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Chapter 6
Bones and Skeletal Tissues
J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.
Functions of Bones• Support – a framework for the body
• Protection– bones protect many internal organs– cranial bones surround the brain; vertebrae
surround the spinal cord; pelvic girdle surrounds the reproductive organs
• Movement - muscles attach to bones
• Mineral homeostasis – Ca2+, PO4- storage
• Site of blood cell production - hematopoiesis in red bone marrow
Macroscopic Bone Structure• Diaphysis
– the shaft of a long bone– contains medullary or
marrow cavity • infants have red
(hematopoietic) bone marrow
• red marrow gradually replaced by yellow (fatty) bone marrow throughout life
• Epiphysis (epiphyses)– ends of a long bone– epiphyseal plate -
growth plate made of cartilage
– nutrient foramen - site of blood vessel entry into bone
– articular cartilage - hyaline cartilage covering epiphysis
Periosteum• Two layers of connective
tissue around bone1. Fibrous layer (outer) -
dense irregular connective tissue
2. Osteogenic layer (inner)• osteoblasts – bone-forming
cells• osteoclasts – bone-
remodeling cells
• Sharpey’s fibers anchor periosteum to the bone
• Site of ligament, tendon attachment
• Large supply of nerves & blood vessels
Endosteum• Lines the medullary
cavity
• Contains osteoblasts and osteoclasts
Bone Tissue Histology
• Much intercellular matrix (osteoid)
• Matrix mineralized– 25% water– 25% protein
fibers– 50%
hydroxyapatites (mineral) salts• Mainly Ca2+
phosphates
Osteoblasts and Osteocytes• Osteoblasts - bone
forming cells– Secrete collagen and
other organic components for bone synthesis
– Found on any bone surface
• Osteocytes - mature bone cells– Embedded in matrix in
lacunae with canaliculi– Maintain daily
activities of bone tissue; nutrient, waste exchange
Osteoclasts
• Osteoclasts– Settle on bone surface – Function in bone resorption (matrix
destruction) for growth, development, maintenance, repair
Histology of Bone Tissue• Maturation
– Matrix - ground substance & collagen– Crystallization = calcification =
mineralization• hydroxyapatite (calcium phosphate salt)• other salts
– Hardness vs. flexibility• collagen fibers • mineralization
– crystallization develops around collagen fibers – stronger than egg shells which have no collagen
– Matrix is not continuous, because many vascular passageways penetrate the mineralized matrix
– Size and distribution of these vascular channels determines the type of bone - spongy or compact
Types of Bone
• Compact– Appears very dense
– Most of the bone mass in the body
• Spongy– Small struts of
bone (trabeculae)
– May appear randomly organized, but the trabeculae, like girders in a building, are generally oriented in the directions of stresses
Compact Bone - Osteons• Osteon - central canal with lamellae,
lacunae, osteocytes, & canaliculi– osteocytes in lacunae – canaliculi – house multiple cytoplasmic
extensions from the osteocytes
Compact Bone - Osteons
• Blood vessels run through perforating (Volkman’s) canals to the central (Haversian) canals
Compact Bone - Interstitial Lamellae • Found in older
bone
• Older osteons are gradually broken down and replaced during the remodeling process
Compact Bone – Microscopic View
Spongy Bone • No true osteon systems –
osteoblasts produce an irregular strutwork of trabeculae
• Osteocytes receive nutrients by diffusion through canaliculi
• Red marrow (1) fills the spaces between the trabeculae (2) (hematopoietic marrow)
• Blood vessels pass through compact bone to spongy bone
• Blood vessels pass through yellow marrow cavities; open out to become red marrow cavities
The Early Embryonic Skeleton
• Embryonic skeleton – composed of fibrous connective tissue membranes and hyaline cartilage
Bonedevelopslater →
Bone Formation and Growth
• Ossification/osteogenesis
• Begins at week 8 of development
• There are two different types of bone formation
– Each process leads to the formation of mature compact and spongy bones
1. Fibrous membrane model - intramembranous ossification – “membrane bones”
2. Hyaline cartilage model - endochondral ossification• the initial cartilage is transformed to
become “endochondral bones”
Intramembranous Ossification
• Results in the formation of cranial bones and the clavicles.
– All are flat bones
• At the site of bone development
– Ossification begins in fibrous connective tissue membranes formed by mesenchymal cells.
– Osteoprogenitor cells (osteoblasts): clusters of embryonic cells • become centers of ossification, secrete matrix until
they are surrounded
Intramembranous Ossification - 1
Intramembranous Ossification -2
Intramembranous Ossification - 3
Intramembranous Ossification - 4
Endochondral Ossification• Forms all bones below the base of the skull
(except clavicle)
• Uses hyaline cartilage “bones” as models for bone construction
• Requires breakdown of hyaline cartilage prior to ossification
• Prepping for ossification: – The perichondrium covering the hyaline
cartilage is infiltrated with blood vessels, converting it to a vascularized periosteum
– Increased nutritional status allows mesenchymal cells to specialize into osteoblasts, creating the primary ossification center.
