lecture Chap 6 skeletal tissue

-avascular and no nerves

-primarily water, accounts for its ability to spring back

-Dense connective tissue girdle of perichondrium contains blood bessels for nutrient delivery to cartilage

• 1. Hyaline Cartilage
• -provides support, flexibility, and resilience (collagen fibers)
• -most abundant type
• -articular cartilage, costal cartilage, respiratory cartilage, nasal cartilage (bones form from this when little)

• 2. Elastic Cartilage
• -similar to hyaline, but contain elastic fibers (bendy)
• -external ear and epiglottus

• 3. Fibrocartilage
• -collagen fibers: have great tensile strength
• -intervertebral discs, menisci, and pubic symphysis

• 1. Appositional
• -cells secrete matrix against external face of existing cartilage

• 2. Interstitial
• -chondrocytes divide and secrete new matric, expanding cartilage from within

• Calcification of cartilage occures during...
• -normal bone growth
• -old age

calcified cartilage is not calcified bone

206 named bones

• 2 Main groups by location
• 1. Axial
• -bones of skull, vertebrak column, and ribs
• -protect, support, and carry other body parts

• 2. Appendicular skeleton
• -upper and lower limbs
• -pectoral and pelvic girdles
• -locomation, manipulation of the enviornment

• Long bones
• -longer than they are wide
• -shaft and 2 ends
• -femur, humerus, phalanges

• Short Bones
• -cube shaped bones (carpals and tarsals)
• -Sesmoid bones: grow within tendons (patella)

• Flat Bones
• -thin, flat, slightly curved
• -sternum, ribs, scapulae, cranium bones

• Irregular Bones
• -complicated shapes
• -vertebrae, hip bones

• 1. Support
• -for the body and the soft organs

• 2. Protection
• -for the brain, spinal cord, and vital organs

• 3. Movement
• -levers for muscle action
• -design of joins determines the type of movement

• 4. Storage
• -minerals (calcium and phosphorus) and growth factors
• - released into the bloodstream when needed for distribution to all parts of the body (continual)

5. Blood cell formation (hematopoiesis) in marrow cavities

6. Triglyceride (energy/fat) storage in bone cavities

• Bulges, depressions, and holes serve as...
• -sites of attachment for muscles, ligaments, and tendons
• -joint surfaces
• -conduits for blood vessels and nerves

Tuberosity: rounded projection

Crest: narrow, prominent ridge

Trochanter: large, blunt, irregular surface

Line: Narrow ridge of bone

Tuberacle: small rounded projection

Epicondyle: raised area above the condyle

Spine: Sharp, slender projection

Process: any bony prominence

Head: bony expansion carroed on a narrow neck

Facet: smooth, nearly flat articular surface

Condyle: rounded articular projection

Ramus: armlike bar

Meatus: canal-like passageway

Sinus: cavity within the bone

Fossa: Shallow, basin like depression

Groove: furrow

Fissure: narrow, slitlike opening

Foramen: round or oval opening through a bone

1. Compact bone: dense outter layer

• 2. Spongey (cancellous) bone
• -honeycomb of trabeculae
• -filled with red or yellow bone marrow

• Diaphysis: shaft
• -compact bone collar surrounds medullary (marrow cavity)
• -medullary cavity in adults contains fat (yellow marrow)

• Epiphysis: expanded ends
• -compact bone exterior and spongey bone interior
• -epiphyseal line (remnant of growth plate) between the diaphysis and each epiphysis
• -articular (hyaline) cartilahe on joint surfaces has the function of cushioning

-covers entire external bone except where joint is (glistening white)

• 2 Layers:
• 1. outer fibrous layer (dense irregular connective tissue) of bone
• 2. Inner osteogenic layer
• *osteoblasts: bone forming cells
• *osteoclasts: bone destroying cells
• *osteogenic cells: stem cells that give rise to osteoblasts

-nerve fibers, nutrient blood vessels, and lymphatic vessels enter the bone via nutrient foramina

-secured to underlying bone by Sharpey's fibers (collagen fibers which are exceptionally dense at point of anchoring for ligaments and tendons)

• Delicate membrane on internal surfaces of bone
• -covers the trabeculae of spongey bone and lines the canals that pass through the compact bone

Also cntain osteoblasts and osteoclasts

No shaft or epiphysis

Periosteum-covered compact bone on outside

• Endosteum-covered spongey bone within
• -called siploe in flat bones
• -like a stiffened sandwhich

-bone marrow between trabecular

• Red marrow cavities in adults
• -trabecular caivites of the heads of femur and humerus
• -trabecular cavities of the diploe of flat bones
• *tend to be more cative in red marrow production, so samples are often drawn from here

• Red Marrow cavities in newborn infants
• -medullary cavities and all spaces of spongey bone

Osteogenic (osteoprogenitor) cells: stem cells in periosteum and endosteum that give rise to osteoblasts

