An Introduction to the Skeletal System

§Skeletal system includes

§Bones of the skeleton

§Cartilages, ligaments, and connective tissues

Functions of the Skeletal System

§Support

§Storage of minerals (calcium)

§Storage of lipids (yellow marrow)

§Blood cell production (red marrow)

§Protection

§Leverage (force of motion)

Classification of Bones

§Bones are classified by

§Shape

§Internal tissue organization

§Bone markings (surface features; marks)

§Bone Shapes

§Long bones

§Are long and thin

§Are found in arms, legs, hands, feet, fingers, and toes

§Flat bones

§Are thin with parallel surfaces

§Are found in the skull, sternum, ribs, and scapulae

§Sutural bones

§Are small, irregular bones

§Are found between the flat bones of the skull

§Irregular bones

§Have complex shapes

§Examples: spinal vertebrae, pelvic bones

§Short bones

§Are small and thick

§Examples: ankle and wrist bones

§Sesamoid bones

§Are small and flat

§Develop inside tendons near joints of knees, hands, and feet

§Bone Markings

§Depressions or grooves

§Along bone surface

§Projections

§Where tendons and ligaments attach

§At articulations with other bones

§Tunnels

§Where blood and nerves enter bone

§Structure of a Long Bone

§Diaphysis

§The shaft

§A heavy wall of compact bone, or dense bone

§A central space called medullary (marrow) cavity

§Epiphysis

§Wide part at each end

§Articulation with other bones

§Mostly spongy (cancellous) bone

§Covered with compact bone (cortex)

§Metaphysis

§Where diaphysis and epiphysis meet

§Structure of a Flat Bone

§The parietal bone of the skull

§Resembles a sandwich of spongy bone

§Between two layers of compact bone

§Within the cranium, the layer of spongy bone between the compact bone is called the diploë

Bone (Osseous) Tissue

§Dense, supportive connective tissue

§Contains specialized cells

§Produces solid matrix of calcium salt deposits

§Around collagen fibers

§Characteristics of Bone Tissue

§Dense matrix, containing

§Deposits of calcium salts

§Osteocytes (bone cells) within lacunae organized around blood vessels

§Canaliculi

§Form pathways for blood vessels

§Exchange nutrients and wastes

§Periosteum

§Covers outer surfaces of bones

§Consists of outer fibrous and inner cellular layers

§Matrix Minerals

§Two thirds of bone matrix is calcium phosphate, Ca3(PO4)2

§Reacts with calcium hydroxide, Ca(OH)2

§To form crystals of hydroxyapatite, Ca10(PO4)6(OH)2

§Which incorporates other calcium salts and ions

§Matrix Proteins

§One third of bone matrix is protein fibers (collagen)

§The Cells of Bone

§Make up only 2% of bone mass

§Bone contains four types of cells

§Osteocytes

§Osteoblasts

§Osteoprogenitor cells

§Osteoclasts

§Osteocytes

§Mature bone cells that maintain the bone matrix

§Live in lacunae

§Are between layers (lamellae) of matrix

§Connect by cytoplasmic extensions through canaliculi in lamellae

§Do not divide

§Functions

§To maintain protein and mineral content of matrix

§To help repair damaged bone

§Osteoblasts

§Immature bone cells that secrete matrix compounds (osteogenesis)

§Osteoid—matrix produced by osteoblasts, but not yet calcified to form bone

§Osteoblasts surrounded by bone become osteocytes

§Osteoprogenitor cells

§Mesenchymal stem cells that divide to produce osteoblasts

§Are located in endosteum, the inner, cellular layer of periosteum

§Assist in fracture repair

§Osteoclasts

§Secrete acids and protein-digesting enzymes

§Giant, multinucleate cells

§Dissolve bone matrix and release stored minerals (osteolysis)

