Biology Reference
In-Depth Information
important ones, digest the different matrix components. Following resorption, the
osteoclast migrates to another area and the bone lining cells enter the site left by
the osteoclast. The bone lining cell cleans the bottom of the pit and deposits a thin
layer of proteins after which osteoblasts start with the deposition of new bone. In
this chapter we will discuss each of the different steps that result in the degradation
of bone and the onset of new bone matrix deposition.
8.2 Definition of the Different Bone-Associated Cells
In this chapter we will use the following definitions for the different cell types:
Osteoclast
is a multinucleated cell that resorbs mineralized tissues, such as bone.
Osteoblast
is the cell type that actively deposits bone and it has a cuboidal shape.
These cells cover the osteoid layer of newly formed bone.
Bone lining cell
is the cell localized at the site where (1) no bone remodelling
activity is found or (2) resorption will occur or (3) resorption is occurring. This
cell is osteoblast-like since it expresses relatively high levels of the enzyme
alkaline phosphatase but is has a flattened morphology and covers the bone
surface from which some non-mineralized bone collagen fibrils protrude.
Osteocyte
is derived from the osteoblast and is, during the formation of bone,
incorporated in bone and finally completely embedded within this tissue.
8.2.1 The Bone Lining Cell and the Attraction of Osteoclast
Precursors
8.2.1.1 The Role of the Osteocyte in Steering the Site Where
Resorption Has to Occur
Of all bone-associated cell types far out the most frequently occurring one is the
osteocyte; being approximately 80-90% of all bone cells. In the past the osteocyte
was not considered to have an important function in processes related to bone
remodelling. Nowadays this view has changed dramatically. Ample data indicate an
essential role of the osteocyte in steering both resorption and deposition of bone
(Burger et al.
1995
; Bonewald and Johnson
2008
).
The osteocytes are surrounded by bone tissue and their cell bodies are localized
in lacunae. The cells contact each other through small channels, canaliculi, in which
their numerous cell extensions are running. The osteocyte has been proved to be
extremely sensitive to small changes in strain exerted upon it. Differences in strain
are sensed by the osteocyte by the flow of fluid surrounding the cellular extensions
and the cell body. This results in synthesis and secretion of a variety of signaling
molecules,
like nitric oxide (NO), prostaglandins (e.g., PGE
2
) and sclerostin
