BIO 2401
BONES & BONE TISSUE
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BONES AND BONE TISSUE
Organization of the Skeletal System
• components:
1) bone
2) skeletal cartilage: surrounded by dense irregular connective tissue which acts to girdle the
cartilage to prevent it from deforming too much under stress
ƒ hyaline cartilage – precursor of endochondral bones; flexible and resilient
ƒ elastic cartilage – cartilage subjected to repeated bending
ƒ fibrocartilage – highly compressible with great tensile strength
Major Functions of skeletal system
1. support – provides hard framework to support body and cradle its soft organs
2. protection – provides protective framework encasing for body structures/organs
3. movement – used as a lever system to move body and its parts; arrangement of bones and design of
joints determines types of movement possible
4. mineral storage – retrievable storage for calcium and phosphate for release into blood
5. blood cell formation – most occurs within marrow cavities of certain bones (ribs, sternum, long bones)
Ways to Classify Bones:
a. Based on Shape
(1) long bones – longer than wide; has shaft plus two ends (ex. limb bones)
(2) short bones – cube shaped bones (ex. carpal and tarsal elements and sesamoid bones that form
within tendons)
(3) flat bones – are thin flattened and usually a bit curved (ex. skull roofing bones, sternum and
scapula)
(4) irregular bones – bones that have complicated shapes (ex. includes vertebrae and brain case)
b. Based on Formation
(1) membrane or dermal bones – bones that form within a collagen membrane (ex. skull roofing
bones frontals and parietals)
(2) endochondral bones – bones that are preformed in hyaline cartilage and then transformed into
bone (ex. long bones, vertebrae)
Structure and Histology of a Long bone
• structure:
• diaphysis – shaft of bone cross section shows from outside toward inside the following layers:
(1) periosteum
(2) compact bone
(3) medullary cavity for marrow
• epiphyses – ends of bones that form articular surfaces; have thin layer of compact bone that is
underlain by cancellous or spongy bone
BIO 2401
BONES & BONE TISSUE
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•
•
periosteum – outer wrapping of bone made up of collagen (dense irregular connective tissue);
inner layers are osteogenic and contain osteoblasts (bone forming cells), osteoclasts (bone
destroying cells; this layer is richly supplied by blood vessels, nerves and lymphatic vessels
• articular cartilage – covers joint surfaces of epiphyses; made up of hyaline cartilage; acts to
cushion stresses during joint movement
• epiphyseal line – remnants of epiphyseal growth plate, a band of actively dividing hyaline
cartilage that acts to lengthen bone
• medullary cavity – marrow cavity that contains blood forming tissue (red marrow) or yellow or
fat marrow; lined with an endosteal membrane
histology:
• outer layer = periosteum – layers of collagen that surround bone; during growth will have bone
forming cells and fibrocytes
• inside epiphyses = membrane that lines medullary cavity and trabecular system inside bone =
endosteum; contains osteoblasts and osteocytes
Structure of Short, Irregular, and Flat Bones
ƒ consist of thin plates of periosteum covered compact bone on outside
ƒ endosteum covered spongy bone is internal
ƒ no shaft or epiphyses
ƒ contain some marrow but no marrow cavity
Microscopic Anatomy of Bone Tissue
1. compact bone – composed of lamellar and Haversian bone
• osteon – concentric cylinders of bone that usually run in the long axis of bone and support
stress and weight of bone
• osteocyte – ameboid bone cells that maintain bone matrix
• lacunae – small cavities in which bones cells reside
• lamellae – each layer of concentric tube of an osteon (Haversian system)
• Haversian canal – central canal of Haverian system that contains blood vessels and nerves
• canaliculi – canals of radiating out from lacunae and housing pseudopods of osteocytes;
mechanism of nutrient transfer from one osteocyte to another
• Volkmann's canal – tranversely arranged canals that bring blood vessels into the Haversian
canals
2. spongy bone
ƒ trabeculae – system of plates and spicules supporting epiphyses of bones; plates and spicules
are arranged along lines of stress and are only a few layers or lamellae thick
Types of Bone Cells
• osteoblasts – embryonic bone cells that lay down bone matrix
• osteocytes – mature bone cells that are derived from osteoblasts and are trapped in bony matrix
• osteoclasts – bone cells that break down and remodel bone; derived from hemopoietic stem cells and
macrophages; use acids to destroy bone; are able to phagocytize demineralized and dead osteocytes
BIO 2401
BONES & BONE TISSUE
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Bone Formation and Remodeling
A. endochondral ossification – all bones from the brain case down except the clavicles
ƒ bone is preformed in hyaline cartilage; ossification begins in shaft at a primary ossification center
(other centers occur in the epiphyses) when chondrocytes near shaft center enlarge and their
surrounding matrix calcifies; this kills the chondryocytes which then disintegrate
ƒ perichondrium becomes infiltrated with blood vessels that break into the eroding cartilage; cells in
the lower layers of the periosteal membrane (formerly the perichondrium) are differentiated into
osteoblasts – this converts the perichondrium into a periosteum and its inner layer is called the
osteogenic layer
ƒ osteoblasts in osteogenic layer form a bony collar around cartilage; chondrocytes hypertrophy
(enlarge) and signal other cartilage cells to secrete osteoid (calcium phosphate matrix); spaces left
by disintegrating chondrocytes are invaded by blood vessels
ƒ most of the cartilage is replaced by bone except in a band on either end of the shaft facing the
epiphyses (called the metaphysis)
ƒ cartilage in the center of forming bone dies and forms the marrow cavity; invading bone cells form
spongy areas under the joint surfaces
