The Skeletal System
• Skeletal system includes:
– bones of the skeleton
– cartilages, ligaments, and connective tissues
Functions of the
Skeletal System
1. Support
2. Storage of minerals (calcium)
3. Storage of lipids (yellow marrow)
Functions of the
Skeletal System
4. Blood cell production (red marrow)
5. Protection
6. Leverage (force of motion)
Classification of Bones
• Bone are identified by:
– shape
– internal tissues
– bone markings
Bone Shapes
1.
2.
3.
4.
5.
6.
Long bones
Flat bones
Sutural bones
Irregular bones
Short bones
Sesamoid bones
Long Bones
Figure 6–1a
Long Bones
• Are long and thin
• Are found in arms, legs, hands, feet,
fingers, and toes
Flat Bones
Figure 6–1b
Flat Bones
• Are thin with parallel surfaces
• Are found in the skull, sternum, ribs, and
scapula
Sutural Bones
Figure 6–1c
Sutural Bones
• Are small, irregular bones
• Are found between the flat bones of the
skull
Irregular Bones
Figure 6–1d
Irregular Bones
• Have complex shapes
• Examples:
– spinal vertebrae
– pelvic bones
Short Bones
Figure 6–1e
Short Bones
• Are small and thick
• Examples:
– ankle
– wrist bones
Sesamoid Bones
Figure 6–1f
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
Bone Markings
Table 6–1 (2 of 2)
Long Bones
• The femur
Figure 6–2a
Long Bones
• Diaphysis:
– the shaft
• Epiphysis:
– wide part at each end
– articulation with other bones
• Metaphysis:
– where diaphysis and epiphysis meet
The Diaphysis
• A heavy wall of compact bone, or dense
bone
• A central space called marrow cavity
The Epiphysis
• Mostly spongy (cancellous) bone
• Covered with compact bone (cortex)
Flat Bones
• The parietal bone of the skull
Figure 6–2b
Flat Bones
• Resembles a sandwich of spongy bone
• Between 2 layers of compact bone
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
– bone cells within lacunae organized around
blood vessels
Characteristics of Bone Tissue
• Canaliculi:
– form pathways for blood vessels
– exchange nutrients and wastes
Characteristics of Bone Tissue
• Periosteum:
– covers outer surfaces of bones
– consist of outer fibrous and inner cellular
layers
Matrix Minerals
• 2/3 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
• 1/3 of bone matrix is protein fibers
(collagen)
Bone Cells
• Make up only 2% of bone mass:
– osteocytes
– osteoblasts
– osteoprogenitor cells
– osteoclasts
Osteocytes
• Mature bone cells
that maintain the
bone matrix
Figure 6–3 (1 of 4)
Osteocytes
• Live in lacunae
• Are between layers (lamellae) of matrix
• Connect by cytoplasmic extensions
through canaliculi in lamellae
• Do not divide
Osteocyte Functions
• To maintain protein and mineral content of
matrix
• To help repair damaged bone
Osteoblasts
• Immature bone cells
that secrete matrix
compounds
(osteogenesis)
Figure 6–3 (2 of 4)
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
Figure 6–3 (3 of 4)
Osteoprogenitor Cells
• Are located in inner, cellular layer of
periosteum (endosteum)
• Assist in fracture repair
Osteoclasts
• Secrete acids and protein-digesting enzymes
Figure 6–3 (4 of 4)
Osteoclasts
• Giant, mutlinucleate cells
• Dissolve bone matrix and release stored
minerals (osteolysis)
• Are derived from stem cells that produce
macrophages
Homeostasis
• Bone building (by osteocytes) and bone
recycling (by osteoclasts) must balance:
– more breakdown than building, bones become
weak
– exercise causes osteocytes to build bone
Compact Bone
Figure 6–5
Osteon
• The basic unit of mature compact bone
• 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
• Binds osteons together
Spongy Bone
Figure 6–6
Spongy Bone
• Does not have osteons
• The matrix forms an open network of
trabeculae
• Trabeculae have no blood vessels
Red Marrow
• 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
Figure 6–7
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
Periosteum and Endosteum
• Compact bone is covered with membrane:
– periosteum on the outside
– endosteum on the inside
Periosteum
