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Adult
bones do not lengthen. Adult bones have fully mineralized. Adult
bones provide solid foundations from which muscles apply force.
Nevertheless, adult bones replenish and respond to mechanical
stresses.
a. Spongy
and Compact Bone
In
long adult bones, spongy bone tissues make up their proximal
and distal ends. Networks of interlacing bone columns (trabeculae)
enclose the small multi-sided fatty marrow spaces in spongy bone
tissue. Parallel bone threads occupy these between trabeculae
spaces. Spongy bone tissues fill the marrow space beneath bones'
articular surfaces. These spongy bone tissues effectively dampen
mechanical stress shocks that transfer across articular surfaces.
Strategically located strut-like Haversian Canals also dampen
and disperse mechanical stress shocks across joints' large cross-sectional
areas.
Compact
bone tissues make up long bone shafts. Compact bone tissues develop
from spongy bone tissues. However, compact bone tissues become
much more rigid than spongy bone tissues. Compact bone tissues
surround the marrow canals in long bone shafts. In bone cortexes,
thin bone sheets (lamellae) arrange in cylindrical layers and
group around narrow axial marrow canals. Marrow canals contain
blood vessels and some loose connective tissues.
b. Mineralized
Bone
Adult
bone cortexes have three segments. The external-most segment
(periosteum) has three layers. From outside to inside, the periosteum
contains layers of fibroblast, pre-osteoblast and osteoblast
bone cells. The middle segment constantly regenerates mineralized
bone tissues (MBT). This regenerating layer contains osteoblasts,
osteocytes, osteoclasts and Haversian Canals. The internal-most
segment (endosteum) is the bone's axial canal. The endosteum
contains osteoclasts and pre-osteoclasts.
c. Adult Bone
Physiology
1. Fibroblasts
Muscles
that arise from bones blend with the periosteum's fibroblastic
layer. Although fibroblasts have rich blood vessel and nerve
supplies, fibroblasts very slowly add new collagenous fibers
to bones' surfaces. Fibroblasts are protein fibers with the same
organic composition as bone, but without the mineral content
that hardens protein fibers into mineralized adult bones. Throughout
adulthood, fibroblastic layers remain relatively stable.
2. Pre-Osteoblasts,
Osteoblasts and Osteocytes
Immediately
beneath fibroblastic layers, pre-osteoblastic layers continually
produce mature osteoblasts. New osteoblasts continually replace
old osteoblasts that line entire external (periosteal) mineralized
bone tissue surfaces. All along mineralized bone tissue's irregular
contours, osteoblasts stand shoulder to shoulder one cell deep.
Every day, osteoblasts add their own volume of new bone tissue
to periosteal surfaces. Every three days, new osteoblasts push
older osteoblasts into mineralized bone tissues and encase them.
New bone tissues continuously push previously added bone tissues
toward marrow canal centers. Consequently, mineralized bone tissues
continually regenerate.
Encased
osteoblasts cannot add new bone tissue. However, they can transfer
nutrients from the periosteum to the new bone tissue that they
added to mineralized bone tissues. Because encased osteoblasts
no longer function as bone tissue manufacturing osteoblasts,
researchers call them osteocytes.
3. Haversian
Canals, Pre-Osteoclasts and Osteoclasts
Osteoclasts
locate in mineralized bone tissue and on the cortexes' endosteal
surfaces. Osteoclasts are many times larger than osteoblasts.
Compression mechanical stresses stimulate mineralized bone tissue
osteoclasts to resorb appropriately aligned microscopic cylinders
(Haversian canals). Consequently, without increasing bones' sizes
or mineralizations, Haversian Canals strengthen bones against
compression mechanical stresses.
Endosteal
osteoclasts cover forty percent of mineralized bone tissues'
endosteal surfaces. Osteoclasts resorb mineralized bone tissues.
Osteoclasts are very mobile. Every eighteen hours, osteoclasts
contact entire endosteal surfaces. Pre-osteoclasts float freely
in bones' marrow canals and mature into osteoclasts. |
