fibrous layer and an inner cellular layer continuous with tendons and joint capsules.

Endosteum

An incomplete cellular membrane that lines the medullary cavity, covers the trabeculae of spongy bone, and lines central canals; contains osteogenic cells, osteoblasts, and osteoclasts.

Hydroxyapatite

Hard mineral crystals with the formula $Ca_{10}(PO_4)_6(OH)_2$, formed when calcium phosphate ($Ca_3(PO_4)_2$) interacts with calcium hydroxide ($Ca(OH)_2$) to comprise approximately $2/3$ of bone mass.

Osteogenic Cell

A mesenchymal stem cell found in the endosteum and the inner cellular layer of the periosteum whose cell divisions give rise to osteoblasts.

Osteoblast

An immature bone cell that produces and secretes osteoid, the organic extracellular matrix component of bone.

Osteoid

The uncalcified organic extracellular matrix composed primarily of collagen fibers secreted by osteoblasts before mineral deposition.

Osteocyte

A mature bone cell residing in a lacuna that maintains protein and mineral matrix turnover and communicates with neighboring cells through canaliculi.

Osteoclast

A large, multinucleated bone-resorbing cell derived from the white blood cell lineage that utilizes a ruffled border to secrete acids and enzymes that dissolve bone matrix.

Osteon (Haversian System)

The primary structural and functional unit of mature compact bone, consisting of concentric lamellae arranged around a central canal parallel to the long axis.

Concentric Lamellae

Circular layers of calcified bone matrix surrounding a central canal, containing collagen fibers aligned in alternating directions between adjacent rings to resist twisting stress.

Interstitial Lamellae

Incomplete remnants of older, partially remodeled osteons that fill the angular gaps between newer osteons in compact bone.

Circumferential Lamellae

Broad sheets of bone matrix encircling the entire inner and outer circumferences of the diaphysis just deep to the periosteum and superficial to the endosteum.

Central (Haversian) Canal

A longitudinal neurovascular channel situated at the axis of an osteon containing blood vessels and nerve fibers.

Perforating (Volkmann's) Canal

A transverse passage lined by endosteum that runs perpendicular to central canals, linking periosteal and medullary vessels to central canals.

Canaliculi

Hairlike micro-tunnels traversing the calcified matrix that connect lacunae to one another and to central canals, facilitating osteocyte nutrient exchange via gap junctions.

Trabeculae

An interlocking meshwork of thin bony plates and struts constituting spongy (cancellous) bone, devoid of osteons and nourished by diffusion from marrow spaces.

Red Bone Marrow

The vascularized tissue located within the trabecular spaces of spongy bone responsible for hemopoiesis (blood cell production).

Yellow Bone Marrow

Fat-storing tissue consisting largely of adipocytes, located inside the medullary cavity and certain spongy bone cavities of adult bones.

Intramembranous Ossification

The direct bone formation mechanism whereby embryonic mesenchymal sheets differentiate into bone tissue, producing cranial bones, facial bones, the mandible, and the medial clavicle.

Endochondral Ossification

The bone formation mechanism wherein a pre-existing hyaline cartilage model is progressively replaced by bone, forming most bones of the human skeleton.

Primary Ossification Center

The initial site of bone tissue replacement of cartilage that develops in the central diaphysis of a forming long bone during endochondral ossification.

Secondary Ossification Center

The site of bone development appearing within the epiphyses of long bones around the time of birth.

Bone Remodeling

A continuous, lifelong physiological cycle balancing osteoclastic bone resorption and osteoblastic bone deposition to maintain, adapt, and repair bone architecture.

Calcitriol

The active hormone synthesized from vitamin $D_3$ (cholecalciferol) that is essential for normal gastrointestinal absorption of calcium and phosphate ions.

Osteopenia

Age-related inadequate ossification and reduction of bone mass, characterized by an approximate loss of $8\%$ of bone mass per decade in women and $3\%$ in men.

Osteoporosis

A severe reduction in bone mass and density that compromises skeletal function, affecting $29\%$ of women and $18\%$ of men over the age of $45$.

Greenstick Fracture

An incomplete break in which one side of the bone shaft breaks while the opposite cortical side bends; commonly seen in children.

Comminuted Fracture

A bone fracture in which the affected bone area shatters into three or more bony fragments.

Colles Fracture

A break in the distal end of the radius typically resulting from bracing against a fall with an outstretched hand.

Pott's Fracture

A fracture affecting the distal end of the fibula, often accompanied by severe injury to the distal articulation of the tibia.

Axial Skeleton

The central longitudinal division of the skeleton consisting of $80$ bones, encompassing the skull and associated bones, the thoracic cage, and the vertebral column.

Cranial Vault (Calvaria)

The superior and lateral dome of the skull that encloses the cranial cavity, formed by the frontal bone, parietal bones, and occipital bone.

Cranial Base

The internal floor of the skull subdivided into three distinct descending steps: the anterior cranial fossa, middle cranial fossa, and posterior cranial fossa.

Suture Bones (Wormian Bones)

Small, flat, irregularly shaped island bones that occasionally form within the fibrous suture joints between flat cranial bones.

Coronal Suture

The transverse cranial joint joining the anterior frontal bone with the bilateral parietal bones.

Sagittal Suture

The midline superior joint running longitudinally between the superior margins of the two parietal bones.

Lambdoid Suture

The posterior arched suture uniting the occipital bone with both the parietal bones and temporal bones.

