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Lecture 6: Osseous Tissue and Bone Structure. Topics:. Skeletal cartilage Structure and function of bone tissues Types of bone cells Structures of the two main bone tissues Bone membranes Bone formation Minerals, recycling, and remodeling Hormones and nutrition Fracture repair

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topics
Topics:
  • Skeletal cartilage
  • Structure and function of bone tissues
  • Types of bone cells
  • Structures of the two main bone tissues
  • Bone membranes
  • Bone formation
  • Minerals, recycling, and remodeling
  • Hormones and nutrition
  • Fracture repair
  • The effects of aging
the skeletal system
The Skeletal System
  • Skeletal system includes:
    • bones of the skeleton
    • cartilages, ligaments, and connective tissues
skeletal cartilage
Skeletal Cartilage
  • Contains no blood vessels or nerves
  • Surrounded by the perichondrium (dense irregular connective tissue) that resists outward expansion
  • Three types – hyaline, elastic, and fibrocartilage
hyaline cartilage
Hyaline Cartilage
  • Provides support, flexibility, and resilience
  • Is the most abundant skeletal cartilage
  • Is present in these cartilages:
    • Articular – covers the ends of long bones
    • Costal – connects the ribs to the sternum
    • Respiratory – makes up larynx, reinforces air passages
    • Nasal – supports the nose
elastic cartilage
Elastic Cartilage
  • Similar to hyaline cartilage, but contains elastic fibers
  • Found in the external ear and the epiglottis
fibrocartilage
Fibrocartilage
  • Highly compressed with great tensile strength
  • Contains collagen fibers
  • Found in menisci of the knee and in intervertebral discs
growth of cartilage
Growth of Cartilage
  • Appositional – cells in the perichondrium secrete matrix against the external face of existing cartilage
  • Interstitial – lacunae-bound chondrocytes inside the cartilage divide and secrete new matrix, expanding the cartilage from within
  • Calcification of cartilage occurs
    • During normal bone growth
    • During old age
functions of the skeletal system
Functions of the Skeletal System
  • Support
  • Storage of minerals (calcium)
  • Storage of lipids (yellow marrow)
  • Blood cell production (red marrow)
  • Protection
  • Leverage (force of motion)
bone osseous tissue
Bone (Osseous) Tissue
  • Supportive connective tissue
  • Very dense
  • Contains specialized cells
  • Produces solid matrix of calcium salt deposits and collagen fibers
characteristics of bone tissue
Characteristics of Bone Tissue
  • Dense matrix, containing:
    • deposits of calcium salts
    • osteocytes within lacunae organized around blood vessels
  • Canaliculi:
    • form pathways for blood vessels
    • exchange nutrients and wastes
characteristics of bone tissue14
Characteristics of Bone Tissue
  • Periosteum:
    • covers outer surfaces of bones
    • consist of outer fibrous and inner cellular layers
    • Contains osteblasts responsible for bone growth in thickness
  • Endosteum
    • Covers inner surfaces of bones
bone matrix
Bone Matrix
  • Solid ground is made of mineral crystals
  • 2/3 of bone matrix is calcium phosphate, Ca3(PO4)2:
    • reacts with calcium hydroxide, Ca(OH)2 to form crystals of hydroxyapatite, Ca10(PO4)6(OH)2 which incorporates other calcium salts and ions
bone matrix16
Bone Matrix
  • Matrix Proteins:
    • 1/3 of bone matrix is protein fibers (collagen)
  • Question: why aren’t bones made of ALL collagen if it’s so strong?
bone matrix17
Bone Matrix
  • Mineral salts make bone rigid and compression resistant but would be prone to shattering
  • Collagen fibers add extra tensile strength but mostly add tortional flexibilitytoresist shattering
chemical composition of bone organic
Chemical Composition of Bone: Organic
  • Cells:
    • Osteoblasts – bone-forming cells
    • Osteocytes – mature bone cells
    • Osteoprogenitor cells – grandfather cells
    • Osteoclasts – large cells that resorb or break down bone matrix
  • Osteoid – unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen; becomes calcified later
there are four major types of cells
There are four major types of cells

periosteum + endo

endosteum only

in matrix only

1 osteoblasts
1. Osteoblasts
  • Immature bone cells that secrete matrix compounds (osteogenesis)
  • Eventually become surrounded by calcified bone and then they become osteocytes

Figure 6–3 (2 of 4)

