Osseous Tissue
and Bone Structure
BIOL241 “Lecture 6”
INTERCONNECTEDNESS
Topics:
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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 skeleton
– cartilages, 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 bones
– Costal – connects the ribs to the sternum
– Respiratory – makes up larynx, reinforces air
passages
– Nasal – 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 occurs
– During normal bone growth
– During old age
Bones and Cartilages of
Homo sapiens
Figure 6.1
Functions of the
Skeletal System
1.
2.
3.
4.
5.
6.
Support
Storage of minerals (calcium)
Storage of lipids (yellow marrow)
Blood cell production (red marrow)
Protection
Leverage (force of motion)
Bone (Osseous) Tissue
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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 salts
– osteocytes within lacunae organized
around blood vessels
• Canaliculi:
– form pathways for blood vessels
– exchange nutrients and wastes
Osteocyte and canaliculi
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
• Solid ground is made of mineral crystals
• ⅔ 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
Hydroxyapatite
Bone Matrix
• Matrix Proteins:
– ⅓ of bone matrix is protein fibers (collagen)
• Question: why aren’t bones made
ENTIRELY of 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 torsional flexibility to resist
shattering
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
The four major types of bone cells
in matrix only
periosteum + endo
endosteum only
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 weak
– exercise 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 vessels
– Lamella – 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 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
• Periosteum – double-layered protective
membrane
– Covers all bones, except parts enclosed in joint
capsules (continuous 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 cavity
– covers 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 Ossification
Note: 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 Genesis 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
• 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
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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
Secondary
ossificaton
center
Hyaline
cartilage
Primary
ossification
center
Deteriorating
cartilage
matrix
Spongy
bone
formation
Epiphyseal
blood vessel
Medullary
cavity
Articular
cartilage
Spongy
bone
Epiphyseal
plate
cartilage
Bone collar
1 Formation of
bone collar
around hyaline
cartilage model.
Blood
vessel of
periosteal
bud
2 Cavitation of
the hyaline cartilage 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 secondary ossification
centers in the epiphyses in preparation
for stage 5.
5 Ossification of the
epiphyses; when
completed, hyaline
cartilage remains only
in the epiphyseal plates
and articular cartilages.
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)
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 calcify
– die, 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
osteoclasts
– spongy bone develops at the
primary ossification center
Figure 6–9 (Step 3)
Endochondral
Ossification: Step 4a (Remodeling)
• 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)
• 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 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
• 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
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 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 ⅓ 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
PTH;
calcitonin
secreted
Calcitonin
stimulates
calcium salt
deposit
in bone
Thyroid
gland
Rising blood
Ca2+ levels
Calcium homeostasis of blood: 9–11 mg/100 ml
Falling blood
Ca2+ levels
Thyroid
gland
Osteoclasts
degrade bone
matrix and release
Ca2+ into blood
Parathyroid
glands
PTH
Parathyroid
glands release
parathyroid
hormone (PTH)
Figure 6.11
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
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
Fractures
• Fractures:
– cracks or breaks in bones
– caused 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 bone
– Osteoblasts 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%
• Can be induced by certain medications
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 osteoclasts
– produces 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
The Major Types of Fractures
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•
•
•
•
•
•
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)
Types of fractures (just FYI)
More fractures