Chapter 6 Bones and bones tissue Flashcards
INTRODUCTION TO BONES AS ORGANS
Skeletal system
Includes:
–Bones, joints, and associated supporting tissue
Bones
Main organs of skeletal system:
Like any organ, composed of more than osseous tissue
Also dense regular, irregular collagenous connective tissue and bone marrow
Functions skeletal system
- Protection
- Mineral storage and acid-base homeostasis
- Blood cell formation
- Fat storage
- Movement
- Support
Protection
Certain bones (skull, sternum (breastbone), ribs, and pelvis) protect underlying organs
Mineral storage and acid–base homeostasis
Bone is most important storehouse for calcium, phosphorus, and magnesium salts
-minerals are also present in blood as electrolytes, acids, and bases
-critical for electrolyte and acid–base maintenance
Blood cell formation
Bones house red bone marrow – specialized connective tissue involved in formation of blood cells (hematopoiesis)
Fat storage
Bones also contain yellow bone marrow; made up of fat cells (adipocytes); store triglycerides; fatty acids from breakdown of triglycerides can be used as fuel by cells
Movement
Bones serve as sites for attachment of most skeletal muscles; when muscles contract, they pull on bones; generates movement at joint
Support
Skeleton supports weight of body; provides its structural framework
Bone Structure
Can be organized into 5 classes despite diversity of bone appearance; all 206 bones fit into categories based on shape:
- Long bones
- Short bones
- Flat bones
- Irregular bones
- Sesamoid bones
Long bones
Named for overall shape; not actual size (some are quite small); longer than they are wide; include most bones in arms and legs
Short Bones
Also named for shape rather than size; roughly cube-shaped or about as long as they are wide.
-wrist or carpals
-ankle or tarsals
Flat bones
Thin and broad bones; include ribs, pelvis, sternum (breastbone), and most skull bones
Irregular bones
Include vertebrae and certain skull bones; do not fit into other classes because of irregular shapes
Sesamoid bones
Specialized bones located within tendons
-usually small, flat, and oval-shaped
-give tendons mechanical advantage
-give muscles better leverage
Ex: patella (kneecap)
Structure of long bones: Periosteum
– membrane composed of dense irregular collagenous connective tissue
-rich with blood vessels and nerves
-surrounds outer surface of long bone
Structure of long bones: Perforating Fibers
(Sharpey’s fibers)
– made of collagen;
– anchors periosteum firmly to underlying bone surface by penetrating deep into bone matrix
Structure of long bones: Diaphysis
– shaft of long bone
-each end is epiphysis; covered with thin layer of hyaline cartilage (articular cartilage) found within joints (articulations) between bones
Structure of long bones: Medullary cavity
(marrow cavity) within diaphysis contains either red or yellow bone marrow, depending on bone and age of individual
Structure of long bones: Compact bone
– one of two bone textures; hard, dense outer region; allows bone to resist linear compression and twisting forces among other stresses
Structure of long bones: Spongy bone
(cancellous bone)
– second bone texture; inside cortical bone; honeycomb-like framework of bony struts; allows long bones to resist forces from many directions; provides cavity for bone marrow
Structure of long bone , endosteum
–Thin membrane that cover bony struts of spongy bone and all inner surfaces of bone
- contain different populations of bone cells involved in maintenance of bone homeostasis
Structure of long bones: Epiphyseal lines
– separate both proximal and distal epiphyses from diaphysis
-remnant of epiphyseal plates (growth plates)