Endochondral Ossification – 1
1. Formation of bone collar
– Osteoblasts secrete osteoid against the hyaline cartilage of the diaphysis
Endochondral Ossification – 2
2. Cavitation of the hyaline cartilage– Chondrocytes in the
diaphysis hypertrophy and start to calcify the surrounding cartilage matrix
– Chondrocytes die and matrix deteriorates
– Cavities open– Cartilage growth
continues
Endochondral Ossification – 3
3. Invasion of internal cavities by the periosteal bud, and spongy bone formation
– Cavities are invaded by periosteal bud
– Osteoclasts partially erode the cartilage matrix
– Osteoblasts secrete osteoid around remaining cartilage matrix, forming spongy bone
Endochondral Ossification – 4
4. Diaphysis elongation and formation of the medullary cavity
– Osteoclasts breakdown new spongy bone, opening the medullary cavity.
– Cartilage growth continues at epiphysis
– Ossification chases cartilage formation
– At birth, secondary ossification centers arise in the epiphysis
Endochondral Ossification – 55. Ossification of the epiphyses
– The epiphyses gain bony tissue, using the same method, except spongy bone remains
– Chondroblasts secrete matrix until they are embedded
– Cartilage cells and matrix begin to disintegrate
– Osteoclasts invade to remove cartilage matrix
– As the matrix forms, trabeculae form, joining together, forming the lattice of spongy bone
– When complete, hyaline cartilage remains only at the epiphyseal plate and the articular cartilages
Endochondral Ossification
• Vascularized connective tissue develops into the periosteum outside and the endosteum inside
• Most of this bone will be remodeled repeatedly over time
Postnatal Bone Growth
Postnatal Bone Growth
• Growth in length and width of long bones– Is accompanied by remodeling in order to
maintain the proper shape of the epiphysis and diaphysis
– Cells of the epiphyseal plate proximal to the resting cartilage form three functionally different zones: growth, transformation, and osteogenic
– Most bone growth stops during adolescence
– Continued growth of nose and lower jaw
Postnatal Bone Growth
• Regulated by hGH and the sex hormones
• In children, cartilage production continues on the epiphyseal (distal) side– cells are destroyed & replaced to increase the
length of bone
Postnatal Long 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
Postnatal Long Bone Growth
• Cells in the growth zone divide quickly, pushing the epiphysis away from the diaphysis
• Cells in hypertrophic zone hypertrophy causing lacunae to erode and enlarge
• Cartilage matrix calcifies and the chondrocytes die
• This leaves long spicules of calcified cartilage at the epiphysis-diaphysis junction
Postnatal Long Bone Growth
• The spicules become the osteogenic zone and are invaded by marrow from the medullary cavity
• The cartilage is eroded by osteoclasts and osteoblasts secrete matrix to form spongy bone
• The spicule tips are removed by osteoclasts
Long Bone Growth
• At the end of adolescence, epiphyseal plates divide less often and plates are replaced by bone tissue
• Longitudal growth ceases and the epiphysis/diaphysis fuse.– Called epiphyseal plate closure– Females at 18 years– Males at 21 years– clavicle is the last bone to stop
growing
Appositional Bone Growth
• Growth in width– From the inside out
– Compact bone lining the medullary cavity is destroyed
– Osteoblasts from periosteum continue to add more bone to the outer surface
Long Bone Growth
• Bone diameter can still increase (appositional)
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter6/animation__bone_growth_in_width.html
Bone Homeostasis - Remodeling• Remodeling - replacement of old bone by
new– Bone is a very metabolically active tissue
• Spongy transforms to compact or vice versa; old to new
• Bone is remodeled along the lines of mechanical stress
– Different rates in different regions • Distal head of the femur is replaced ~ every 4 months• Other areas never are replaced• Bone is replaced every 3 to 10 years
– Delicate balance between breakdown and synthesis• Too much bone tissue, bones become thick and heavy• Too much mineral causes bumps or spurs which
interfere with joint function• Too much Ca2+ loss or crystallization makes bones
brittle, breakable
Bone Growth and Remodeling
• Remodeling – bone is resorbed and added by appositional growth
• Occurs at periosteum and endosteum
• Remodeling units: packets of osteoblast and osteoclast cells that coordinate remodeling
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’s cell where it is secreted into the interstitial fluid and then into the blood
• Two control mechanisms
Hormonal Mechanism• Falling blood Ca2+
levels signal the parathyroid glands to release PTH
• PTH signals osteoclasts to degrade bone matrix and release Ca2+ into the blood
• Rising blood Ca2+ levels trigger the thyroid to release calcitonin
• Calcitonin stimulates calcium salt deposit in bone
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
Response to Mechanical Stress
• Trabeculae form along lines of stress
• Large, bony projections occur where heavy, active muscles attach
• 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 longitudinal bone growth
Hormonal Regulation of Bone Growth During Youth
Bone Homeostasis - Nutrition• Minerals needed for remodeling
– Ca2+ - matrix– PO4- - matrix– magnesium - needed for osteoblast function– manganese - needed for lamellae formation
• Vitamins needed for remodeling - D, C, A, B12 – D (calcitrol) – encourages Ca2+ removal from
