Ch 7 Bone Structure Pt2 & LAB Vertebra n Ribs
Bone tissue: overview and development
Bone is a living, dynamic, organic tissue that changes continually even after growth in height ends.
Bones respond to daily mechanical stresses (loads from activities like walking/running) by remodeling – forming new bone where needed.
The surface and internal architecture of bone are shaped during development by muscles and tendons exerting pull, producing bumps and features (e.g., greater trochanter) on bones.
Bones are vascular and populated with various cell types; cartilage is avascular and relies on diffusion for nourishment.
Bone cells: the four main players
Osteoprogenitor cells (bone stem cells)
Blank-slate cells that reside around bone and in periosteum and Endosteum; receive signals to become osteoblasts.
derived from mesenchyme
Osteoblasts (bone-forming cells)
Immature osteoprogenitor cells that secrete organic matrix (osteoid) which later mineralizes (calcifies).
Initially produce osteoid, a semi-solid organic matrix rich in collagen.
As matrix calcifies, osteoblasts become trapped within matrix into osteocytes.
Osteocytes (mature bone cells)
reside in lacunae within the calcified matrix; extend processes through canaliculi to communicate with other osteocytes.
Maintain the bone matrix and respond to mechanical stresses and strains; communicate chemically (not electrically) to coordinate remodeling.
Osteoclasts (bone-resorbing cells)
Large, multinucleated cells with many cytoplasmic extensions; resorb bone by phagocytosis to release minerals.
Located: Found within or next to a resorption lacuna (Howship’s lacuna) within a depression of bone
Help remove bone where remodeling is needed or to reshape bone in response to load and injury.
How the cells relate during remodeling:
Mechanical stimulus or fracture triggers osteoprogenitor cells to become osteoblasts, which form new matrix and eventually osteocytes.
Osteoclasts remove bone where resorption is required, releasing calcium and phosphate for bloodstream use.
Bone matrix: organic and inorganic components
Organic component (mostly collagen and osteoid)
Osteoid is secreted by osteoblasts and forms a collagen protein fibers matrix that later calcifies.
Semisolid ground substance consists of glycoproteins and proteoglycans that help provide the bone with structure and support.
Proteoglycans: protein core + complex carbohydrates (e.g., chondroitin sulfate)
Glycoproteins: protein with attached carbohydrate chains
Collagen fibers provide toughness and a small amount of elasticity to prevent brittle fractures.
Inorganic component (mineral content)
Bone matrix composed of Hydroxyapatite crystals, which are formed from calcium phosphate and calcium hydroxide.
these crystals, along with other salts + ions, deposit around collage fiber, give bone its rigidity and compressional strength.
o Other substances incorporated into crystals
§ calcium carbonate, sodium, magnesium, sulfate, fluoride
The bone matrix is a composite: a resilient organic scaffold reinforced by hard mineral crystals.
The role of vitamins and nutrients in bone formation:
Vitamin D: enhances intestinal absorption of calcium and phosphate; activated through skin exposure to sunlight, then processed in liver and kidneys to support mineralization.
Vitamin C (ascorbic acid): essential for collagen synthesis; deficiency leads to weak bone matrix and scurvy.
Vitamin A and other nutrients influence bone growth and remodeling.
Key terms in matrix biology:
Osteoid: the organic, protein-rich precursor matrix secreted by osteoblasts before mineralization.
Calcification: the process by which mineral crystals deposit in the organic matrix, hardening it.
Lacunae: the little spaces that house osteocytes.
Canaliculi: tiny channels that connect osteocytes to each other for chemical signaling.
Endosteum and Periosteum: inner and outer connective tissue layers surrounding bone that house progenitor cells and supply vasculature.
Blood supply, marrow, and bone remodeling dynamics
Blood supply is rich in bone; nutrients reach bone via nutrient foramina (tiny openings in the shaft).