2 osteocytes
2.Osteocytes
  • Mature bone cells that maintain the bone matrix

Figure 6–3 (1 of 4)

osteocytes
Osteocytes
  • Live in lacunae
  • Found between layers (lamellae) of matrix
  • Connected by cytoplasmic extensions through canaliculi in lamellae (gap junctions)
  • Do not divide (remember G0?)
  • Maintain protein and mineral content of matrix
  • Help repair damaged bone
3 osteoprogenitor cells
3. Osteoprogenitor Cells
  • Mesenchyme stem cells that divide to produce osteoblasts
  • Are located in inner, cellular layer of periosteum
  • Assist in fracture repair
4 osteoclasts
4. Osteoclasts
  • Secrete acids and protein-digesting enzymes

Figure 6–3 (4 of 4)

osteoclasts
Osteoclasts
  • Giant, mutlinucleate cells
  • Dissolve bone matrix and release stored minerals (osteolysis)
  • Often found lining in endosteum lining the marrow cavity
  • Are derived from stem cells that produce macrophages
homeostasis
Homeostasis
  • Bone building (by osteocytes and -blasts) and bone recycling (by osteoclasts) must balance:
    • more breakdown than building, bones become weak
    • exercise causes osteocytes to build bone
bone cell lineage summary
Osteoprogenitor cells

osteoblasts

osteocytes

Osteoclasts are related to macrophages (blood cell derived)

Bone cell lineage summary
gross anatomy of bones bone textures
Gross Anatomy of Bones: Bone Textures
  • Compact bone – dense outer layer
  • Spongy bone – honeycomb of trabeculae filled with yellow bone marrow
compact bone
Compact Bone

Figure 6–5

osteon
Osteon
  • The basic structural unit of mature compact bone
  • Osteon = Osteocytes arranged in concentric lamellae around a central canal containing blood vessels
    • Lamella – weight-bearing, column-like matrix tubes composed mainly of collagen
three lamellae types
Three Lamellae Types
  • Concentric Lamellae
  • Circumferential Lamellae
    • Lamellae wrapped around the long bone line tree rings
    • Binds inner osteons together
  • Interstitial Lamellae
    • Found between the osteons made up of concentric lamella
    • They are remnants of old osteons that have been partially digested and remodeled by osteoclast/osteoblast activity
compact bone32
Compact Bone

Figure 6–5

spongy bone
Spongy Bone

Figure 6–6

spongy bone tissue
Spongy Bone Tissue
  • Makes up most of the bone tissue in short, flat, and irregularly shaped bones, and the head (epiphysis) of long bones; also found in the narrow rim around the marrow cavity of the diaphysis of long bone
spongy bone39
Spongy Bone
  • Does not have osteons
  • The matrix forms an open network of trabeculae
  • Trabeculae have no blood vessels
bone marrow
Bone Marrow
  • The space between trabeculae is filled with marrow which is highly vascular
    • Red bone marrow
      • supplies nutrients to osteocytes in trabeculae
      • forms red and white blood cells
    • Yellow bone marrow
      • yellow because it stores fat
  • Question: Newborns have only red marrow. Red changes into yellow marrow in some bones as we age. Why?
location of hematopoietic tissue red marrow
Location of Hematopoietic Tissue (Red Marrow)
  • In infants
    • Found in the medullary cavity and all areas of spongy bone
  • In adults
    • Found in the diploë of flat bones, and the head of the femur and humerus
bone membranes
Bone Membranes
  • Periosteum – double-layered protective membrane
    • Covers all bones, except parts enclosed in joint capsules (continuois w/ synovium)
    • Made up of:
      • outer, fibrous layer (tissue?)
      • inner, cellular layer (osteogenic layer) is composed of osteoblasts and osteoclasts
    • Secured to underlying bone by Sharpey’s fibers
  • Endosteum – delicate membrane covering internal surfaces of bone
sharpy s perforating fibers
Sharpy’s (Perforating) Fibers
  • Collagen fibers of the outer fibrous layer of periosteum, connect with collagen fibers in bone
  • Also connect with fibers of joint capsules, attached tendons, and ligaments
  • Tendons are “sewn” into bone via periosteum
periosteum
Periosteum

Figure 6–8a

functions of periosteum
Functions of Periosteum
  • Isolate bone from surrounding tissues
  • Provide a route for circulatory and nervous supply
  • Participate in bone growth and repair
endosteum
Endosteum

