Chapter 6 (Bones and Skeletal Tissue) Flashcards
Skeletal cartilage
made of highly resilient, molded cartilage tissue that consists primarily of water
– Contains no blood vessels or nerves
Perichondrium
layer of dense connective tissue surrounding cartilage like a
girdle
– Helps cartilage resist outward expansion
– Contains blood vessels for nutrient delivery to cartilage
Cartilage is made up of
chondrocytes, cells encased in small cavities
(lacunae) within jelly-like extracellular matrix
Three types of cartilage:
hyaline, elastic, cartilage
Hyaline cartilage
Provides support, flexibility, and resilience
Most abundant type; contains collagen fibers only
Articular (joints), costal (ribs), respiratory (larynx), nasal cartilage (nose
tip)
Elastic cartilage
Similar to hyaline cartilage, but contains elastic fibers
External ear and epiglottis
Fibrocartilage
Thick collagen fibers – has great tensile strength
Menisci of knee; vertebral discs
Cartilage grows in two ways:
appositional and interstitial
Appositional growth
Cartilage-forming cells in perichondrium secrete matrix against
external face of existing cartilage
– New matrix laid down on surface of cartilage
Interstitial growth
Chondrocytes within lacunae divide and secrete new matrix, expanding
cartilage from within
– New matrix made within cartilage
There are seven important functions of bones
- Support
For body and soft organs - Protection
Protect brain, spinal cord, and vital organs - Movement
Levers for muscle action - Mineral and growth factor storage
Calcium and phosphorus, and growth factors reservoir - Blood cell formation
Hematopoiesis occurs in red marrow cavities of certain bones - Triglyceride (fat) storage
Fat, used for an energy source, is stored in bone cavities - Hormone production
Osteocalcin secreted by bones helps to regulate insulin secretion,
glucose levels, and metabolism
206 named bones in human skeleton
Divided into two groups based on location
Axial skeleton
Long axis of body
Skull, vertebral column, rib cage
Appendicular skeleton
Bones of upper and lower limbs
Girdles attaching limbs to axial skeleton
Bones are also classified according to one of four shapes:
- Long bones
Longer than they are wide
Limb bones - Short bones
Cube-shaped bones (in wrist and ankle)
Sesamoid bones form within tendons (example: patella)
Vary in size and number in different individuals - Flat bones
Thin, flat, slightly curved
Sternum, scapulae, ribs, most skull bones - Irregular bones
Complicated shapes
Vertebrae and hip bones
Bones are organs because they contain different types of tissues
Bone (osseous) tissue predominates, but a bone also has nervous
tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial
cells in its blood vessels
Three levels of structure
– Gross
– Microscopic
– Chemical
Structure of short, irregular, and flat bones
– Consist of thin plates of spongy bone (diploe) covered by compact bone
– Compact bone sandwiched between connective tissue membranes
Periosteum covers outside of compact bone, and endosteum covers
inside portion of compact bone
– Bone marrow is scattered throughout spongy bone; no defined marrow
cavity
– Hyaline cartilage covers area of bone that is part of a movable joint
Structure of typical long bone
All long bones have a shaft (diaphysis), bone ends (epiphyses), and
membranes
Diaphysis: tubular shaft that forms long axis of bone
– Consists of compact bone surrounding central medullary cavity that
is filled with yellow marrow in adults
Epiphyses: ends of long bones that consist of compact bone
externally and spongy bone internally
– Articular cartilage covers articular (joint) surfaces
Between diaphysis and epiphysis is epiphyseal line
– Remnant of childhood epiphyseal plate where bone growth occurs
Membranes: two types (periosteum and endosteum)
– Periosteum: white, double-layered membrane that covers external
surfaces except joints
» Fibrous layer: outer layer consisting of dense irregular
connective tissue consisting of Sharpey’s fibers that secure to
bone matrix
» Osteogenic layer: inner layer abutting bone and contains
primitive osteogenic stem cells that gives rise to most all bone
cells
» Contains many nerve fibers and blood vessels that continue on
to the shaft through nutrient foramen openings
» Anchoring points for tendons and ligaments
Endosteum
