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Bone Structure
Individual bones can be classified according to their shapes. Long bones are longer
than they are wide. Short bones are about as broad as they are long. Flat bones
have a relatively thin, flattened shape and are usually curved. Irregular bones have
shapes that do not fit into these three categories.
The ends of a long bone are called the epiphyses (singular,
epiphysis), which are covered by a layer of hyaline
cartilage called the articular cartilage and articulate with
other bones. The internal area of the epiphyses contains
red bone marrow where erythrocytes (red blood cells),
leukocytes (white blood cells), and thrombocytes
(platelets) are produced. The shaft of a bone is called the
diaphysis. The diaphysis contains an internal medullary
cavity that is lined by the endosteum and usually filled
with yellow marrow, where fat is stored. Except for the
articular cartilage, a bone is covered by a tough outer
membrane called the periosteum.
In children, an epiphyseal plate of hyaline cartilage
separates the epiphysis and diaphysis. It allows for
growth in length. The epiphyseal plate will eventually
ossify, at which point bone growth will stop.
There are two types of osseous tissue; compact and spongy bone tissue. The composition of the matrix is the same in both types,
however compact bone has fewer spaces than spongy bone. Both compact and spongy bone are strong and resist bending.
Bone Histology
Bone tissue consists of bone cells surrounded by extracellular
matrix. The bone matrix consists primarily of hydroxyapatite,
calcium phosphate, and calcium carbonate crystals. The bone cells
produce the matrix, and become entrapped within the bone matrix.
Other bone cells break down the bone matrix so that new matrix
can replace the old matrix. There are four principal types of bone
cells:
Osteogenic cells are located in the endosteum, the inner surface of
the periosteum, and within haversian canals. They are immature
bone cells capable of mitosis that give rise to osteoblasts.
Osteoblasts produce and surround themselves with bone matrix.
They are usually found only on the inner surface of the periosteum
and are incapable of mitosis.
Osteocytes are mature bone cells that maintain exisiting bone tissue. They are connected to each other through canaliculi. As the
osteoblast secretes bone matrix, the cell becomes trapped within lacunae (tiny spaces) at which time they become known as osteocytes.
Like osteoblasts, they are nonmitotic.
Osteoclasts are bone-dissolving cells that form by the fusion of monocytes, a type of white blood cell.
Compact bone is more dense and has fewer spaces than cancellous bone.
Blood vessels that run parallel to the long axis of the bone are contained
within central, or Haversian, canals. Haversian canals are lined with
endosteum and contain blood vessels, nerves, and loose connective tissue.
Concentric lamellae are circular layers of matrix that surround a common
center, the haversian (central) canal.
Spongy bone consists of interconnecting rods or plates of bone called trabeculae. Between the trabeculae are spaces that in living
bone are filled with bone marrow and blood vessels. Spongy bone gets its name from its porous appearance.