§Are derived from stem cells that produce macrophages

§Homeostasis

§Bone building (by osteoblasts) and bone recycling (by osteoclasts) must balance

§More breakdown than building, bones become weak

§Exercise, particularly weight-bearing exercise, causes osteoblasts to build bone

Compact and Spongy Bone

§ The Structure of Compact Bone

§Osteon is the basic unit

§Osteocytes are arranged in concentric lamellae

§Around a central canal containing blood vessels

§Perforating Canals:

– perpendicular to the central canal

– carry blood vessels into bone and marrow

§Circumferential Lamellae

§Lamellae wrapped around the long bone

§Bind osteons together

§The Structure of Spongy Bone

§Does not have osteons

§The matrix forms an open network of trabeculae

§Trabeculae have no blood vessels

§The space between trabeculae is filled with red bone marrow:

§Which has blood vessels

§Forms red blood cells

§And supplies nutrients to osteocytes

§Yellow marrow

§In some bones, spongy bone holds yellow bone marrow

§Is yellow because it stores fat

§Weight-Bearing Bones

§The femur transfers weight from hip joint to knee joint

§Causing tension on the lateral side of the shaft

§And compression on the medial side

§Compact bone is covered with a membrane

§Periosteum on the outside

§Covers all bones except parts enclosed in joint capsules

§Is made up of an outer, fibrous layer and an inner, cellular layer

§Perforating fibers: collagen fibers of the periosteum:

–connect with collagen fibers in bone

–and with fibers of joint capsules; attach tendons, and ligaments

§Functions of Periosteum

§Isolates bone from surrounding tissues

§Provides a route for circulatory and nervous supply

§Participates in bone growth and repair

§Compact bone is covered with a membrane:

§Endosteum on the inside

§An incomplete cellular layer:

–lines the medullary (marrow) cavity

–covers trabeculae of spongy bone

–lines central canals

–contains osteoblasts, osteoprogenitor cells, and osteoclasts

–is active in bone growth and repair

Bone Formation and Growth

§Bone Development

§Human bones grow until about age 25

§Osteogenesis

§ Bone formation

§Ossification

§The process of replacing other tissues with bone

§Bone Development

§Calcification

§The process of depositing calcium salts

§Occurs during bone ossification and in other tissues

§Ossification

§The two main forms of ossification are

– intramembranous ossification

– endochondral ossification

§Endochondral Ossification

§Ossifies bones that originate as hyaline cartilage

§Most bones originate as hyaline cartilage

§There are six main steps in endochondral ossification

§Appositional growth

§Compact bone thickens and strengthens long bone with layers of circumferential lamellae

[Insert Animation Endochondral Ossification]

Bone Formation and Growth

§Epiphyseal Lines

§When long bone stops growing, after puberty

§Epiphyseal cartilage disappears

§Is visible on X-rays as an epiphyseal line

§Mature Bones

§As long bone matures

§Osteoclasts enlarge medullary (marrow) cavity

§Osteons form around blood vessels in compact bone

§Intramembranous Ossification

§Also called dermal ossification

§Because it occurs in the dermis

§Produces dermal bones such as mandible (lower jaw) and clavicle (collarbone)

§There are three main steps in intramembranous ossification

Bone Formation and Growth

§Blood Supply of Mature Bones

§Three major sets of blood vessels develop

§Nutrient artery and vein:

– a single pair of large blood vessels

– enter the diaphysis through the nutrient foramen

– femur has more than one pair

§Metaphyseal vessels:

– supply the epiphyseal cartilage

– where bone growth occurs

§Periosteal vessels provide:

