Exam 2: Bones and Skeletal Tissue

Chapter 6a

Characteristics of skeletal cartilages?

-no blood vessels (avascular, 80% water)

-no nerves

-perichondrium: dense irregular connective tissue girdle= contains blood vessels for nutrient delivery to cartilage

What are hyaline cartilages?

-most abundant type

-provide support, flexibility, and resilence

-types: articular (joint connection), costal (ribs to sternum), respiratory (skeleton to larynx), and nasal (external nose)

What are elastic cartilages?

-similar to hyaline cartilages, but contain elastic fibers

-found in epiglottis and pinna (outer ear)

What are fibrocartilages?

collagen fibers providing great tensile strength

-absorbs compressive shock

-in vertebral discs and pubic symphysis

Types of cartilage growth?

-appositional (to apply)

-interstitial (in between)

-calcification of cartilage

What is appositional growth?

cells secrete matrix against the external face of existing cartilage

-gets thicker and thicker

What is interstitial growth?

chondrocytes divide and secrete new matrix, expanding cartilage from within

When does calcification of cartilage occur?

-during normal bone growth

-during old age

-But it’s NOT BONE!!!

Two main groups for bones of the skeleton?

-axial skeleton (skull, vertebral column, rib cage): protects and supports

-appendicular skeleton (appendages, limbs, hips, and shoulder bones; helps in movement)

What are the classifications of bones by shape?

-long bones

-short bones

-flat bones

-irregular bones

What are long bones?

-longer than they are wide

-all limb bones (except the few short bones)

What are short bones?

-cube shaped bones in wrist and ankle

-sesamoid bones (within tendons, like patella): these bones reduce friction

What are flat bones?

-thin, flat, slightly curved

-found in sternum, scapulae, ribs, most skull bones

What are irregular bones?

-complicated shapes fit no other category

-found in vertebrae, hip bones, etc.

Functions of bones?

-support: for body and soft organs

-protection: for brain, spinal cord, and vital organs

-movement (anchorage of muscles): levers for muscle action

-storage: minerals (calcium and phosphorus) and growth factors (metabolism)

-blood cell formation (hematopoiesis): in red marrow cavities

-triglyceride (energy) storage: yellow marrow in long bone cavities

-Hormone production: osteocalcin (regulates insulin, secretion, glucose, homeostasis, and energy expenditure)

What are bone markings?

-bulges, depressions, and holes

-sites of attachment for muscles, ligaments, and tendons

-joint surfaces

-conduits for blood vessels and nerves

Bone marking projections that are sites of muscle and ligament attachment?

-tuberosity: rounded projection

-crest: narrow, prominent ridge

-trochanter: large, blunt, irregular surface

-line: narrow ridge of bone

-tubercle: small rounded projection

-epicondyle: raised area above a condyle

-spine: sharp, slender projection 

-process: any bony prominence

Bone marking projections that help to form joints

-head: bony expansion carried on a narrow neck

-facet: smooth, nearly flat articular surface

-condyle: rounded articular projection

-ramus: armlike bar

Bone marking depressions and openings: for blood vessels and nerves?

-groove: furrow

-fissure: narrow, slitlike opening

-foramen: round or oval opening through a bone

-notch: indentation at edge of structure

What are other bone marking depressions and openings?

-meatus: canal-like passageway

-sinus: cavity within a bone: filled with air, lined with mucosae

-fossa: shallow, basinlike depression

What are the different textures of bone?

-compact bone: dense outer layer, smooth and solid

-spongy (trabecular) bone: honeycomb of small needle-like pieces called trabeculae, open spaces between trabeculae filled with bone marrow

What are the structures of long bone?

-shaft (diaphysis)

-bone ends (epiphysis)

-membranes

What is diaphysis (shaft)?

-compact bone collar surrounds medullary (marrow) cavity, contains no bone tissue

-medullary cavity in adults contains fat (yellow marrow)

What is epiphyses?

-expanded ends (bone ends)

-spongy bone interior (compact bone exterior)

-epiphyseal line (remnant of growth plate)

-flared portion of bone where diaphysis and epiphysis meet is the metaphysis

-articular (hyaline) cartilage on joint surfaces (cushion/shock absorber)

What are the different membranes of bone?

-Periosteum: around the bone

-Endosteum

What is included in the periosteum membrane?

