Chapter 6 - The Skeletal System
What is the epiphyseal growth plate
A layer of hyaline cartilage in the metaphysis of a growing bone that consists of a zone of resting cartilage, zone of proliferating cartilage, zone of hypertrophic cartilage and zone of calcified cartilage
Where do the nutrient arteries supply
Near center of diaphysis the nutrient artery passes through hole called nutrient foramen, entering medullary cavity it splits into proximal and distal branches and supply both the inner part of compact bone in diaphysis and spongy bone and red bone marrow as far up as the epiphyseal plates; small bones such as tibia may have only one nutrient artery, long bones such as femur can have many
Vitamin A in bone growth
Needed for activity of osteoblasts in bone remodelling; deficiency stunts bone growth; too much is toxic
Vitamin K and B12
Needed for synthesis of bone proteins, abnormal protein production in bone extracellular matrix and decreased bone density if deficient
Vitamin C
Needed for synthesis of collagen, the main bone protein; deficiency leads to decreased collagen production which slows down bone growth and delays repair of broken bones
What is the nerve supply for bone
Nerves accompany blood vessels; periosteum rich in sensory nerves and some carry pain sensations and are sensitive to tearing or tension; some pain from bone marrow needle biopsy
Development of secondary ossification center in endochondral ossification of fetal bone development
Occurs when branches of epiphyseal artery enter epiphyses, usually around time of birth. Spongy bone remains in interior of epiphyses (no medullary cavity forms here) and proceeds outward from center of epiphyses toward outer surface of bone
What are circumferential lamillae
areas between osteons are interstitial Surrounds the entire inner and outer circumference of the shaft of long bone and develops during initial bone formation, circumferential lamellae deep to the periosteum is the outer circumferential lamellae and are connected to the periosteum by perforating (Sharpey's) fibers, the circumferential lamillae that lines the medullary cavity are the inner circumferential lamellae.
Zone of hypertrophic cartilage
Large maturing chondrocytes arranged in columns
Calcitonin (CT)
Secreted by thyroid gland; inhibits bone resorption by osteoclasts
Closed (Commute) fracture
Broken ends of bone do not poke through the skin
Open (Compound) fracture
Broken ends of bone protrude through the skin
What are lacunae
Located between concentric lamellae, contain osteocytes
Repair of bone fracture
1) Bone formation of fracture hematoma 2) Fibrocartilaginous callus formation 3) Bony callus formation 4) Bone remodeling
What is the extracellular matrix in bone
15% water, 30% collagen fibers, 55% crystallized mineral salts (calcium phosphate is most abundant mineral salt and forms with calcium hydroxide to become hydroxyapatite). Hydroxyapatite crystals form they join with calium carbonate, magnesium, flouride, potassium, and sulfate. Calcification is when the mineral salts deposit into the framework formed by the collagen fibers of the extracellular matrix, they crystallize and harden and is initiated by bone building cells called osteoblasts.
How much weight does the bone tissue make up in the human body
18%
What happens in a bone marrow needle biopsy
A needle is inserted into the middle of the bone to withdraw a sample of red bone marrow and examine it for conditions such as leukemia, metastatic neoplasms, lymphoma, Hodgkins disease, aplastic anemia, pain is felt when the needle passes through the periosteum
Stress fracture
A series of microscopic fissures that bone forms from stress; result from repeated strenuous activities such as running, jumping or aerobic dancing; very painful; can result from disease processes that disrupt normal bone calcification such as osteoporosis; 25% involve the tibia, show clearly in bone scan
Manganese in bone growth
Activates enzymes in synthesis of bone extracellular matrix
Vitamin D
Active form of calcitriol is produced by the kidneys, helps build bone by increasing absorption of calcium from GI tract into blood; deficiency causes faulty calcification and slows down bone growth; may reduce the risk of osteoporosis, but is toxic if taken in high doses
Bone deposition
Addition of minerals and collagen fibers to bone from osteoblasts, formation of bone extracellular matrix
How do sex hormones affect bone growth and remodelling?