Figure 6–8b

endosteum47
Endosteum
  • An incomplete cellular layer:
    • lines the marrow cavity
    • covers trabeculae of spongy bone
    • lines central canals
  • Contains osteoblasts, osteoprogenitor cells, and osteoclasts
  • Is active in bone growth and repair
bone development
Bone Development
  • Human bones grow until about age 25
  • Osteogenesis:
    • bone formation
  • Ossification:
    • the process of replacing other tissues with bone
  • Osteogenesis and ossification lead to:
    • The formation of the bony skeleton in embryos
    • Bone growth until early adulthood
    • Bone thickness, remodeling, and repair through life
calcification
Calcification
  • The process of depositing calcium salts
  • Occurs during bone ossification and in other tissues
formation of the bony skeleton
Formation of the Bony Skeleton
  • Begins at week 8 of embryo development
  • Ossification
    • Intramembranous ossification – bone develops from a fibrous membrane
    • Endochondral ossification – bone forms by replacing hyaline cartilage
intramembranous ossification note you don t have to know the steps of this process in detail
Intramembranous OssificationNote: you don’t have to know the steps of this process in detail
  • Also called dermal ossification (because it occurs in the dermis)
    • produces dermal bones such as mandible and clavicle
  • Formation of most of the flat bones of the skull and the clavicles
  • Fibrous connective tissue membranes are formed by mesenchymal cells
the birth of bone
The Birth of Bone
  • When new bone is born, either during development or regeneration, it often starts out as spongy bone (even if it will later be remodeled into compact bone)
endochondral ossification note you do have to know this one
Endochondral OssificationNote: you DO have to know this one
  • Begins in the second month of development
  • Uses hyaline cartilage “bones” as models for bone construction then ossifies cartilage into bone
  • Common, as most bones originate as hyaline cartilage
  • This is like a “trick” the body uses to allow long bones to grow in length when bones can only grow by appositional growth
bone formation in a chick embryo
Bone formation in a chick embryo
  • Stained to represent hardened bone (red) and cartilage (blue)
  • : This image is the cover illustration from The Atlas of Chick Development by Ruth Bellairs and Mark Osmond, published by Academic Press (New York) in 1998
stages of endochondral ossification
Stages of Endochondral Ossification
  • Bone models form out of hyaline cartilage
  • Formation of bone collar
  • Cavitation of the hyaline cartilage
  • Invasion of internal cavities by the periosteal bud, and spongy bone formation
  • Formation of the medullary cavity; appearance of secondary ossification centers in the epiphyses
  • Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates
stages of endochondral ossification57

Secondary

ossificaton

center

Articular

cartilage

Spongy

bone

Epiphyseal

blood vessel

Deteriorating

cartilage

matrix

Hyaline

cartilage

Epiphyseal

plate

cartilage

Spongy

bone

formation

Primary

ossification

center

Medullary

cavity

Bone collar

Blood

vessel of

periosteal

bud

1

Formation of

bone collar

around hyaline

cartilage model.

2

Cavitation of

the hyaline carti-

lage within the

cartilage model.

3

Invasion of

internal cavities

by the periosteal

bud and spongy

bone formation.

4

Formation of the

medullary cavity as

ossification continues;

appearance of sec-

ondary ossification

centers in the epiphy-

ses in preparation

for stage 5.

5

Ossification of the

epiphyses; when

completed, hyaline

cartilage remains only

in the epiphyseal plates

and articular cartilages.

Stages of Endochondral Ossification

Figure 6.8

endochondral ossification step 1 bone collar
Blood vessels grow around the edges of the cartilage

Cells in the perichondrium change to osteoblasts:

producing a layer of superficial bone (bone collar) around the shaft which will continue to grow and become compact bone (appositional growth)

Endochondral Ossification: Step 1 (Bone Collar)

Figure 6–9 (Step 2)

endochondral ossification step 2 cavitation
Endochondral Ossification: Step 2 (Cavitation)
  • Chondrocytes in the center of the hyaline cartilage of each bone model:
    • enlarge
    • form struts and calcify
    • die, leaving cavities in cartilage

Figure 6–9 (Step 1)

endochondral ossification step 3 invasion
Endochondral Ossification: Step 3 (Invasion)
  • Periosteal bud brings blood vessels into the cartilage:
    • bringing osteoblasts and osteoclasts
    • spongy bone develops at the primary ossification center

