Bone Tissue and Skeletal System Overview

Support

Provides structural support for the entire body; individual bones or groups of bones provide a framework for the attachment of soft tissues and or organs.

Protection

Delicate tissues and organs are often surrounded by skeletal elements; the ribs protect the heart and lungs, the skull encloses the brain, the vertebrae shield the spinal cord, and the pelvis cradles delicate urinary and reproductive organs.

Leverage

Many bones of the skeleton function as levers that can change the magnitude and direction of the forces generated by skeletal muscles.

Storage of minerals

The calcium salts of bone represent a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in the body fluids.

Blood cell production

Also known as hematopoiesis; red blood cells, white blood cells, and platelets are produced in the red bone marrow, which fills the internal cavities of many bones.

Axial skeleton

Consists of 80 bones including the skull, hyoid, sternum, rib cage, vertebral column, sacrum, and coccyx.

Appendicular skeleton

Includes 126 bones of the limbs and the pectoral and pelvic girdles that attach the limbs to the axial skeleton.

Flat bones

Thin, roughly parallel surfaces that form the roof of the skull, sternum, ribs, and scapulae; they provide protection from underlying soft tissues and offer an extensive surface for the attachment of skeletal muscles.

Sutural bones

Also known as Wormian bones; small, flat, irregularly shaped bones between the flat bones of the skull with individual variations in number, shape, and position.

Long bones

Relatively long and slender bones located in the arm, forearm, thigh, lower leg, palms, soles, fingers, and toes; the femur is the largest and heaviest bone in the body.

Irregular bones

Have complex shapes with short, flat, notched, or ridged surfaces; examples include spinal vertebrae, bones of the pelvis, and several skull bones.

Sesamoid bones

Generally small, flat, and shaped somewhat like a sesame seed; they develop inside of tendons and are most commonly located near joints.

Short bones

Small and boxy bones; examples include bones of the wrist (carpals) and bones of the ankles (tarsals).

Diaphysis

Long tubular shaft that forms the axis of a typical long bone; the walls of the shaft are made primarily of compact bone.

Epiphyses

Ends of the bones composed primarily of spongy bone, also called trabecular bone, covered by a thin layer of compact bone and articular cartilage.

Proximal epiphyses

End closest to the origin of attachment.

Distal epiphyses

End furthest from the origin of attachment.

Metaphysis

A narrow zone that connects the diaphysis to the epiphyses.

Epiphyseal plate

A thin layer of hyaline cartilage more commonly called the growth plate, important for growth in the length of bones.

Medullary cavity

A cavity within the shaft of a long bone where bone marrow is located.

Red bone marrow

Important for hematopoiesis, fills the medullary cavity in childhood.

Yellow bone marrow

No longer hematopoietic, stores fat as an important energy source.

Periosteum

The outermost covering of bone made primarily of dense irregular tissue and held on by Sharpey's fibers.

Endosteum

The internal membrane of bone made of connective tissue, lining canals that supply blood and nerves.

Nutrient foramen

A tunnel that penetrates the diaphysis and provides access for blood vessels into the shaft of the bone.

Nutrient artery

Transports oxygenated, nutrient-rich blood to the bone.

Nutrient vein

Transports deoxygenated, waste-laden blood from the bone.

Metaphyseal artery

Carries blood to the area of the metaphysis.

Metaphyseal vein

Carries blood from the area of the metaphysis.

Articular cartilage

Covers portions of the epiphysis that articulate with other bones, relying on diffusion from synovial fluid.

Fossa

A shallow depression or recess in the surface of a bone.

Fissure

A narrow, slit-like opening or an elongated cleft or gap.

Foramen

A round or oval opening through the bone.

Canal or meatus

A large passageway through a bone.

Sulcus or groove

A furrow or narrow trough in a bone.

Sinus

A chamber within a bone filled with air and lined with a mucous membrane.

Tuberosity

A large, round or rough projection that may cover a broad area.

Crest

A narrow ridge of bone; usually prominent.

Trochanter

A very large, irregularly shaped projection.

Line

Narrow ridges of bone; less prominent than a crest.

Tubercle

A small, rounded projection.

Epicondyle

A raised area above a condyle.

Spine

A sharp, slender, and often pointed process.

Head

The expanded proximal end of a bone carried on a narrow neck.

Facet

A smooth, flat articular surface.

Condyle

A smooth, rounded articular surface.

Ramus

An arm-like bar of a bone.

Osteon

The basic structural and functional unit of bone consisting of bone cells organized around a central canal.

Central canal

Also known as the Haversian canal, runs parallel to the axis of bone and contains an artery, vein, lymph vessel, and nerve.

Perforating canals

Passageways that extend perpendicular to the axis of the bone and connect the central canals of adjacent osteons.

Lamellae

Nested, concentric rings of matrix surrounding the central canal.

Circumferential lamellae

Specialized lamellae found at the outer and inner surfaces of bone.

Interstitial lamellae

Fill in the spaces between adjacent osteons of compact bone.

