Although they may seem lifeless and
simple, bones are in fact complex
structures filled with live
cells and
tissues. Bones interact with one another at joints, and different bones and joints are specialized for different types of movements.
Long bone structure
Long bones are the most common bones found in the mammalian body. They are composed mostly of
compact bone, and are roughly cylindrical in shape with enlarged ends filled with
spongy bone.
At each end of the bone, the
proximal and distal epiphyses are where
long bones make contact with other bones at joints. Each
epiphysis (also called the
head of the
long bone) is covered in
articular cartilage to allow bones to slide past one another more easily. The interior of epiphyses is filled with
spongy bone, and is differentiated from the interior of the
diaphysis by the
epiphyseal line (in adults) or
plate (in juveniles). Some epiphyses are also sites of red blood cell formation in adults.
The
diaphysis or
shaft of a
long bone makes up most of the length of the bone. It is roughly cylindrical in shape, and is separated from the epiphyses by the
epiphyseal line/
plate. The
diaphysis is composed of a thick layer of
compact bone surrounding the
medullary cavity. In adults, the medullary cavity can also be called the
yellow marrow cavity, but in infants it is called the
red marrow cavity, as it is filled with newly forming
red blood cells. The medullary cavity is lined with a thin layer of
connective tissue called the
endosteum.
The
diaphysis is covered by the
periosteum, a protective layer of fibrous
connective tissue. The periosteum is anchored to the underlying bone by
perforating or
Sharpey’s fibers.
Microscopic structure of compact bone
Trapped within dense
compact bone, mature
osteocytes (bone forming cells) are found within small cavities called
lacunae. The lacunae are arranged in layers of concentric circles surrounding
central (Haversian) canals, which run lengthwise through the
diaphysis. Each cylindrical section of layers of lacunae (called
lamellae) together with the
central canal they surround is called an
osteon or
Haversian system.
All lacunae are connected to the
central canals by
canaliculi, which extend outward from the
central canals. The canaliculi allow the osteocytes access to nutrients and other materials that would not otherwise be able to reach them through the hard
compact bone matrix. Also serving to supply the bone with nutrients, the
perforating (Volkmann’s) canals run transversely from the outside to the inside of the bone and create passageways for
blood vessels to enter the bone.
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1 - Anatomy and Physiology Basics