Adrenal glands in woman and men secrete androgens and other tissues such as adipose tissue can convert androgens to estrogens, these hormones are responsible for increased osteoblast activity and synthesis of bone extracellular matrix and sudden growth spurt. Estrogens promote changes in the skeleton such as widening of the hips also shuts down growth at the epiphyseal growth plate causing bone elongation to stop and ends earlier in females than in men due to higher levels of estrogens; during adulthood sex hormones slow down bone resorption and stimulates bone deposition, estrogens stimulate oestoclast apoptosis
What is spongy bone
Also called trabecular or cancellous bone tissue does not contain osteons; always located in the interior of bone; lamellae arranged in irregular patterns of thin columns called trabeculae; each trabeculae contains concentric lamellae; between trabeculae are spaces seen by the unaided eye and are filled with red bone marrow in bones that produce red blood cells and yellow bone marrow (adipose tissue) in other bones; makes up most of the interior part of bone tissue in short, flat, sesamoid or irregularly shaped bones, forms core of epiphyses beneath the paper thin layer of compact bone, forms a variable rim bordering the medullary cavity of the diaphysis; always covered by a layer of compact bone for protection; located where bones are not heavily stressed or where stress is applied from many different angles
Fracture
Any break in bone, named according to severity, shape or position of the fracture line, or physician who first described them
Bony callus formation in bone repair
Areas of well-vascularized healthy bone tissue osteogenic cells develop into osteoblasts that produce spongy bone trabeculae, trabeculae join living and spongy bone tissue and the fibrocartilage is converted to spongy bone, callus is then called a bony (hard) callus. The bony callus lasts about 3-4 months
Formation of trabeculae in intramembranous fetal bone development
As bone extracellular matrix forms it changes into trabeculae that fuse together to form spongy bone around network of blood vessels in a connective tissue, this connective tissue forms red bone marrow.
How does the acitivity of the epiphyseal plate in infancy, childhood and adolescence help increase bone length
As bone grows chondrocytes proliferate on the epiphyseal side of the growth plate and new chondrocytes replace old ones and are destroyed by calcification. The cartilage on the diaphysis side of the epiphyseal growth plate is replaced by bone. This helps maintain the size of the epiphyseal growth plate but still increases the length of the bone
Development of ossification center in intramembranous ossification fetal bone development
At site where bone is to harden, specific chemical messages cause the mesenchymal cells to cluster (ossification center) and differentiate into osteogenic cells and then into osteoblasts that secretes the extracellular matrix that forms bone until they are surrounded by it.
Development of cartilage model in endochondral ossification in fetal bone development
At site where the bone is going to form, Chemical messages cause the mesenchymal cells to form together in a general shape of the future bone and develop into chondroblasts. Chondroblasts secrete cartilage extracellular matrix, producing a cartilage model consisting of hyaline cartilage; a covering called the perichondrium develops around the cartilage model.
Development of periosteum in intramembranous fetal bone development
At the same time as the formation of trabeculae, mesenchyme condenses at periphery of bone and develops into periosteum, thin layer of compact bone replaces the surface layers of spongy bone, but spongy bone remains in center, most of newly formed bone is remodeled and transformed into adult size and shape
Bone formation of fracture hematoma in bone repair
Blood vessels crossing fracture line is broken, as a result of loss of blood to the area bone cells die, blood lost in this area form a hematoma that takes about 6-8 hours to form, bone cells dying triggers inflammation that brings phagocytes such as neutrophils and macrophages and osteoclasts into the area to remove dead and damaged tissue in and around the fracture hematoma. This may last a couple weeks
What are osteoblasts
Bone building cells, synthesize and secrete collagen fibers and other organic components needed to make extracellular matrix of bone tissue, initializes calcification; surrounds themselves with extracellular matrix, trapped in their secretions and become osteocytes. When ending in -blast it means that the bone cell or any other connective tissue it secretes extracellular matrix. Do not undergo cell division. Osteoblasts build bone
Comminuted fracture
Bone is splintered, crushed or broken into pieces at site of impact and smaller pieces sit between the two main fragments
Reduction of bone
Bones to unite properly, fractured ends must be brought into alignment. Following reduction the limb or bone may be kept immobilized by cast, sling, splint, elastic bandage, external fixation device or combination of these
Why is Maintaining level of calcium (Ca+) so important
Bone stores 99% the body's calcium; nerve and muscle cells depend on a stable level of calcium ions in extracellular fluid to function properly; blood clotting requires calcium and blood plasma levels are closely regulated between 9-11mg and can be fatal if outside these normal ranges (concentration too high - heart may stop, too low - respiratory arrest).
