8. Skeletal System Anatomy and Bone Physiology

Comprehensive vocabulary flashcards covering skeletal anatomy, bone classifications, bone histology, cartilaginous and osseous growth mechanisms, calcium homeostasis, and fracture types.

Compact bone

Also called dense or cortical bone; a relatively dense, smooth, and solid osseous tissue that makes up approximately $80\%$ of total bone mass.

Spongy bone

Also called cancellous or trabecular bone; a porous osseous tissue located internal to compact bone that makes up the remaining $20\%$ of total bone mass.

Hyaline cartilage

A flexible, semi-rigid connective tissue that covers articular ends of long bones, forms costal and nasal cartilages, composes epiphyseal growth plates, and serves as the model for fetal skeletal development.

Fibrocartilage

A durable cartilage type situated at sites subjected to heavy pressure and stretching, including intervertebral discs, the pubic symphysis, and menisci of the knee joints.

Ligaments

Dense regular connective tissue bands that anchor bone to bone.

Tendons

Dense regular connective tissue cords that anchor muscle to bone.

Hematopoiesis

Also termed hemopoiesis; the process of blood cell production that takes place within red bone marrow.

Flat bones

Bones with thin, flat, and slightly curved surfaces that protect underlying soft tissue and provide large surface area for muscle attachment, such as the cranial vault, scapulae, sternum, and ribs.

Long bones

The most common bone shape, characterized by being longer than wide with an elongated shaft (diaphysis) and two knobby ends (epiphyses), found in the upper and lower limbs (excluding wrists and ankles).

Short bones

Bones whose length is approximately equal to their width, such as carpals, tarsals, and sesamoid bones.

Sesamoid bones

Small, flat, oval-shaped bones embedded within tendons that act to increase muscle leverage, such as the patella.

Irregular bones

Bones characterized by complex, elaborate shapes that do not fit into other morphological classifications, including vertebrae, hip bones, and specific skull bones.

Diaphysis

The elongated, cylindrical shaft of a long bone that provides leverage and weight-bearing support, enclosing the central medullary cavity.

Epiphysis

The expanded, knobby end of a long bone composed of an outer thin layer of compact bone and an inner core of stress-resisting spongy bone, covered by articular cartilage at movable joints.

Metaphysis

The region of a mature long bone located between the diaphysis and epiphysis that contains the epiphyseal (growth) plate.

Medullary cavity

The hollow internal cylinder within the diaphysis of a long bone, filled with hematopoietic red marrow in children and yellow marrow in adults.

Periosteum

A double-layered sheath covering the external surface of bone, consisting of an outer fibrous layer of dense irregular connective tissue and an inner cellular layer of osteogenic cells.

Perforating fibers

Collagen bundles that securely attach and anchor the periosteum to the underlying bone matrix.

Endosteum

An incomplete cellular layer containing osteoprogenitor cells, osteoblasts, and osteoclasts that lines the internal surface of bone and the medullary cavity.

Diploë

The internal layer of spongy bone found sandwiched between the inner and outer compact bone layers of flat skull bones.

Red bone marrow

Soft hematopoietic connective tissue found in adult spongy bone and flat bone diploë, as well as the medullary cavities of children, responsible for blood cell production.

Yellow bone marrow

A fatty connective tissue consisting primarily of adipocytes that replaces red marrow in the medullary cavities of adult long bones, retaining the ability to revert to red marrow during severe anemia.

Osteoprogenitor cells

Mitotic stem cells located in the periosteum and endosteum that divide to give rise to bone-forming osteoblasts.

Osteoblasts

Bone-building cells that produce and secrete osteoid (bone matrix); once entrapped in calcified matrix, they differentiate into osteocytes.

Osteocytes

Mature bone cells situated within lacunae that maintain the existing bone matrix and detect mechanical strain.

Osteoclasts

Large, multinucleated phagocytic cells derived from hematopoietic progenitors that break down and resorb bone matrix.

Resorption lacuna

A shallow depression or pit on the bone surface where an osteoclast adheres and enzymatically breaks down bone matrix.

Osteoid

The organic fraction of bone matrix (roughly $\frac{1}{3}$ of matrix) composed of collagen fibers and semi-solid ground substance, giving bone tensile strength to resist stretching and twisting.

Hydroxyapatite

Inorganic mineral crystals formed from calcium phosphate ($\text{Ca}_3(\text{PO}_4)_2$) and other mineral salts that deposit along collagen fibers to provide hardness, rigidity, and compressional strength.

Osteon

The structural, weight-bearing functional unit of compact bone running parallel to the long axis; displays a characteristic concentric bullseye appearance in cross section.

Central canal

The cylindrical longitudinal opening at the core of an osteon through which small blood vessels and nerves travel to supply bone cells.

