Squids can be very large [up to 50 feet long] but have no bones. They are soft-bodied animals, i.e. mollusks--like snails--and are invertebrates. Squids have an internal skeleton of cartilages, impregnated with calcium salts. They also have a sort of brain box of cartilage-like material protecting their brain, eyes etc. Their endoskeleton provides the rigidity and support. Their internal skeleton is on their dorsal, i.e. backside and is in the form of a shell which looks like a flat, thick leaf. Though called cuttlebone it is NOT really a bone. (Want to see a picture of cuttlebone? Please click here -http://www.cuttleboneplus.com/cuttlebone... ). The cuttlebone gives both buoyancy and acts like a rigid framework. Sharks too have no bones. They have only cartilages which are forerunners of bones. Cartilages are flexible but not as strong and rigid as bones are.
Vertebrate bones look like rigid columns and beams but are active and dynamic tissues really. Bones are remodeled in adults all the time. The bone is deposited and removed. Osteoblasts are the cells lying at the outer and inner surfaces of bone and they make the bones. An osteoblast has a single nucleus unlike the bone-dissolving osteoclast, which is multinucleate. The osteoblasts lie in between periosteum, (a layer that covers sides of a long bone) and endosoteum, (the layer that lines the bone marrow). They make bone in two ways. They lay down around them new pliable bony matrix, called osteoid.{Take a look at the osteoid - http://depts.washington.edu/bonebio/bonA... please be patient and skip 12 slides and see clearly the osteoid in the 13th.} The osteoid is soft as it contains collagen fibers and has no minerals. The osteoblasts deposit minerals and get trapped in the mass of matrix they have deposited. Then they are called osteocytes. Though trapped in the bony matrix the osteocytes are not isolated, because they send out long processes that connect to the other osteocytes around. Osteocytes maintain bone and are arranged in mammals in concentric circles around the bone marrow called Haversian lamellae. . [see the osteobalsts and osteocytes - http://www.medes.fr/Eristo/Osteoporosis/... ]
Osteoblasts also help in healing fractures of bones by forming new bony tissue at the site of injury.
If a person takes tetracycline, an antibiotic, the drug is deposited where calcium is newly laid down in bones. One can count rings in a bone with tetracycline labelling and fluorescence technique. Follow the link below and click on 22 times on next slide...hmm...but the patience will pay when you see the golden yellow bone rings -http://depts.washington.edu/bonebio/bonA...
Go To Page: 1