Medullary cavity (marrow cavity) contains marrow:
In children: red marrow (blood-forming elements) is present throughout many bones.
In adults: red marrow is mostly replaced by yellow marrow (fatty tissue) in long bones; red marrow persists in certain areas (e.g., skull, ribs, sternum, pelvis, vertebrae).
Bone marrow transplants typically draw from red marrow-rich sites (e.g., iliac crest, sometimes vertebral sites) because those sites contain hematopoietic stem cells.
Nutrient foramen and vascular supply are critical for delivering nutrients and removing waste from bone tissue.
The bone marrow environment supports hematopoiesis in children and can revert to red marrow under severe anemia in adults, though this is not common.
Compact bone vs. spongy (trabecular) bone
Compact bone (cortical bone)
Dense, with osteons (Haversian systems) as the basic functional unit.
Each osteon contains a central (Haversian) canal, concentric lamellae, osteocytes in lacunae, and interconnecting canaliculi.
Central canal houses blood vessels and nerves.
The outer shell of bones consists primarily of compact bone.
Spongy bone (cancellous/trabecular bone)
Porous, composed of trabeculae (rod- or plate-like elements) arranged to withstand multidirectional forces.
Lattice-like structure with spaces filled by bone marrow and blood vessels.
Endosteum lines all surfaces within the medullary cavity and trabeculae.
Growth and remodeling across these tissues:
Remodeling occurs throughout life: osteoblasts form bone, osteoclasts resorb bone, and osteocytes coordinate responses to loading via canaliculi.
Stress and strain (e.g., exercise) stimulate localized bone formation to strengthen areas under higher load (periosteal response).
Growth, remodeling, and the response to mechanical stress
Mechanical loading stimulates bone formation and thickness increases via appositional growth (growth in thickness) and osteogenesis around the periosteum.
Osteoprogenitor cells respond to signals that indicate the need for more bone in areas under stress; they differentiate into osteoblasts and secrete osteoid.
Osteocytes sense mechanical stress and communicate with osteoblasts/osteoclasts through chemical signaling to remodel bone architecture.
Osteoclasts remove bone at sites where resorption is needed, releasing minerals into the bloodstream as part of homeostasis.
The concept of homeostasis in bone: maintaining calcium and phosphate levels to support nerve conduction and muscle function; when blood calcium is low, osteoclasts resorb bone to release calcium.
Calcium and phosphate homeostasis example:
Low blood Ca^{2+} triggers osteoclast-mediated bone resorption, releasing Ca^{2+} and phosphate to increase blood calcium levels; this is part of systemic homeostasis.
Calcium relevance in physiology:
Calcium is essential for nerve conduction, muscle contraction, and bone mineralization.
The body maintains tight control of serum Ca^{2+} levels via bone remodeling and renal and intestinal handling.
Endochondral ossification vs intramembranous ossification
Intramembranous ossification (membranous bone formation)
Occurs in flat bones of the skull (and some facial bones): skull bones form directly from a primitive membrane.
Process: mesenchymal stem cells → osteoprogenitor cells → osteoblasts → secretion of osteoid → matrix calcification → osteocytes trapped in lacunae.
The skull begins as a membranous membrane with ossification centers that expand; partial fusion of centers forms the cranial bones.
Fontanels (soft spots): gaps between growing skull bones in infants that allow for molding during birth and growth; anterior fontanelle, posterior fontanelle, sphenoid fontanelle, and mastoid fontanelle exist in a newborn and gradually close as bones fuse.
Appositional growth adds bone to the surface; endochondral elements may also contribute as ossification centers expand.
Endochondral ossification (bone formation from cartilage)
Most bones form from a hyaline cartilage model that is gradually replaced by bone.
Growth in length occurs at epiphyseal plates; growth in thickness occurs through periosteal apposition.
Growth in cartilage: two mechanisms
Interstitial growth: growth within the cartilage matrix via chondroblasts dividing within lacunae (cartilage lengthening).