What is the diaphysis
Bones shaft or body
Bone growth in infancy, childhood, and adolescence
Bones throughout body grow in thickness by appositional growth. Long bones lengthen by addition of bone material on diaphyseal side of epiphyseal growth plate by interstitial growth
What constitutes the skeletal system
Cartilage, Bones, Tendons, Ligaments
What is concentric lamellae in compact bone
Circular plates of mineralized extracellular matrix of increasing diameter surrounding a network of blood vessels, lymphatics and nerves in the central canal, arranged around a central haversian canal
Initial bone formation in embryo and fetus
Composed of mesenchyme in general shape of bones is site where cartilage and bone formation occurs in sixth week of embryonic development. Two types: Intramembranous ossification and endochondral ossification
What is compact bone
Comprises 80% of bone tissue. Contains few spaces, is the strongest of the types of bone tissue, located beneath the periosteum of all bones and in the bulk of the diaphyses of long bones, provides protection and support, resists stresses produced by body weight and movement. Comprised of repeating structural units called Osteons or Haversian systems,
What is inside each trabecula
Concentric lamellae, osteocytes that lie in lacunae and canaliculi that radiate outward from the lacunae
What are the transverse perforating canals (Volkmann's canals)
Contain the blood vessels, nerves and lymphatic vessels from the peritoneum, these vessels and nerves connect with those of the medullary cavity, periosteum, and central canals
Bone remodeling of callus in bone repair
Dead portions of original fragments of broken bone are resorbed by osteoclasts, compact bone replaces spongy bone around periphery of fracture. Sometimes repair process is so thorough that the broken fracture line cannot be seen by x-ray
Where do the epiphyseal arteries supply
Enter epiphyses of long bones and supply red bone marrow and bone tissue of the epiphyses
Where do the metaphyseal arteries supply
Enter metaphysis of long bone and with the nutrient artery supply the red bone marrow and bone marrow of the metaphyses
What is the epiphyseal line
Epiphyseal plate closes between the ages of 18 and 21 which is caused by the epiphyseal cartilage cells no longer dividing and bone replaces all remaining cartilage. All bone growth stops at this point. Happens 1-2 years earlier in females
Sex hormones (estrogens and testosterone)
Estrogens secreted in ovaries, testosterone secreted in testes, stimulates osteoblasts and growth spurt in adolescence; shuts down growth at the epiphyseal growth plate around 18-21 years of age stopping length wise growth of bone; promoting bone deposition in adult years by stimulating osteoblasts and slowing down osteoclasts
Fibrocartilaginous callus formation in bone repair
Fibroblasts from periosteum invade fracture site and produce collagen fibers. Cells from periosteum develop into chondroblasts and begin producing fibrocartilage that leads to fibrocartilaginous (soft) callus (a mass of repair tissue consisting of collagen fibers and cartilage that bridges the broken bone. This takes about 3 weeks
Zone of calcified cartilage
Final zone of epiphyseal plate is only a few cells thick and consists mostly of chondrocytes that are dead because the extracellular matrix around them has calcified. Osteoclasts dissolve the calcified cartilage and the osteoblasts and capilleries of the diaphysis invade the area and lay down extracellular matrix, replacing the calcified cartilage with bone. The zone of calcified cartilage now becomes the new diaphysis that is firmly cemented into the rest of the diaphysis of the bone
Intramembranous ossification in embryo and fetus bone development
Flat bones of skull, medial portion of clavicle, mandible, facial bones and soft spots in the fetus skull to aid delivery of baby from womb also harden in this way. 1) Development of ossification center At site where bone is to harden, specific chemical messages cause the mesenchymal cells to cluster (ossification center) and differentiate into osteogenic cells and then into osteoblasts that secretes the extracellular matrix that forms bone until they are surrounded by it. 2) Calcification Secretion of extracellular matrix stops and the osteocytes lie in lacunae and extend narrow cytoplasmic processes into canaliculi that radiates in all directions. Within a few days, Calcium and other mineral salts are deposited and extracellular matrix hardens or calcifies. 3) Formation of trabeculae As bone extracellular matrix forms it changes into trabeculae that fuse together to form spongy bone around network of blood vessels in a connective tissue, this connective tissue forms red bone marrow. 4) Development of periosteum At the same time as the formation of trabeculae, mesenchyme condenses at periphery of bone and develops into periosteum, thin layer of compact bone replaces the surface layers of spongy bone, but spongy bone remains in center, most of newly formed bone is remodeled and transformed into adult size and shape
