lecture 6: osseous tissue and bone structure. topics: skeletal cartilage structure and function of...
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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 The effects of aging
The Skeletal System
Skeletal system includes:bones of the skeletoncartilages, ligaments, and connective tissues
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
Provides support, flexibility, and resilience Is the most abundant skeletal cartilage Is present in these cartilages:
Articular – covers the ends of long bonesCostal – connects the ribs to the sternumRespiratory – makes up larynx, reinforces air
passagesNasal – supports the nose
Elastic Cartilage
Similar to hyaline cartilage, but contains elastic fibers
Found in the external ear and the epiglottis
Fibrocartilage
Highly compressed with great tensile strength
Contains collagen fibers Found in menisci of the knee and in
intervertebral discs
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 occursDuring normal bone growthDuring old age
Bones and Cartilages of the Human Body
Figure 6.1
Functions of the Skeletal System1. Support
2. Storage of minerals (calcium)
3. Storage of lipids (yellow marrow)
4. Blood cell production (red marrow)
5. Protection
6. Leverage (force of motion)
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
Dense matrix, containing:deposits of calcium saltsosteocytes within lacunae organized around
blood vessels Canaliculi:
form pathways for blood vesselsexchange nutrients and wastes
Osteocyte and canaliculi
Characteristics of Bone Tissue
Periosteum: covers outer surfaces of bones consist of outer fibrous and inner cellular
layersContains osteblasts responsible for bone
growth in thickness Endosteum
Covers inner surfaces of bones
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 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 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 flexibility to resist shattering
Chemical Composition of Bone: Organic Cells:
Osteoblasts – bone-forming cellsOsteocytes – mature bone cellsOsteoprogenitor cells – grandfather cellsOsteoclasts – 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
in matrix only
endosteum only
periosteum + endo
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
Mature bone cells that maintain the bone matrix
Figure 6–3 (1 of 4)
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 Mesenchyme
stem cells that divide to produce osteoblasts
Are located in inner, cellular layer of periosteum
Assist in fracture repair
4. Osteoclasts
Secrete acids and protein-digesting enzymes
Figure 6–3 (4 of 4)
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
Bone building (by osteocytes and -blasts) and bone recycling (by osteoclasts) must balance:more breakdown than building, bones become
weakexercise causes osteocytes to build bone
Bone cell lineage summary
Osteoprogenitor cells
osteoblasts
osteocytes
Osteoclasts are related to macrophages (blood cell derived)
Gross Anatomy of Bones: Bone Textures Compact bone – dense outer layer Spongy bone – honeycomb of trabeculae
filled with yellow bone marrow
Compact Bone
Figure 6–5
Osteon
The basic structural unit of mature compact bone
Osteon = Osteocytes arranged in concentric lamellae around a central canal containing blood vesselsLamella – weight-bearing, column-like matrix
tubes composed mainly of collagen
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 Bone
Figure 6–5
Microscopic Structure of Bone: Compact Bone
Figure 6.6a, b
Microscopic Structure of Bone: Compact Bone
Figure 6.6a
Microscopic Structure of Bone: Compact Bone
Figure 6.6b
Microscopic Structure of Bone: Compact Bone
Figure 6.6c
Spongy Bone
Figure 6–6
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 Bone
Does not have osteons The matrix forms an open network of
trabeculae Trabeculae have no blood vessels
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)
In infantsFound 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 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
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
Figure 6–8a
Functions of Periosteum
1. Isolate bone from surrounding tissues
2. Provide a route for circulatory and nervous supply
3. Participate in bone growth and repair
Endosteum
Figure 6–8b
Endosteum
An incomplete cellular layer: lines the marrow cavitycovers trabeculae of spongy bone lines central canals
Contains osteoblasts, osteoprogenitor cells, and osteoclasts
Is active in bone growth and repair
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
The process of depositing calcium salts Occurs during bone ossification and in
other tissues
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 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
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 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
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
Fetal Primary Ossification Centers
Figure 6.15
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 Ossification
Figure 6.8
Formation ofbone collararound hyalinecartilage model.