Appositional growth: growth from the surface via perichondrium giving rise to chondrocytes that lay down new matrix on the outside (bone thickening).
Fontanels and sutures illustrate ongoing intramembranous ossification in the skull; skull bones remain connected by flexible sutures and fontanels during early development.
Fetal development and fetal skull features
In utero (around 2–4 months): skeleton begins as a cartilage model; bones undergo intramembranous ossification to form flat bones (e.g., skull).
Fontanels (soft spots) are gaps between membranous skull bones that allow for brain growth and birth canal passage; uncommon in later life as sutures fuse.
Occipital condyles and other skull landmarks are used to orient the skull with the vertebral column.
The perichondrium around cartilage is a source of cells that can give rise to chondrocytes and later to osteoblasts in endochondral ossification.
Vertebral column: structure and key features
The vertebral column consists of 24 vertebrae in regions plus sacrum and coccyx:
Cervical: C1–C7
Thoracic: T1–T12
Lumbar: L1–L5
Sacrum (fusion of 5 sacral vertebrae in adulthood)
Coccyx (tailbone; fusion of 3–5 coccygeal vertebrae in adulthood)
Major functions:
Supports head weight and maintains upright posture.
Protects the spinal cord within the vertebral canal.
Typical vertebral features (examples explained here for lumbar vertebrae):
Body: large, weight-bearing portion of vertebra
Vertebral foramen: opening that houses the spinal cord
Spinous process: posterior projection felt along the back; in many specimens it is palpable
Transverse process: lateral projections; in cervical vertebrae there is a transverse foramen through which the vertebral artery passes
Superior and inferior articulating facets (facets): surfaces that form joints with adjacent vertebrae; allow articulation and stability between vertebrae
Special cases:
Atlas (C1): supports the skull; no body; has large vertebral foramen, superior articular facets to cradle the occipital condyles; contains a transverse foramen; supports nodding movement (yes).
Axis (C2): has the dens (odontoid process) that acts as a pivot for C1; articulates with the atlas via the dens; facilitates side-to-side head movement (no).
Cervical vertebrae (C1–C7) characteristics:
Small bodies; split (bifid) spinous processes; transverse foramina in each cervical vertebra; atlas and axis are specialized.
Thoracic vertebrae (T1–T12) characteristics:
Long spinous processes; costal facets on transverse processes indicating rib attachment; each thoracic vertebra articulates with a rib via costal facets.
Lumbar vertebrae (L1–L5) characteristics:
Large, thick bodies; short, blunt spinous processes; designed to bear most of the body's weight.
Spinal curvatures and development:
Primary curvatures: thoracic and sacral (present during fetal development in utero).
Secondary curvatures: cervical (develops with head control) and lumbar (develops with upright walking); these provide the spine’s S-shaped spring-like flexibility.
Curvature function:
The S-curve distributes mechanical loads and reduces impact forces when walking or running.
The rib cage and sternum
Ribs:
True ribs (ribs 1–7) attach directly to the sternum by their own costal cartilages.
False ribs (ribs 8–10) attach to the sternum indirectly via shared costal cartilage connections.
Floating ribs (ribs 11–12) do not attach anteriorly to the sternum; they end in the posterior abdominal wall.
Rib anatomy:
Each rib has a head articulating with the vertebral body and a neck that leads to a tubercle that articulates with the transverse process of the corresponding thoracic vertebra.
The sternal (anterior) end articulates with costal cartilage, which connects to the sternum.
Sternum (breastbone) features:
Manubrium: upper portion with the jugular notch at the top; articulates with the clavicles.
Body: the central long portion; connects with the ribs via costal cartilage.
Xiphoid process: small inferior tip; clinically important for CPR landmarking.
Clinically relevant points:
Jugular notch: palpable notch at the superior border of the manubrium.
