Anatomy Module 2: The Skeletal System

functions of the skeletal system

1. Structural support — without bone, the body would be an unsupported mass of soft tissue.

2. Protection of internal organs

   • Skull → protects the brain

   • Vertebral column → protects the spinal cord

   • Rib cage → protects the heart and lungs

3. Mineral storage — reservoir for calcium, phosphorus, and other minerals

   • Thyroid & parathyroid glands (endocrine system) regulate reabsorption of minerals from bone into the bloodstream when levels are low

4. Blood cell formation (hematopoiesis)

   • Occurs in bone marrow (inside long bones)

   • In early childhood, the spleen plays a larger role in red blood cell formation before the skeletal system fully matures (skeletal maturity generally reached by late teens, ~17–19 years old)

   • After maturity, the spleen's hematopoietic role diminishes (it still filters blood, but no longer significantly produces RBCs)

5. Motion / Mobility & Balance

   • Achieved through joints (the "articular system," a subdivision of the skeletal system)

   • Joints absorb ground reaction forces and enable balance and movement

epiphysis

Small junction region between the diaphysis and metaphysis; site of the growth plate; most visible in children (still cartilaginous) — less distinct in adults

metaphysis

The flared ends of a long bone

diaphysis

The long central shaft — the largest part of the bone

articular cartilage

Covers the epiphyseal ends of bones at joints to reduce friction

anatomy of a long bone

ex. humerus

cortical bone

dense outer layer; gives structural rigidity

spongy (cancellous) bone

porous inner bone; absorbs shock and mechanical stress, preventing fractures from concentrated forces

medullar cavity

hollow cavity inside spongy bone containing bone marrow, the site of red blood cell formation

periosteum

• covers the outside of bone

  • Latin: peri = "on top of"

  • Function: provides attachment sites for tendons/ligaments (soft tissue "blends in" with the periosteum at attachment points)

endosteum

• lines the inside of the medullary cavity

  • Latin: endo = "inside"

  • Function: provides a smooth lining so blood cells can move through the marrow cavity without friction

composition of bone tissue

• ~50% minerals (calcium, phosphorus — provide strength)

• ~25% protein

• ~25% water (mostly interstitial fluid, not "drinking water")

Haversian canal

Central vertical canal running through bone; carries blood vessels/nutrients

Volkmann’s canal

Horizontal canals connecting Haversian canals

lamellae

Concentric rings of bone tissue (osteocytes) surrounding the Haversian canal

lacunae

Small spaces between lamellae containing interstitial fluid (a mix of lymph, proteins, blood vessels, WBCs, RBCs)

osteocytes

Mature bone cells arranged in the lamellar rings

osteogenic cells

"Blueprint"/stem-like precursor cells present from birth; differentiate into osteoblasts (and are associated with chondrocytes in early cartilage)

chondrocytes

Cartilage cells — present throughout the initially cartilaginous skeleton

osteoblasts

Bone-forming cells; combine with calcium (delivered via blood vessels) to form calcified matrix → become osteocytes

osteoclasts

Bone-destroying/resorbing cells; break down bone to release calcium/minerals into the bloodstream

osteocytes

Mature bone cells that make up the lamellar rings of formed bone

osteoporosis

caused by increased osteoclastic activity relative to bone formation → brittle bones due to low calcium levels. Treatment includes calcium supplementation.

osteophytes

(bone spurs) — form when arthritis causes chronic bone erosion; osteoblasts respond by forming new bone haphazardly (not in an organized fashion), creating bony outgrowths.

bone remodeling

(post-fracture) — osteoblasts initially form bone in a disorganized/bulging fashion; osteoclasts then reshape it back toward the original contour. This remodeling phase takes roughly 1 year, even though initial fracture union takes only ~3–6 weeks.

bone width growth

as osteoblasts add bone on the outside, osteoclasts resorb bone from the inside, allowing the medullary cavity to widen proportionally as the bone thickens.