Squamous Suture

The bilateral suture running between the squama of each temporal bone and the inferior edge of the adjacent parietal bone.

Sella Turcica

A saddle-shaped bony prominence on the superior surface of the sphenoid body containing the hypophyseal fossa, which cradles the pituitary gland.

Cribriform Plate

A perforated horizontal plate of the ethmoid bone that forms part of the cranial floor and nasal roof, perforated by olfactory foramina for cranial nerve I.

Crista Galli

A prominent, vertical crest projecting superiorly from the ethmoid midline between the cribriform plates, serving as an anchor for the falx cerebri.

Optic Canal

An aperture within the lesser wing of the sphenoid bone providing passage for the optic nerve (CN II) and the ophthalmic artery into the orbit.

Superior Orbital Fissure

A jagged diagonal slit between the greater and lesser wings of the sphenoid transmitting cranial nerves III, IV, VI, and the ophthalmic branch of V ($V_1$).

Foramen Rotundum

A circular aperture in the greater wing of the sphenoid permitting exit of the maxillary division ($V_2$) of the trigeminal nerve.

Foramen Ovale

An oval-shaped opening through the greater wing of the sphenoid through which the mandibular division ($V_3$) of the trigeminal nerve exits.

Foramen Lacerum

A jagged, irregular canal bounded by the sphenoid, temporal, and occipital bones, containing the auditory tube and transmitting the internal carotid artery after leaving the carotid canal.

Carotid Canal

A curved passage within the petrous portion of the temporal bone transmitting the internal carotid artery to the brain.

Internal Acoustic Meatus

A channel situated on the posterior aspect of the petrous temporal bone carrying the facial nerve (CN VII), vestibulocochlear nerve (CN VIII), and the internal acoustic artery.

Foramen Magnum

The large circular opening in the base of the occipital bone that connects the cranial cavity to the vertebral canal, conveying the medulla oblongata, accessory nerve (CN XI), and vertebral arteries.

Jugular Foramen

An irregular passageway formed between the temporal and occipital bones that transmits the internal jugular vein and cranial nerves IX, X, and XI.

Paranasal Sinuses

Mucous-lined, air-filled cavities located in the frontal, sphenoid, ethmoid, and maxillary bones that reduce skull mass, humidify inhaled air, and resonate speech.

Fontanels

Fibrous, unossified mesenchymal membranes in an infant skull that enable cranial compression during birth and generally close by intramembranous ossification by $2$ years of age.

Hyoid Bone

An isolated, U-shaped bone located anteriorly in the neck between the mandible and larynx that articulates with no other bone; consists of a body, greater horns, and lesser horns.

Primary Curves

Anteriorly concave spinal curvatures (thoracic and sacral) that are present at birth to provide space for developing thoracic and abdominopelvic organs.

Secondary Curves

Anteriorly convex spinal curvatures (cervical and lumbar) that appear postnatally; the cervical curve develops when balancing the head, and the lumbar curve forms with standing and walking.

Scoliosis

An abnormal lateral displacement or bending of the vertebral column, most frequently encountered in the thoracic region.

Kyphosis

An exaggerated posterior thoracic curvature of the vertebral column that produces a rounded, hunchbacked posture.

Lordosis

An exaggerated anterior curvature of the lumbar vertebral column, often termed swayback.

Nucleus Pulposus

The soft, gelatinous inner core of an intervertebral disc that provides high elasticity and compressibility to absorb shock.

Anulus Fibrosus

The durable outer ring of fibrocartilage and collagen fibers that confines the nucleus pulposus within an intervertebral disc.

Herniated Disc

A painful pathological condition caused by the tearing of the anulus fibrosus, enabling protrusion of the nucleus pulposus onto nearby spinal nerves or the spinal cord.

Vertebral Arch

The posterior bony structure of a vertebra extending from the body, composed bilaterally of two pedicles and two laminae to enclose the vertebral foramen.

Transverse Foramen

A round aperture found exclusively in the transverse processes of cervical vertebrae ($C_1-C_7$) that accommodates the vertebral artery and vein.

Costal Facets

Articular indentations located on the bodies (demifacets) and transverse processes of thoracic vertebrae that articulate with the heads and tubercles of ribs.

Manubrium

The wide, superior triangular bone of the sternum that features the suprasternal (jugular) notch and paired clavicular notches for articulation with the clavicles.

Sternal Angle

A palpable horizontal clinical ridge formed at the joint between the manubrium and the sternal body, marking the attachment level of the second rib.

Xiphoid Process

The inferior cartilaginous tip of the sternum that fully ossifies in adulthood and can lacerate underlying organs such as the liver if subjected to incorrect CPR compressions.

True Ribs (Vertebrosternal Ribs)

Rib pairs $1$ through $7$, which connect directly and independently to the sternum through individual strips of costal cartilage.

False Ribs (Vertebrochondral Ribs)

Rib pairs $8$ through $10$, whose anterior costal cartilages merge into the costal cartilage of rib $7$ instead of articulating directly with the sternum.

Floating Ribs (Vertebral Ribs)

Rib pairs $11$ and $12$, classified as false ribs that have no anterior attachment to the sternum or costal cartilage and end in lateral abdominal musculature.

Costal Groove

A longitudinal groove running along the inferior internal border of a rib shaft that accommodates and protects the intercostal nerves and blood vessels.