Skeletal System

Skeletal System

Consists of bones, cartilage, tendons, and ligaments; the adult skeleton contains 206 bones.

Major functions of the skeletal system

Supports the body, protects organs, allows movement, stores minerals and lipids, and produces blood cells.

Major bones listed in the module

Skull, clavicle, sternum, ribs, vertebral column, humerus, radius, ulna, pelvis, femur, tibia, and fibula.

Extracellular matrix

Material surrounding cells whose composition helps determine the characteristics of connective tissues.

Components of bone extracellular matrix

Collagen, ground substance, organic molecules, water, and minerals.

Collagen

A tough, ropelike protein that provides flexible strength.

Proteoglycans

Large molecules that attract water and help give tissues their properties.

Tendon and ligament extracellular matrix

Contains large amounts of collagen fibers, making these structures tough like ropes or cables.

Cartilage extracellular matrix

Contains collagen and proteoglycans.

Why is cartilage tough?

Collagen makes cartilage tough.

Why is cartilage resilient?

Water-filled proteoglycans allow cartilage to spring back after being bent or slightly compressed.

Function of cartilage as a shock absorber

Its extracellular matrix makes it smooth and resilient, allowing it to absorb shock.

Bone extracellular matrix

Contains collagen and minerals including calcium and phosphate.

Function of collagen in bone

Provides flexible strength.

Function of minerals in bone

Provide compression or weight-bearing strength.

Hydroxyapatite

Inorganic material consisting primarily of calcium phosphate crystals.

Appositional growth

Addition of new bone to the surface of older bone or cartilage.

How do bones increase in size?

By appositional growth.

Osteoblast

Bone-building cell involved in bone formation, repair, and remodeling.

Ossification

Formation of new bone by osteoblasts.

Osteocyte

A mature bone cell that maintains the bone matrix.

Osteoclast

Cell that removes existing bone during bone repair and remodeling.

Bone resorption

Removal or breakdown of existing bone by osteoclasts.

Woven bone

Bone with collagen fibers oriented in many directions.

What happens to woven bone?

It is remodeled to form lamellar bone.

Lamellar bone

Bone arranged in thin layers called lamellae with collagen fibers oriented parallel to one another.

Four shape classifications of bones

Long, short, flat, and irregular.

Long bone

A bone that is longer than it is wide; found in the upper and lower limbs.

Short bone

A bone approximately as wide as it is long.

Examples of short bones

Bones of the wrists and ankles.

Flat bone

A relatively thin and flattened bone.

Examples of flat bones

Skull bones and sternum.

Irregular bone

A bone with a shape that does not fit into the long, short, or flat categories.

Examples of irregular bones

Vertebrae and facial bones.

Diaphysis

Shaft of a long bone.

Epiphysis

End of a long bone containing spongy bone tissue.

Articular cartilage

Covers the epiphysis and reduces friction.

Epiphyseal plate

Site of bone growth between the diaphysis and epiphysis.

Medullary cavity

Center of the diaphysis containing red or yellow marrow.

Periosteum

Membrane surrounding the outer surface of a bone.

Endosteum

Membrane lining the medullary cavity.

Spongy bone

Bone containing many spaces with lamellae forming trabeculae.

Trabeculae

Interconnected beams of bone forming a lattice-like structure.

What fills the spaces of spongy bone?

Bone marrow and blood vessels.

Function of trabeculae

They are oriented along lines of stress and provide structural strength.

Compact bone

Dense bone containing relatively few spaces.

Lamellae

Thin layers of bone tissue.

Circumferential lamellae

Lamellae that form the outer surface of compact bones.

Concentric lamellae

Lamellae surrounding the central canal and forming osteons.

Interstitial lamellae

Remnants of lamellae left after bone remodeling.

Osteon

Structural unit formed by concentric lamellae surrounding a central canal.

Perforating canals

Carry blood vessels toward the central canals.

Canaliculi

Connect central canals to osteocytes according to the module.

Intramembranous ossification

Bone development from membranes.

Bones formed from membranes in the module

Some skull bones, part of the mandible, and the diaphyses of the clavicles.

Center of ossification

Area within a membrane where osteoblasts begin producing bone.

Fontanels

Areas of membrane that have not ossified at birth.

Cartilage bone development

Process in which cartilage matrix calcifies, chondrocytes die, and osteoblasts form bone on the calcified cartilage matrix.

Primary ossification center

Forms in the diaphysis during fetal development.

Secondary ossification center

Forms in the epiphysis.

Structures that do not ossify in developing long bones

Articular cartilage and the epiphyseal plate.

Bone remodeling

Converts woven bone to lamellar bone and allows bone to change shape, adjust to stress, repair itself, and regulate calcium levels.

How does bone adjust to stress?

By adding new bone and realigning bone through remodeling.

First event in fracture repair

Formation of a hematoma.

Hematoma

A collection formed at the beginning of fracture repair.

Internal callus

Structure consisting of fibers and cartilage that replaces the hematoma.

External callus

Bone-cartilage collar that stabilizes the ends of a broken bone.

What happens to the internal and external calluses?

They ossify and become woven bone.

Final stage described in fracture repair

Woven bone is replaced by compact bone.

Fracture repair sequence

Hematoma → internal and external callus → woven bone → compact bone.

Calcium homeostasis

Regulation of calcium levels in the blood.

PTH effect on blood calcium

Increases blood Ca²⁺ by increasing bone breakdown, intestinal calcium absorption, and calcium reabsorption from urine.

Calcitonin effect on blood calcium

Decreases blood Ca²⁺ by decreasing bone breakdown.

Effects of aging on bone

Bone matrix is lost and becomes more brittle.

Spongy bone loss

Results from thinning and loss of trabeculae.

Compact bone loss

Mainly occurs from the inner surface of bones and involves formation of fewer osteons.

Consequences of bone loss

Increased fracture risk, deformity, loss of height, pain, stiffness, and loss of teeth.

Osteoporosis

Bone disease in which bone mineral density and bone mass decrease or bone structure and strength change.