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ppt7 bone formation
Spring 2012
72
Anatomy
Undergraduate 4
02/01/2012

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Term
what is ossification?
Definition
formation of bone
Term
what is osteogenesis?
Definition
another term for the formation of bone
Term
what are the two methods of ossification?
Definition
intramembranous
endochondra
Term
what is the result of intramembranous ossification?
Definition
production of flat bones (of skull and most of clavicle)
Term
what is the result of endochondral ossification?
Definition
responsible for producing most of the skeleton
Term
what does endochondral ossification create bone out of?
Definition
bone develops from hyaline cartilage model
Term
when does endochondral ossification occur?
Definition
six weeks fetal development to early 20s
Term
what are the first two steps for intramembranous ossification?
Definition
1. An area of the embryonic connective tissue (mesenchyme) condenses into a layer of soft tissue with a dense supply of blood capillaries. The mesenchymal cells enlarge and differentiate into osteogenic cells, and regions of mesenchyme become a network of soft sheets called trabeculae.
2. Osteogenic cells gather on these trabeculae and differentiate into osteoblasts. These cells deposit an organic matrix called osteoid tissue—soft collagenous tissue similar to bone except for a lack of minerals . As the trabeculae grow thicker, calcium phosphate is deposited in the matrix. Some osteoblasts become trapped in the matrix and are now osteocytes. Mesenchyme close to the surface of a trabecula remains uncalcified, but becomes denser and more fibrous, forming a periosteum.
Term
what do osteoblasts do?
Definition
deposit soft osteoid tissue and then calcify it into bone
Term
what do calicified trabeculae become?
Definition
spongy bone
Term
how is compact surface bone formed?
Definition
filling in the spaces between trabeculae (forming spongy bone) with osseous tissue
Term
what is mesenchyme?
Definition
embryonic connective tissue
Term
what are the last two steps of intramembranous bone development?
Definition
3. Osteoblasts continue to deposit minerals, producing a honeycomb of bony trabeculae. Some trabeculae persist as permanent spongy bone, while osteoclasts resorb and remodel others to form a marrow cavity in the middle of the bone.
4. Trabeculae at the surface continue to calcify until the spaces between them are filled in, converting the spongy bone to compact bone. This process gives rise to the sandwichlike arrangement typical of mature flat bones.
Term
how is the primary ossification center formed in endochondral ossification?
Definition
formed by chondrocytes in the middle of the cartilage model that enlarge
Term
what is a lacuna?
Definition
small space containing an osteocyte in bone or a chondrocyte in cartilage
Term
how is marrow cavity formed in endochondral bone development?
Definition
cartilage lacunae (the small spaces containing chondrocytes in cartilage) break down as the chondrocytes enlarge to form the primary ossification center
Term
what happens to the chondrocytes as their lacunae break down to form the primary marrow cavity?
Definition
they die
Term
how do stem cells enter the primary marrow cavity? what happens once they're there?
Definition
enter via blood vessels

differentiate into osteoblasts and osteoclasts
Term
what do osteoclasts do in endochondral bone development?
Definition
dissolve cartilage remnants and enlarge the marrow cavity, which grows toward the ends of the bone.
Term
what is the secondary marrow cavity? when does it start to form?
Definition
ossification proceeds from secondary ossification center that appears near birth in the middle of the epiphysis. This creates a secondary marrow cavity at ends of new bones and leaves a layer of articular cartilage over the end of th ebone
Term
do primary and secondary marrow cavities ever connect?
Definition
no, they remain separated by a cartilaginous epiphyseal plate
Term
how do bones become longer?
Definition
by interstitial growth of cartilage in a transitional zone
Term
what is the metaphysis?
Definition
the transitional zone in interstitial cartilage growth

lies between the primary marrow cavity and the cartilage of an epiphysis or epiphyseal plate
Term
what are the five zones of the metaphysis in order of farthest to closest to marrow space?
Definition
1) zone of reserve cartilage
2) zone of cell proliferation
3) zone of cell hypertrophy
4) zone of calcification
5) zone of bone deposition
Term
when are epiphyseal plates depleted/stop appearing completely?
Definition
depleted from the end of adolesence into the 20s