– blood to superficial osteons

– secondary ossification centers

§Lymph and Nerves

§The periosteum also contains

§Networks of lymphatic vessels

§Sensory nerves

Bone Remodeling

§Process of Remodeling

§The adult skeleton

§Maintains itself

§Replaces mineral reserves

§Recycles and renews bone matrix

§Involves osteocytes, osteoblasts, and osteoclasts

§Bone continually remodels, recycles, and replaces

§Turnover rate varies

§If deposition is greater than removal, bones get stronger

§If removal is faster than replacement, bones get weaker

Exercise, Hormones, and Nutrition

§Effects of Exercise on Bone

§Mineral recycling allows bones to adapt to stress

§Heavily stressed bones become thicker and stronger

§Bone Degeneration

§Bone degenerates quickly

§Up to one third of bone mass can be lost in a few weeks of inactivity

§Normal bone growth and maintenance requires nutritional and hormonal factors

§A dietary source of calcium and phosphate salts

§Plus small amounts of magnesium, fluoride, iron, and manganese

§The hormone calcitriol

§Is made in the kidneys

§Helps absorb calcium and phosphorus from digestive tract

§Synthesis requires vitamin D3 (cholecalciferol)

§Vitamin C is required for collagen synthesis, and stimulation of osteoblast differentiation

§Vitamin A stimulates osteoblast activity

§Vitamins K and B12 help synthesize bone proteins

§Growth hormone and thyroxine stimulate bone growth

§Estrogens and androgens stimulate osteoblasts

§Calcitonin and parathyroid hormone regulate calcium and phosphate levels

Calcium Homeostasis

§The Skeleton as a Calcium Reserve

§Bones store calcium and other minerals

§Calcium is the most abundant mineral in the body

§Calcium ions are vital to:

–membranes

–neurons

–muscle cells, especially heart cells

Calcium Homeostasis

§Calcium Regulation

§Calcium ions in body fluids

§Must be closely regulated

§Homeostasis is maintained

§By calcitonin and parathyroid hormone

§Which control storage, absorption, and excretion

§Calcitonin and parathyroid hormone control and affect

§Bones

§Where calcium is stored

§Digestive tract

§Where calcium is absorbed

§Kidneys

§Where calcium is excreted

§Parathyroid Hormone (PTH)

§Produced by parathyroid glands in neck

§Increases calcium ion levels by

§Stimulating osteoclasts

§Increasing intestinal absorption of calcium

§Decreasing calcium excretion at kidneys

§Calcitonin

§Secreted by C cells (parafollicular cells) in thyroid

§Decreases calcium ion levels by

§Inhibiting osteoclast activity

§Increasing calcium excretion at kidneys

Fractures

§Cracks or breaks in bones

§Caused by physical stress

Fractures

§Fractures are repaired in four steps

§Bleeding

§Produces a clot (fracture hematoma)

§Establishes a fibrous network

§Bone cells in the area die

§Cells of the endosteum and periosteum

§Divide and migrate into fracture zone

§Calluses stabilize the break:

– external callus of cartilage and bone surrounds break

– internal callus develops in medullary cavity

Fractures

§Fractures are repaired in four steps

§Osteoblasts

§Replace central cartilage of external callus

§With spongy bone

§Osteoblasts and osteocytes remodel the fracture for up to a year

§Reducing bone calluses

§The Major Types of Fractures

§Pott fracture

§Comminuted fractures

§Transverse fractures

§Spiral fractures

§Displaced fractures

§Colles fracture

§Greenstick fracture

§Epiphyseal fractures

§Compression fractures

Osteopenia

§Bones become thinner and weaker with age

§Osteopenia begins between ages 30 and 40

§Women lose 8% of bone mass per decade, men 3%

§The epiphyses, vertebrae, and jaws are most affected:

§Resulting in fragile limbs

§Reduction in height

§Tooth loss

§Osteoporosis

§Severe bone loss

§Affects normal function

§Over age 45, occurs in

§29% of women

§18% of men

Aging

§Hormones and Bone Loss

§Estrogens and androgens help maintain bone mass

§Bone loss in women accelerates after menopause

§Cancer and Bone Loss

§Cancerous tissues release osteoclast-activating factor

§That stimulates osteoclasts

§And produces severe osteoporosis