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Make No Bones About It!
Bones can tell us a lot about a persons physical
and medical history.
Bone structure and connective tissue account for
about 25 of the bodys weight.
Bone Formation and Growth Early embryo - bones
start out as cartilage or fibrous
structures. The cartilage is gradually
replaced by calcified bone matrix.
Ossification - the process where mineral matter
starts to replace previously formed cartilage,
thus creating bone. (Usually starts during the
8th week of embryonic development.) Infant
bones are soft because ossification is not
complete. Continues through childhood.
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You are born with about 300 bones bones but by
the time you are an adult, you have about 206
bones. Many bones fuse together as you grow.
Soft Spot (Fused)
Soft Spot
The 206 bones are divided into 4 types based on
their form.
1. Long Bones - constructed for weight-
bearing and movement. Examples are the
femur and the humerus
2. Short Bones - make flexible movement
possible. Examples are wrist bones
(carpals) and foot bones (tarsals).
3. Flat Bones - these bones help provide
organ protection and are important sites
for hemopoiesis (blood cell formation.)
Examples are skull bones, sternum, and ribs.

4. Irregular Bones - odd shapes that do not
fit into the other three categories. They
have unique shapes related to their
function. Examples are the vertebrae and
the ear bones.
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There are two major types of bone based on
their histological (tissue) structure
1. Compact or Cortical Bone - mostly solid
bone matrix and cells with few spaces.
2. Spongy or Cancellous bone - has many
spaces within a lacy network of bone.
Long Bones


Long bones are hollow and shaped like rods or
shafts with rounded ends Parts of the long bone
are
Epiphyses
Epiphyseal Plate
1. Diaphysis ? Also called the shaft. ?
Mostly compact or cortical bone. ? Strong yet
light enough in weight to permit easy
movement.
Diaphysis
Epiphyseal Plate
2. Epiphyses ? Mostly spongy or cancellous
bone. ? Has many spaces filled with red
bone marrow which produces red and
white blood cells.
Epiphyses
3. Epiphyseal Plate ? Cartilage between the
epiphyses and the diaphysis. ? Site of growth
in bone length. ? Growth ceases when all
epiphyseal cartilage is transformed into
bone.
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4. Periosteum ? Peri means around and
osteum means bone ? Tough, white, vascular,
fibrous membrane on the outside of a
bone ? Contains blood vessels, lymph
vessels, and nerves ? Responsible for bone
growth, bone repair, and nutrition. ? If
periosteum is removed, bone will die.
Medullary Cavity
Periosteum
Endosteum
5. Endosteum ? Endo means within and
osteum means bones ? Membrane within the
bone that lines the medullary cavity. ?
Osteoclasts (tear down bones) are located in
this cavity. They dissolve bone to
keep the cavity. 6. Medullary Cavity ?
Hollow space or cavity inside the diaphyses of
the bone that contains the yellow bone
marrow. ? Yellow marrow started out as red
marrow, but gradually turns to fat
cells in long bones. ? These fat cells can be
converted to energy when needed and some
WBCs are made here.
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7. Articular Cartilage ? Thin layer of
cartilage covering each epiphysis ?
Acts like a shock absorber between
bones
8. Haversian System (HC) ? Circulatory
system within bone ? Contains blood vessels
that run parallel to the long axis of
the bone ? Provides nutrients to
the osteocytes (OC) and removes
waste.
.
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How Bones Grow
Our bones are constantly renewing themselves. Up
to 10 of your bones is eaten away and replaced
each year. There are 3 basic types of bone
cells
Osteoblasts ? Are the bone-builders and are
responsible for building new bone ? These
cells secrete bone matrix
Osteocytes ? Living bone cell located in the
lacunae (gaps) in the bone matrix. ? These
are mature bone cells ? These cells live for
about 20 years
Osteoclasts ? Are the bone breakers ? Are
large phagocytic cells ? Tear down bones by
excavating channels within the bone ?
Eat away bone in the medullary cavity,
preventing bone from becoming too thick
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Gerontological Conditions
? Bone mass peaks at about age 35, after
which there is a universal, gradual loss
of bone. ? Decreased activities and metabolic
changes (such as menopause and
decreased estrogen levels) contribute to
bone loss. ? Women lose more bone mass than
men. ? Bones also change in shape and
have reduced strength - leads to increased
change of fractures. ? Fractures are slower to
heal because new fibrous tissue develops
more slowly in the elderly. ? The
ability of the collagen structure to
absorb energy is reduced - leads to
osteoarthritis. ? The articular cartilage
decreases in weight-bearing areas and has
reduced ability to heal.
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Two Major Divisions Of The Skeletal System
I. Axial Skeleton ? 80 bones of the head and
trunk ? Many of these bones protect the
major organs of the body ? Consists of the bones
which run down the middle of the body
(its axis!)
II. Appendicular Skeleton ? Appendicular means
to hang and these bones are
attached to or hang from the bones of the
axial skeleton. ? Consists of the 126 bones of
the arms (upper appendages), the legs
(lower appendages), as well as the bones of
the hips and shoulders.
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Functions Of The Skeletal System
1. Support Provides the framework to
support the bodys fat, muscles, and skin.
Gives shape to the body.
2. Protection Protect delicate structures
within them (heart, lungs, brain) as well as
protecting bone marrow which is
responsible for forming blood cells.
3. Movement Serves as a point of attachment
for skeletal muscles. As muscles contract
and shorten, they pull on bones and move
them.
4. Storage Stores most of the bodys
calcium supply. 98 of the bodys
extracellular calcium is stored in bones.
Excess calcium in the blood, calcium goes
into bone for storage. Lack of calcium in
blood and it goes from bone to blood.
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Functions Of The Skeletal System (continued)
5. Hemiopoiesis (hee-mo-poy-EE-sis) ?
Hemo means blood and poiesis means to make.
? Blood cell formation takes place in the red
bone marrow. ? The average life
of a RBC is 120 days so your body must
produce 3 million new RBCs every
second! ? If your bodys need for red blood
cells is greater than what the body
can supply, some of the fatty yellow
matter can be converted to red marrow.