embryological origin of bone

1. Fertilization → single cell → rapid division

2. Gastrulation: cells organize into 3 germ (dermal) layers:

   • Ectoderm (outer layer) → skin/epidermis, nervous system

   • Mesoderm (middle layer) → bone, muscle, cartilage, ligaments

   • Endoderm (inner layer) → GI tract, respiratory tract (internal "tube" structures)

3. Bone-forming cells originate from mesenchymal cells of the mesoderm, which differentiate into osteogenic cells and chondroblasts.

intramembranous ossification

• Used for: flat bones (e.g., skull bones)

• Process: osteogenic cells become trapped between two membranous layers; compression + blood supply → calcium delivery → osteoblasts form a single concentric ring of bone → compact, calcified flat bone

• Weaker than endochondral bone — appropriate because flat bones (like the skull) are built for protection, not for withstanding heavy mechanical load

• Begins in the embryo around 6–8 weeks gestation

• In infants, skull bones are not yet fully fused (sutures remain cartilaginous) — full calcification of the skull occurs within about the first 2 years of life

endochondral ossification

• Used for: most bones of the body, especially long bones

• Process: cartilage is replaced by bone (not converted into bone)

• Two ossification centers per long bone:

  1. Primary ossification center — located in the diaphysis; ossification proceeds outward, this is the main growth driver in length

  2. Secondary ossification center — located in the epiphysis

• Mechanism:

  1. Blood vessels proliferate near the cartilaginous template

  2. Osteoblasts form rings around blood vessels (providing entry points into the cartilage)

  3. Blood vessels deliver calcium → osteoblasts combine with calcium → form calcified matrix (concentric rings, similar to Haversian system)

  4. Cartilage is progressively pushed toward the epiphysis/metaphysis as new bone forms at the diaphysis

• Growth in length: occurs at the diaphyseal side, pushing the cartilage further toward the epiphysis over time (correction noted: bone length grows on the diaphyseal side, not "toward" the diaphysis)

• Growth in width: occurs via the same peripheral osteoblast/calcification process, expanding outward

primary ossification center

 located in the diaphysis; ossification proceeds outward, this is the main growth driver in length

endochondral ossification

secondary ossification center

located in the epiphysis

endochondral ossification

endochondral ossification mechanism

1. Blood vessels proliferate near the cartilaginous template

2. Osteoblasts form rings around blood vessels (providing entry points into the cartilage)

3. Blood vessels deliver calcium → osteoblasts combine with calcium → form calcified matrix (concentric rings, similar to Haversian system)

4. Cartilage is progressively pushed toward the epiphysis/metaphysis as new bone forms at the diaphysis

The Epiphyseal (Growth) Plate — 4 Zones (conceptual, not required to memorize by name)

1. Zone of proliferation — osteoblasts proliferating from the diaphyseal side

2. Two transitional zones — osteoblasts receiving blood supply and calcium, becoming calcified matrix

3. Cartilaginous zone (near metaphysis) — remains cartilaginous; the "incentive" driving continued bone growth

• Once growth is complete, all 4 zones fuse into a single epiphyseal line.

• Articular cartilage remains cartilaginous for life (never fully ossifies) — reduces joint friction.

• "Double-jointedness" = a colloquial term referring to persistent cartilage at the epiphyseal line, allowing extra mobility/flexibility at that joint (not a fully fused, rigid union).

fracture healing

(Same General Process as Ossification)

1. hematoma formation

2. cartilaginous callus formation

3. bony union

4. remodeling

hematoma formation

~6-8 hours post-fracture

Ruptured blood vessels bleed → clot forms around fracture site for protection; brings in phagocytes (WBCs that clear debris) and osteoblasts

cartilaginous callus formation

following hematoma

Osteogenic cells → osteoblasts form a cartilaginous matrix around the fracture site

bony union

~6 weeks

Blood vessels deliver calcium; osteoblasts form calcified matrix; bone is united across the fracture — but the healed area may look bulkier/thicker than the original bone (osteoblasts lack "organizational" precision)

remodeling

~1 year

Osteoclasts reshape and smooth the newly formed bone back toward its original structure

growth plate fractures

pediatric concern

• In children/pre-pubertal individuals, fracture of the epiphysis/growth plate is a serious concern because it can disrupt future bone growth.