- line of hyaline cartilage found in developing bones of children
Structure of short, flat, irregular, and sesamoid bones:
Do not have diaphyses, epiphyses, medullary cavities, epiphyseal lines, or epiphyseal plates:
–Covered by periosteum, with associated perforating fibers, blood vessels, and nerves
–Internal structure – two outer layers of thin compact bone with middle layer of spongy bone (diploë) and associated bone marrow
–Some flat and irregular bones of skull contain hollow, air-filled spaces (sinuses), which reduce bone weight
Blood and nerve supply to bone
–Blood supply to short, flat, irregular, and sesamoid bones is provided mostly by vessels in periosteum that penetrate bone
–Long bones get third of their blood supply from periosteum; mostly supplies compact bone
Red bone marrow
loose connective tissue; supports islands of blood-forming hematopoietic cells
–Amount of red marrow decreases with age
–Red marrow in adult is only in pelvis, proximal femur and humerus, vertebrae, ribs, sternum, clavicles, scapulae, and some bones of skull
–Children need more red marrow to assist in growth and development
Yellow bone marrow
Triglycerides, blood vessels, and adipocytes
MICROSCOPIC STRUCTURE OF BONE TISSUE
Bone or Osseous tissue
Primary tissue found in bone; composed mostly of extracellular matrix with a small population of cells scattered throughout
Extracellular matrix of bones
–Inorganic matrix – minerals make up about 65% of bone’s total weight
–Organic matrix – makes up remaining 35%; consists of collagen fibers and usual ECM components
Inorganic Matrix
Predominantly calcium salts; bone stores around 85% of total calcium ions as well as large amount of phosphorus:
–Calcium and phosphorus salts exist as large molecules of hydroxyapatite crystal
–Crystalline structure makes bone one of hardest substances in body; strong and resistant to compression
–Allows bone to be both protective and supportive
–Bicarbonate, potassium, magnesium, and sodium are also in inorganic matrix
Organic Matrix
Known as osteoid; consists of protein fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins
–Collagen – predominant protein fiber; forms cross-links with one another; helps bone resist torsion (twisting) and tensile (pulling or stretching) forces
–Collagen fibers align themselves with hydroxyapatite crystals; enhances hardness of bone
–Glycosaminoglycans and proteoglycans create an osmotic gradient; draw water into osteoid; help tissue resist compression
–Glycoproteins in osteoid bind different components of osteoid and inorganic matrix together
Bone Cells
Responsible for bone’s dynamic nature:
–Osteoblasts
–Osteocytes
–Osteoclasts
Osteoblasts
Metabolically active bone cells in periosteum and endosteum:
–Osteogenic cells – flattened cells; differentiate into osteoblasts when stimulated by specific chemical signals
–Osteoblasts – bone-building cells; perform bone deposition
–Bone deposition –osteoblasts secrete organic matrix materials; assist in formation of inorganic matrix
Osteocytes
-Osteoblasts eventually surround themselves with matrix in small cavities (lacunae); become osteocytes that no longer actively synthesize bone matrix
–No longer metabolically active except for maintaining bone extracellular matrix
–Appear to have ability to recruit osteoblasts to build up or reinforce bone under tension
Osteoclasts
–Responsible for bone resorption; cell secretes hydrogen ions and enzymes; break down bone matrix
–Completely different overall cell structure than other two cell types; large multinucleated cells; resemble jellyfish; derived from fusion of cells from bone marrow
–Eventually located in shallow depressions on internal and external surfaces of bone
Osteoclasts cont.