Osteoblasts: bone forming cells

Osteoclasts: break down or resorb matrix

• Osteocytes: mature bone cells
• -help to maintain bone matrix and act as stress sensors

Structural Unit: Haversian System or Osteon

• Lamella:
• -weight bearin
• -column like matrix tubes like rings on a tree
• -collagen fibers in lamellae always go in the same direction, but the next one (ring) goes in a different direction
• *this reinforces to prevent twisting

• Central/haversian Canal:
• -through the lamllae
• -contains blood vessels and nerves

• Perforating (Volkmann's) Canals
• -Covered with endosteum
• -at right angles to the central canal
• -connects blood vessels and nerves of the periosteum and central canal

• Lacunae:
• -small cavities that contain oesteocytes (spider shaped)

• Canaliculi:
• -hairlike canals thatr connect lacunae to each other and the central canal
• -allows bone to be nurished through connection of osteocytes

• Trabeculae:
• -align along lines of stress (Stress resistors)
• -no osteons
• -contain irrefularly arranged lamellae, osteocytes, and canaliculi
• -cappillaries in endosteum supply nutrients (diffuses thru canaliculi)

• -Osteogenic cells, osteoblasts, osteocytes, osteoclasts
• -Osteoid: organic bone matrix secreted by osteoblasts (1/3 of matrix)
• *ground substance (proteoglycans, glycoprotiens)
• *collagen fibers: provide tensile strength and flexibility

• Hydroxyapatites (mineral salts)
• -65% of bone by mass
• -mainly calcium phosphate crystals
• -responsible for hardness and resistance to compression
• -why bones remain hard after death

Osteogenesis (ossification)-bone tissue formation

• Stages:
• -bone formation: begins 2nd month of development
• -postnatal bone growth: until early childhood
• -bone remodeling and repair: lifelong

• 1. Intramembranous ossification (from mesenchyme)
• -membrane bone develops from fibrous membrane
• -forms flat bones, e.g. clavicles and cranial bones

• 2. Endochondral ossification
• -cartilage (endochondral) bone forms by replacing hyaline cartilage
• -forms most of the rest of the skeleton

• 1. Ossification center appears in the fibrous connective tissue membrane
• -selected centrally located to mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center

• 2. Bone matrix (osteoid) is secreted within the fibrous membrane and calcifies
• -osteoblasts begin to secrete osteoid, which is calcified within a few days
• -trapped ostepblasts become osteocytes

• 3. Woven bone and periosteum form
• -accumulating osteoid is laid down between embryonic blood vessels ina random manner. This creates a network of woven trabeculae

• 4. Lamellar Bone replaces woven bone, just deep to the periosteum. Red marrow appears
• -trabeculae jus deep to the periosteum thicken, and are later replaced with mature lamellar bone, forming compact bone plates
• -spongey bone (diploe) consisting of distinct trabeculae persists internall and its vascular tissue becomes red marrow

1.Bone Collar forms around hyaline cartilage model (periosteal bone collar) (week 9 of development)

• 2. Cartilage in the center of disaphysis calcified and then dvelops cavities
• -chondrocytes within the shaft enlarge (hypertrophy) and signal surrounding cartilage to calcify
• *these condrocytes die due to lack of nutrients so cavitiy forms
• -elsewhere, cartilage remains healthy and grows for the catrilage model to enlongate

• 3. The periosteal bud invades the internal cavities and spongey bone begins to form (month 3 of development)
• -nutrient artery and vein, lymphatic vessels, nerve fibers, red marrow elements, osteoblasts, osteoclasts
• -osteoclasts partially erode calcified cartilage and steoblasts secrete osteroid, beginning the creation of spongey bone

• 4. The diaphysis elongates and a medullary cavitiy form as ossification continues. Secondary ossification center appears in epiphysis in prep for stage 5 (at birth)
• -osteoclasts create the medullary cavoty and "chase" cartilahe formation along the shaft

• 5. The epiphysises ossify. When completed, hyaline cartilage reamins only in the epiphyseal plates and articular cartilage
• -stages 1-4 same at secondary epiphysis site until ossification is complete

• Interstitial growth: length of long bones
• -growth ends when bone of epiphysis and diaphysis fuse, around 18 for f and 23 for m

Appositional growth: thickness and remodeling of all bones by osteoblasts and osteoclasts on bone surfaces throughout life

• Epiphyseal plate cartilage organizes into four important functional zones:
• 1. Proliferation (Growth)
• -cartilage abutting the plate on the diaphysis side is in the growth zone

• 2. Hypertropic:
• -older chondrocytes enlarge, leaving spaces

• 3. Calcification:
• -surrounding cartilage die and calcify

• 4. Ossification (osteogenic)
• -calcification leaves long spondle like spincules. Will be invaded with marrow elements from medullary cavity

• Growth hormone stumulates epiphyseal plate activity
• -released by anterior pituitary gland

