SKELETAL SYSTEM

Notes from the skeletal system packet

Five functions of the skeletal system

1. Support

2. Protection

3. Movement

4. Storage

5. Blood Cell

How does the skeletal system offer support?

Bones form the internal framework that supports and anchors all soft organs.

How does the skeletal system offer protection?

Bones protect soft body organs.

How does the skeletal system help our body move?

Skeletal muscles, attached to bones by tendons, use the bones as levers to move the body and its parts.

How does the skeletal system store?

Bones serve as storehouses for minerals such as Calcium and Phosphorus.

• The stored minerals are deposited and reabsorbed continuously

• Mineralized bone matrix stores growth factors such as insulin-like growth factors, etc.

• Adipose is stored in the internal cavities of bone

How does the skeletal system form blood cells?

Hematopoiesis occurs within the red marrow cavities of certain bones.

How are bones classified?

The 206 bones are divided into two groups.

• Axial skeleton

• Appendicular skeleton

Axial skeleton

skull, vertebral column, thoracic cage, sternum

Appendicular skeleton

limbs and bones connecting the limbs to the pectoral girdle (scapula, clavicle) and the pelvic girdle

Bone Shapes

Long, Short, Flat, & Irregular

Long bones

longer than they are wide; have a shaft plus two ends

• Ex. bones of limbs

Short bone (cube)

roughly cubes shape

• Ex. wrist, ankle, tarsals & carpals

Flat bone (plate)

thin, flattened & usually curved

• Ex. scapula, ribs, sternum, skull (frontal & occipital)

Irregular bone

shape that doesn’t fit into other groups

• Ex. vertebrae, hip, tarsals, carpals , hyoid

Bone markings

Projections for muscle and ligament attachment. Depressions and openings allow blood vessels and nerves to pass.

Osseous Tissue

• Compact Bone

• Spongy Bone

Compact bone

dense outer layer

Spongy bone (cancellous or trabecular)

honeycomb of flat pieces called trabeculae; open spaces between trabeculae are filled with red or yellow bone marrow.

Skeletal Cartilages

Consist primarily of water which accounts for its resilience.

Three types of of skeletal cartilage

hyaline, elastic and fibrocartilage

• Each type has chondrocytes encased in lacunae within an extracellular matrix containing ground substances and fibers.

Structure of Short, Irregular and Flat Bones

Consist of the plates of compact bone on the outside and spongy bone within.

• They contain bone marrow between their trabeculae. Hyaline cartilage covers articular surfaces.

• It does NOT contain a shaft or epiphyses

Structure of a Long Bone

Diaphysis, Epiphyses, Periosteum & Endosteum

Diaphysis

shaft of bone constructed of compact one that surrounds a central medullary cavity

• The medullary cavity contains yellow marrow (fat) in adults and red marrow in infants

• Yellow marrow can revert to red marrow if a person becomes very anemic and needs enhanced red blood cell production

Epiphyses

Ends of bone with exterior compact bone and interior spongy bone

• The joint surface of each epiphysis is covered with a thin layer of articular (hyaline) cartilage which cushions the opposing bone ends during movement and absorbs stress.

• Red marrow is found within trabecular cavities of spongy bone. In adults, red marrow is only found in head of femur and humerus, trabeculae of flat bones and some irregular bones.

• Between the diaphysis and each epiphysis of adult long bones in an epiphyseal line, a remnant of the epiphyseal plate. The epiphyseal plate is a disc of hyaline cartilage that grows during childhood to lengthen bones.

Periosteum

double-layered membrane covering entire bone except articular cartilage

• Outer fibrous layer is dense irregular connective tissue

• Inner osteogenic layer composed of osteoclast (bone destroying cells) and osteoblast (bone forming cells)

• The periosteum is richly supplied with nerve fibers, blood, and lymphatic vessels, which enter the diaphysis via a nutrient foramina

• Membrane is secured to underlying bone by perforating (Sharpey) fibers, collagen fibers that extend into fibrous layer

• Membrane provides anchoring points for tendons and ligaments. At these points, the fibers are exceptionally dense

Endosteum

membrane covering the internal bone surface

• Membrane covers the trabeculae and lines the canals that pass through compact bone.

• Inner osteogenic layer is composed of osteoclast and osteoblast

• inside medullary cavity

Microscopic anatomy - Structure of bone

Cells found in bone - osteogenic cells, osteoblasts, osteocytes and osteoclast.