Lacunae

Open spaces where mature bone cells, called osteocytes, are trapped.

Canaliculi

Processes of osteocytes that extend into narrow crevices, connecting lacunae to the central canal.

Osteocytes

Mature bone cells that maintain the protein and mineral content of the surrounding matrix through the turnover of matrix components.

Osteoblasts

Immature bone cells located on the surface of bone; produce new bone matrix in a process called osteogenesis, or ossification.

Osteoprogenitor cells

Mesenchymal cells located with the periosteum and endosteum that divide to produce daughter cells that differentiate into osteoblasts.

Osteoclasts

Bone digesting cells that remove and recycle bone matrix; these are giant cells with 50 or more nuclei.

Osteolysis

The process of bone resorption, important in bone remodeling.

Organic Osteoid

Roughly 1/3 of the weight of bone is contributed by collagen fibers, which are strong and flexible.

Inorganic Hydroxyapatites

Mineral salts that account for almost 2/3 of the weight of bone, formed by calcium phosphate and calcium hydroxide.

Endochondral Ossification

Formation of most bones using a hyaline cartilage model, beginning approximately 6 weeks after fertilization.

Cavitation of hyaline shaft

The process where chondrocytes within the shaft hypertrophy and the surrounding matrix begins to calcify.

Primary ossification center

The area within the shaft of hyaline cartilage where blood vessels penetrate and invade during endochondral ossification.

Periosteal bud

A collective term for migrating fibroblasts, lymph vessels, nerve fibers, and red marrow elements that accompany blood vessels during ossification.

Trabeculae

The spongy bone formed by osteoblasts secreting osteoid around remaining fragments of hyaline cartilage.

Bony collar

A superficial layer of bone produced by osteoblasts during the conversion of perichondrium.

Collagen fibers

Strong and flexible fibers that contribute to the organic component of bone.

Calcium carbonate

A calcium salt that is incorporated into hydroxyapatite crystals during bone formation.

Calcium phosphate

A mineral that interacts with calcium hydroxide to form hydroxyapatite crystals in bone.

Ossification

The process of bone formation that begins during embryonic development.

Hypertrophy

The enlargement of chondrocytes within the hyaline shaft during endochondral ossification.

Phagocytic white blood cells

Cells derived from the same stem cells as osteoclasts, known as monocytes.

Bone remodeling

The ongoing process of bone resorption and formation that maintains bone strength and integrity.

Acids and proteolytic enzymes

Substances secreted by osteoclasts that dissolve the bone matrix and release stored minerals.

Bone weight composition

Approximately 1/3 organic (collagen fibers) and 2/3 inorganic (mineral salts) components.

Spongy Bone

The spongy bone is NOT broken down during the remodeling process.

Intramembranous Ossification

Formation of bones without a cartilage model. Typical in flat bones, mandible, clavicles, and patella. Begins approximately 8 weeks after fertilization.

Mesenchyme Cells

Mesenchyme cells differentiate into osteoblasts within fibrous connective tissues.

Ossification Center

The location where mesenchymal cells cluster and secrete organic components of the matrix.

Osteoid

The resulting osteoid mineralizes and the mesenchymal cells differentiate into osteoblasts.

Compact Bone Plate

Initially, the intramembranous bone consists of spongy bone only.

Osteons

Subsequent remodeling around trapped blood vessels can produce osteons typical of compact bone.

Fibrous Layer of the Periosteum

As the rate of growth slows at the surface, the connective tissue around the bone becomes organized into

Longitudinal Growth

Hyaline cartilage cells form tall columns at the epiphyseal plate (or growth plate) and within the articular cartilage.

Chondrocytes

Older chondrocytes closer to the shaft enlarge, die, and the surrounding cartilage matrix deteriorates.

Calcified Cartilage

The deterioration leaves spicules of

Osteoblasts in the Medullary Cavity

ossify the cartilage spicules, forming spongy bone.

Epiphyseal Line

Once completely replaced with bone, the epiphyseal plate is now called the epiphyseal line.

Appositional Growth

Osteoprogenitor cells beneath the periosteum differentiate into osteoblasts and form new osteons on the external bone surface.

Hematoma Formation

Blood vessels in bone tear and hemorrhage occurs, leading to the development of a large blood clot, or hematoma.

Fibrocartilage Callus Formation

Capillaries grow into the hematoma and phagocytic cells invade the area, leading to

Bony Callus Formation

Osteoblasts and osteoclasts continue to migrate inward and multiply rapidly in the fibrocartilaginous callus, leading to the formation of

Fractures Classification

Fractures are classified on the basis of whether the bone penetrates the skin.

Simple Fracture

when the bone breaks cleanly, but does not penetrate the skin.

Compound Fracture

fracture when the broken ends of bone protrude through the tissue and skin.

Compound (open) fracture

Broken ends of bone protrude through the tissue and skin.

Transverse fracture

Break occurs perpendicular to the long axis of a bone.

Linear fracture

Breaks parallel to the long axis of the bone.