-outer fibrous layer: dense irregular connective tissue

-inner osteogenic layer: mostly osteogenic cells (stem cells): give rise to all bone cells except bone-destroying cells (osteoclasts) and bone forming cells (osteoblasts)

-nerve fibers, nutrient blood vessels, and lymphatic vessels enter the bone via nutrient foramina (bone marrow→ spongy bone→ compact bone)

-secured to underlying bone by perforating (Sharpey’s) fibers

What is included in the endosteum membrane?

-delicate membrane on internal surfaces of bone

-also contains osteogenic cells

What is the structure of short, irregular, and flat bones?

-periosteum-covered compact bone on the outside

-endosteum-covered spongy bone within

-spongy bone called diploe in flat bones

-bone marrow between the trabeculae

Where is red marrow, ie, hematopoietic tissue found?

-cavities in adults: trabecular cavities of the heads of the femur and humerus, trabecular cavities of the diploe of flat bones (more productive than in long bones)

-cavities in infants: medullary cavities and all spaces in spongy bone

What are the different cells of bone?

-osteogenic (osteoprogenitor) cells

-osteoblasts

-osteocytes

-osteoclasts

-bone lining cells

What are osteogenic (osteoprogenitor) cells?

stem cells in periosteum and enosteum that give rise to osteoblasts and bone lining cells

-flattened squamous, self-renewing

What are osteoblasts?

bone forming cells

secrete bone matrix

responsible for bone growth

-cube shaped when secreting, matrix is composed of collagen

What are osteocytes?

mature bone cells, maintain matrix

-spider-looking cells, “sensing” what goes on with bone and communicates with other cells on what the bone needs

What are osteoclasts?

cells that break down (resorb) bone matrix

-multinucleated

-has ruffled border

What are bone-lining cells?

found on bone surfaces not being remodeled— look like stem cells, act like osteocytes (might be involved with maintaining the matrix)

-squamous

What is included in microscopic anatomy of compact bone?

-osteon (aka Haversian system): structural unit

-perforating (Volkmann’s) canals

-lacunae

-canaliculi

What is included in Osteon (Haverian system)?

-lamellae: weight bearing (withstand torsional stress), column like matrix tubes (run parallel with bone)

-central (Haversian) canal: contains blood vessels and nerves

What are perforating (Volkmann’s) canals?

-at right angles to the central canal

-connects blood vessels and nerves of the periosteum and central canal

-not surrounded by lamellae, lined with endosteum

What are lacunae?

small cavities that contain osteocytes

What are canaliculi?

hairlike canals that connect lacunae to each other and the central canal

-contain osteocyte extensions with gap junctions

What is the interstitial lamellae?

-fills gaps between forming osteons

-remnants of osteons that have been cut through by bone remodeling

What is the cirumferential lamellae?

-deep to periosteum, superficial to endosteum

-resist twisting of the long bone

What is included in the microscopic anatomy of spongy bone?

trabeculae

• align along lines of stress

• no osteons

• contain irregularly arranged lamellae, osteocytes, and canaliculi

• capillaries in endosteum supply nutrients

What is the chemical composition of organic bone?

-osteogenic cells, osteoblasts, osteocytes, and osteoclasts

-osteoid (ground substance and collagen fibers)

What is an osteoid?

organic bone matrix secreted by osteoblasts (1/3 of matrix)

What is included in osteoid?

-ground substance: proteoglycans, glycoproteins

-collagen fibers: provide tensile strength and flexibility, sacrificial bonds break easily on impact to dissipate energy—prevent force rising to fracture level

What is the chemical composition of inorganic bone?

-resist compressive forces

-hydroxyapatite (mineral salts)

-65% of bone by mass

-mainly calcium phosphate crystals

-responsible for hardness and resistance to compression

What happens during bone development?

-referred to as osteogenesis (ossification): bone tissue formation

-stage 1= bone formation: begins in the 2nd month of development

-stage 2= postnatal bone growth: until early adulthood (~18 in females, ~21 in males)

-stage 3= bone remodeling and repair: lifelong

What are the two types of ossification?

intramembranous and endochrondral

Intramembranous ossificaiton

-intra= within, inside

-membrane bone develops from fibrous membrane

-forms most flat bones, e.g. cranial bones and clavicle

Endochondral ossification

-endo=within, inner

-cartilage (Endochondral) bone forms by replacing hyaline cartilage

-forms most of the rest of the skeleton

Step 1 of intramembranous ossification?

ossification center appear in fibrous connective tissue membrane

-centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming ossification center

Step 2 of intramembranous ossification?

bone matrix (Osteoid) secreted within fibrous membrane and calcifies

-osteoblasts begin to secrete osteoid—calcified in a few days

-trapped osteoblasts become osteocytes

Step 3 of intramembranous ossification?

woven bone and periosteum form

-osteoid laid down between blood vessels randomly (but usually form along lines of stress) → network of trabeculae called woven bone or immature spongy bone

-vascularized mesenchyme condenses on external surface of woven bone→ becomes periosteum

Step 4 of intramembranous ossification?