do not appear in adults 30 and older
Term
what occurs in the zone of cell proliferation?
Definition
chondrocytes multiply and form longitudinal columns of cells
Term
what occurs in the zone of cell hypertrophy?
Definition
chondrocytes in this zone enlarge
Term
what occurs in the zone of calcifications?
Definition
the matrix becomes temporarily calcified
Term
what occurs in the zone of bone deposition?
Definition
lacunae break down, chondrocytes die, and bone is deposited
Term
what is the significance of epiphyseal plate depletion in late adolesence/early 20s?
Definition
a bone cannot grow longer (a person cannot grow taller either) once the epiphyseal plates are depleted and the gaps between the epiphyses and the diaphyses close
Term
what are epiphyseal lines?
Definition
surface markings on bone where epiphyseal plates used to be before they undergo depletion in late adolescence/early 20s
Term
what is appositional growth?
Definition
growth of bone thickness and width; adds more matrix to the surface
Term
to what other ossification process is appositional growth similar?
Definition
appositional growth is similar to intramembranous ossification
Term
what are the roles of calcium and phosphate in bone development?
Definition
raw materials for calcified ground substance
Term
what is the role of vitamin A in bone development?
Definition
promotes formation of glycosaminoglycans (protein-carb fibers)
Term
what is the role of vitamin C in bone development?
Definition
promotes collagen cross linking
Term
what is the role of vitamin D in bone development?
Definition
necessary for calcium absorption by small intestine and reduces urinary calcium loss
Term
what hormones stimulate bone growth?
Definition
calcitonin
growth hormone
estrogen
testosterone
thyroid hormone
insulin
Term
what does calcitonin do?
where does it come from?
who does it primarily affect?
Definition
a hormone secreted by the thyroid gland
stimulates osteoblast activity
functions chiefly in children and pregnant women
little significance in nonpregnant adults
Term
how does growth hormone contribute to bone development?
Definition
promotes intestinal absorption of calcium
proliferation of cartilage at the epiphyseal plates
elongation of bones
Term
how do sex steroids (estrogen and testosterone) contribute to bone development?
Definition
stimulate osteoblasts
promote growth of long bones, esp. in adolesence
Term
what factors promote bone deposition?
Definition
thyroid hormone
insulin
local growth factors
parathyroid hormone (main player)
Term
where are local growth factors produced?
Definition
from within the bone itself
Term
what stimulates the four small parathyroid glands to secrete PTH? Why?
Definition
Secrete PTH in response to a drop in blood calcium level
PTH stimulates osteoblasts which then secrete an osteoclast-stimulating factor that promotes bone resorption by the osteoclasts, thus maintains an appropriate blood calcium level

also reduces urinary calcium loss
promotes calcitrol synthesis
Term
what is anchondroplastic dwarfism a result of?
Definition
a spontaneous mutation that can arise any time DNA is replicated (dominant mutant allele)
Term
do bones change throughout lifetime?
Definition
yes, they are remodeled to accommodate bodily growth and changes in force applied to the skeleton
Term
what is Wolff's law of bone?
Definition
osteoblasts and osteoclasts reshape bones throughout life to adapt to the stresses placed on them
Term
how does bone repair a fracture?
Definition
callus formation at the fracture site is composed of clotted blood. it eventually is replaced by connective tissue and cartilage
endochondral ossification follows
Term
how long does it take for endochondral ossification to replace the cartilaginous plug of a fracture?
Definition
6-8 weeks
Term
which method of bone formation is referred to as being direct and why?
Definition
intramembranous ossification- because there is no cartilaginous template as in endochondral ossification
Term
which method of bone formation is referred to as being indirect and why?
Definition
endochondral ossification- because a cartilaginous template is formed prior to ossification
Term
Explain the sequence of intramembranous bone formation?
Definition
1. mesenchymal cells condense into a connective tissue membrane
2. mesenchymal cells differentiate into osteoblasts, osteoid is secreted and forms bone spicules
3. calcium phosphate salts are incorporated into osteoid matrix, osteoblasts are surrounded and become osteocytes
4. spicules continue to develop, the spaces inbetween form the marrow cavity, the outer and inner membranes undergo compaction
5. remodelling
Term
What types of bones are normally formed by intramemrabous formation?
Definition
flatbones, ie. of the skull
Term
what is meant by endochondral ossification?
Definition
bones are formed by a cartilage model
Term
where does endochondral ossification initially occur?
Definition
in the primary ossification center
Term
which type of bone normally exhibits endochondral ossification?
Definition
long bones
Term
what is the first step in endochondral ossification?
Definition
a fetal template of hyaline cartilage is formed
Term
why is the first step in endochondral ossification important?
Definition
must have template of cartilage for ossification to occur, the template is a model for what is to come
Term
what is the critical second step in endochondral ossification?
Definition
calcification of the perichondrium
Term
where is the primary center of ossification located?
Definition
in the diaphysis
Term
what occurs at the primary center of ossification?
Definition
osteoclasts resorb bone spicules to form marrow cavity, osteoblasts form new bone
Term
how does the marrow cavity form in endochondral ossification?
Definition
by the action of osteoclasts resorbing bone spicules
Term
where do the secondary centers of ossification occur?
Definition
epiphysis
Term
what is the region dividing the primary and secondary centers of ossification called?
Definition
epiphyseal plate
Term
what steps form the general process involved in growth of long bones?
Definition
continued production of cartilage at the epiphyseal plate in the metaphysis region causes elongation - endochondral ossification
Term
what type of bone growth is responsible for increase in width?
Definition
appositional
Term
what type of bone growth is responsible for increase in length?
Definition
endochondral
Term
why do long bones stop growing in length?
Definition
hormonal control of cartilage production at the epiphyseal plate stops
Term
how does the stopping of long bones growing in length occur?
Definition
the epiphyseal plate stops producing new cartilage cells, the ossification front from the primary ossification center ossifies the plate which is then resorbed by osteoclasts
Term
what is formed as a result of the stoppage of long bones growing in length?
Definition
a confluent marrow cavity
Term
how is the stoppage of long bones growing in length controlled?
Definition
hormones: gonadal and growth
Term
what is involved in the healing of bone fractures?
Definition
callus formation: clotted blood replaced by connective tissue, then endochondral ossification
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