BIOL 2304
Bones and Joints
The Skeletal System
Functions of the Skeletal System
Support against gravity
Movement - leverage for muscle action
Protection of soft internal organs
Blood cell production
Storage - calcium, phosphorous, fat
The Skeletal System
The skeletal system includes:
Bones
Cartilages
Joints
Ligaments
Other connective tissues (CT)
Tissues in Bone
Bones contain several types of tissues:
Primarily bone (CT)
Contain nervous tissue (NT) and blood (CT)
Contain cartilage (CT) in articular cartilages that
articulate with other bones, forming joints
Contain connective tissue proper (CT) in the form
of periosteum, the outermost covering of bones
Contain epithelial tissue (ET) lining blood vessels
that permeate bone tissue
Bone (Osseous Tissue)
Bone Types:
Compact Bone
Dense outer layer
Spongy Bone (cancellous bone)
Trabeculae – honeycomb, needle-like, or flat
pieces filled with bone marrow
Specialized cells - 2% of bone weight
Strong flexible matrix:
Calcium phosphate crystals - 2/3 of bone weight
Collagen fibers
3 Types of Cartilage
Hyaline cartilage –
Most abundant cartilage
Glassy appearance
Provides support through flexibility
Articular cartilages and costal cartilage, larynx,
trachea, and nose
Elastic cartilage –
Contains many elastic fibers
Able to tolerate repeated bending
Ear and epiglottis
Fibrocartilage –
Resists immense compression and tension
An intermediate between hyaline and elastic
cartilage
Intervertebral discs and pubic symphysis
General Shapes Of Bones
Long bones
e.g., humerus, femur
Short bones
e.g., carpals, tarsals, patella
Flat bones
e.g., parietal bone, scapula, sternum
Irregular bones
e.g., vertebrae, hip bones
Structure of Typical Long Bone
Diaphysis – tubular shaft forming the axis of long bones
Composed of compact bone
Central medullary cavity
Contains bone marrow
Epiphysis – expanded end of long bones
Composed mostly of spongy bone
Joint surface is covered with articular (hyaline) cartilage
Epiphyseal plate (line) separates the diaphysis from the
epiphyses
Metaphysis –
Where epiphysis and diaphysis meet
Contains the epiphyseal plate (epiphyseal line)
Bone Membranes
Periosteum
“Peri-” = around; enclosing
Outermost, protective membrane
Double-layered
Innervated (supplied with nerve fibers)
Vascularized (supplied with blood and lymphatic
vessels) which enter the bone via nutrient foramina
Provides anchoring points for tendons and ligaments
Inner osteogenic layer is composed of osteoblasts and
osteoclasts
Endosteum
“Endo-” = inside; internal
Delicate CT membrane covering internal surfaces of
bone
Covers trabeculae of spongy bone
Also lines canals in compact bone
Also contains both osteoblasts and osteoclasts
Gross Anatomy of Bones
External Features of Bones:
Projections and depressions for muscle, ligament, & tendon attachment
Surfaces that form joints
Openings as conduits for blood vessels and nerves
Superficial surfaces of bones reflect stresses on them
Depression & Openings
Meatus – canal like passageway
Foramen – round or oval opening
Sinus – air-filled cavity
Fissure – narrow, slit-like opening
Fossa – shallow basin
Facet – smooth, nearly flat articular surface
Sulcus, Groove, or Furrow – shallow groove
Processes (projections):
Condyle – rounded, articular process
Epicondyle – elevated area above condyle
Head – rounded epiphysis
Crest – prominent narrow ridge
Line – small narrow ridge
Spine – sharp, narrow, pointed end
Ramus – arm like bar of bone
Trochanter – large, blunt process (femur only)
Tubercle – small, rounded process
Tuberosity – large, rounded, rough process
Histology of Compact Bone
Osteon – the structural unit of compact bone
Lamellae – column-like matrix tubes composed of collagen and crystals of bone salts
Central canal (Haversian canal) – canal containing blood vessels and nerves
Lacunae - cavities in bone containing osteocytes
Canaliculi - hairlike canals that connect lacunae to each other and the central canal
Perforating canal (Volkmann’s canal) – channels lying at right angles to the central canal, connecting
blood and nerve supply of the periosteum to the central canal
Cells in Bone
Osteoprogenitor cells – precursors to osteoblasts
Osteocytes - mature bone cells between lamellae
Osteoclasts - bone-destroying cells, break down bone matrix for remodeling and release of calcium
Source of acid, enzymes for osteolysis (the breakdown of bone)
Lysosomal enzymes & HCl