• Potential consequence: limb length discrepancy (e.g., femur growth plate fracture → leg length difference)

• Once the growth plate is fully ossified (post-puberty), fractures there heal like any other fracture, with no further growth-related concerns

bony landmarks

Soft tissue (muscles, tendons, blood vessels, nerves) exerts force on bone even while it is still cartilaginous in early development, creating characteristic surface markings

condyle

Rounded bony prominence

Medial/lateral femoral condyles

tuberosity

Elevated bump (often muscle/tendon attachment)

Tibial tuberosity (quadriceps tendon attachment)

foramen

An opening/hole (for vessels/nerves)

Obturator foramen (pelvis)

groove

Depression/gutter (often for a tendon or vessel)

facet

Flat surface formed where two bones articulate

Patellar facets (articulate with femur); vertebral facet joints

head/neck of bone

Rounded proximal end / narrowed region below the head

Head of femur, head of humerus, neck of femur

tendons

Connect muscle to bone (muscle fibers do NOT attach directly to bone)

ligaments

Connect bone to bone; often layered on top of the articular capsule

articular capsule

Surrounds each synovial joint

cartilage

Covers articular (joint) surfaces to reduce friction

hyaline cartilage

Thin, bluish tinge; found on the surface of virtually every bone; relatively weaker/less tough

General bone surfaces (jaw, clavicle, etc.)

fibrocartilage

Strongest type; fibrous + cartilaginous — built to withstand significant joint forces

Articular cartilage (joint surfaces), menisci (knee), intervertebral discs

elastic cartilage

Highly elastic, flexible

External ear, nose (nares), epiglottis, trachea

long bone

Length > width

Most extremity bones (humerus, femur, etc.) — but NOT carpals/tarsals

flat bones

Flat, plate-like structure

Skull bones — frontal bone, parietal bones, occipital bone (cranial vault)

irregular bones

No consistent/standard shape; adapted to surrounding structures

Maxilla, nasal bone, ethmoid bone, sphenoid bone, vertebrae

short bones

Width > length; small, compact

Carpal bones (wrist), tarsal bones (foot)

Sesamoid bones

Small, pea/sesame-shaped; embedded within a tendon/soft tissue

Patella (largest example), sesamoid bones at base of the great toe and thumb

• Acts like a pulley within a tendon, improving the mechanical efficiency of the associated muscle.

• Example: The patella, embedded in the quadriceps tendon, provides mechanical advantage during knee extension.

• Clinical note: Without a patella (e.g., after removal due to fracture or tumor), the quadriceps must generate ~60% more effort to produce the same knee extension force.

synarthrosis

immobile

Skull sutures (parietal-parietal, parietal-frontal)

functional joint classification

diarthrosis

Highly mobile — most common joint type in the body

Shoulder, elbow, hip, knee, AC joint, sternoclavicular joint, costovertebral joints

functional joint classification

amphiarthrosis

Slightly mobile (mobility + stability)

Symphysis pubis (junction of the two pubic bones); allows slight alternating pelvic motion during gait

functional joint classification

fibrous joints

Bones connected by fibrous connective tissue

Generally synarthrotic (immobile)

Skull sutures

synovial joints

Joint enclosed by an articular capsule + synovial membrane, which secretes synovial fluid to reduce friction

Generally diarthrotic (highly mobile)

Hip, knee, shoulder

cartilaginous

Bones connected by cartilage

Generally amphiarthrotic

Two subtypes of cartilaginous joints:

1. Primary cartilaginous joints — temporary; cartilage present only until full ossification occurs

   • Example: Epiphyseal plate (cartilage between diaphysis and epiphysis during growth)

2. Secondary cartilaginous joints — cartilaginous for life

   • Example: Intervertebral discs (fibrocartilaginous discs between vertebral bodies); slightly mobile

primary cartilaginous joints

temporary; cartilage present only until full ossification occurs

• Example: Epiphyseal plate (cartilage between diaphysis and epiphysis during growth)

secondary cartilaginous joints

cartilaginous for life

• Example: Intervertebral discs (fibrocartilaginous discs between vertebral bodies); slightly mobile