Figure 6–9 (Step 3)

endochondral ossification step 4a remodelling
Endochondral Ossification: Step 4a (Remodelling)
  • Remodeling creates a marrow (medullary) cavity:
    • bone replaces cartilage at the metaphyses
    • Diaphysis elongates

Figure 6–9 (Step 4)

endochondral ossification step 4b 2 ossification
Endochondral Ossification: Step 4b (2° Ossification)
  • Capillaries and osteoblasts enter the epiphyses:
    • creating secondary ossification centers (perinatal)

Figure 6–9 (Step 5)

endochondral ossification step 5 elongation
Epiphyses fill with spongy bone but cartilage remains at two sites:

ends of bones within the joint cavity = articular cartilage

cartilage at the metaphysis = epiphyseal cartilage (plate)

Endochondral Ossification: Step 5 (Elongation)

Figure 6–9 (Step 6)

postnatal bone growth
Postnatal Bone Growth
  • Growth in length of long bones
    • Cartilage on the side of the epiphyseal plate closest to the epiphysis is relatively inactive
    • Cartilage abutting the shaft of the bone organizes into a pattern that allows fast, efficient growth
    • Cells of the epiphyseal plate proximal to the resting cartilage form three functionally different zones: growth, transformation, and osteogenic
functional zones in long bone growth
Functional Zones in Long Bone Growth
  • Growth zone – cartilage cells undergo mitosis, pushing the epiphysis away from the diaphysis
  • Transformation zone – older cells enlarge, the matrix becomes calcified, cartilage cells die, and the matrix begins to deteriorate
  • Osteogenic zone – new bone formation occurs
postnatal bone growth67
Postnatal bone growth
  • Remember that bone growth can only occur from the outside (appositional growth). So this type of endochondral growth is a way for bones to grow from the inside and lengthen because it is the cartilage that is growing, not the bone
key concept
Key Concept
  • As epiphyseal cartilage grows through the division of chondrocytes it pushes the ends of the bone outward in length.
  • At the “inner” (shaft) side of the epiphyseal plate, recently born cartilage gets turned into bone, but as long as the cartilage divides and extends as fast or faster than it gets turned into bone, the bone will grow longer
long bone growth and remodeling
Long Bone Growth and Remodeling
  • Growth in length – cartilage continually grows and is replaced by bone as shown
  • Remodeling – bone is resorbed and added by appositional growth as shown
    • compact bone thickens and strengthens long bones with layers of circumferential lamellae
epiphyseal lines
Epiphyseal Lines
  • When long bone stops growing, between the ages of 18 – 25:
    • epiphyseal cartilage disappears
    • epiphyseal plate closes
    • visible on X-rays as an epiphyseal line
  • At this point, bone has replaced all the cartilage and the bone can no longer grow in length
epiphyseal lines73
Epiphyseal Lines

Figure 6–10

hormonal regulation of bone growth during youth
Hormonal Regulation of Bone Growth During Youth
  • During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone
  • During puberty, testosterone and estrogens:
    • Initially promote adolescent growth spurts
    • Cause masculinization and feminization of specific parts of the skeleton
    • Later induce epiphyseal plate closure, ending long bone growth
remodeling
Remodeling
  • Remodeling continually recycles and renews bone matrix
  • Turnover rate varies within and between bones
  • If deposition is greater than removal, bones get stronger
  • If removal is faster than replacement, bones get weaker
  • Remodeling units – adjacent osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces
bone deposition
Bone Deposition
  • Occurs where bone is injured or added strength is needed
  • Requires a diet rich in protein, vitamins C, D, and A, calcium, phosphorus, magnesium, and manganese
  • Alkaline phosphatase is essential for mineralization of bone
  • Sites of new matrix deposition are revealed by the:
    • Osteoid seam – unmineralized band of bone matrix
    • Calcification front – abrupt transition zone between the osteoid seam and the older mineralized bone
effects of exercise on bone
Effects of Exercise on Bone
  • Mineral recycling allows bones to adapt to stress
  • Heavily stressed bones become thicker and stronger
response to mechanical stress
Response to Mechanical Stress
  • Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon it
  • Observations supporting Wolff’s law include
    • Long bones are thickest midway along the shaft (where bending stress is greatest)
    • 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
bone resorption
Bone Resorption
  • Accomplished by osteoclasts
  • Resorption bays – grooves formed by osteoclasts as they break down bone matrix
  • Resorption involves osteoclast secretion of:
    • Lysosomal enzymes that digest organic matrix
    • Acids that convert calcium salts into soluble forms
  • Dissolved matrix is transcytosed across the osteoclast cell where it is secreted into the interstitial fluid and then into the blood
bone degeneration
Bone Degeneration
  • Bone degenerates quickly
  • Up to 1/3 of bone mass can be lost in a few weeks of inactivity
minerals vitamins and nutrients
Minerals, vitamins, and nutrients

