Bones, in the form of cartilage, begin to grow on an embryo in the womb. naulicrea/Shutterstock
Before we talk about bone formation, we need to discuss how cartilage turns into bone. When you’re floating around in the womb, your developing body is just beginning to take its shape, and it’s creating cartilage to do so. Cartilage is a tissue that isn’t as hard as bone, but much more flexible and, in some ways, more functional. Cartilage is pretty good stuff to use if you’re going to mold a human — good enough for the finer work, especially, such as your nose or your ear.
A large amount of that fetus cartilage begins transforming into bone, a process called ossification. When ossification occurs, the cartilage begins to calcify; that is, layers of calcium and phosphate salts begin to accumulate on the cartilage cells. These cells, surrounded by minerals, die off. This leaves small pockets of separation in the soon-to-be-bone cartilage, and tiny blood vessels grow into these cavities.
Specialized cells called osteoblasts begin traveling into the developing bone by way of these blood vessels. These cells produce a substance consisting of collagen fibers, and they also aid in the collection of calcium, which is deposited along this fibrous substance.
Eventually, the osteoblasts become part of the mix, turning into lower-functioning osteocytes. This osteocyte network helps form the spongelike lattice of cancellous bone. Cancellous bone isn’t soft, but it does look spongy. Its spaces help transfer the stress of external pressures throughout the bone, and these spaces also contain marrow. Little channels called canaliculi run all throughout the calcified portions of the bone, enabling nutrients, gases and waste to make their way through.
Before turning into osteocytes, osteoblasts produce cortical bone. One way to imagine this process is to picture a bricklayer trapping himself inside a man-sized brick chamber of his own construction. After forming the hard shell (cortical bone), the bricklayer himself fills the chamber. Air makes its way through the brick and decays the bricklayer.
In bone, this part of the process is accomplished by osteoclasts, which make their way into the calcifying cartilage and take bone out of the middle of the shaft, leaving room for marrow to form. Osteoclasts do this by engulfing and digesting the bone matrix using acids and hydrolytic enzymes. So, our bricklayer (osteoblast) made the tomb (cortical bone), died inside the tomb (became an osteocyte), decayed over time (dissolved by osteoclasts) and left behind his remains that formed a network of mass and space inside the brick tomb.
Eventually, all the cartilage has turned to bone, except for the cartilage on the end of the bone (articular cartilage) and growth plates, which connect the bone shaft on each side to the bone ends. These cartilage layers help the bone expand and finally calcify by adulthood.
So, right now in your body, there are osteoclasts hard at work absorbing old bone cells and osteoblasts helping to build new bone in its place. This cycle is called remodeling. When you’re young, your osteoblasts (the builders) are more numerous than the osteoclasts, resulting in bone gain. When you age, the osteoblasts can’t keep up with the osteoclasts, which are still efficiently removing bone cells, and this leads to loss of bone mass (and a condition called osteoporosis, which we’ll discuss shortly).