» Delicate connective tissue membrane covering internal bone
surface
» Covers trabeculae of spongy bone
» Lines canals that pass through compact bone
» Like periosteum, contains osteogenic cells that can differentiate
into other bone cells
Bone markings
– Three types of markings:
Projection: outward bulge of bone
– May be due to increased stress from muscle pull or is a
modification for joints
Depression: bowl- or groove-like cut-out that can serve as
passageways for vessels and nerves, or plays a role in joints
Opening: hole or canal in bone that serves as passageways for blood
vessels and nerves
Cells of bone tissue
– Five major cell types, each of which is a specialized form of the same
basic cell type
1. Osteogenic cells
2. Osteoblasts
3. Osteocytes
4. Bone-lining cells
5. Osteoclasts
Osteogenic cells
– Also called osteoprogenitor cells
– Mitotically active stem cells in periosteum and endosteum
– When stimulated, they differentiate into osteoblasts or bone-lining cells
– Some remain as osteogenic stem cells
- Stem cell
Osteoblasts
– Bone-forming cells that secrete unmineralized bone matrix called osteoid
Osteoid is made up of collagen and calcium-binding proteins
Collagen makes up 90% of bone protein
– Osteoblasts are actively mitotic
- Matrix-synthesizing cell responsible for bone growth
Osteocytes
– Mature bone cells in lacunae that no longer divide
– Maintain bone matrix and act as stress or strain sensors
Respond to mechanical stimuli such as increased force on bone or
weightlessness
Communicate information to osteoblasts and osteoclasts (cells that
destroy bone) so bone remodeling can occur
- Mature bone cell that monitors and maintains the mineralized bone matrix
Bone-lining cells
– Flat cells on bone surfaces believed to also help maintain matrix (along
with osteocytes)
– On external bone surface, lining cells are called periosteal cells
– On internal surfaces, they are called endosteal cells
Osteoclasts
– Derived from same hematopoietic stem cells that become macrophages
– Giant, multinucleate cells function in bone resorption (breakdown of bone)
– When active, cells are located in depressions called resorption bays
– Cells have ruffled borders that serve to increase surface area for enzyme
degradation of bone
Also helps seal off area from surrounding matrix
- Bone-reabsorbing cell
Compact bone
– Also called lamellar bone
– Consists of:
Osteon (Haversian system)
Canals and canaliculi
Interstitial and circumferential lamellae
Osteon (Haversian system)
– An osteon is the structural unit of compact bone
– Consists of an elongated cylinder that runs parallel to long axis of bone
Acts as tiny weight-bearing pillars
– An osteon cylinder consists of several rings of bone matrix called lamellae
Lamellae contain collagen fibers that run in different directions in
adjacent rings
Withstands stress and resist twisting
Bone salts are found between collagen fibers
Canals and canaliculi
– Central (Haversian) canal runs through core of osteon
Contains blood vessels and nerve fibers
– Perforating (Volkmann’s) canals: canals lined with endosteum that
occur at right angles to central canal
Connect blood vessels and nerves of periosteum, medullary cavity,
and central cana
Canals
Lacunae: small cavities that contain osteocytes
– Canaliculi: hairlike canals that connect lacunae to each other and to
central canal
– Osteoblasts that secrete bone matrix maintain contact with each other and
osteocytes via cell projections with gap junctions
– When matrix hardens and cells are trapped the canaliculi form
Allow communication between all osteocytes of osteon and permit
nutrients and wastes to be relayed from one cell to anothe
Interstitial and circumferential lamellae
– Interstitial lamellae
Lamellae that are not part of osteon
Some fill gaps between forming osteons; others are remnants of
osteons cut by bone remodeling
– Circumferential lamellae
Just deep to periosteum, but superficial to endosteum, these layers of
lamellae extend around entire surface of diaphysis
Help long bone to resist twisting
Spongy bone
– Appears poorly organized but is actually organized along lines of stress to
help bone resist any stress
– Trabeculae, like cables on a suspension bridge, confer strength to bone
No osteons are present, but trabeculae do contain irregularly arranged
lamellae and osteocytes interconnected by canaliculi