Thyroid hormone modulates activity of growth hormone

• Testosterone and estrogens (at puberty)
• -promote adolescent growth spurts as well as feminization/masculization
• -end growth by epiphyseal plate closure

Spongey bone replaced every 3-5 years

Compact bne replaced every 10 years

• bone remodeling: bone deposit and bone resorbtion
• *remodeling does not occur uniformly, some faster than others

-Occues where bone is injured or added strength is needed

-requires a diet rich in protien, vits A, C, and D, calcium, phosphorus, magnesium, and manganese

• Sites of new matrix deposit are revealed by...
• *Osteoid Seam
• - Unmineralized band of the matrix
• *calcification front
• - the abrupt transition zone between the osteoid seam and the older mineralized bone

• Osteoclasts secrete...
• -Lysosomal enzymes: digest organic matrix
• -Acids: convert calcium salts into soluable forms (hydrocloric acid)
• -creates grooves in bone

Dissolved matrix is transcytossed across osteoclast, enters interstitual fluid and then blood

• Hormonal mechanisms that maintain calcium homeostasis in the blood
• -negative feedback system
• -determines whether and when

• Mechanical and gravitational forces
• -determines where

• Calcium is necessary for...
• -transmission of nerve impulses
• -muscle contraction
• -blood coagulation
• -secretion by glands and nerve cells
• -cell division

*99% of bodies calcium is found in bones

Primarily control by parathyroid hormone (PTH)

Levels decline-->parathyroid gland releases PTH-->PTH stimulates osteoclasts to degrade bone matrix and release Calcium--> blood Ca²⁺ levels raise

-May be affected to a lesser extent by calcitonin

Levels up-->parafollicular cells of thyroid release calcitonin-->osteoblasts deposit calcium salts--> blood Ca²⁺ levels decrease

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

Wolff's Law: a bone grows or remodels in response to forces or demands placed upon it

• Observations supporting wolff's law:
• -handedness (right or left) 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, boney projections occur where heavy, active muscles attach

May be classfified by 4 "either/or" categories

• 1. Position of bone ends after fracture
• A. nondisplaced: ends retain normal position
• B. Displaced: ends out of normal allignemnt

• 2. Completeness of Break
• A. Complete: broken all the way through
• B. Incomplete: not broken all the way through

• 3. Orientation of the break long the long axis of bone
• A. Linear: Parallell to long axis of bone
• B. Transverse: perpendicular to lone axis of the bone

• 4. Whether or not ends penetrate skin
• A. Compound (open)- bone ends pentrate
• B. Simple (closed)- bone ends do not penetrate

All fractures can be described in terms of the location, external appearance, and nature of the break

• 1. Hematoma forms
• -torn blood vessels hemorage
• -clot forms
• -site becomes swollen, painful, and inflamed (bone cells starve and die)

• 2. Fibrocartilaginous callus forms
• -phagocytic cells clear debris
• -osteoblasts behin forming spngey bone within 1 week
• -fibroblasts secrete collagen fibers to connect bone ends
• -mass of repair tissue now calls fibrocartilaginous callus (splints the broken bone)

• 3. Bony Callus formation
• -new trabeculae form a bony (hard) callus
• -bony callus continues until firm union is formed (about 2 mo)

• 4. Bone remodeling
• -in response to mechanical stressors over several months
• -final structure resembles original

Osetomalacia is adult form of rickets

Almost non-existent in US except in certain situations

• -Calcium salts not deposited
• -Rickets (childhood disease) causes bowed legs and other born deformities

Cause: Vit D deficience or dietary calcium deficience

Symptoms: pain when weight on affected area

Treatment: Vit D, Calcium, and Sun

Loss of bone mass: bone resoption outpaces deposit

Bones become porous and light

Spongey bone of spine and neck femur become most susceptible to fracture

Risk factors: lack of estrogen, calcium or vit D, petite body form, immobility, low levels of TSH, diabetes mellitis

• Prevent:
• -calcium, vit d, and flouride suppliments
• -increase weight-bearing exercise throughout life

• Treat:
• -hormone (Estrogen) replacement therapy (HRT) slows bone loss, but is controversial due to side effects
• -some drugs increase bone mineral density

Excessive and haphazard bone formtion and breakdown, usually in spine, pelvis, femur, or skull

Pegetic bone has very high ratio of spongey to compact bone and reduced mineralization

Unknown cause (possibly viral)

Treatment includes calcitonin and biosphonates

Embyronic skeleton ossifies predictably so fetal age is easily determined from x-rays or sonograms

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

In children and adolescents, bone formation exceeds bone resorption

• nearly all bones are completely ossified by age 25
• Bone mass decreases with age beginning at 40 (except in skull)

Rate of loss determined by genetics and enviornmental factos

In old age, bone resoption predominates leading to fractures (that heal more slowly)