• These cells are surrounded by extracellular matrix.

• The matrix is composed of mineral salts (calcium & phosphorus) and collagenous fibers

Osteogenic cells

actively mitotic stem cells found in the membranous periosteum and endosteum

• Osteogenic cells differentiate and develop into osteoblasts

Osteoblasts

bone forming cells

• Cells secrete the unmineralized bone matrix which includes collagen and calcium-binding proteins

• remove calcium from blood to form new bone

• eventually surrounded by matrix and trapped in lacunae; they the mature into osteocytes

Osteocytes

Mature bone cells that monitor and maintain bone matrix

• Osteoblasts become osteocytes once they are enclosed by extracellular substances

• worn cells and deposition

Osteoclasts

Cells break down bone and phagocytize bone fragments.

• They are derived from hematopoietic stem cells that become macrophages

Parts of compact bone

Osteocytes, Lacunae, Lamellae, Haversian/central canal, Canaliculi, & Volkmann’s (perforating) canals

Osteocytes

mature bone cells

Lacunae

small cavities in bone cells

Lamellae

ring of column-like matrix tubes composed mainly of collagen formed by lacunae arranged in concentric rings

• The collagen fibers to help withstand torsion stress

Haversian/central canal

central channel containing vessels and nerves that serve the needs of the osteons cells

Canaliculi

canals that connect lacunae to each other and the central canal

Volkmann’s (perforating) canals

connecting blood and nerves of the periosteum to the Haversian canal and the medullary cavity

Spongy bone

The trabeculae align precisely along lines of stress and help the bone resist stress

• Trabeculae contain irregularly arranged lamellae and osteocytes interconnected by canaliculi

• No osteons are present. Nutrients reach the osteocytes by diffusing through canaliculi from capillaries in the endosteum surrounding the trabeculae

Chemical composition of bone

• Organic components

• Inorganic components

Organic components

include bone cells and osteoid secreted by osteoblasts

• Osteoid contains ground substances and collagen fibers

• Organic components contribute to structure, provides tensile strength and flexibility

Inorganic compounds

mostly hydroxyapatites or mineral salts (65% of bone mass)

• Calcium salts surround the collagen fibers in the matrix and are responsible for hardness and resistance to compression

Osteogenesis and Ossification

The bone formation process leads to the bony skeleton, bone growth, bone thickness, remodeling and repair

• The formation of the bony skeleton begins at week 8 of embryo development

Intramembranous ossification

osteoblast develops bone from a fibrous membrane

• occurs in flat bones of skull and clavicle

• spongy bone forms between two sheets

Endochondral ossification

Hyaline cartilage gradually is replaced by bone tissue; begins near the middle of the diaphysis and then in the epiphyses

• Diaphysis elongates, medullary cavity forms and epiphyses ossify

• Hyaline cartilage remains only in the epiphyseal plate and articular cartilages

Interstitial growth

increase in length at the epiphyseal plate

• The epiphyseal plate maintains a constant thickness

• The rate of cartilage growth on one side of the plate is balanced by bone replacement on the other side

1st step of Interstitial Growth

The cartilage on the diaphysis side of plate are rapidly dividing, pushing the epiphysis away from the diaphysis - result lengthening

2nd step of Interstitial Growth

• Chondrocytes die and matrix calcifies

• Osteoblasts cover area with new bone matrix

• Once the osteoblast is enclosed in matrix, it becomes an osteocyte

3rd step of Interstitial Growth

Bones increase in length as these cartilage cells continue reproduce and ossify

• As long as growth is occurring, the plate remains between the diaphysis and epiphysis

4th step of Interstitial Growth

The process continues until the cartilage growth slows and finally stops

• The epiphyseal plate completely ossifies and only a thin epiphyseal line remains

• The bones can no longer grow in length

What do growth hormones promote?

general bone growth and stimulate the epiphyseal plate

• Thyroid hormones modulate the activity of growth hormone to ensure proper bone proportions

• At puberty, sex hormones are released to increase osteoblast activity and stimulate bone growth

Appostitional Growth

increase thickness; occurs throughout life

• Osteoblast beneath the periosteum secret bone matrix as osteoclasts under the endosteum remove bone

• There is normally less breaking down than building up -- results in thicker; stronger ones that are not too heavy

Remodeling

• adjacent osteoblast and osteoclast deposit and resorb bone at the periosteal and endosteal surface