–Hydrogen ions dissolve components of inorganic matrix; enzymes break down organic matrix
Structure of compact bone
Continuously subjected to great deal of stress; tends to strain or deform objects like bone; must be able to withstand these forces or suffer damage:
–Compact bone cross section resembles forest of tightly packed trees; each tree is a unit called Osteon or Haversian system
–Rings of each tree are made up of thin layers of bone called Lamellae
Osteon Strcuture
–Each osteon contains 4 to 20 lamellae arranged in layered ring structures (concentric lamellae)
–Lamellar arrangement is very stress resistant
–Collagen fibers of neighboring lamellae run in opposite directions; resist twisting and bending forces from variety of directions
Central Canal (osteon structure)
Endosteum-lined hole in center of each osteon
-blood vessels and nerves supply bone
Osteocytes in lacunae
Small cavities between lamellae; filled with extra cellular fluid
Canaliculi
Network of small passageways (canals) in matrix that connect neighboring lacunae
Overall compact bone structure
Osteons are not permanent structures; osteoclasts break down and osteoblasts rebuild bone matrix depending on needs of bone or body
Characteristic features:
1. Interstitial lamellae
2. Circumferential lamellae
3. Perforating canals (Volkmann’s canals)
Interstitial lamellae
Fill spaces between circular osteons; represent remnants of old osteons
Circumferential lamellae
Outer and inner layers of lamellae just inside periosteum; at boundary with spongy bone; add strength
Perforating canals (Volkmann’s canals)
Originate from blood vessels in periosteum; travel at right angles (perpendicular) to central canals of neighboring osteons; connect them to one another
Structure of spongy bone
–Spongy bone – usually not weight-bearing like compact bone; much less densely packed
–Network of struts reinforces compact bone; resists forces from variety of directions
–Provides protective structure for bone marrow tissue
Structure of spongy bone:–Trabeculae
– struts or ribs of bone; covered with endosteum
Usually not arranged into osteons
Composed of concentric lamellae with osteocytes in lacunae; communicate through canaliculi
Osteopetrosis
(“marble bone disease”) – defective osteoclasts; do not properly degrade bone; cause bone mass to increase and become weak and brittle:
- Infantile
- Adult
Infantile osteopetrosis
Predominately inherited, more severe form; openings of skull and marrow cavities fail to enlarge with growth; traps nerves causing blindness and deafness; decreases blood cell production; can be fatal; must be treated with drugs to stimulate osteoclasts and red marrow
Adult Osteopetrosis
Also inherited; develops during adolescence or later
Symptoms:
bone pain, recurrent fractures, nerve trapping, joint pain; treated symptomatically only
Ossification (osteogenesis)
Process of bone formation
-begins in embryonic period; continues through childhood with most bones completing process by age 7
2 mechanisms of ossification
First bone formed is immature primary (woven) bone; irregularly arranged collagen bundles, osteocytes, and sparse inorganic matrix
Usually primary bone is broken down by osteoclasts and replaced with mature secondary or lamellar bone; more inorganic matrix and increased strength
Intramembranous ossification
Formation of bones that are built on model (starting material) made of membrane of embryonic connective tissue
-Many flat bones (skull and calvicles)
Endochondral ossification
Formation of bones that are built on model of hyaline cartilage
Process of Intramembranous Ossification
- Osteoblasts develop in the primary ossification center from mesenchymal cells
- Osteoblasts secrete organic matrix, which calcifies and trapped osteoblasts become osteocytes
- Osteoblasts lay down trabeculae of early spongy bone, and some of the surrounding mesenchyme differentiate into the periosteum
- Osteoblasts in the periosteum lay down early compact bone
Osteogenic cells-osteoblasts-osteocytes
Fontanels
An example of early incomplete ossification
-Soft spots in skulls of newborn babies
Endochondral Ossification
–Bone development for all bones below head except clavicles
–Begins in fetal stage of development for most bones; some bones (wrist and ankle) ossify much later
–Many bones complete ossification by age 7
Hyaline cartilage mode (of Endochondral ossification)
Chondrocytes, collagen, and ECM all surrounded by connective tissue membrane (perichondrium) and immature cartilage cells (chondroblasts)
