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