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CTO 10 bone development


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Primary Bone
first type of bone produced in the fetus or during repair of a fracture
Secondary Bone
mature bone, both types (compact/cortical or spongy/trabeccular) laid out in well defined layers
2 Mechanisms of Bone Development
Intramembranous Ossification
Endochondral Ossification
Intramembranous Ossification
osteoblasts secrete the matrix which is then mineralized
Endochondral Ossification
bone is deposited on a pre-exisiting cartilage matrix
bone tissue that appears first in either mechanism of bone formation
woven bone
Characteristics of Primary Bone
non-lamellar, random weave of bone, abundant osteocytes and low mineral conent, temporary, will be replaced by secondary bone tissue
Primary Unit of Compact Bone, lamellae arranged in circular pattern around Haversian Canal
Osteon or Haversion System
arrangement of collagen fibers in one lamella
parallel (but perpindicular in adjacent lamella)
lacunae between and sometimes within the lamellae contain...
small channels between the lacunae that house the cellular processes of osteocytes and facilitate communication.
lamellae associated with the osteon
outer circumferential
inner circumferential
Outer circumferential lamellae
found deep to the periosteum
inner circumferential lamellae
surround the marrow cavity
intertitial lamellae
triangular or irregularly shaped lamellae that are remnats of osteons that were destroyed during growth and remodeling
Volksman's Canals
connections between Haversion canals and the periosteum/endosteum
characteristics of spongy bone
trabeculae or spicules
trabeculae contain irregular or circular lamellae
does not usually contain osteons
intramembranous ossification is responsible for...
process responsible for most flat bones including those in cranial vault
where does intramembranous ossification takes place?
highly vascularized mesenchymal tissues
in intramembranous ossification what cells migrate to region where bone is to form
mesenchymal cells
in intramembranous ossification.. into what types of cells do groups of mesenchymal cells differentiate?
osteoblasts (via an intermediate osteoprogenitor stage)
what do osteoblast secrete?
what determines the location of the primary ossification center?
region of initial osteogenesis
at what point are osteocytes formed?
when osteoblasts become trapped in surrounding newly-formed matrix
islands of devleoping bone
orientation of collagen in primary bone spicules
what happens in the connective tissue among the newly formed spicules?
growing blood vessels penetrate the CT and undifferentiated mesenchymal cells give rise to bone marrow cells
in what manner to the ossification centers grow?
radially, fusing to replace original connective tissue
what happens to the portion of the bone that does not undergo ossification
this part of the bone becomes the periosteum and the endosteum
type of ossification that forms short and long bones
endochondral ossification
hyaline cartilage model
model that begins the process of endochondral ossification, resembles small version of bone to be formed
how are trabeculae formed in hyaline cartilage model?
chondracytes within the shaft hypertrophy, reducing the cartilage matrix to slender trabeculae, cartilage matrix calcifies with large spaces
first bone is formed by...
intramembranous ossification within the perichondrium surrounding the diaphysis
at what point and how does the periosteum form?
when the perichondrium becomes vascularized and devleops osteogenic potential, osteblasts begin secreting bone, forming bone collar
how does the the central cavity in cartilage form?
periosteum prevents diffusion of nutrients to the condrocytes causing them to die forming...
what is the osteogenic bud?
osteoprogenitor cells, hematopoetic cells and blood vessels
how does the osteogenic bud enter the concavities of the cartilage model?
osteoclasts form homes in the bone collar
what forms a continuous layer on calcified cartilage?
osteoblasts, secrete osteoid onto it
primary ossification center is where?
in the diaphysis, progresses toward epihysis
what forms the marrow cavity?
osteoclastic activity in teh center of the forming bone.
secondary ossification center
in epiphysis, pgoresses much like the other, but does not form a bondy collar
where does cartilage remain?
1. articular cartilage- hyaline cartilage covering joint surfaces
2. epiphyseal plate (continues to grow and is continuously replaced by bone matrix- resulting in elongation of bone)
proliferation occurs at what aspect of the epiphyseal plate?
epiphyseal aspect (top)
replacement by bone occurs on what aspect of the epiphyseal plate?
diaphyseal aspect
Zones of epitheseal plate
resting zone
zone of proliferation
zone of hypertrophy
zone of ossification
resting zone
hyaline cartilage without morphological changes
zone of proliferation
chondrocytes dividing rapidly that form columns of stacked cells parallel to the long axis of the bone
zone of hypertrophy and calcification
large chondrocytes whose cytoplasmhas accumulated gylcogen and narrow areas of matrix between lacunae
zone of ossification
osteoprogenitor cells invade the area and differentiate into osteoblasts, secrete bone matrix onto the calcified cartilage matrix
Bone length depends upon what
activity in the epiphyseal plate
most important stimulus of bone growth (epiphyseal plate activity) in infancy and childhood
growth hormone (somatotropin) released from anterior pituitary
pituitary gigantism
excessive height- resulting from excessive amount of growth hormone
diminished height (deficits of growth hormone)
what determines the end of bone growth?
when the cartilage of the epiphyseal plate ceases proliferation and bone development continues to unite the diaphysis and epiphysis
appositional grwoth
increase in bone width, pone is produced by the periosteum (by intramembranous ossification) on the external surface of bone collar
vitamin D deficiency
prevents calcium absorption from teh GI tract resulting in rickets (kids) osteomalacia (adults) osteoid is produced, but calcium salts are not deposited so bones soften and weaken
bone remodeling
process of selective bone resorption and esposition- essential for continually reshaping bones in response to changing forces
in a growing person bone deposition is less than, greater than, or equal to bone resportion
what causes osteopeorosis?
bone resorption exceeds bone deposition
How is fracture repair initiated
1. blood from damaged vessels forms a 2. clot, damaged matrix, dead cells removed by macrophages.
3. granulation tissue forms in the site of clot and condenses into CT and later into a fibrocartilagenous callus
(at same time..)
Osteoprogenitor cells of periosteum are activated and become osteoblasts that begin to deposit new bone (meshwork of trabeculae of primary bone) callus around fracture site
4. Endosteum cells activated and deposit bone around cartilagenous callus, that is slowly eroded and replaced by bone (endochondral ossification)
5. spongy bone is transformed into combact bone by osteoblastic deposition of matrix
6. osteoclasts reabsorp excess bone- and reestablish marrow cavity
calcium homeostasis
bones release calcium when blood levesls decrease below normal, store calcium in matrix when blood leves are high
parathyroid hormone, produced in response to depressed calcium levels, triggers and increase in blood calcium through multiple paths
Vitamin D
may also play a role in calcium regulation at times of depressed levels
acts directly on osteoclasts to inhibit their resportive activity (lower free calcium levels)

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