Pott fracture
Fracture of distal end of lateral leg bone with serious injury to the distal tibial articulation
Colles fracture
Fracture of distal end of lateral radius in which the distal fragment is pushed posteriorly
Open reduction
Fractured ends of bone are brought into alignment by surgical methods using internal fixation devices such as screws, plates, pins, rods and wires
Closed reduction
Fractured ends of bone brought into alignment by manual manipulation
Bone growth in thickness
Grows by appositional growth. Surface of bone periosteal cells differentiate into osteoblasts that secrete collagen fibers and other organic molecules that form extracellular matrix; osteoblasts surrounded by extracellular matrix turn into osteocytes; ridges are formed on either side of periosteal blood vessels and slowly enlarge creating groove for periosteal blood vessel; ridges fold together and fuse, groove becomes a tunnel that encloses the blood vessel, periosteum then becomes the endosteum that lines the tunnel; osteoblasts deposit bone extracellular matrix that forms new concentric lamellae that forms inward producing a new osteon that houses the blood vessel in its center; growth process continues as more blood vessels are enclosed and more osteons are formed in this way
What are osteons in compact bone
Have haversian systems that are repeating structural units and contains concentric lamellae arranged around a central haversian canal
What is bones role in maintaining homeostasis
Helps buffer the blood Ca+ level, releases Ca+ into blood plasma using osteoclasts and absorbing Ca+ when the levels rise
Magnesium in bone growth
Helps form bone extracellular matrix
Fluoride in bone growth
Helps strengthen bone extracellular matrix
How does a fracture at the epiphyseal growth plate limit bone growth
If a fracture damages the epiphyseal growth plate it will cause damage to the cartilage which is avascular will reduce the cartilage cell division which inhibits the lengthwise growth of bone, therefore the fractured bone may be shorter than normal as an adult
What is an osteoclast
Huge cells derived from 50 or more monocytes (type of white blood cell) and are concentrated in the endosteum. Plasma membrane is deeply folded into a ruffled border on the side of the cell that faces the bones surface and releases powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying extracellular bone matrix (termed resorption) is part of the normal development, maintenance and repair of bone. Ending clast means that the cell breaks down the extracellular matrix. Help regulate blood calcium levels. Osteoclasts carve bone
How are IGFs produced during bone growth and remodelling?
Human growth hormone is released from the anterior lobe of the pituitary gland and stimulates production of Insulinlike growth factors (IGFs) Insulinlike growth factors enhance synthesis of bone proteins, stimulate osteoblasts and promote cell division at the epiphyseal growth plate and periosteum
Formation of cartilage in endochondral ossification in fetal bone development
Hyaline cartilage that covers the epiphyses becomes articular cartilage, and hyaline cartilage remains in the epiphyseal growth plate prior to adulthood
Growth in length of bone in infancy, childhood and adolescence
Involves interstial growth of cartilage on the epiphyseal side of the epiphyseal growth plate and replacement of cartilage on the diaphyseal side of the epiphyseal growth plate with bone by endochondral ossification
Zone of resting cartilage in the epiphyseal growth plate
Layer that is nearest the epiphysis and consists of small, scattered chondrocytes (these cells are resting because they do not function in bone growth) chondrocytes anchor the epiphyseal plate to the epiphsis of the bone
What is the metaphyses
Located between the diaphysis and epiphyses of bone; each metaphyses has an epiphyseal plate where the bone grows surrounded by hyaline cartilage that allows the bone to grow in length. At around 18-21 years of age the epiphyseal plate cartilage will stop growing and will be replaced by a bony structure called the epiphyseal line.