• bone is continually being broken down & built up

Bone Deposition

osteoblast secrete new matrix and become trapped within to become osteocytes

• Phosphorus, matrix proteins that blind, concentrate calcium salts and the enzyme alkaline phosphatase is essential for mineralization

• Deposition occurs were bone is injured or added strength is needed

Bone Resorption

Osteoclasts secrete lysosomal enzymes that digest organic matrix and acids that convert calcium salts into soluble forms

• Osteoclasts phagocytize mineralized matrix and dead osteocytes

• These digested materials are secreted into the interstitial fluid and then into the blood

• Once resorption is complete, osteoclasts undergo apoptosis (cell death)

Factors that influence remodeling

Hormones

the thyroid gland to release the hormone, calcitonin

What does rising blood Ca levels trigger?

What does calcitonin do?

accelerates Ca absorption and inhibits osteoclast

What do falling Ca levels signal?

the parathyroid glands to release the hormone, parathyroid hormone (PTH)

What does PTH signal?

osteoclasts to break down bone and release Ca into bloodstream

What does the hormonal controls do?

acts not to preserve the skeleton’s strength or well-being, but rather to maintain blood calcium homeostasis

Why can minute changes in blood calcium be dangerous?

cause severe neuromuscular problems

What do hormonal controls determine?

Whether and when remodeling occurs to changing blood calcium levels

What determines where remodeling occurs?

Mechanical stress

What are the three ways bone fractures are classified?

• The position of the bone ends after fracture

• Completeness of the break

• Whether or not the bones ends penetrate the skin

What is the first way bone fractures are classified?

The position of the bone ends after fracture

• nondisplaced (ends retain normal position) or displaced (bone ends out of alignment)

What is the second way bone fractures are classified?

Completeness of the break

• compolet (broken through) or incomplete (not broken all the way)

What is the third way bone fractures are classified?

Whether or not the bone ends penetrate the skin

• simple/closed (sin is not penetrated) or compound/open (skin is penetrated)

Complete/Comminuted fracture

• Bone fragments into pieces

• Common in elderly because of brittle bones

Compression fracture

• Bone is crushed

• Common in porous bones

Spiral fracture

• Ragged break occurs when excessive twisting forces are applied to bone

• Common sports fracture

incomplete/greenstick fracture

• Bone breaks incompletely

• Common in children, whose bones have more organic matrix and are more flexible

Impacted fracture

• Broken bone ends are forced into each other

• Commonly occurs when one attempts to break a fall with outstretched arms

4 stages of healing a bone fracture

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bone callus formation

4. Bone remodeling

Hematome formation

a mass of clotted blood forms at fracture site and tissue becomes swollen, painful and inflamed

Fibrocartilage callus formation

• Capillaries regrow, phagocytic cells clean debris, fibroblasts and osteoblasts invade site and begin reconstructing bone

• The callus splints the broken bone

Bone callus formation

Within a week, new bone appears and is converted into bony callus

Bone remoldeling

Compact bone is laid down to replace callus

Type of joints (articulations)

Synarthrotic (fibrous) joints, Amphiarthrotic (cartilaginous) joint, & Diarthrotic (synovial) joint

Synarthrotic (fibrous) joints

• immovable joint such as skull

• junctions are called sutures

Amphiarthrotic (cartilaginous) joint

slightly moveable joint such as vertebrae

Diarthrotic (synovial) joint

• freely moveable joint such as shoulders, hips, knees, elbows, wrist, etc.

• plane, hing, picot, condyloid, saddle, & ball-and-socket joints

Homeostatic Imbalances/Diseases

Osteoporosis, Rickets, Osteomalacia, & Pagets diseases

Osteoporosis

• bone reabsorption outpaces bone deposit; age-correlated with lifestyle contributors such as smoking, poor nutrition and lack of exercise

• proper Calcium levels needed to prevent it

Rickets

• vitamin D deficiency in children

• inadequately mineralized bones resulting in bowed legs, deformities of pelvis, skull and rib cage

Osteomalacia

• vitamin D deficiency in adults

• inadequately mineralized causing softened, weak bones

Pagets disease

overactive osteoclasts and osteoblast causing irregular thickening and softening of bones

Surface features

epicondyle, condyle, fermen, head, tubercle, fossa, crest, & sinus

epicondyle

smaller knob above condyle

condyle

rounded knob of bone

fermen

hole in bone

head

large knob or bone that creates joint

tubercle

small, round knob

fossa

depression in bone

crest

a slender ridge

sinus

hollow cavity in bone