Rewired for bone growth

  • A dietary source of calcium and phosphatesalts:
    • plus small amounts of magnesium, fluoride, iron, and manganese
  • Protein, vitamins C, D, and A
calcitriol
Calcitriol
  • The hormone calcitriol:
    • synthesis requires vitamin D3 (cholecalciferol)
    • made in the kidneys (with help from the liver)
    • helps absorb calcium and phosphorus from digestive tract
the skeleton as calcium reserve
The Skeleton as Calcium Reserve
  • Bones store calcium and other minerals
  • Calcium is the most abundant mineral in the body
  • Calcium ions in body fluids must be closely regulated because:
  • Calcium ions are vital to:
    • membranes
    • neurons
    • muscle cells, especially heart cells
    • blood clotting
calcium regulation hormonal control
Calcium Regulation: Hormonal Control
  • Homeostasis is maintained by calcitonin and parathyroid hormone which control storage, absorption, and excretion
  • Rising blood Ca2+ levels trigger the thyroid to release calcitonin
  • Calcitonin stimulates calcium salt deposit in bone
  • Falling blood Ca2+ levels signal the parathyroid glands to release PTH
  • PTH signals osteoclasts to degrade bone matrix and release Ca2+ into the blood
hormonal control of blood ca
Hormonal Control of Blood Ca

PTH;

calcitonin

secreted

Calcitonin

stimulates

calcium salt

deposit

in bone

Thyroid

gland

Imbalance

Rising blood

Ca2+ levels

Calcium homeostasis of blood: 9–11 mg/100 ml

Falling blood

Ca2+ levels

Imbalance

Thyroid

gland

Osteoclasts

degrade bone

matrix and release

Ca2+ into blood

Parathyroid

glands

Parathyroid

glands release

parathyroid

hormone (PTH)

PTH

Figure 6.11

calcitonin and parathyroid hormone control
Calcitonin and Parathyroid Hormone Control
  • Bones:
    • where calcium is stored
  • Digestive tract:
    • where calcium is absorbed
  • Kidneys:
    • where calcium is excreted
parathyroid hormone pth
Produced by parathyroid glands in neck

Increases calcium ion levels by:

stimulating osteoclasts

increasing intestinal absorption of calcium

decreases calcium excretion at kidneys

Parathyroid Hormone (PTH)
calcitonin
Secreted by cells in the thyroid gland

Decreases calcium ion levels by:

inhibiting osteoclast activity

increasing calcium excretion at kidneys

Actually plays very small role in adults

Calcitonin
fractures
Fractures
  • Fractures:
    • cracks or breaks in bones
    • caused by physical stress
  • Fractures are repaired in 4 steps
fracture repair step 1 hematoma
Fracture Repair Step 1: Hematoma
  • Hematoma formation
    • Torn blood vessels hemorrhage
    • A mass of clotted blood (hematoma) forms at the fracture site
    • Site becomes swollen, painful, and inflamed
  • Bone cells in the area die

Figure 6.13.1

fracture repair step 2 soft callus
Fracture Repair Step 2: Soft Callus
  • Cells of the endosteum and periosteum divide and migrate into fracture zone
  • Granulation tissue (soft callus) forms a few days after the fracture from fibroblasts and endothelium
  • Fibrocartilaginous callus forms to stabilize fracture
    • external callus of hyaline cartilage surrounds break
    • internal callus of cartilage and collagen develops in marrow cavity
  • Capillaries grow into the tissue and phagocytic cells begin cleaning debris

Figure 6.13.2

stages in the healing of a bone fracture
Stages in the Healing of a Bone Fracture
  • The fibrocartilaginous callus forms when:
    • Osteoblasts and fibroblasts migrate to the fracture and begin reconstructing the bone
    • Fibroblasts secrete collagen fibers that connect broken bone ends
    • Osteoblasts begin forming spongy bone
    • Osteoblasts furthest from capillaries secrete an externally bulging cartilaginous matrix that later calcifies
fracture repair step 3 bony callus
Fracture Repair Step 3: Bony Callus
  • Bony callus formation
    • New spongy bone trabeculae appear in the fibrocartilaginous callus
    • Fibrocartilaginous callus converts into a bony (hard) callus
    • Bone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later