Steps of Endochondral ossification
- The chondroblasts in the perichondrium differentiate into osteoblasts
- Osteoblasts build the bone collar on the bones external surface as the bone begins to ossify from the outside
- simultaneously, the internal cartilage begins to calcify and the chondrocytes die
- In the primary ossification center, osteoblasts replace the calcified cartilage with early spongy bone; the secondary ossification centers and medullary cavity develop
- As the medullary cavity enlarges, the remaining cartilage is replaced by bone; the epiphyses finish ossifying
Osteoporosis
Most common bone disease in United States
-bones become weak and brittle due to inadequate inorganic matrix
-increases risk of fractures with decreased rate of healing
Causes of Osteoporosis
Dietary (calcium and/or vitamin D deficiency), female gender, advanced age, lack of exercise, hormonal (lack of estrogen in postmenopausal women), genetic factors, and other diseases
Diagnosis, prevention and treatment of osteoporosis
*Diagnosis – bone density measurement
*Prevention – balanced diet, with supplementation as needed, weight-bearing exercise, and estrogen replacement if appropriate
*Treatment – drugs that inhibit osteoclasts or stimulate osteoblasts
Bone growth in length
*Long bones lengthen by longitudinal growth; involves division of chondrocytes (not osteocytes or osteoblasts) in epiphyseal plate
*Bone growth takes place at epiphysis on side closest to diaphysis
Epiphyseal plate
Composed of hyaline cartilage that did not ossify; five different zones of cells:
- Zone of reserve cartilage
- Zone of proliferation
3.Zone of hypertrophy and maturation - Zone of calcification
- Zone of ossification
Zone of reserve cartilage
(closest to epiphysis) cells that are not directly involved in bone growth but can be recruited for cell division if needed
Zone of proliferation
(next region)
– actively dividing chondrocytes in lacunae
Zone of hypertrophy and maturation
(next region closer to diaphysis)
– mature chondrocytes
Zone of calcification
(second to last region)
– dead chondrocytes; some calcified
Zone of ossification
(last region)
– calcified chondrocytes and osteoblasts
Process of Longitudinal Growth (@ epiphyseal plate)
All zones are involved in this process except zone of reserve cartilage
- Chondrocytes divide in the zone of proliferation
- Chondrocytes that reach the next zone enlarge and mature
- Chondrocytes die and their matrix cells calcifies
- Calcified cartilage is replaced with bone
Mitotic Rate
–Mitotic rate slows around ages of 12-15 years while ossification continues; epiphyseal plates shrink as zone of proliferation is overtaken by zone of calcification and ossification
–Between ages of 18-21, zone of proliferation is completely ossified; longitudinal growth stops; epiphyseal plate is closed
Epiphyseal line
Is calcified remnant of epiphyseal plate
Bone growth in width
(appositional growth)
–Osteoblasts, in between periosteum and bone surface, lay down new bone
–Appositional growth does not result in immediate formation of osteons; instead, new circumferential lamellae are formed
–As new lamellae are added, older deeper circumferential lamellae are removed or restructured into osteons
–Bone growth in width may continue after bone growth in length ceases; depends on factors such as hormones, forces to which bone is subjected, and diet
Achondroplasia
*Most common cause of dwarfism; gene defect inherited from parent or caused by new mutation
*Defective gene produces abnormal growth factor receptor on cartilage; interferes with hyaline cartilage model used in endochondral ossification
*Bones form and grow abnormally; results in short limbs, disproportionately long trunk, and facial abnormalities
*Long-term problems include joint disorders, respiratory difficulties, and spinal cord compression; may be managed with medication
Hormones
Factor that affects bone growth
- are secreted by cells of endocrine glands
Growth hormone
Secreted by anterior pituitary gland; enhances protein synthesis and cell division in nearly all tissues, including bone
Growth hormones effects on both longitudinal and appositional growth
–Increases rate of cell division of chondrocytes in epiphyseal plate
–Increases activity of osteogenic cells, including activity in zone of ossification
–Directly stimulates osteoblasts in periosteum; triggers appositional growth
Hormone Testosterone effects on bone growth
–Increases appositional growth; bones in males become thicker with more calcium salt deposition than females