Lamellar bone replaces woven bone just deep to periosteum and red marrow appears

-trabeculae just deep to periosteum thicken—later replaced with mature lamellar bone→ compact bone plates

-spongy bone (diploe) made of distinct trabeculae persist internally, its vascular tissue becomes red marrow

What is different with endochondral ossification?

-uses hyaline cartilage models

-requires breakdown of hyaline cartilage prior to ossification

step 1: endochondral ossification?

bone collar forms around hyaline cartilage model

-osteoblasts of new periosteum secrete osteoid against hyaline cartilage

step 2: endochondral ossification?

cartilage in the center of the diaphysis calcifies and then develops cavities

-chondrocytes enlarge, signal calcification, then die when calcification cuts off nutrients, matrix deteriorates but bone callar stabilizes cavities/cartilage model

Step 3: endochondral ossification?

the periosteal bud invades the internal cavities and spongy bone begins to form

-bud= nutrient artery and vein, nerve, red marrow elements, osteogenic cells, osteoclasts

-osteoclasts partially erode calcified cartilage matrix, osteoblasts secrete osteoid to cover rest with bone

Step 4: endochondral ossification?

the diaphysis elongates and a medullary cavity forms as ossification continues; secondary ossification centers appear in the epiphyses in preparation for stage 5

• primary ossification center enlarges

• osteoclasts break down spongy bone and open up medullary cavity

• cartilaginous epiphyses: hyaline cartilage proliferates and elongates by division

• ossification chases cartilage formation (along length of shaft)

Step 5: endochondral ossification?

epiphyses ossify, when completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages

• secondary ossification centers appear in epiphyses near birth

• cartilage in center calcifies → cavities → periosteal bud → bone trabeculae (no medullary cavity in epiphyses)

• ossification centers: only primary in short bone, several in irregular bone

What are the two types of postnatal bone growth?

-interstitial growth: length of bones (bone growth length-wise stops during adolescence)

-appositional growth: thickness and remodeling of all bones by osteoblasts and osteoclasts on bone surfaces

How does the length of long bones grow?

The epiphyseal plate cartilage organizes into four important functional zones

-the epiphyseal plate stays relatively same thickness wise during the zones

1. resting zone: relatively inactive on side of epiphyseal plate

2. proliferation (growth): cartilage cells undergo mitosis, pushes epiphyses away from diaphyses and lengthens bone

3. Hypertrophic: older cartilage cells enlarge, lacunae erode and enlarge; leaves large interconnecting spaces

4. calcification: matrix becomes calcified, cartilage cells die, matrix begins deteriorating, blood vessels invade

5. Ossification (osteogenic): new bone is forming, medullary cavity is growing

How are hormones regulated during bone growth?

-growth hormones stimulate epiphyseal plate activity (from internal pituitary)

-thyroid hormone modulates activity of growth hormone

-testosterone and estrogens (at puberty)

• promote adolescent growth spurts (low level estrogen)

• end growth by inducing epiphyseal plate closure (high level estrogen)

Bone deposit

5-10% of skeleton is replaced annually, spongy bone replaced faster than compact (3-5 years compard to 10), important for non-brittle bones

-occurs where bone is injured or added strength is needed

-requires a diet rich in protein, vitamins C,D,A, calcium, phosphorus, magnesium, and manganese

How are sites of new matrix bone deposits revealed?

-osteoid seam: unmineralized band of matrix

-calcification front: abrupt transition zone between the osteoid seam and the older mineralized bone

What is bone resorption?

osteoclasts in resorption bays

• seal off area with ruffled border against bone

• secrete: lysosomal enzymes (digest organic matrix) and acids (convert calcium salts into soluble forms)

transcytosis moves dissolved matrix and minerals across osteoclast to opposite side, enters interstitial fluid then enters blood

What controls remodeling of bone?

-hormonal mechanisms that maintain calcium homeostasis in the blood (negative feedback loop)

-mechanical and gravitational forces

Hormonal Control of Blood Ca2+?