Concentric lamellae

Concentric rings of bone matrix encircling a central canal, featuring alternating directions of collagen fibers in adjacent rings to resist rotational torsion.

Canaliculi

Tiny microscopic channels traversing bone matrix that connect lacunae, allowing osteocyte cytoplasmic extensions to pass nutrients, waste, and signals via gap junctions.

Perforating canals

Also called Volkmann canals; transverse canals oriented at right angles to the long axis of bone that deliver blood vessels and nerves to central canals and the medullary cavity.

Circumferential lamellae

Rings of bone matrix extending around the entire external boundary beneath the periosteum or internal boundary beneath the endosteum to protect against torsion.

Interstitial lamellae

Incomplete matrix rings positioned between fully formed osteons that lack central canals, representing remnants of partially resorbed older osteons.

Trabeculae

Narrow rods and plates of spongy bone organized along lines of stress into an open lattice without osteons.

Perichondrium

A dense irregular connective tissue layer enveloping hyaline cartilage that helps maintain its shape and houses stem cells for appositional growth.

Chondroblasts

Mesenchyme-derived cells that actively secrete extracellular cartilage matrix during cartilage growth.

Chondrocytes

Mature cartilage cells that occupy lacunae within the matrix and maintain the surrounding cartilage tissue.

Interstitial growth (cartilage)

Growth in cartilage length occurring from within internal tissue regions as chondrocytes undergo mitosis and deposit new matrix, separating from each other.

Appositional growth (cartilage)

Growth in cartilage width or diameter occurring along the outer periphery as stem cells in the perichondrium differentiate into chondroblasts and synthesize new matrix.

Zone of proliferating cartilage

The layer in the epiphyseal plate where chondrocytes undergo rapid mitotic division, organizing into longitudinal columns.

Zone of hypertrophic cartilage

The epiphyseal plate layer where chondrocytes cease mitotic division, enlarge substantially (hypertrophy), and begin resorbing surrounding matrix.

Zone of calcified cartilage

A narrow epiphyseal plate layer where minerals precipitate into the cartilage matrix, causing nutrient deprivation and chondrocyte death.

Zone of ossification

The epiphyseal plate layer nearest to the diaphysis where invading capillaries and osteogenic cells lay down bone matrix upon calcified cartilage remnants.

Intramembranous ossification

Bone development occurring directly within a fibrous mesenchymal connective tissue membrane, forming flat bones of the skull, zygomatic, and clavicle.

Woven bone

Immature, disorganized bone tissue produced rapidly during initial intramembranous ossification and fracture repair, which is subsequently remodeled into lamellar bone.

Endochondral ossification

The process of bone development wherein a pre-existing hyaline cartilage model is progressively replaced by osseous tissue, forming most skeletal bones.

Primary ossification center

The initial site of endochondral bone formation established in the central diaphysis of the hyaline cartilage model during fetal development.

Secondary ossification center

Sites of bone formation that develop in the epiphyses of long bones around the time of birth.

Wolff's law

The physiological principle stating that bone grows and remodels in direct response to the physical stresses and mechanical loads applied to it.

Somatomedins

Liver-produced peptide hormones stimulated by Growth Hormone (GH) that directly promote chondrocyte division and cartilage growth.

Calcitriol

The hormonally active form of vitamin D, converted by sequential enzymatic reactions in the liver and kidneys, that enhances intestinal absorption of calcium ($\text{Ca}^{2+}$) and phosphate ($\text{PO}_4^{3-}$).

Parathyroid hormone (PTH)

A peptide hormone secreted by the parathyroid glands in response to decreased blood $\text{Ca}^{2+}$ levels; acts to increase osteoclast activity, decrease renal calcium excretion, and promote calcitriol synthesis.

Calcitonin

A hormone produced by thyroid C cells in response to elevated plasma $\text{Ca}^{2+}$; counteracts PTH by inhibiting osteoclasts and stimulating renal calcium excretion.

Colles fracture

A fracture of the distal end of the radius producing a characteristic "dinner fork" dorsal deformity of the wrist.

Comminuted fracture

A bone fracture characterized by splintering into multiple small fragments between the primary fractured sections.

Greenstick fracture

A partial fracture where one side of the bone breaks while the opposing cortical side bends; most common in pediatric patients.

Pott fracture

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

Stress fracture

A fine, hairline crack in a bone resulting from repetitive, forceful impact (such as running), frequently requiring a bone scan to detect.

Fibrocartilaginous (soft) callus

A temporary splint of collagen fibers and fibrocartilage formed at a fracture site by fibroblasts and chondroblasts to bridge broken bone ends.

Hard (bony) callus

A mass of trabecular bone formed during fracture repair as osteoblasts replace the soft fibrocartilaginous callus with woven bone.