Osteocytes: mechanosensation and remodelling
Osteocytes are mature bone cells that sense mechanical strain (the small deformation of bone under load) and coordinate remodelling, the building and breaking down of bone.
Remember
Osteocytes are the sensors of bone: they detect strain and signal to osteoblasts and osteoclasts, which then build or break bone down where it is needed.
Memory aid
osteoblasts build, osteoclasts break down, osteocytes sense
Use it to recall which bone cell builds, which breaks down and which senses.
Every osteocyte began life as an osteoblast (a bone-building cell). When an osteoblast has surrounded itself with the matrix it made (the hard material of bone), it is trapped in a small space called a lacuna and becomes an osteocyte. From then on the cell cannot divide, and it no longer makes osteoid (the new collagen-rich matrix of bone).
Instead, the osteocyte puts its effort into communication through long cytoplasmic processes (thin extensions of the cell). These run through tiny channels in the matrix called canaliculi and link each osteocyte to its neighbours and to cells on the bone surface, forming one connected network.
That network makes osteocytes the mechanosensors of bone, meaning cells that detect mechanical force such as stretch or compression. When load on the bone changes, the network senses the strain and the osteocytes release signalling molecules (chemical messages). These act on osteoblasts and osteoclasts (the cells that break bone down), which are the effectors: they carry out the instructions by building or removing bone where it is needed. The osteocyte itself neither builds nor breaks down bone.
Bone needs regular load to keep its mass. When load falls, for example when a limb is kept in a cast for several weeks, the signals shift remodelling towards resorption (the breakdown of bone), and bone mass is lost. This is how bone mass and shape adapt to the demands placed on them.
Key facts
- Origin
- Osteocytes are osteoblasts that have become trapped in the bone matrix they made.
- Lacuna
- Each osteocyte sits in a small space called a lacuna.
- Network
- Long processes run through canaliculi and link osteocytes to each other and to cells on the bone surface.
- Job
- Osteocytes detect mechanical strain and signal to osteoblasts and osteoclasts.
- Limits
- Osteocytes cannot divide, make osteoid or break down bone.
- Disuse
- Reduced load, such as weeks in a cast, shifts bone towards resorption and loss of bone mass.
Exam traps
- Laying down osteoid at the bone surface is the job of osteoblasts, not osteocytes, because an osteocyte stops making osteoid once it is trapped in the matrix.
- Osteocytes do not divide to supply new osteoblasts, because they are fully mature cells that cannot divide; the osteogenic cells (cells that divide and develop into osteoblasts) do that.
- Osteocytes do not turn into osteoclasts to remove bone under load, because osteoclasts develop from monocytes and macrophages (types of white blood cell); osteocytes influence osteoclasts by sending signals to them.
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