ƒ as diaphysis enlarges osteoclasts erode the central portion (filled with spongy bone) and create the
marrow cavity
ƒ length increases then occur at metaphyses; at shaft end of metaphyses, osteoblasts are continually
invading cartilage and replacing it with bone; at epiphyseal end of metaphyses, new cartilage is
produced at same rate
ƒ at time of birth, centers of epiphyses begin to calcify and capillaries and osteoblasts migrate into
these areas (called secondary ossification centers); this fills epiphyses with spongy bone, but at the
proximal or distal-most end of the bone, cartilage remains to form articulating cartilage protecting
bones from grinding against each other; at metaphyses, cartilage band remains to permit bone
length growth (called epiphyseal plate) with ossification near shaft and cartilage growth near
epiphysis
B. intramembranous (dermal) ossification – bone is formed within a fibrous membrane (the periosteum)
ƒ embryonic cells within the periosteum form osteoblasts within the connective tissue
ƒ osteoblasts cluster together and secrete organic components of matrix including collagen fibers
that form the scaffolding or framework for bone formation and osteoid
ƒ ossification occurs in the eighth week of development via a process of crystallizing calcium salts
and forms an ossification center
ƒ developing bone grows outward in small struts or spicules and osteoblasts become entrapped and
entombed within the bone; they are then called osteocytes
ƒ new osteoblasts continue to be formed from embryonic cells to continue process; are supplied by
blood vessels that grow between the spicules
ƒ this forms spongy bone which is then remodeled into compact bone as marrow cavities are formed
ƒ examples of these bones are clavicles, mandible, patella and the skull roofing bones such as the
frontal, parietal and zygomatic
C. bone growth
ƒ post-natal growth:
ƒ long bones lengthen by interstitial growth of epiphyseal plates
ƒ all bones grow in thickness by appositional growth
ƒ all bones except facial bones stop growth in early adulthood
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BONES & BONE TISSUE
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ƒ
length increases in bone mimics endochondral ossification
ƒ cartilages stack up at the epiphyseal plate; those cells on top undergo rapid mitosis and
push epiphyses away from diaphysis
ƒ those on bottom, hypertrophy (lacunae enlarge and then erode); ossification occurs and
leaves long spicules of calcified cartilage at epiphysis/diaphysis junction
ƒ spicules are invaded by marrow elements from medullary cavity; osteoclasts erode
them and they are then covered with bone matrix by osteoblasts to form spongy bone
ƒ chondroblasts divide less often in plate region at close of adolescence (18 for females;
21 for males)
ƒ when bone of epiphysis and bone of diaphysis fuse (“epiphyseal closure”), growth ends
• effect of hormones on bone growth:
1.
growth hormone – released by anterior pituitary gland and modulated by thyroid hormone;
acts to stimulate epiphyseal plate activity
ƒ too much growth hormone = giantism
ƒ too little growth hormone = dwarfism
2.
sex hormones – initially promotes growth spurts and masculinization or feminization of
specific parts of the skeleton
D. bone remodeling – occurs continually in response to:
1.
Ca+2 levels in the blood
• Ð blood calcium causes release of parathyroid hormone
• stimulates osteoclasts to reabsorb bone and release clacium into blood
• turns off calcitonin production
• Ï blood calcium shuts off release of parathyroid hormone
• stimulates secretion of calcitonin
• inhibits bone reabsorption and stimulates calcium deposition in bone matrix
2.
stress on bones from gravity and muscles
• bone grows or remodels in response to forces placed upon it
• weight is put on bones in an assymetrical way so that bone is stretched on one side and
compressed on other
• in long bones bending stresses are midway down shaft
• the neck regions also bear the most stress
• cancellous bone best supports compression under joints
Types of Bone Fractures
a. classified by position of bone ends after fracture:
• non displaced –bone ends are in natural position
• displaced bone ends are out of alignment
b. by completeness of break – complete fracture or incomplete fracture.
c. by orientation of break to long axis of bone: linear = parallel to long axis or transverse = perpendicular
to long axis
d. whether bone ends penetrate the skin: open (compound fracture), closed (simple)
BIO 2401
BONES & BONE TISSUE
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Healing of fractures
a. fracture hematoma
• occurs because blood vessels in bone, periosteum, and surrounding tissues are torn and
hemorrhage
• hematoma is the mass of clotted blood that forms at fracture site
• bone cells deprived of nutrition die and site becomes swollen and painful
b. fibrocartilage callus
• granulation tissue forms
• capillaries grow into the hematoma; phagocytocic cells clean up debris
• fibroblasts and osteoblasts migrate into fracture site from periosteum and endosteum and begin
reconstructing bone
• fibroblasts form collagen fibers that connect broken end
• osteoblasts form woven bone
c. bony callus
• osteoblasts lay down new bone, trabeculae in the fibrocartilage callus gets converted into bony
callus
• takes 3-4 weeks
• continues for 2-3 months before stopping
Effects of aging on skeletal system
a.
b.
c.
d.
e.
f.
estrogen – estrogen levels drop during menopause; affects calcium absorption
insufficient exercise – no stress leads to bone reabsorption
diet poor in calcium and protein leads to osteomalacia – bones inadequately mineralized
vitamin D and calcitonin metabolism also leads to osteomalacia
smoking – reduces estrogen levels
hormone related conditions (corticosteroid drugs)
Osteoporosis
•
•
•
•
bone reabsorption outpaces bone deposition – bone mass is reduced
estrogen and testosterone help restrain osteoclast activity and promote bone growth
peak density is reached between 35-40 years
occurs more frequently in menopausal females because males secrete testosterone throughout life