Download intramembranous bones

Skeletal System: Day Three
Bone Formation, Growth, and Remodeling, Bone Diseases and Disorders

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Bone Development and Growth
 Bones form by replacing connective tissues in
the fetus.
 Some form within sheetlike layers of connective
tissue (intramembranous bones), while others
replace masses of cartilage (endochondral
bones).
 Ossification is another word for bone building.
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Intramembranous Bones
 The flat bones of the skull form as intramembranous
bones that develop from layers of connective tissue.
 Osteoblasts deposit bony tissue around themselves.
 Once osteoblasts deposit bone and are located in
lacunae, they are called osteocytes.
 Cells of the membranous connective tissue that lie
outside the developing bone give rise to the
periosteum.
Intramembranous Bone
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Endochondral Bone Growth
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Endochondral Bones
 Most of the bones of the
skeleton fall into this category.
1.
They first develop as hyaline
cartilage models and are
then replaced with bone.
2.
Cartilage is broken down in
the diaphysis and
progressively replaced with
bone while the periosteum
develops on the outside.
3. Cartilage tissue is invaded by
blood vessels and
osteoblasts that first form
spongy bone at the primary
ossification center in the
diaphysis.
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5. Osteoblasts beneath the
periosteum lay down compact
bone outside the spongy bone.
6. Secondary ossification centers
appear later in the epiphyses.
7. A band of hyaline cartilage, the
epiphyseal plate, forms between
the two ossification centers.
8. Layers of cartilage cells
undergoing mitosis make up the
epiphyseal plate.
9. Osteoclasts break down the
calcified matrix and are replaced
with bone-building osteoblasts
that deposit bone in
place
of calcified cartilage.
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10. Epiphyseal plates are
responsible for lengthening
bones while increases in
thickness are due to
intramembranous ossification
underneath the periosteum.
11. A medullary cavity forms in
the region of the diaphysis due
to the activity of osteoclasts.
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Bone Formation: Ossification
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By birth, most hyaline cartilage have been converted to bone
except for two regions:
1. Articular cartilages
• Cover long bone ends (epiphysis)
• Persist for lifetime
• Reduce friction at the joint surfaces
2. Epiphyseal Plate
• Allows for longitudinal growth of long bones during
childhood
• Will eventually disappear leaving only the epiphyseal
line
Bone Growth
 New cartilage is continuously formed
 Old cartilage is broken down and replaced by bony matrix
 Process of long-bone growth
 controlled by hormones: growth hormone and sex
hormone
 Ends during adolescence when epiphyseal plate is converted
to bone
New cartilage added on the
external part of the articular
cartilage and on the
epiphyseal plate surface away
from medullary cavity
Old cartilage in broken down
and replaced by bone on the
internal part of the articular
cartilage and on the
epiphyseal plate near the
medullary cavity
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Homeostasis of Bone Tissue
 Osteoclasts tear down and osteoblasts
build bone throughout the lifespan with
the processes of resorption and deposition,
with an average of 3% to 5% of bone
calcium exchanged annually.
Bone Remodeling
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Bone is very active tissue
Bones are remodeled continuously in response to
two factors:
1. Calcium levels in the blood
2. Pull of gravity and muscles on the skeleton
Bone Remodeling:
Calcium levels in blood
 Blood calcium low
 Parathyroid glands release parathyroid hormone (PTH)
 PTH activate osteoclasts (destroy bone cells)
 Osteoclasts break down bone and release calcium into the
blood
 Blood calcium high
 Thyroid gland releases calcitonin
 Calcitonin stimulates Calcium to be deposited from the
blood into the bone
Bone Remodeling:
Pull of gravity and muscles on the skeleton
 Bones become thicker to increase strength where bulky muscles
attached
 Bones of physically inactive people tend to lose mass
Rickets
 Disease of children where bones
fail to calcify
 Bones soften and a bowing of the
weight-bearing bones of the legs
occur
 Due to lack of calcium in the diet
or a lack of Vitamin D (necessary
to absorb calcium into the blood)
 Not seen much in U.S., but is a
problem in other parts of the
world.
Osteoporosis
 A loss in bone mass leading
to thin, fragile bones
 Common consequence of
aging, esp. in women
 Estrogen helps to maintain
skeleton in women
 Lack of estrogen during
menopause in women as well
as a diet poor in calcium and
protein, lack of Vitamin D,
smoking, insufficient weightbearing exercise can
contribute to osteoporosis.