Capillaries in endosteum supply nutrients
Chemical Composition of Bone
Bone is made up of both organic and inorganic components
– Organic components
Includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells,
osteoclasts, and osteoid
– Osteoid, which makes up one-third of organic bone matrix, is
secreted by osteoblasts
» Consists of ground substance and collagen fibers, which
contribute to high tensile strength and flexibility of bone
Organic components
– Resilience of bone is due to sacrificial bonds in or between collagen
molecules that stretch and break to dissipate energy and prevent fractures
– If no additional trauma, bonds re-form
Inorganic components
– Hydroxyapatites (mineral salts)
Makeup 65% of bone by mass
Consist mainly of tiny calcium phosphate crystals in and around
collagen fibers
Responsible for hardness and resistance to compression
– Bone is half as strong as steel in resisting compression and as strong as
steel in resisting tension
– Lasts long after death because of mineral composition
– Can reveal information about ancient people
Ossification (osteogenesis)
is the process of bone tissue formation
– Formation of bony skeleton begins in month 2 of development
– Postnatal bone growth occurs until early adulthood
– Bone remodeling and repair are lifelong
Endochondral ossification
– Bone forms by replacing hyaline cartilage
– Bones are called cartilage (endochondral) bones
– Form most of skeleton
– Forms essentially all bones inferior to base of skull, except clavicles
– Begins late in month 2 of development
– Uses previously formed hyaline cartilage models
– Requires breakdown of hyaline cartilage prior to ossification
– Begins at primary ossification center in center of shaft
Blood vessels infiltrate perichondrium, converting it to periosteum
Mesenchymal cells specialize into osteoblasts
Intramembranous ossification
– Bone develops from fibrous membrane
– Bones are called membrane bones
begins within fibrous connective tissue
membranes formed by mesenchymal cells
– Forms frontal, parietal, occipital, temporal, and clavicle bone
Formation of the Bony Skeleton
Five main steps in the process of ossification:
1. Bone collar forms around diaphysis of cartilage model
2. Central cartilage in diaphysis calcifies, then develops cavities
3. Periosteal bud invades cavities, leading to formation of spongy bone
Bud is made up of blood vessels, nerves, red marrow, osteogenic
cells, and osteoclasts
4. Diaphysis elongates, and medullary cavity forms
Secondary ossification centers appear in epiphyses
5. Epiphyses ossify
Hyaline cartilage remains only in epiphyseal plates and articular
cartilages
Four major steps are involved:
- Ossification centers are formed when mesenchymal cells cluster and
become osteoblasts - Osteoid is secreted, then calcified
- Woven bone is formed when osteoid is laid down around blood vessels,
resulting in trabeculae
Outer layer of woven bone forms periosteum - Lamellar bone replaces woven bone, and red marrow appear
Postnatal Bone Growth
Long bones grow lengthwise by interstitial (longitudinal) growth of epiphyseal
plate
Bones increase thickness through appositional growth
Bones stop growing during adolescence
– Some facial bones continue to grow slowly through life
Growth in Length of Long Bones
Interstitial growth requires presence of epiphyseal cartilage in the epiphyseal
plate
Epiphyseal plate maintains constant thickness
– Rate of cartilage growth on one side balanced by bone replacement on
other
Epiphyseal plate consists of five zones:
1. Resting (quiescent) zone
2. Proliferation (growth) zone
3. Hypertrophic zone
4. Calcification zone
5. Ossification (osteogenic) zone
Epiphyseal plate consists of five zones:
- Resting (quiescent) zone
– Area of cartilage on epiphyseal side of epiphyseal plate that is relatively
inactive - Proliferation (growth) zone
– Area of cartilage on diaphysis side of epiphyseal plate that is rapidly
dividing
– New cells formed move upward, pushing epiphysis away from diaphysis,
causing lengthening - Hypertrophic zone
– Area with older chondrocytes closer to diaphysis
– Cartilage lacunae enlarge and erode, forming interconnecting spaces - Calcification zone
– Surrounding cartilage matrix calcifies; chondrocytes die and deteriorate - Ossification zone
– Chondrocyte deterioration leaves long spicules of calcified cartilage at
