September 26, 2011
Lecture 6 – Bone
Bone is used for physical support, leverage, protection of internal organs (like the brain),
storage of minerals (calcium phosphate) and lipids), and the protection of blood cells
Bone connective tissue is mostly matrix. A third of your bone is organic matter, it’s not
all calcium. There’s osteoid and it’s maid mostly of collagen type I (eosinophilic and it’ll
stain very pink). There are also components of glycoproteins and proteoglycans. These
molecules are much smaller compared to collagen, meaning there is less tissue fluid.
There is tissue fluid (fluid between blood and cells). The organic component of the bone
matrix resists pull (provides tensile strength) and it endows flexibility. Bones will bend
before it breaks. Without the organic component, it’ll be very fragile.
Two thirds of the bone is inorganic. It is mainly calcium and phosphate (hydroxyapatite;
hydroxyl molecule). This component of the bone provides compressional strength and
mineral storage (where calcium phosphate is stored).
There are four cell types (osteoprogenitor, osteoblasts and osteocytes are the same cells
but are at different stages:
Osteoprogenitor cells come from embryonic mesenchyme, and are capable of
differentiating into active osteoid producing cells (into osteoblasts).
Osteoblasts are located on the surface of the bone matrix and secrete osteoids into their
surroundings, calcified extracellularly. The appearance of the osteoblast range from being
cuboidal to squamous. They are purple-blue in color.
Osteoblasts mature into osteocytes. It’s a lot like cartilage. They are living in a little
space inside the matrix, called the lacunar (space occupied by cell); there is no calcium
deposition in it (halo of unmineralized matrix). Osteocytes have all kinds of growths
extending through the matrix (matrix is solid, so there has to be little tunnels in the matrix
called canaliculi). Osteocytes form this network where they can communicate with each
other via gap junctions. The job of the osteocyte is to maintain surrounding bone tissue.
They sense mechanical stresses and they are remodeled or reorganized. Bone is more
dynamic and is constantly being remodeled, reorganized or improved (for ex. to maintain
optimal blood calcium levels). If bone experiences constant pressure, it will resorb (ex.
getting braces: constant pressure forcing teeth a certain direction will cause the bone
socket [gums] to resorb). Constant tension (ex. muscle attachment to bone via tendon: if
there is force to generate a greater amount of tension) will deposit more bone, making it
stronger. Bone from males have more bulk and can generate more power (bones are
larger), bones from females are smaller thus generate less power and are lighter.
We get a new femur every few years because of the matrix turnover. Osteoclasts are not from the same original cell as osteoprogenitor cells. Osteoclasts are
completely different cell types. They are large multinuclear cells from the macrophage
cell lineage. Osteoclasts are found always at the surface of bone matrix, in Howship’s
(resorption) lacunae. It secretes acids and enzymes for a process called osteolysis which
releases calcium and phosphate, which is added to tissue fluid and blood (bone
breakdown). Osteoclasts have a ruffled border. Osteoclasts form from multiple
monocytes, fusing together. They are full of lysosomes (sacks of enzymes) which can be
released onto the matrix to dissolve the bone. Howship’s lacuna is a pivot where the
osteoclast lies. Ruffled border increases surface area to facilitate processes.
Osteoblasts are being left behind and left into the matrix and will mature into the