The osteocyte

Published on January 19, 2015   34 min
Hello, my name is Lynda Bonewald. And I'm from the University of Missouri at Kansas City. And the title of my talk is The Osteocyte.
First, I would like to give you an outline of the presentation. I'll talk about osteocytes in history, their first proposed function, that of osteocytic osteolysis, their second proposed function, that of mechanosensation, then about how osteocytes can serve as secretory cells regulating skeletal remodeling through molecules, such as sclerostin and RANKL, fifth, how osteocytes are endocrine cells that can target kidney and muscle. And finally, I will summarize the functions of osteocytes.
Now, there are three types of bone cells. There are the osteoclasts that resorb bone and the osteoblasts that make bone. And then there is the third the cell type called the osteocyte.
Osteoclasts and osteoblasts are defined by their function. Osteoclasts resorb bone and osteoblasts make bound. However, osteocytes are defined by their location. So this implies a lack of knowledge regarding their function.
This slide shows osteocyte ontogeny. The osteocyte is descended from the pre-osteoblast, which differentiates into the matrix-producing osteoblast. And by some unknown mechanism, some of these cells become osteocytes. And then once they're surrounded by osteoid, they begin to mineralize. And once surrounded by mineral, they are called mature osteocytes.
So osteocytes make up over 90% to 95% of all bone cells in the adult skeleton. And they express these long dendritic processes that we think play a role in communication and viability. And the interesting thing about these cells is that unlike osteoclasts that may be viable for days and blasts maybe weeks, osteocytes are viable for decades in the bone matrix.
This slide shows the osteocyte lacuno-canalicular network inside of a bone. And this was taken from a mouse in which they tail vein was injected with Procian red, a small dye. And what you can see is that the dye has totally penetrated the osteocyte lacuno-canalicular network. This shows that the circulation can reach the osteocyte netowrk and that anything that is made by osteocytes can reach the circulation.
This is further emphasized by this image. This is a resin cast acid etched bone sample where all of the pores in the bone are filled with resin and then acid is used to remove the surface of the mineral. And what this reveals is a blood vessel running through the bone and an osteocyte lacunae. And what you can see is that these are intimately connected, suggesting an intimate connection between the osteocyte lacuno-canalicular our system and the vascular system.
This is a drawing of the osteocyte, shown in the middle. It's connected to and communicating with other osteocytes. Osteocytes connect with the vasculature, as shown with the blood vessel. And they can extend their dendritic processes into the vascular space. Not only that, osteocytes can communicate with cells on the bone's surface, such as osteoblasts or lining cells. And they can send their dendritic processes into them marrow space. But I'd like to also emphasize the perilacunar matrix as shown here. This is the kind of lavender shaded matrix around the osteocyte. And I'll talk about this in a little more detail.
So there were a number of osteocyte pioneers in history. And one of the earliest pioneers was von Recklinghousen in 1851. And he came up with the first proposed function of osteocytes, that as removal of their perilacunar matrix. Another osteocyte pioneer was Herald Frost, who talked about the mechanostat in the skeleton and how osteocytes mechanosensory cells. After that, in 1966, Belanger talked about osteocyte osteolysis. Baylink about how osteocytes could resorb and form matrix. Moarrotti talked about osteocyte remodeling. Parfitt talked about osteocytic osteolysis. And then Kumegawa was the first to actually make a movie of osteocytes in 1983. Peter Nijweidi first described avian osteocytes and avian osteocyte markers in 1986. Carla Palumbo talked about osteoid osteocytes. And Lance Lanyon studied mechanosensation. So these were there early osteocyte pioneers.