Osteoprogenitor cells are the 'stem' cells of bone, and are the source of new osteoblasts.
Osteoblasts, lining the surface of bone, secrete collagen and the organic matrix of bone (osteoid), which becomes calcified soon after it has been deposited. As they become trapped in the organic matrix, they become osteocytes.
Osteocytes maintain bone tissue. Fine processes from these cells ramify through bone, and form gap junctions with other osteocytes.
This diagram shows how an osteocyte sits in the calcified matrix.
Osteocytes sit in the calcified matrix, in small
spaces called lacunae (lacuna - singular). Long processes from the
osteocyte lie in small channels called canaliculi (small canals).
These are channels for the transport for nutrients and waste. The
osteocyte processes contact other ostocytes, forming gap junctions,
so that they can communicate with each other.
This photo shows a scanning electron micrograph of an osteocyte sitting in the bone matrix.
Click here to find out how these cells are organised in bone.
Cells that are involved in remodelling bone: Osteoclasts
This is a picture of an osteoclast sitting in
a 'Howship's lacuna' in the endosteum.
These recesses are areas of bone that the osteoclasts
have eroded away. Can you identify the osteoclast, and its 'ruffled
border' which is full of fine finger like projections that insert
into the bone matrix. How man nuclei does the osteoclast have? (Taken
from Essential Histology, David NcCormack).
Osteoclasts are secretory, and have prominent
Golgi apparatus, and vesicles. They secrete enzymes such as carbonic
anhydrase which acidifies the matrix, and causes it to decalcify,
and hydrolyses, which break down the matrix once it is decalcified.
Other cell types help to phagocytose and get rid of the debris.
Osteoclasts are large multinucleated cells, with a 'ruffled border' that resorb bone matrix, as shown in the diagram above. They are important for remodelling, growth and repair of bone. (clast - greek 'to break').
Osteoclasts are not derived from osteoprogenitor cells. They are derived from blood monocytes/macrophages which are derived from haemopeoitic cells in the bone marrow. The precursors are often released as monocytes into the blood stream and they then collect at sites of bone resorption, where they fuse to form multinucleated osteoclasts, stick to the surface of bone, and break down the bone matrix.
Bone re-modelling is necessary for growth: -
1. Mechanical stresses on the skeleton cause release of calcium, that stimulate bone-remodelling.
2. Hormones also control bone re-modelling. Parathyroid hormone stimulates bone resorption. and calcitonin inhibits resorption.