epiphysis-diaphysis junction
– Spicules are then eroded by osteoclasts and are covered with new bone
by osteoblasts
– Ultimately replaced with spongy bone
– Medullary cavity enlarges as spicules are eroded
Three “either/or” fracture classifications
Three “either/or” fracture classifications
– Position of bone ends after fracture
Nondisplaced: ends retain normal position
Displaced: ends are out of normal alignment
– Completeness of break
Complete: broken all the way through
Incomplete: not broken all the way through
– Whether skin is penetrated
Open (compound): skin is penetrated
Closed (simple): skin is not penetrated
Repair involves four major stages
- Hematoma formation
- Fibrocartilaginous callus formation
- Bony callus formation
- Bone remodeling
Hematoma formation
– Torn blood vessels hemorrhage, forming mass of clotted blood called a
hematoma
– Site is swollen, painful, and inflamed
Fibrocartilaginous callus formation
– Capillaries grow into hematoma
– Phagocytic cells clear debris
– Fibroblasts secrete collagen fibers to span break and connect broken
ends
– Fibroblasts, cartilage, and osteogenic cells begin reconstruction of bone
Create cartilage matrix of repair tissue
Osteoblasts form spongy bone within matrix
– This mass of repair tissue is called fibrocartilaginous callus
Bony callus formation
– Within one week, new trabeculae appear in fibrocartilaginous callus
– Callus is converted to bony (hard) callus of spongy bone
– Bony callus formation continues for about 2 months until firm union forms
Bone remodeling
– Begins during bony callus formation and continues for several months
– Excess material on diaphysis exterior and within medullary cavity is
removed
– Compact bone is laid down to reconstruct shaft walls
– Final structure resembles original structure
Responds to same mechanical stressors
Three major bone diseases:
– Osteomalacia and rickets
– Osteoporosis
– Paget’s disease
Osteomalacia
– Bones are poorly mineralized
– Osteoid is produced, but calcium salts not adequately deposited
– Results in soft, weak bones
– Pain upon bearing weight
Rickets (osteomalacia of children)
– Results in bowed legs and other bone deformities because bones ends
are enlarged and abnormally long
– Cause: vitamin D deficiency or insufficient dietary calcium
Osteoporosis
Osteoporosis is a group of diseases in which bone resorption exceeds
deposit
Matrix remains normal, but bone mass declines
– Spongy bone of spine and neck of femur most susceptible
Vertebral and hip fractures common
Tuberosity
Large rounded projection; may be roughened
Crest
Narrow ridge of bone; usually prominent
Trochanter
Very large; blunt, irregularly shaped process (the only examples are on the femur)
Line
Narrow ridge of bone; less prominent than a crest
Tubercle
Small rounded projection or process
Epicondyle
Raised area on or above a condyle
Spine
Sharp, slender, often pointed projection
Process
Any bony prominence
Head
Bony expansion carried on a narrow neck
Facet
Smooth, nearly flat articular surface
Condyle
Rounded articular projection
Ramus
Armlike bar of bone
Groove
Furrow
Fissure
Narrow, slitlike opening
Foramen
Round or oval opening through a bone
Notch
Indentation at the edge of a structure
Meatus
Canal-like passageway
Sinus
Cavity within a bone, filled with air and lined with mucous membrane
Fossa
Shallow, basinlike depression in a bone, often served as an articular surface
Spiral
Ragged break occurs when excessive twisting forces are applied to a bone
- common sports fracture
Epiphyseal
Epiphysis separates from the diaphysis along the epiphyseal plate
Tends to occur where cartilage are dying and calcification of the matrix is occuring
Depressed
Broken bone portion is pressed inward
- typical of skull fracture
Greenstick
Bone breaks incompletely, much in the way a green twig breaks. Only one side of the shaft breaks; the other side bends
- common in children, whose bones have relatively more organic matrix and are more flexible than those of adults
Paget’s Disease
Excessive and haphazard bone deposit and resorption cause bone to be
made fast and poorly
– Called Pagetic bone
– Very high ratio of spongy to compact bone and reduced mineralization
Usually occurs in spine, pelvis, femur, and skull
Rarely occurs before age 40
Cause unknown: possibly viral
Treatment includes calcitonin and bisphosphonates
