The Skeletal System

• Bone provides rigid support for the soft tissues of the body.

• Cartilage provides semi-rigid support in limited sites (respiratory tree, external ear) and as a template for bone formation.

• The functional differences are due to differences in the nature and proportions of the fibrous elements and ground substance.

Cartilage

Composed of chondrocytes separated by abundant matrix.– Proteoglycan aggregates (chondroitin sulfate,

keratan sulfate) plus hyaluronic acid. – Plus collagen and elastin in varying proportions

depending on type.

Types of Cartilage

• Hyaline cartilage.

• Fibrocartilage.

• Elastic cartilage.

• Varying proportions of fibers including collagen and elastin determine type.

Hyaline Cartilage

• Most common type in the body.– Tracheal rings, nasal septum, articular surfaces,

sternal ends of ribs.– Precursor to bone formation.

• Contains type II collagen which is not cross banded (articular cartilage is type I).

       

Fibrocartilage

• Intermediate in form between dense fibrous connective tissue and hyaline cartilage.

• Alternating layers of hyaline cartilage matrix and collagen fibers oriented in the direction of functional stresses.

Elastic Cartilage

• Occurs in external ear, epiglottis and larynx.

• Similar in appearance to hyaline cartilage but contains numerous elastic fibers.

• Elastic fibers provide flexibility and give the tissue a yellow hue.

Bone

• Function– Provides a rigid support structure for the soft

tissues of the body– Points of attachment for muscle-movement– A resevoir of calcium used to maintain calcium

homeostasis in the body

Bone

• Composed of cells and collagenous extracellular matrix (osteoid) that becomes mineralized.

• Cells:– Osteoblasts line the surfaces of bone, produce matrix

(osteoid) and mediate its mineralization.

– Osteocytes are inactive osteoblasts trapped in bone, may assist in nutrition of bone.

– Osteoclasts are phagocytic cells capable of resorbing bone; Important in constant turnover of bone.

Bone

Woven bone

1. Immature bone formed rapidly as in the fetus or repair of an injury.

2. Collagen is irregularly arranged.

Lamellar bone

1. Mature bone that is physically stronger.

2. Collagen is regularly arranged

Types of Bone

• Cancellous– Found in marrow

cavity

– No Haversian systems

– Surface lined by endosteum

• Compact– Forms the dense shell

of bones

– Has Haversian systems

– Outer surface lined by periosteum

– Inner surface lined by endosteum

• Morphology of compact bone.

• Note the arrangements of the Haversian canals, osteocyte lacunae, cannaliculi and cortical bone beneath the periosteum. Cortical bone is also deposited by the endosteum

Osteoblasts and osteoid formation

Bone Formation In The Developing Skeleton

• Endochondral– Forms on a cartilage

scaffold

– Mineralized cartilage replaced by bone

– Long bones, pelvis and base of skull.

– Method by which long bones grow in length

• Intramembranous– Forms from condensed

mesenchyme

– No cartilage template

– Method by which bones of skull (except skull base) form

Type of Joints

• Synovial.– Bone ends held in apposition by a joint capsule and

ligaments.

– Surface lubricated by synovial fluid.

• Non-synovial.– Syndesmoses-joints between bones of skull.

– Synchondroses-where sternum and ribs join.

– Sympheses-pelvic symphysis and intervertebral disks.

Symphyses: intervertebral discs

Syndesmoses

Synchondroses

Tendons

Ligaments

• Connect bone to bone and reinforce joint capsules.

• Provide stability to joints.

• Dense bands of fibrous tissue resembling tendons but not as well ordered.