bone, also increases intestinal absorption of Ca2+
– C - maintains matrix of connective tissues and for collagen synthesis
– A - controls activity, distribution, coordination of osteoblasts and osteoclasts during development
– B12 – for osteoblast metabolism and activity
Bone Homeostasis - Regulation• Hormonal regulation of bone growth
and remodeling– hGH (human growth hormone)
• responsible for general growth of all body tissues
• becoming tall or short depends on hGH levels
• works with the sex hormones • aids in the growth of new bone• causes degeneration of cartilage cells in
epiphyseal plates
– Sex hormones – androgens and estrogens - important for normal bone growth & development
– Insulin and thyroid hormones - important for bone and connective tissue growth & metabolism
Calcium Homeostasis
• Bones are important for Ca2+ homeostasis
– Bone tissue is the main reservoir for Ca2+ ions in the body (500-1000 times more calcium is in bone than in the rest of the tissues)
– Blood levels are regulated very tightly by the endocrine system
– Bone serves as a “buffer” to prevent sudden changes in blood Ca2+ levels• too much blood Ca2+ (hypercalcemia) - heart
stops• too little blood Ca2+ (hypocalcemia) -
breathing stops
Bone Fractures (Breaks)
• Bone fractures are classified by:
– The position of the bone ends after fracture
– The completeness of the break
– The orientation of the bone to the long axis
– Whether or not the bones ends penetrate the skin
Types of Bone Fractures
• 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
Types of Bone Fractures
• Linear – the fracture is parallel to the long axis of the bone
• Transverse – the fracture is perpendicular to the long axis of the bone
• Compound (open) – bone ends penetrate the skin
• Simple (closed) – bone ends do not penetrate the skin
Common Types of Fractures
• Comminuted – bone fragments into three or more pieces; common in the elderly
Common Types of Fractures• Spiral – ragged break when bone
is excessively twisted; common sports injury
Common Types of Fractures• Compression – bone is
crushed; common in porous bones
Common Types of Fractures• Depressed – broken bone
portion pressed inward; typical skull fracture
Common Types of Fractures– Epiphyseal – epiphysis separates from
diaphysis along epiphyseal line; occurs where cartilage cells are dying• plate fracture increases calcification; ends growth
in length• growth ceases• bone shows epiphyseal lines
Common Types of Fractures
• Greenstick – incomplete fracture where one side of the bone breaks and the other side bends; common in children
Repair of Bone Fractures• Fracture - any
break in a bone• Surgical repair by:
– closed reduction - manipulation without making an incision during surgery
– open reduction - manipulation after making an incision during surgery
fractured radius ↑
Stages in the Healing of a Bone Fracture
1. Hematoma formation– Torn blood vessels
hemorrhage– A mass of clotted blood
(hematoma) forms at the fracture site
– Site becomes swollen, painful, and inflamed
– Cells in the area die & have to be removed – inflammation, swelling, pain
– Osteoclasts begin to break down damaged portions of bone
Stages in the Healing of a Bone Fracture
2. Fibrocartilaginous callus formation– Capillaries grow into
the tissue and phagocytic cells begin cleaning debris
– Fibroblasts secrete collagen to stimulate new connective tissue formation
– Chondrocytes develop in avascular areas
– The hematoma is transformed a few days into a fibrocartilaginous (soft) callus
3. Bony (hard) callus formation– Bone callus begins 3-4
weeks after injury, and continues until firm union is formed 2-3 months later
– Osteoprogenitor cells turn into osteoblasts in the vascular areas
– Osteoblasts begin to produce spongy bone
– New bone trabeculae appear in the fibrocartilaginous callus
– Fibrocartilaginous callus converts into a bony (hard) callus
Stages in the Healing of a Bone Fracture
Stages in the Healing of a Bone Fracture
4. Bone remodeling - replacing spongy bone with compact bone where appropriate
– Excess material on the bone shaft exterior and in the medullary canal is removed
– Compact bone is laid down to reconstruct shaft walls
– Remodeling - May be accelerated by electrical stimuli (pulsating electromagnetic fields) which increase osteoblast activity
Homeostatic Imbalances
• Osteomalacia (adults)
– Bones are inadequately mineralized causing softened, weakened bones
– Main symptom is pain when weight is put on the affected bone
– Caused by insufficient calcium in the diet, or by vitamin D deficiency
Rickets• Bones of children
are inadequately mineralized causing softened, weakened bones
• Bowed legs and deformities of the pelvis, skull, and rib cage are common
• Caused by insufficient calcium in the diet, or by vitamin D deficiency
Osteoporosis• Group of diseases in which bone
reabsorption outpaces bone deposit– Spongy bone of the spine is most
vulnerable– Occurs most often in postmenopausal
women– Bones become so fragile that sneezing or
stepping off a curb can cause fractures
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
Paget’s Disease• Characterized by excessive bone
formation and breakdown
• Pagetic bone with an excessively high ratio of spongy to compact bone is formed
• Pagetic bone, along with reduced mineralization, causes spotty weakening of bone
• Osteoclast activity wanes, but osteoblast activity continues to work
End Chapter 6.