What are osteocytes
Located inside canaliculi, slender fingerlike processes that communicate via gap junctions with neighboring osteocytes
Calcium and phosphorous in bone growth
Make bone extracellular matrix
What is the medullary cavity
Marrow cavity; cylinderical, narrow space in the diaphysis, contains yellow bone marrow and numerous blood vessels in adults; reduces the weight of bone by minimizing the dense bony material where it needs it least, the long tubular design provides maximum strength with minimum weight
What are osteocytes
Mature bone cells, main cells in bone tissue and maintains its daily metabolism, such as exchange of nutrients and wastes with the blood. Do not undergo cell division. Cyte in the name of a bone cell or any other tissue means that the cell is maintaining the tissue.
Factors affecting bone growth and remodelling
Minerals: lots of calcium, phosphorous; small amounts of magnesium, flouride, and manganese Vitamins: Vitamin A (stimulates action of osteoblasts); Vitamin C (synthesis of collagen, main bone protein); Vitamin D (build bone by increasing the absorption of calcium from food in the gastrointestinal tract into blood; Vitamins K and B12 (synthesis of bone proteins) Hormones: Insulinlike growth factors (IGFs) that are produced by the liver and bone tissue; IGFs stimulate osteoblasts, promote cell division at the epiphyseal growth plate and in the periosteum, enhance synthesis of proteins to build new bone; Thyroid gland hormones T3 and T4 stimulate osteoblasts; insulin from the pancreas increases synthesis of bone proteins; Parathyroid hormone, calcitriol (active form of vitamin D) also affects bone remodelling
Growth of cartilage model of endochondral ossification of fetal bone development
Once chondroblasts are firmly embedded into the carilage extracellular matrix they are called chondrocytes; cartilage model grows in length by continual cell division of chondrocytes and further secretion of cartilage extracellular matrix also called interstitial (endogenous) growth (from within) results in increase in length; growth of cartilage in thickness is from apositional (exogenous) growth from the outer surfaces by chondroblasts (that developed from perichondrium) depositing extracellular matrix material on the cartilage surface; as cartilage grows the chondrocytes increase in size and surrounding cartilage extracellular matrix calcify and other chondrocytes within the calcifying cartilage die because of lack of nutrients; holes left by dead chondrocytes form small spaces that develop into lacunae
Impacted fracture
One end of bone is forcefully driven into the interior of the other
Where are veins that carry blood away from long bones present
One or two nutrient veins accompany the nutrient artery and exit through the diaphysis; numerous epiphyseal and metaphyseal veins accompany their arteries and exit through the epiphyses; many small periosteal veins accompany their arteries and exit through the persiosteum
Remodeling of bone
Ongoing replacement of new bone tissue with old bone tissue. 5% of total bone mass in body is being remodeled at any given time. Renewal rate for compact bone is 4% per year and 20% per year for spongy bone. Some bones and areas of bone remodel at different rates. Removes old or damaged bone. Triggered by excercise, sedentary lifestyle and changes in diet. If bone is introduced to heavy loads it will grow thick and stronger than the old bone and resistant to fracture than old bone.