–Increases rate of mitosis in epiphyseal plate; leads to “growth spurts” in teenage years
–Accelerates closure of epiphyseal plate
Hormone Estrogen effects on bone growth
–Increases rate of longitudinal bone growth; inhibits osteoclast activity
–When estrogen levels spike in teen years an accompanying “growth spurt” occurs in females
–Accelerates closure of epiphyseal plate at much faster rate than testosterone; leads to average height differences between genders
Childhood – gigantism
Epiphyseal growth plates have yet to close; individuals get very tall due to excessive longitudinal and appositional bone growth
Adulthood – acromegaly
Epiphyseal growth plates have closed; no increase in height, but enlargement of bone, cartilage, and soft tissue
–Skull, bones of face, hands, feet, and tongue affected
–Can cause heart and kidney malfunction; associated with development of diabetes
Bone remodeling
Continuous process of bone formation and loss after growth in length is finished; new bone formed by bone deposition; old bone removed by bone resorption
Reasons of the cycle of Bone remodeling
–Maintenance of calcium ion homeostasis
–Replacement of primary bone with secondary bone
–Bone repair
–Replacement of old brittle bone with newer bone
–Adaptation to tension and stress
Bone remodeling rate/process
–In healthy bone of adults, process of formation and loss occur simultaneously; bone breakdown by osteoclasts matches bone formation by osteoblasts
–In childhood, deposition proceeds at much faster rate than resorption; once epiphyseal plates close and longitudinal growth is complete, deposition and resorption become roughly equivalent
Bone deposition
Carried out by osteoblasts
Found in both periosteum and endosteum; make organic matrix and facilitate formation of inorganic matrix
Secrete proteoglycans and glycoproteins that bind to calcium ions
Secrete vesicles containing calcium ions, ATP, and enzymes; bind to collagen fibers; calcium ions eventually crystallize, rupturing vesicle and beginning calcification process
Bone Resorption
Osteoclasts secret hydrogen ions on bone ECM :
Hydroxyapatite crystals in inorganic matrix are pH-sensitive; break down in acidic environment created by osteoclasts
Calcium ions and other liberated minerals can be reused elsewhere in body
Osteoclasts secrete enzymes:
Degrade organic matrix, including proteoglycans, glycosaminoglycans, and glycoproteins
Breakdown products of matrix are taken into osteoclasts for reuse
Bone remodeling in response to tension and stress
- Compression
- Tension
- Pressure
Compression
Squeezing or pressing together; occurs when bones are pressed between body’s weight and ground; stimulates bone deposition
Tension
Stretching force; bone deposition occurs in regions of bone exposed to tension
Pressure
Continuous downward force; bone resorption is stimulated in regions of bone exposed to continuous pressure
Other factors influencing bone remodeling
-Hormones- testosterone promotes bone deposition; estrogen inhibits osteoclast activity
-Age
-Calcium ion intake
-Vitamin D intake
-Vitamin C intake
-Vitamin K intake
-Protein intake
Bone remodeling and calcium ion homeostasis
–Bone stores most of calcium ions in body
–Negative feedback loop maintains calcium ion homeostasis in blood
–Calcium ion levels in blood are closely monitored; both high and low levels can lead to major homeostatic disruptions (even death)
Bone repair
Most dramatic bone injury is fracture (broken bone) :
–Simple fractures – skin and tissue around fracture remain intact
–Compound fractures – skin and tissues around fracture are damaged
Process of fracture healing Step 1
–Hematoma (blood clot) fills in gap between bone fragments
Mass of blood cells and proteins form due to ruptured blood vessels
Bone cells in surrounding area die
Process of fracture healing Step 2
Fibroblasts and chondroblasts (from periosteum) infiltrate hematoma and form soft callus (mixture of hyaline cartilage and collagenous connective tissue); bridges gap between fragments
Fibroblasts form dense irregular collagenous connective tissue
Osteogenic cells become chondroblasts; secrete hyaline cartilage
Process of fracture healing Step 3
Osteoblasts build bone callus (hard callus); collar of primary bone made by osteoblasts in periosteum; forms bridge between fragments
Process of fracture healing Step 4
Bone callus is remodeled and primary bone is replaced with secondary bone; bone regains previous structure and strength after several months
