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Microscopic Structure of Bone
There are 2 basic kinds of bone tissue: compact and
spongy. As the name suggests, spongy bone is lighter and has spaces (where we
find red bone marrow and blood cell production). Compact bone is so dense that
nutrients/wastes cannot diffuse through the tissue. Cells must arrange
themselves in patterns for nutrient/waste exchange (called Osteons or Haversion
systems).
Haversion system parts: Haversion (central) canals
which contain blood vessels; Lamellae - rings of bone matrix; Osteocytes (bone
cells) in lacunae; Canaliculi - microscopic tunnels that connect osteocytes.
Long Bone Structure
Epiphysis (end): thicker; spongy bone/red bone marrow.
Diaphysis (middle shaft): hollow with compact bone/yellow bone marrow. Long bone
structure is well suited for support: a cylinder is the most efficient
weight- bearing shape.
Bone growth
1. Intramembranous (fetal-infancy)
2. Endochondral (fetal-infancy)
3. Longitudinal (Childhood)
4. Appositional growth/Remodeling (lifelong)
5. Fracture healing
Bone Growth: Intramembranous
Bone Grows within two membranes
Cells within the membrane develop into osteoblasts or
bone- forming cells. Osteoblasts form an ossification center within the fibrous
CT of the two membranes. The irregular channels of bone that grow outward are
called trebeculae of woven bone. Woven bone resembles spongy bone. Cells
at the top and bottom of the membrane join the bone- building process. The bone
tissue produced at the top and bottom resembles immature compact bone.
This process continues throughout fetal development,
and beyond. The remaning ”soft spots” of connective tissue between the growing
ossification centers are called fontanelles. Remodeling of this bone eventually
produces a layer of spongy bone in between two layers of compact bone. Most flat
bones, such as the bones of the skull, grow this way.
Bone Growth: Endochondral
Bone growth using cartilage as a model to speed up the
process.
1. A mold or model is made of cartilage similar to
hyaline cartilage.
2. Blood vessels bring nutrients to the edge of the
“cartilage bone.” This activates osteoblasts, and a bony collar forms.
Cartilage inside the collar begins to die/deteriorate.
3. Blood vessels push through into the diaphysis,
bringing nutrients and osteoblasts to the interior. This area of growth in
the diaphysis is called the primary ossification center. Osteoclasts or bone
absorbing cells, also become active, eat decaying cartilage, and reshape the
inside of the bone.
4. Late in fetal development or in early childhood,
blood vessels push through into the epiphyses, bringing nutrients and
osteoblasts to the interior. This is called the secondary ossification
center(s). Mainly spongy bone is produced here. The epiphyseal plate remains
stuck between the ossification centers, and plays an important role in continued
growth throughout childhood.
Bone Growth: Longitudinal
1. Cartilage cells multiply rapidly along top edge of
the epiphyseal plate.
2. Cartilage tissue growth occurs rapidly in the
middle of the epiphyseal plate.
3. Osteoblasts convert old decaying cartilage tissue
into bone on the bottom edge of the epiphyseal plate.
Longitudinal growth is affected by hormones and
nutrition. Growth hormone and thyroid hormone promote normal bone growth in
childhood. The Sex hormones synergize bone growth, especially osteoblast
activity. Eventually leads to closure of the epiphyseal plate into the
epiphyseal line.
Bone Growth: Appositional/Remodeling
Appositional growth: growth in width. Osteoblasts
deposit bone on the outside, osteoclasts digest bone on the inside. Eventually,
it will need to make new osteons on the outside.
Remodeling and appositional growth are also stimulated
by stress placed on a bone during normal use and exercise. Bone generally
thickens, and points of muscle attachment grow, in response to stress.
This is why exercise is important to strengthening
bones and preventing osteoporosis.
Fracture Healing
1. Hematoma formation: Blood and fribrous tissue
accumulates at the fracture site. White blood cells are also attracted and
eventually clean up the fractured site.
2. Callus formation: Fibrous tissue is converted to
cartilage, and then gradually into spongy bone. Notice how this is similar
to the strategy used for rapid growth in the fetus. Use fibrous tissue or
cartilage first to quickly establish the area you will convert to bone.
This is sometimes categorized as two separate steps - fibrocartliage callus
formation, and bony callus formation - but both parts are occuring at the same
time.
3. Remodeling: Callus formation is inexact, and
produces spongy bone in places where compact bone is required. The weeks that
follow will find osteoblasts and osteoclasts reshaping the bone back to its
original dimensions and composition.
Fractures
Simple: Usually a single, clean break that doesn’t
break through the skin.
Compound - broken bone breaks through skin.
Comminuted - Bone breaks into 3 or more pieces.
Compression: crushed inward.
Depressed: pressed inward.
Impacted: bones forced into each other (a bit like a
telescope).
Spiral: irregular break caused by twisting.
Greenstick: Incomplete break occuring in softer bones
(children).
Osteoporosis
Reduced bone density due to faster loss of calcium
from bone tissue as we age and sex hormone levels decrease. Building extra
bone matrix with exercise is one of the best ways to prevent this.
Joints
Named for degree of movement or type of tissue that
makes the joint.
1. Synarthroses -immovable joints (e.g. the
sutures of the skull).
2. Amphiarthroses -Slightly movable joints (e.g.
the intervertebral joints).
3. Diarthroses - Freely movable joints.
Examples:
Ball and socket (shoulder)
Hinge (elbows and knees)
Gliding/sliding (wrist and ankle)
Saddle (thumb)
Diarthroses have a: joint capsule of tough fibrous
tissue, joint cavity, reinforcing ligaments (and sometimes tendons), and
synovial membrane that makes synovial fluid (lubrication and shock absorption).
Arthritis
Simply means inflammation of the joints.
Osteoarthritis - Wear and tear arthritis common in
older age
Rheumatoid arthritis - autoimmune disease; the
person’s immune system attacks joint tissues
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