How do Bones Grow?
How do bones grow?
Bones are made of a network of calcium laid down by cells. As kids grow, special cells at the end of bones add new calcium to the network of bone. Children have layers of these cells in the shape of plates at the ends of their bones. These are called “growth plates,” and they close up when kids reach their full adult height.
Do you love to go to amusement parks in the summer? The warm rays of the Sun on your skin and the taste of the sweet cotton candy make great memories. Of course, the best memories probably come from the thrilling rides that take your breath away.
When you're little, though, you don't always get to ride the most exciting rides. As you step up to the front of the roller coaster entrance, you're usually greeted by a sign indicating you have to be THIS tall to ride. If you're under that mark, you probably yearn for the day when you'll grow tall enough to ride.
Most kids spend time when they're little dreaming about what life will be like when they grow bigger. But have you ever WONDERed exactly what happens inside your body to make you grow? Specifically, have you ever thought about how your bones grow?
When you were born, every part of you was tiny — even your bones! Babies are born with about 300 bones. This is approximately 100 more than the 206 bones you'll have as a full-grown adult. So what happens? Do you lose some bones?
Not at all! Many of the tiny bones you're born with fuse together over time to form bigger, longer bones as you mature. They also go through significant changes as they get bigger and grow into mature adult bones.
When babies are born, many of their bones are made partly or mostly of a soft, flexible substance called cartilage. Pinch the end of your nose and the top of your ear. Feel how they're soft and flexible? That's because those parts of your body are made of cartilage.
As you grow, the cartilage in your bones grows. Over time, it slowly gets replaced by bone with the help of calcium. This process is called ossification. During ossification, layer upon layer of calcium and phosphate salts begin to accumulate on cartilage cells.
Once encased in these minerals, the cartilage cells die, leaving tiny pockets behind. Blood vessels grow into these tiny pockets, delivering specialized cells called osteoblasts. The osteoblasts help to collect additional calcium and also produce a substance full of collagen fibers.
Osteoblasts also produce layers of cortical bone that surround the cartilage. After making the cortical bone, osteoblasts become cells called osteocytes that work to form a sponge-like lattice of marrow and a substance called cancellous bone inside the developing bone.
Eventually, other cells known as osteoclasts make their way into the middle of developing bones. They use hydrolytic enzymes and acids to dissolve the cancellous bone and make room for more marrow. This process continues until all the cartilage has turned to bone.
The process of ossification is usually complete by your mid-twenties. At that point, your bones are as big as they'll ever be. Although they won't continue to grow bigger, they can heal and repair themselves in the case of fractures.
Ever noticed how a baby's bones are very small and fragile compared to adult bones? The fingers and toes on a baby are so very much smaller than ours yet they become quite huge over time. How do bones get so much bigger over time?
Bones grow longer over time but they also get thicker. There are mechanism that allow for bones to grow in both length and width.
Growth in length
To make a bone longer just add bone tissue to the ends. The problem is, we can't add directly to the end of a bone, it has to move at the joints. If tissue is added at the end of the bone the skeleton could not move properly. It's like closing both lanes on an interstate until the bridge is repaired.
For this to work, bone tissue must be added below the joint somewhere along the length of the bone. This occurs at the epiphyseal plate, or growth plate. Here chondrocytes first produce hyaline cartilage. The cartilage then becomes calcified or ossified to form hard bone tissue (involves addition of Ca+ and Phosphorous ions). The condrocytes produce cartilage on one side of the plate and push the end of the bone up. The other side of the epiphyseal plate gradually becomes calcified.
Once a person reached adulthood and the bones have reached maximum length, and the whole plate gets calcified. It forms a visible line called the epiphyseal line.
Growth in diameter
Making a bone grow in diameter is a more straightforward process. To make a bone thicker, just add new bone tissue to the outside. It's like taking a pipe and adding layer after layer of duct tape to it. The pipe would get thicker and heavier as you go. The problem is the bone gets thicker and heavier as you go. In fact if you started with a bone the size of a baby's and make it as large as an adult's, you would have a bone that's essentially solid. Baby bone have a very small marrow cavity.
Since we don't want to weigh an extra couple hundred of pounds more than we do now, it's important to take excess bone tissue away from the inside. That way the bone can grow in diameter, get thicker and not completely fill up with bone tissue.
Using proper terms, we then add tissue to the periosteal side of bone and remove it from the endosteal side. In growing bone we find Osteoblasts on the periosteal side adding bone, and Osteoclasts on the endosteal side removing some bone tissue.
Bone Construction Zone: How Bones Grow
Right now, the bones in your body are undergoing renovation. There are wrecking crews blasting into the bone quarry and carting off debris while an entirely different work crew hauls bags of concrete to the blast site and patches the newly made holes with stronger, newer, better material.
Before we talk about replacing bone with bone, we'd better learn 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 (which doesn't have salts or minerals in it) 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. (One common analogy for this design is reinforced concrete, which is a grid of metal rods covered with concrete mix.)
After a while, the osteoblasts themselves become part of the mix, turning into lower-functioning osteocytes, sort of a retired version of osteoblasts that continue to putter along but don't stray too far from the blood vessels. 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.
But we're not through with the bone-growing process yet.
Bone Growth, Continued
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).