Xiphoid process: small pointed cartilage that ossifies with age; important CPR landmark (two fingerbreadths above it).
Costosternal joints and costal cartilage connections allow for some flexibility of rib cage during respiration.
Fontanels and infant skull growth
Fontanels are soft spots between growing skull bones in infants.
They are sites of intramembranous ossification where bone forms and expands to fuse bone plates over time.
Fontanels allow for skull molding during birth and accommodate rapid brain growth in infancy.
Practical mechanisms: examples and clinical relevance
Mechanical loading example:
A person weighing 70 kg running creates very high impact forces when the foot strikes the ground; a single strike can be about depending on body weight and contact area.
The bone responds by increasing localized bone formation to strengthen regions under stress (a demonstration of Wolff’s law in action).
Vitamin D and calcium:
Vitamin D enhances gut calcium absorption; without sufficient Vitamin D, calcium availability for bone mineralization is reduced.
Vitamin C and collagen:
Vitamin C is required for collagen synthesis; a deficiency weakens the bone matrix and can lead to bone conditions like scurvy.
Scurvy and historical note:
Historical sailors who lacked Vitamin C suffered from bleeding gums, poor wound healing, and weakened bones; lime juice provided a practical source of Vitamin C and helped sailors recover.
Bone marrow and transplantation:
In adults, red marrow is largely replaced by yellow marrow in long bones; red marrow remains in specific bones (e.g., certain skull bones, sternum, ribs, pelvis, vertebrae) and can be targeted for marrow transplants.
For anatomy exams and practice:
Be able to identify: atlas (C1) and axis (C2) and their distinctive features (atlas lacks a vertebral body; axis has the dens), the presence of transverse foramina in cervical vertebrae, bifid spinous processes in some cervical vertebrae, costal facets in thoracic vertebrae, and the robust bodies of lumbar vertebrae.
X-ray interpretation tips:
Compact bone appears denser on X-ray; spongy bone appears with a trabecular pattern; the central canal of an osteon is visible in cross-section of compact bone.
Quick reference terms and key definitions
Osteoprogenitor cells: bone stem cells awaiting signals to differentiate into osteoblasts.
Osteoblasts: cells that secrete osteoid and initiate mineralization.
Osteocytes: mature bone cells maintaining bone matrix and communicating signals via canaliculi.
Osteoclasts: bone-resorbing cells that digest bone material.
Osteoid: organic matrix secreted by osteoblasts before mineralization.
Canaliculi: small channels connecting osteocytes.
Endosteum: membrane lining inner surfaces of bone, including medullary cavity.
Periosteum: fibrous outer membrane covering bone; site of osteoprogenitor cells and vascular supply.
Diploë (diploë): the spongy bone layer found between the inner and outer layers of flat bones like the skull.
Fontanels: soft spots on an infant’s skull due to unresolved sutures; allow molding during birth.
True ribs: ribs 1–7; False ribs: 8–10; Floating ribs: 11–12.
Atlas (C1) and Axis (C2): the first two cervical vertebrae with distinctive features; atlas supports the skull; axis contains the dens for pivot motion.
Vertebral foramen: opening through which the spinal cord passes.
Transverse foramen: unique to cervical vertebrae, transmitting the vertebral artery.
Primary curvatures: thoracic and sacral (present in fetal development).
Secondary curvatures: cervical and lumbar (develop after birth with postural changes).
Nutrient foramen: artery and nerve entry point into the bone shaft, critical for blood supply.
Cartilage vs bone growth:
Interstitial growth: growth of cartilage from within.
Appositional growth: growth on the surface, increasing bone diameter or cartilage thickness.
Notes: The content above mirrors the lecture's emphasis on bone biology, development, and anatomy, including cell types, matrix composition, marrow, ossification processes, and detailed vertebral and rib anatomy. Where numerical examples were given in the transcript (e.g., the force from running and the notion of vitamins), those have been included with LaTeX formatting for clarity.