Calcium homeostasis (bones contain 99% of body calcium)
Osteoblasts - bone-forming cells
Responsible for osteogenesis (new bone)
Source of collagen, calcium salts
The Structure of Spongy Bone
No osteons
Lamellae as trabeculae
Arches, rods, plates of bone
Branching network of bony tissue
Strong in many directions
Red marrow (blood forming) spaces
Short, Irregular, and Flat Bones
Plates of periosteum-covered compact bone on the outside
Endosteum-covered spongy bone, diploë, on the inside
Diploë – the spongy bone within short, irregular, and flat bones; found between the inner and
outer compact layers
Have no diaphysis or epiphyses
Contain red bone marrow between the trabeculae
Bone Development
Osteogenesis or Ossification – the process of bone tissue formation that leads to:
The formation of the skeleton in embryos
Bone growth until early adulthood
Bone thickness, remodeling, and repair
Osteolysis – active resorption of bone matrix by osteoclasts during the
natural formation of health bones
Bone Growth and Development
Before week 8: Skeleton of embryo made of fibrous CT membranes
and hyaline cartilage
After week 8: Embryo undergoes ossification (converting other tissues
to bone)
Processes of Development:
Intramembranous ossification –
Bone develops from a fibrous connective tissue
membrane
Endochondral ossification –
Bone forms by replacing hyaline cartilage; uses hyaline cartilage “bones” as patterns
Intramembranous Ossification
Intramembranous Ossification
Forms flat bones of skull, mandible, clavicle
Stem cells differentiate to osteoblasts
Produces spongy bone, then compact bone
An ossification center appears in the fibrous connective tissue membrane
Osteoblasts secrete bone matrix within the fibrous membrane
Osteoblasts mature into osteocytes
Osteoblasts are trapped in the matrix they secrete
The bone matrix develops into trabeculae
The trabeculae formed from various ossification centers fuse with one another to create spongy bone
Eventually, the spaces between trabeculae fill with red bone marrow
Endochondral Ossification
Endochondral Ossification
Most bones formed this way
Hyaline cartilage model replaced by bone
Replacement begins in middle (diaphysis)
Replacement follows in ends (epiphyses)
Endochondral Ossification
Appositional Bone Growth
Growing bones widen as they lengthen
Appositional growth – growth of a bone by addition of bone tissue to its surface
Bone is resorbed at endosteal surface and added at periosteal surface
Osteoblasts – add bone tissue to the external surface of the diaphysis
Osteoclasts – remove bone from the internal surface of the diaphysis
Longitudinal Bone Growth
Longitudinal Growth (interstitial) – cartilage continually grows and is replaced by bone
Bones lengthen entirely by growth of the epiphyseal plates
Cartilage is replaced with bone as quickly as it grows
Epiphyseal plate maintains constant thickness
Epiphyseal Plate
Cartilage is organized for quick, efficient growth
1) Chondroblasts (cartilage cells) divide quickly forming tall stacks of daughter cells
Chondroblasts at the top of stacks are highly mitotic during growth periods
Pushes the epiphysis away from the diaphysis
Lengthens entire long bone
2) Older chondrocytes at bottom of stacks signal surrounding matrix to calcify, then die and disintegrate
3) Leaves long trabeculae (spicules) of calcified cartilage on diaphysis side
Trabeculae are partly eroded by osteoclasts
Osteoblasts then cover trabeculae with bone tissue
4) Trabeculae finally eaten away from their tips by osteoclasts
Bone - Remodeling/Homeostasis
Role of Remodeling in Support
Remodeling – Continuous breakdown and reforming of bone tissue
Shapes reflect applied loads
Mineral turnover enables adapting to new stresses
Remodeling Units – adjacent osteoblasts and osteoclasts deposit and reabsorb bone at periosteal and
endosteal surfaces
Bone Remodeling
Bone is active tissue – small changes in bone architecture occur continuously
5 to 7% of bone mass is recycled weekly
Spongy bone is replaced every 3-4 years
Compact bone is replaced every 10 years
What you don’t use, you lose
The stresses applied to bones during exercise are essential to maintaining bone strength and
bone mass
Bone Remodeling
Bone Deposition - the formation of new bone by osteoblasts
Occurs when bone is injured or extra strength is needed
Requires a healthy diet - protein, vitamins C, D, and A, and minerals (calcium, phosphorus,
magnesium, manganese, etc.)