Hyalinecartilage
Cavitation ofthe hyaline carti-lage within thecartilage model.
Invasion ofinternal cavitiesby the periostealbud and spongybone formation.
Formation of themedullary cavity asossification continues;appearance of sec-ondary ossificationcenters in the epiphy-ses in preparationfor stage 5.
Ossification of theepiphyses; whencompleted, hyalinecartilage remains onlyin the epiphyseal platesand articular cartilages.
Deterioratingcartilagematrix
Epiphysealblood vessel
Spongyboneformation
Epiphysealplatecartilage
Secondaryossificatoncenter
Bloodvessel ofperiostealbud
Medullarycavity
Articularcartilage
Spongybone
Primaryossificationcenter
Bone collar
1
2
34
5
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) Figure 6–9 (Step 2)
Endochondral Ossification: Step 2 (Cavitation)
Chondrocytes in the center of the hyaline cartilage of each bone model:enlarge form struts and calcifydie, leaving cavities in cartilage
Figure 6–9 (Step 1)
Endochondral Ossification: Step 3 (Invasion)
Periosteal bud brings blood vessels into the cartilage:bringing osteoblasts and
osteoclastsspongy bone develops at the
primary ossification center
Figure 6–9 (Step 3)
Endochondral Ossification: Step 4a (Remodelling)
Figure 6–9 (Step 4)
Remodeling creates a marrow (medullary) cavity:bone replaces cartilage at the
metaphysesDiaphysis elongates
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)
Figure 6–9 (Step 6)
Postnatal Bone Growth
Growth in length of long bonesCartilage on the side of the epiphyseal plate
closest to the epiphysis is relatively inactiveCartilage 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 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
Growth in Length of Long Bone
Figure 6.9
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
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 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
Long Bone Growth and Remodeling
Figure 6.10
Appositional Growth
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 Lines
Figure 6–10
During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone
During puberty, testosterone and estrogens: Initially promote adolescent growth spurtsCause masculinization and feminization of
specific parts of the skeletonLater induce epiphyseal plate closure, ending
long bone growth
Hormonal Regulation of Bone Growth During Youth
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 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
Mineral recycling allows bones to adapt to stress
Heavily stressed bones become thicker and stronger
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
Response to Mechanical Stress
Figure 6.12
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 degenerates quickly Up to 1/3 of bone mass can be lost in a
few weeks of inactivity
Minerals, vitamins, and nutrients
Rewired for bone growth A dietary source of calcium and phosphate
salts: plus small amounts of magnesium, fluoride,
iron, and manganese Protein, vitamins C, D, and A
Hormones for Bone Growth and Maintenance
Table 6–2
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 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 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
Figure 6.11
PTH;calcitoninsecreted
Calcitoninstimulatescalcium saltdepositin bone
Parathyroidglands releaseparathyroidhormone (PTH)
Thyroidgland
Thyroidgland
Parathyroidglands
Osteoclastsdegrade bonematrix and releaseCa2+ into blood
Falling bloodCa2+ levels
Rising bloodCa2+ levels
Calcium homeostasis of blood: 9–11 mg/100 ml
PTH
Imbalance
Imbalance
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
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:cracks or breaks in bonescaused by physical stress
Fractures are repaired in 4 steps
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 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 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 boneOsteoblasts furthest from capillaries secrete an
externally bulging cartilaginous matrix that later calcifies
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 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 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
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
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
Notice what happens in osteoporosis
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
Estrogens and androgens help maintain bone mass
Bone loss in women accelerates after menopause
Cancer and Bone Loss
Cancerous tissues release osteoclast-activating factor:stimulates osteoclastsproduces severe osteoporosis
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 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 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
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
Figure 6–16 (1 of 9)
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
Types of fractures (just FYI)
More fractures