What are the types of cells found in bone
Osteogenic cells; Osteoblasts; Osteocytes; Osteoclasts
What is the purpose of the arrangement of the trabeculae in spongy bone
Precisely orientated along lines of stress which helps bone resist stresses and transfer forces without breaking; are not arranged in finality until true locomotion is learned completely and can change due to a poorly healed fracture or deformity
Compact bone from outside layers to inside layers
Osteons or haversian systems; osteon consists of concentric lamellae arranged around central haversian canal; between concentric lamellae are lacunae that contain osteocytes; from lacunae are canaliculi filled with extracellular fluid; inside canaliculi are osteocytes that communicate via gap junctions; between neighboring osteons are interstitial lamellae which have lacunae, osteocytes and canaliculi; transverse perforating canals contain blood vessels, lymphatic vessels, nerves from periosteum and connect with the vessels and nerves with the medullary cavity, periosteum and central canals; arranged in the entire outer and inner circumference of the shaft of long bone are lamellae, called outer circumferential lamellae they are connected to periosteum by perforating (sharpey's) fibers and inner circumferential lamillae line the medullary cavity.
What hormones regulate calcium exchange in a negative feedback system
Parathyroid hormone (PTH) is most important, is secreted by the parathyroid gland cells (effectors), increases blood Ca+ level by increasing production of cyclic adenosine monophosphate (cyclic AMP), the gene for the PTH in the nucleus within the parathyroid gland cell (control center) detects the cyclic AMP in the intracellular fluid this speeds up PTH synthesis (the output) into the blood, high levels of PTH hormone increases the number and activity of osteoclasts (effectors) which increases the bone resorption, resulting release of bone into blood returns blood calcium levels back to normal. PTH acts on kidneys (effectors) to decrease loss of Ca+ in urine so more is retained in the blood. PTH stimulates formation of calcitriol (active form of vitamin D) to increase the absorption of dietary calcium from the gastrointestinal tract into the blood increasing blood calcium levels
Greenstick fracture
Partial fracture where one side of bone is broken and other side bends, bends occur only in children because they are not fully ossified and contain more organic material than inorganic material
Exercise and bone tissue
Placed under stress bone has the ability become stronger, deposition of mineral salts and production of collagen fibers by osteoblasts; running, jumping stimulates bone growth more dramatically than walking; main mechanical stresses on bone: pull of skeletal muscles, pull of gravity
What is red bone marrow
Present in bones of fetus and some adult bones such as hip, ribs, sternum, vertebrae, skull and ends of long bones of humerus, femur; in newborn all bone marrow is red and is involved in hemopoiesis; increasing age red bone marrow changes from red to yellow
Development of medullary (marrow) cavity in endochondral ossification of fetal bone development
Primary ossification grows towards ends of bone, osteoclasts break down trabeculae and form holes called the medullary cavity in the diaphysis (shaft) of bone and eventually most of the wall of the diaphysis is replaced by compact bone.
Development of primary ossification center in endochondral ossification in fetal bone development
Proceeds inward from the external surface of bone; nutrient artery penetrates the perichondrium and calcifying cartilage model through a nutrient foramen in the mid region of cartilage model, this stimulates the osteogenic cells in the perichondrium to differentiate into osteoblasts. Perichondrium starts to form bone then it is called the periosteum. Near middle of the cartilage model where the cartilage is disintegrating periosteal capillaries grow inducing the growth of the primary ossification center, a region where bone tissue replaces most of cartilage. Osteoblasts deposit bone extracellular matrix over remnants of calicified cartilage, forming spongy bone trabeculae and spreads to both ends of cartilage model
What is the purpose of red and yellow bone marrow in spongy bone
Produce many blood vessels and provides nourishment to osteocytes
What is hemopoiesis
Production of red and white blood cells and platelets in red bone marrow
What is the epiphyses
Proximal and distal ends of the bone
What is Canaliculi
Radiate from the lacunae, are tiny and filled with extracellular fluid, connect lacunae with one another and with central canals, forms intricate, miniature system of interconnected canals that allow nutrients and oxygen to reach osteocytes and remove waste
Bone resorption