Bone Resorption - the process by which osteoclasts break down bone and release the minerals
Osteoclasts are multinucleate, phagocytic cells
Osteoclasts secrete:
Lysosomal enzymes that digest organic matrix
HCl that converts calcium salts into soluble forms
Dissolved matrix is removed by interstitial fluid and blood
Bone - Remodeling/Homeostasis
Homeostasis and Mineral Storage
Bones store calcium
Contain 99% of body calcium
Store up to two kg calcium
Hormones control storage/release
PTH, calcitriol release bone calcium (through osteolysis)
Calcitonin stores bone calcium (through osteogenesis)
Blood levels kept relatively constant
Joints
Rigid elements of the skeleton meet at joints or articulations
Greek root “arthro” means joint
Functions of joints
Hold bones together
Allow for mobility
Articulations can be
Bone to bone
Bone to cartilage
Teeth in bony sockets
Classification of Joints
Joints can be classified by function or structure
Functional:
Synarthroses – immovable joints
Amphiarthroses – slightly moveable joints
Diarthroses – freely moveable joints
Structural:
Fibrous joints – generally immovable
Cartilaginous joints – immovable or slightly moveable
Synovial joints – freely moveable
Functional Classification
Functional classification – based on amount of movement
Synarthroses – immovable joints
Suture – very short CT fibers, e.g. between cranial bones
Gomphosis – teeth in sockets
Synchondrosis – hyaline cartilage unites bones, e.g. epiphyseal plate, costal cartilage
of 1st rib and manubrium
Amphiarthroses – slightly moveable joints
Syndesmosis – bones connected by ligaments, e.g. between tibia and fibula
Symphysis - bones are covered by hyaline cartilage fused with fibrocartilage, e.g.
between vertebrae, pubic bones of the hip
Diarthroses – freely moveable; knee, elbow, etc
Classifications of Joints
Structural classification based on
Material that binds bones together
Presence or absence of a joint cavity
Structural classifications include
Fibrous
Cartilaginous
Synovial
Fibrous Joints
Bones are connected by fibrous connective tissue
Primarily dense regular CT
Do not have a joint cavity
Most are immovable or slightly movable
Types:
Sutures
Syndesmoses
Gomphoses
Fibrous Joints – Sutures
Bones are tightly bound by a minimal amount of fibrous tissue
Only occur between the bones of the skull
Allow bone growth so the skull can expand with brain during childhood
Fibrous tissue ossifies in middle age
Synostoses – closed sutures
Fibrous Joints – Syndesmoses
Bones are connected exclusively by ligaments
Amount of movement depends on length of fibers
Tibiofibular joint – immovable synarthrosis
Interosseous membrane between radius and ulna
Freely movable diarthrosis
Fibrous Joints – Gomphoses
Tooth in a socket
Periodontal ligament – the connecting ligament
Cartilaginous Joints
Bones are united by cartilage
Lack a joint cavity
Two types:
Synchondroses - hyaline cartilage unites bones
Epiphyseal plates
Rib and sternum
Symphyses – permanent, slightly moveable fibrocartilaginous fusion between two bones
The pubic symphysis
Intervertebral disc between two vertebrae
Synovial Joints
Most movable type of joint
All are diarthroses
Each contains a fluid-filled joint cavity
General Structure of Synovial Joints
Articular cartilage
Ends of opposing bones are covered with
hyaline cartilage
Absorbs compression
Joint cavity (synovial cavity)
Unique to synovial joints
Cavity is a potential space that holds a small
amount of synovial fluid
Articular capsule – joint cavity is enclosed in a twolayered capsule
Fibrous capsule – dense irregular
connective tissue, which strengthens joint
Synovial membrane – loose connective
tissue
Lines joint capsule and covers internal
joint surfaces
Functions to make synovial fluid
Synovial fluid
A viscous fluid similar to raw egg white
A filtrate of blood
Arises from capillaries in synovial membrane
Contains glycoprotein molecules secreted by fibroblasts
Reinforcing ligaments
Often are thickened parts of the fibrous capsule
Sometimes are extracapsular ligaments, located outside the capsule
Sometimes are intracapsular ligaments, located within the capsule
Structures Associated with the Synovial Joint
Tendon sheath – elongated bursa that
wraps around a tendon
Bursae – flattened fibrous sacs
Lined with synovial membranes
Filled with synovial fluid
Not actually part of the joint
Menisci
Fat pads
Structural Classification of Synovial Joints
Gliding (Plane joint)
Example: vertebra–vertebra
Hinge
Example: knee
Pivot
Example: Atlas-axis vertebral articulation
Ellipsoidal (Condyloid joint)
Example: distal radius
Saddle
Example: thumb
Ball-and-Socket
Example: hip
Types of Synovial Joints Based on Shape
Summary of Joint Classes