Chapter 6: Skeletal System

Bone

A hard and rigid component of the skeletal system.

Cartilage

A flexible yet strong tissue that comes in three types: hyaline, fibrocartilage, and elastic.

Tendons

Connective tissues that attach muscles to bones.

Ligaments

Connective tissues that connect bones to other bones and allow some movement while preventing excessive movement.

Support

One of the functions of the skeletal system, providing a framework for the body.

Protection

Function of the skeletal system that safeguards vital organs.

Movement

Produced by muscles acting on bones via tendons.

Storage

Function of the skeletal system that includes storing calcium, phosphorus, and adipose tissue in marrow cavities.

Blood cell production

Process that occurs in red bone marrow.

Chondroblasts

Specialized cells that form the cartilage matrix.

Chondrocytes

Cells surrounded by cartilage matrix and located in lacunae.

Matrix

The extracellular material in cartilage that contains collagen fibers for strength and proteoglycans for resiliency.

Perichondrium

A double-layered connective tissue sheath that covers cartilage except at articulations.

Articular cartilage

Cartilage that covers bones at joints and lacks perichondrium, blood vessels, or nerves.

Appositional growth

Type of cartilage growth where new chondrocytes and matrix are laid down at the periphery.

Interstitial growth

Type of cartilage growth where chondrocytes within the tissue divide and add more matrix between the cells.

Bone matrix

Similar to reinforced concrete, consisting of collagen fibers (steel rods) and hydroxyapatite (cement).

Osteogenesis Imperfecta

A condition characterized by a deficiency of collagen, leading to fragile bones.

Osteochondral Progenitor Cells

Stem cells that develop into either chondroblasts or osteoblasts, located in the periosteum.

Osteoblasts

Cells responsible for the formation of bone through ossification.

Osteocytes

Mature bone cells that are surrounded by matrix and reside in lacunae.

Canaliculi

Canals occupied by osteocyte cell processes that connect to other osteocytes.

Osteoclasts

Multinucleated cells involved in the resorption of bone.

Resorption

The breakdown of bone into its constituent parts.

Woven bone

Bone with collagen fibers randomly oriented, laid down during fetal development and fracture repair.

Lamellar bone

Mature bone organized in sheets called lamellae, providing strength through fiber orientation.

Spongy bone

A type of bone that has a porous structure, providing lightweight support.

Trabeculae

Interconnecting rods or plates of bone.

Compact bone

Dense bone that forms the outer layer of bone structures.

Osteoblast

Bone-forming cell.

Osteoclast

Bone-resorbing cell.

Osteocyte

Mature bone cell that maintains bone tissue.

Canaliculus

Small channels in bone that connect lacunae.

Lamellae

Concentric layers of bone matrix.

Osteon

Structural unit of compact bone, consisting of a central canal and concentric lamellae.

Central canals

Canals that run parallel to the long axis of the bone.

Perforating canal

Canals that run perpendicular to the length of bone.

Circumferential lamellae

Layers of bone matrix that encircle the bone.

Interstitial lamellae

Remnants of osteons that fill spaces between osteons.

Perforating (Volkmann's) canals

Blood vessels from periosteum that penetrate bone.

Diaphysis

Shaft of a long bone.

Epiphysis

End of the bone, primarily composed of spongy bone.

Epiphyseal plate

Growth plate composed of hyaline cartilage.

Epiphyseal line

Ossified remnant of the epiphyseal plate.

Medullary cavity

Hollow center of the diaphysis of long bones that contains bone marrow.

Endosteum

Membrane that lines all internal spaces including spaces in spongy bone.

Periosteum

Outer layer of bone that is fibrous and contains bone cells.

Red marrow

Connective tissue in the spaces of spongy bone or in the medullary cavity; the site of blood cell production.

Yellow marrow

Fat stored within the medullary cavity or in the spaces of spongy bone.

Intramembranous ossification

Takes place in connective tissue membrane.

Endochondral ossification

Forms from a pre-existing cartilage model.

Centers of ossification

Locations in membrane where ossification begins.

Fontanels

Large membrane-covered spaces between developing skull bones; unossified.

Bone collar

A layer of compact bone that forms around the diaphysis during endochondral ossification.

Secondary ossification centers

Form in the epiphyses of long bones; the original cartilage model is almost completely ossified.

Cartilage formation

Begins at the end of the fourth week of development.

Ossification

The process of bone tissue formation.

Interstitial cartilage growth

A type of growth where new cartilage is formed internally.

Closure of epiphyseal plate

The epiphyseal plate is ossified, becoming the epiphyseal line, which occurs between 12 and 25 years of age.

Proliferation zone

New cartilage is produced on the epiphyseal side of the plate as the chondrocytes divide and form stacks of cells.

Hypertrophic zone

Chondrocytes mature and enlarge.

Calcified cartilage zone

Matrix is calcified and chondrocytes die.

Ossification zone

Cartilage on the diaphyseal side of the plate is replaced by bone.

Appositional growth in bone width

Occurs only on old bone and/or on cartilage surface; interstitial growth cannot occur because the matrix is solid.

Factors affecting bone growth

Size and shape of a bone are determined genetically but can be modified by nutrition and hormones.

Vitamin D

Necessary for absorption of calcium from intestines; can be eaten or manufactured in the body.

Rickets

A condition resulting from a lack of vitamin D during childhood.

Osteomalacia

A condition resulting from a lack of vitamin D during adulthood leading to softening of bones.

Vitamin C

Necessary for collagen synthesis by osteoblasts; deficiency leads to scurvy.

Growth hormone

Stimulates interstitial cartilage growth and appositional bone growth.

Thyroid hormone

Required for growth of all tissues.

Sex hormones

Cause growth at puberty and closure of the epiphyseal plates.

Bone remodeling

Converts woven bone into lamellar bone, caused by migration of osteoclasts and osteoblasts.

Mechanical stress and bone strength

Stress causes bone remodeling to increase bone mass and align trabeculae with stress.

Osteoporosis

A condition characterized by decreased bone density resulting from an imbalance between bone resorption and formation.

Hematoma formation

Localized mass of blood released from blood vessels but confined within an organ or space.

Clot formation

The process of blood coagulation that occurs after hematoma formation.

Callus

mass of tissue that forms at a fracture site and connects the broken ends of the bone

Internal Callus

forms between the ends of the bones

Macrophages

clean up debris at the fracture site

Fibroblasts

produce collagen

External Callus

collar around opposing ends that stabilizes two pieces

Callus ossification

Callus replaced by woven bone

Calcium Homeostasis

Bone is major storage site for calcium

Blood calcium levels

depends upon movement of calcium into or out of bone

Parathyroid hormone (PTH)

Released when blood calcium levels are low; stimulates osteoclasts to resorb bone

Calcitonin

Released when blood calcium levels are high; inhibits osteoclasts and allows osteoblasts to take up calcium from blood

Effects of Aging on Skeletal System

Bone matrix decreases, leading to brittleness and decreased bone mass

Bone mass in men

Denser bone mass due to testosterone and greater weight

Bone mass by ethnicity

African Americans and Hispanics have higher bone masses than Caucasians and Asians

Rate of bone loss after menopause

Increases 10-fold

Bone loss effects

Causes deformity, loss of height, pain, stiffness, stooped posture, and loss of teeth