Figure 6–8a
Periosteum
• Covers all bones:
– except parts enclosed in joint capsules
• It 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, attached
tendons, and ligaments
Functions of Periosteum
1. Isolate bone from surrounding tissues
2. Provide a route for circulatory and
nervous supply
3. Participate in bone growth and repair
Endosteum
Figure 6–8b
Endosteum
• An incomplete cellular layer:
– lines the marrow cavity
– covers trabeculae of spongy bone
– lines central canals
Endosteum
• Contains osteoblasts, osteoprogenitor
cells, and osteoclasts
• Is active in bone growth and repair
Bone Development
• Human bones grow until about age 25
• Osteogenesis:
– bone formation
• Ossification:
– the process of replacing other tissues with
bone
Ossification
• The 2 main forms of ossification are:
– intramembranous ossification
– endochondral ossification
Endochondral Ossification
• Ossifies bones that originate as hyaline
cartilage
• Most bones originate as hyaline cartilage
Endochondral Ossification
• Growth and ossification of long bones
occurs in 6 steps
Endochondral
Ossification: Step 1
• Chondrocytes in the center of
hyaline cartilage:
– enlarge
– form struts and calcify
– die, leaving cavities in cartilage
Figure 6–9 (Step 1)
Endochondral
Ossification: Step 2
Figure 6–9 (Step 2)
Endochondral
Ossification: Step 2
• Blood vessels grow around the edges of
the cartilage
• Cells in the perichondrium change to
osteoblasts:
– producing a layer of superficial bone around
the shaft which will continue to grow and
become compact bone (appositional growth)
Endochondral
Ossification: Step 3
• Blood vessels enter the
cartilage:
– bringing fibroblasts that
become osteoblasts
– spongy bone develops at the
primary ossification center
Figure 6–9 (Step 3)
Endochondral
Ossification: Step 4
• Remodeling creates a marrow
cavity:
– bone replaces cartilage at the
metaphyses
Figure 6–9 (Step 4)
Endochondral
Ossification: Step 5
• Capillaries and osteoblasts
enter the epiphyses:
– creating secondary
ossification centers
Figure 6–9 (Step 5)
Endochondral
Ossification: Step 6
Figure 6–9 (Step 6)
Endochondral
Ossification: Step 6
• Epiphyses fill with spongy bone:
– cartilage within the joint cavity is articulation
cartilage
– cartilage at the metaphysis is epiphyseal
cartilage
Endochondral Ossification
• Appositional growth:
– compact bone thickens and
strengthens long bone with
layers of circumferential
lamellae
PLAY
Endochondral Ossification
Figure 6–9 (Step 2)
Epiphyseal Lines
Figure 6–10
Epiphyseal Lines
• When long bone stops growing, after
puberty:
– epiphyseal cartilage disappears
– is visible on X-rays as an epiphyseal line
Remodeling
• The adult skeleton:
– maintains itself
– replaces mineral reserves
• Remodeling:
– recycles and renews bone matrix
– involves osteocytes, osteoblasts, and
osteoclasts
Functions of Calcium
• Calcium ions are vital to:
– membranes
– neurons
– muscle cells, especially heart cells
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
• Bones:
– where calcium is stored
• Digestive tract:
– where calcium is absorbed
• Kidneys:
– where calcium is excreted
Calcitonin
• Secreted by C cells (parafollicular cells) in
thyroid
• Decreases calcium ion levels by:
– inhibiting osteoclast activity
– increasing calcium excretion at kidneys
The Major Types of Fractures
• Pott’s fracture
Figure 6–16 (1 of 9)
The Major Types of Fractures
• Comminuted fractures
Figure 6–16 (2 of 9)
The Major Types of Fractures
• Transverse fractures
Figure 6–16 (3 of 9)
The Major Types of Fractures
• Spiral fractures
Figure 6–16 (4 of 9)
The Major Types of Fractures
• Displaced fractures
Figure 6–16 (5 of 9)
The Major Types of Fractures
• Colles’ fracture
Figure 6–16 (6 of 9)
The Major Types of Fractures
• Greenstick fracture
Figure 6–16 (7 of 9)
The Major Types of Fractures
• Epiphyseal fractures
Figure 6–16 (8 of 9)
The Major Types of Fractures
• Compression fractures
Figure 6–16 (9 of 9)