Removal of minerals and collagen fibers from bone by oestoclasts, destruction of extracellular matrix. Osteoclasts attach to the endosteum or periosteum and forms a leakproof seal at the ruffled border, releases protein digesting lysosomal enzymes and several acids into sealed pockets; acids digest organic molecules and collagen fibers while the acids desolve the bone minerals; osteoclasts carve out the a small tunnel in the old bone; degraded bone proteins and extracellular matrix minerals, mainly calcium and phosphorous, enter osteoclast by endocytosis, cross the cell in a vesicle, and undergo exocytosis on opposite side of ruffled border; products of bone resorption diffuse into nearby blood capillaries; once small area of bone has been resorbed, osteoclasts depart and osteoblasts move in to mend the bone
Endochondral ossification in fetal and embryonic bone for development
Replacement of cartilage by bone, usually observed in long bones 1) Development of cartilage model: At site where the bone is going to form, Chemical messages cause the mesenchymal cells to form together in a general shape of the future bone and develop into chondroblasts. Chondroblasts secrete cartilage extracellular matrix, producing a cartilage model consisting of hyaline cartilage; a covering called the perichondrium develops around the cartilage model. 2) Growth of the cartilage model: Once chondroblasts are firmly embedded into the carilage extracellular matrix they are called chondrocytes; cartilage model grows in length by continual cell division of chondrocytes and further secretion of cartilage extracellular matrix also called interstitial (endogenous) growth (from within) results in increase in length; growth of cartilage in thickness is from apositional (exogenous) growth from the outer surfaces by chondroblasts (that developed from perichondrium) depositing extracellular matrix material on the cartilage surface; as cartilage grows the chondrocytes increase in size and surrounding cartilage extracellular matrix calcify and other chondrocytes within the calcifying cartilage die because of lack of nutrients; holes left by dead chondrocytes form small spaces that develop into lacunae 3) Development of primary ossification center: Proceeds inward from the external surface of bone; nutrient artery penetrates the perichondrium and calcifying cartilage model through a nutrient foramen in the mid region of cartilage model, this stimulates the osteogenic cells in the perichondrium to differentiate into osteoblasts. Perichondrium starts to form bone then it is called the periosteum. Near middle of the cartilage model where the cartilage is disintegrating periosteal capillaries grow inducing the growth of the primary ossification center, a region where bone tissue replaces most of cartilage. Osteoblasts deposit bone extracellular matrix over remnants of calicified cartilage, forming spongy bone trabeculae and spreads to both ends of cartilage model 4) Development of medullary (marrow) cavity: Primary ossification grows towards ends of bone, osteoclasts break down trabeculae and form holes called the medullary cavity in the diaphysis (shaft) of bone and eventually most of the wall of the diaphysis is replaced by compact bone. 5) Development of secondary ossification center: Occurs when branches of epiphyseal artery enter epiphyses, usually around time of birth. Spongy bone remains in interior of epiphyses (no medullary cavity forms here) and proceeds outward from center of epiphyses toward outer surface of bone 6) Formation of articular cartilage: Hyaline cartilage that covers the epiphyses becomes articular cartilage, and hyaline cartilage remains in the epiphyseal growth plate prior to adulthood
Human growth hormon (hGH)
Secreted by anterior lobe of pituitary gland, promotes growth in all body tissues, including bone, mainly stimulates insulinlike growth factors
Insulinlike growth factors (IGFs)
Secreted by liver, bones and other tissues mainly upon stimulation of human growth hormone; promotes normal bone growth by stimulating osteoblasts and by decreasing synthesis of proteins to build new bone
Parathyroid hormone (PTH)
Secreted by parathyroid glands; promotes bone resorption by osteoclasts; enhances recovery of calcium ions from urine; promotes formation of active form of vitamin d (calcitriol)
Thyroid hormones (thyroxine and triiodothyronine)
Secreted by thyroid gland; promote normal bone growth by stimulating osteoblasts
Calcification in intramembranous ossification in fetal bone development
Secretion of extracellular matrix stops and the osteocytes lie in lacunae and extend narrow cytoplasmic processes into canaliculi that radiates in all directions. Within a few days, Calcium and other mineral salts are deposited and extracellular matrix hardens or calcifies.
