Chapter 6: The Skeletal System

Osteology

Study of the skeleton

Skeletal System

Composed of bones, cartilage, & ligaments

• Cartilage

  • precursor of most bones in childhood & embryonic development

  • covers many joint surfaces

• Ligaments

  • holds bones together at joints

• Bone Marrow

  • soft material enclosed in bones

Functions of the Skeletal system

1. Support

   1. bones of limbs & vertebral column support body

   2. mandible and maxilla support teeth

   3. some viscera supported by bones

2. Protection

   1. brain, spinal cord, lungs, heart, & pelvic viscera

3. Movement

   1. provide attachment & leverage for muscles

   2. allow for limb movement & breathing

4. Blood Formation

   1. bones are the major producer of blood cells

5. Storage

   1. body’s main reservoir of calcium and phosphorus; released when needed

   2. Bone marrow is a reserve of stored fuel

Osseous (Bone) Tissue

Connective tissue with a hard, calcified matrix

2 types:

1. Compact Bone

2. Spongy bone

Compact Bone (dense bone)

Made up of osteons

• Visible, central canals

• found in greater proportion in bone diaphyses

• Solidly filled with an opaque matrix

• Encloses spongy bone

Spongy Bone

Porous lattice, with spaces found in the interior of the bone

• composed of trabeculae

• found in greater proportion in bone epiphyses & flat bones

• provides strength with light weight

• arranged along the lines of forces that are encountered

• gaps b/w ossified material are filled with marrow

Bone Cells

1. Osteogenic cells

2. Osteoblasts

3. Osteocytes

4. Osteoclasts

Osteogenic Cells

• occurs on bone surface

• stem cells that give rise to osteoblasts

Osteoblasts

• Bone-forming cells that synthesize the organic matter of the bone and promote its mineralization

• lie in a single layer on the bone surface

• Overactivity = increased bone density

• Bone matrix deposition

• abundant in mitochondria

Osteocytes

• former osteoblasts that are inecase in deposit matrix

• reside in cavites (lacunae), which are connected by channels (canaliculi)

Osteoclasts

• bone-dissolving/resorbing cells

• large, with multiple nuclei

• lie on pits of bone surface

• have infoldings of plasma membrane (ruffled border) on the side facing bone

  • secret hydrochloric acid & enzymes here

• overactivity = decreased bone density

• greatest # of lysosomes

Bone Matrix

surrounds osteocytes & lacunae

• 1/3 organic by weight; mostly collagen & protein carbohydrate complexes

• 2/3 inorganic; mostly calcium phosphate

When bones are deficient in minerals, they are soft & bend easily

• EX: in rickets & osteomalacia, bones are mineral deficient & easily deformed

• Collagen in matrix gives bones a degree of flexibility so they can bend without breaking

Spongy (cancellous) bone

trabeculae: porous lattice rods & plates

• sponge like appearance; spaces are filled with bone marrow & blood vessels

• imparts strength without adding too much weight

Compact (dense) bone

Forms outer, hard shell of bone

• provides solid attachments for muscles, tendons, & ligaments

• organized into osteons - made of concentric layers (lamellae) arranged around central canals

Long bones

Most important bones for movement

• platforms for skeletal muscle attachment

Diaphysis (Shaft)

Elongated midsection

• provides leverage

Epiphysis (head)

Expanded end; strengthens joints

• provides area for tendon & ligament attachment

• filled with spongy bone

Articular Cartilage

hyaline cartilage covering the ends where bones meet

• eases joint movements

Medullary cavity

long cavity inside diaphysis

• contains bone marrow

Epiphyseal lines

in adults

• remnant of childhood growth zone

Flat Bones

Shield like plates that protect delicate organs

• EX: stenum protects heart

  • cranial bones protect brain

• May provide attachment surfaces for muscles

  • EX: scapula

• 2 layers of compact bone, middle layer of spongy bone

Other Bone Shapes

Short bones:

• wrist

Irregular bones:

• vertebrae

Periosteum

Fibrous sheath covering the bone

• collagen fibers are continuous with tendons & penetrate into bone itself

• no periosteum over the articular cartilage

Endosteum

Thin layer of reticular CT

• covers internal surface of bone

Bone Marrow

2 types:

1. Red Bone Marrow: produces blood cells & platelets

• fills nearly every bone of a child’s skeleton

  • limited distribution in adults

• soft network of delicate blood vessels surrounded by reticular tissue & blood forming cells

2. Yellow Bone Marrow: replaces much of red bm during maturity

• contains fat

• dominates long bones in limbs of adults

Ossification

formation of bone

2 types:

1. Intramembranous

2. Endochondral

Intramembranous Ossification

Produces flat bones of the skull

Begins with a soft embryonic membrane

• embryonic cells deposit collagen into membrane

Mineral salts are then deposited on the fibers, hardening the tissue

• creates the trabeculae of spongy bone

The the inner & outer surfaces calcify , producing the sandwich like appearance of skull bones (compact or spongy bone)

Fontanels

soft areas in cranium

• like when a baby is born

Endochondral Ossification

Produces most other bones in the body

Steps:

1. Future bone begins as hyaline cartilage covered with perichondrium

2. Formation of primary ossification center near middle of cartilage model

   1. osteoblasts deposit layer of bone around cartilage

   2. fibrous sheath is now called periosteum

3. Blood vessels pierce the periosteum at the primary ossification center

   1. secondary ossification center develops at one end of bone; forms a second marrow cavity

4. Primary & Secondary cavities separated by a wall epiphyseal plate

5. The reserve cartilage is depleted

What is the primary reservoir of calcium & phosphate, which can be withdrawn when needed?

The skeleton

Mineral deposition v. Mineral resorption

Deposition by osteoblasts

• extract calcium phosphate from blood & deposit them in osseous tissue

Resorption by osteoclasts

• they dissolve bone & release minerals into blood

• critical balance; if not can cause bone abnormalities

Hypocalcemia

Calcium deficiency

• muscle tremors, inability of muscle to relax, life-threatening

Hypercalcemia

excess calcium

• Depresses nervous, muscular, & cardiac function

Calcium homeostasis

Regulated by 2 main hormones:

1. Calcitrol: most active form of vitamin

• raises blood calcium by

  • food

  • reduces urinary loss of calcium

  • stimulates release of calcium from bones

2. Parathyroid hormone: secreted by parathyroid glands when blood calcium levels fall

• raises blood calcium by

  • stimulating bone resorption by osteoclasts

  • promoting calcium reabsorption by the kidneys

  • calcitriol synthesis

Substances affecting osseous tissue

Calcitonin

• From thyroid gland; role in bone metabolism.

Growth hormone, estrogen, testosterone

• Promote ossification and bone growth.

Thyroid hormone, insulin, growth factors

• Promote bone deposition.

Vitamins D, A & C

• Needed for bone deposition and repair.

Fractures

Types

Stress: From unusual stress, such as a fall or auto accident.

Pathological: Bone fractures only because it was weakened by disease.

Open: breaks the skin

Closed: doesn’t break skin

Healing

Takes 4-6 weeks

Fracture hematoma - forms from broken blood vessels

Soft callus - deposition of collagen & fibrocartilage turns the hematoma into this

Hard callus - soft callus ossifies

Closed reduction - alignment of broken bones without surgery

Open reduction - uses surgery

Osteoporosis

Most common bone disease

Loss of bone tissue, especially spongy

• bones fracture easily

• Most commonly affects post-menopausal white women

• After menopause, estrogen levels decrease, estrogen stimulates bone deposition