-calcium necessary for transmission of nerve impulses, muscle contraction, blood coagulation, secretion by glands and nerve cells, and cell division

-primarily controlled by parathyroid hormone (PTH)

-may be affected to a lesser extent by calcitonin

-leptin has also been shown to influence bone density by inhibiting osteoblasts

How does control by parathyroid hormone work?

blood ca2+ levels go down→ parathyroid glands release PTH→ PTH stimulates osteoclasts to degrade bone matrix and release Ca2+ into blood → blood ca2+ levels go up

How is hormonal control of blood Ca2+ affected by calcitonin?

blood ca2+ levels go up→ parafollicular cells of thyroid release calcitonin→ osteoblasts deposit calcium salts→ blood ca2+ levels go down

What are responses to mechanical stress?

-wolff’s law: a bone grows or remodels in response to forces or demands placed upon it

observations supporting the law:

• handedness (right or left handed) results in bone of one upper limb being thicker and stronger

• curved bones are thickest where they are most likely to buckle

• trabeculae form along lines of stress

• large, bony projections occur where heavy, active muscles attach

What are the different classifications of bone fractures?

position of bone ends after fracture

• nondisplaced= ends retain normal position

• displaced= ends out of normal alignment

completeness of the break

• complete= broken all the way through

• incomplete= not broken all the way through

whether or not the bone ends penetrate the skin

• compound (open)= bone ends penetrate the skin

• simple (closed)= bone ends do not penetrate the skin

Bone fracture: comminuted

-bone fragments into three or more pieces

-particularly common in the aged, whose bones are more brittle

Bone fracture: compression

-bone is crushed

-common in porous bones (osteoporotic bone) subjected to extreme trauma, as in a fall

Bone fracture: spiral

-ragged break occurs when excessive twisting forces are applied to a bone

-common sports fracture

Bone fracture: epiphyseal

-epiphysis separates from the diaphysis along the epiphyseal plate

-tends to occur where cartilage cells are dying and calcification of the matrix is occurring

Bone fracture: depressed

-broken bone portion is pressed inward

-typical of skull fracture

Bone fracture: greenstick

-bone breaks incompletely, much in the way a green twig breaks. only one side of the shaft breaks; the other side bends

-common in children, whose bones have relatively more organic matrix and are more flexible than those of adults

What are the stages of healing a bone fracture?

1. hematoma forms

2. fibrocartilaginous callus forms

3. bony callus formation

4. bone remodeling

Stage 1: hematoma forms

-torn blood vessels hemorrhage

-clot (hematoma) forms

-site becomes swollen, painful, and inflamed

Stage 2: fibrocartilaginous callus forms

-phagocytic cells clear debris

-osteoblasts begin forming spongy bone within 1 week

-fibroblasts secrete collagen fibers to connect bone ends

-mass of repair tissue now called fibrocartilaginous callus

Stage 3: bony callus formation

-new trabeculae form a bony (hard) callus

-bony callus formation continues until firm union is formed in ~2 months

Stage 4: bone remodeling

-in response to mechanical stressors over several months

-final structure resembles original

What are the types of homeostatic imbalances?

-osteomalacia and rickets

-osteoporosis

-paget’s disease

Osteomalacia and rickets

-rickets (childhood disease) causes bowed legs and other bone deformities

-calcium salts not deposited

-cause: vitamin D deficiency or insufficient dietary calcium

Osteoporosis

-loss of bone mass: bone resorption outpaces deposition

-Spongy bone of the spine and the neck of the femur become most susceptible to fracture

-risk factors: lacks of estrogen, calcium or vitamin D; petite body form; immobility; low levels of TSH; diabetes mellitus

-treatment and prevention: calcium, vitamin D, and fluoride supplements; increase weight bearing exercise throughout life

-hormone (estrogen) replacement therapy (HRT) slows bone loss (but increases cancer risk)

-some drugs (Fosamax, SERMs, stains) increase bone mineral density

Paget’s disease

-excessive and haphazard bone formation and breakdown, usually in spine, pelvis, femur, or skull

-pagetic bone has very high ratio of spongy to compact bone and reduced mineralization

-unknown cause (possibly viral)

-treatment includes calcitonin and biphosphonates

What are the developmental aspects of bones?

-embryonic skeleton ossifies predictably so fetal age easily determined from X rays or sonograms

-at birth, most long bones are well ossified (except epiphyses)

-nearly all bones completely ossified by age 25

-bone mass decreases with age beginning in 4th decade

-rate of loss determined by genetics and environmental factors

-in old age, bone resorption predominates: homeostatic imbalance