Aging and bone tissue
Sex hormone levels dimish from age bone resorption by osteoclasts outpace bone deposition by osteoblasts which can lead to ostoporosis in old age; loss of bone mass resulting from demineralization (loss of calcium, other minerals from bone extracellular matrix) calcium bone loss starts after 30 in woman (70% total bone mass lost by age 70) and after 60 in men (3% of calcium bone loss every 10 years); Brittleness resulting from decreased protein synthesis, loss of tensile strength mainly from collagen fiber loss, susceptible to fracture, deformity, pain, loss of height and loss of teeth
Aging
Sex levels diminish especially in woman in later years, especially after menopause, bone resorption by osteoclasts out paces bone deposition by osteoblasts resulting in decreased bone mass which can lead to osteoporosis
Zone of proliferating cartilage
Slightly larger chondrocytes are arranged like stacked coins, they undergo interstitial growth as they divide and secrete extracelllular matrix, chondrocytes divide and replace those that die at the diaphyseal side of the epiphyseal growth plate
What are periosteal arteries
Small arteries accompanied by nerves that enter diaphysis through the perforating Volksmanns canals and supply the outer periosteum and outer part of compact bone
What is osteology
Study of bone structure and the treatment of bone disorders
What are the functions of the skeletal system
Support (soft tissues, structural framework, attachment for tendons of most skeletal muscles) Protection (internal organs) Assistance with movement (most skeletal muscles attach to bones when the muscles contract the bones move to produce movement) Mineral homeostasis - storage and release (99% of the bodies calcium is stored in bone along with many other minerals such as calcium and phosphorous. When the body needs it it is released into the blood stream to be used in the body and maintain homeostasis) Blood cell production (hemopoiesis in red bone marrow, producing blood cells, adipocytes, fibroblasts, macrophages within network of reticular fibers) Triglyceride storage (yellow bone marrow stores mostly triglycerides which are a potential energy reserve)
What is ossification
The process which bone is formed. Also called osteogenesis. Occurs in four principal situations 1) initial formation of bones in an embryo or fetus 2) growth of bones in infancy, childhood and adolescence 3) remodeling (replacing of old bone with new bone) of bone throughout life 4) repair of fractures
What is the periosteum
Thick connective tissue sheath and its associated blood supply that surrounds bone wherever it is not covered by hyaline cartilage. Outer fibrous layer (dense irregular connective tissue) and inner osteogenic layer (consists of cells); some cells enable bone to grow in thickness, but not in length. Protects, assists in fracture repair, nourish bone tissue, serves as attachment point for some ligaments and tendons. Attached to underlying bone by perforating (sharpey's) fibers (thick bundles of collagen that extend the periosteum into the bone extracellular matrix)
What is the articular cartilage
Thin layer of hyaline cartilage covering part of the epiphyses where bone forms an articulation (joint) with another bone. Reduces friction, absorbs shock at freely movable joints; lacks perichondrium and blood vessels so repair is minimal
What is the endosteum
Thin membrane that lines the medullary cavity; single layer of bone-forming cells and a small amount of connective tissue
What are osteogenic cells
Unspecialized bone stem cells that derived from mesenchyme (tissue from which almost all connective tissues are formed), only bone cells that undergo cell division resulting in osteoblasts. Found along the inner portion of periosteum, in the endosteum and the canals within bone that contain blood vessels
Treatment for fractures
Vary according to age, type of fracture, bone involved; goals are immobilization, realignment, restoration of function
Exercise
Weight-bearing exercise helps to strengthen and thicken bone by stimulating osteoblasts, slows bone loss as people age
What hormone works to decrease blood calcium levels
When blood calcium levels rise above normal parafollicular cells in the thyroid gland secrete calcitonin which inhibits activity of osteoclasts, speeds blood Ca+ uptake into bone and accelerates Ca+ deposition into bone, slows bone resorption
What is calcification
When the mineral salts are deposited among the collagen fibers they begin to crystallize in the microscopic spaces between the collagen fibers, after the spaces are filled the mineral crystals accumulate around the collagen fibers. The combination of crystallized salts and collagen fibers is responsible for the characteristics of bone. Collagen fibers give bone its flexibilty while the cyrstallized minerals give bone its hardness. Collagen fibers with other organic molecules provide tensile strength (resistance to being stretched or torn apart)
Are bones considered organs
Yes