Figure 6.13.3

fracture repair step 4 remodeling
Fracture Repair Step 4: Remodeling
  • Bone remodeling
    • Excess material on the bone shaft exterior and in the medullary canal is removed
    • Compact bone is laid down to reconstruct shaft walls
    • Remodeling for up to a year
      • reduces bone callus
      • may never go away completely
    • Usually heals stronger than surrounding bone

Figure 6.13.4

clinical advances in bone repair
Clinical advances in bone repair
  • Electrical stimulation of fracture site.
    • results in increased rapidity and completeness of bone healing
    • electrical field may prevent  parathyroid hormone from activating osteoclasts at the  fracture site thereby increasing formation of bone and minimizing breakdown of bone,
  • Ultrasound. 
    • Daily treatment results in decreased healing time of  fracture by about 25% to 35% in broken arms and shinbones. Stimulates cartilage cells to make bony callus.
  • Free vascular fibular graft technique.
    • Uses pieces of fibula to replace bone or splint two broken ends of a bone.  Fibula is a non-essential bone, meaning it does not play a role in bearing weight; however, it does help stabilize the ankle.
  • Bone substitutes.
    • synthetic material or crushed bones from cadavers serve as bone fillers (Can also use sea coral).
aging and bones
Aging and Bones
  • Bones become thinner and weaker with age
  • Osteopenia begins between ages 30 and 40
  • Women lose 8% of bone mass per decade, men 3%
osteoporosis
Osteoporosis
  • Severe bone loss which affects normal function
  • Group of diseases in which bone reabsorption outpaces bone deposit
  • The epiphyses, vertebrae, and jaws are most affected, resulting in fragile limbs, reduction in height, tooth loss
  • Occurs most often in postmenopausal women
  • Bones become so fragile that sneezing or stepping off a curb can cause fractures
  • Over age 45, occurs in:
    • 29% of women
    • 18% of men
osteoporosis treatment
Osteoporosis: Treatment
  • Calcium and vitamin D supplements
  • Increased weight-bearing exercise
  • Hormone (estrogen) replacement therapy (HRT) slows bone loss
  • Natural progesterone cream prompts new bone growth
  • Statins increase bone mineral density
  • PPIs may decrease density
hormones and bone loss
Hormones and Bone Loss
  • Estrogens and androgens help maintain bone mass
  • Bone loss in women accelerates after menopause
cancer and bone loss
Cancer and Bone Loss
  • Cancerous tissues release osteoclast-activatingfactor:
    • stimulates osteoclasts
    • produces severe osteoporosis
paget s disease
Paget’s Disease
  • Characterized by excessive bone formation and breakdown
  • An excessively high ratio of spongy to compact bone is formed
  • Reduced mineralization causes spotty weakening of bone
  • Osteoclast activity wanes, but osteoblast activity continues to work
developmental aspects of bones
Developmental Aspects of Bones
  • Mesoderm gives rise to embryonic mesenchymal cells, which produce membranes and cartilages that form the embryonic skeleton
  • The embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonograms
  • At birth, most long bones are well ossified (except for their epiphyses)
developmental aspects of bones106
Developmental Aspects of Bones
  • By age 25, nearly all bones are completely ossified
  • In old age, bone resorption predominates
  • A single gene that codes for vitamin D docking determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life
summary
SUMMARY
  • Skeletal cartilage
  • Structure and function of bone tissues
  • Types of bone cells
  • Structures of compact bone and spongy bone
  • Bone membranes, peri- and endosteum
  • Ossification: intramembranous and endochondral
  • Bone minerals, recycling, and remodeling
  • Hormones and nutrition
  • Fracture repair
  • The effects of aging
the major types of fractures
The Major Types of Fractures
  • Simple (closed): bone end does not break the skin
  • Compound (open): bone end breaks through the skin
  • Nondisplaced – bone ends retain their normal position
  • Displaced – bone ends are out of normal alignment
  • Complete – bone is broken all the way through
  • Incomplete – bone is not broken all the way through
  • Linear – the fracture is parallel to the long axis of the bone
  • Transverse – the fracture is perpendicular to the long axis of the bone
  • Comminuted – bone fragments into three or more pieces; common in the elderly

Figure 6–16 (1 of 9)