By: Annie Hu
We all know that in early years, as we grow older, we grow taller. The growth of the entire body as a whole process is extremely fascinating and complex. Although there are many processes involved in a child’s development, there is much to examine in how growth occurs in the skeleton, even though it is only a part of the big picture.
There are 206 bones total in the adult human body and each one has a unique shape, structure, and function. Does that mean that the way they grow is different as well? Yes and no. Bone growth does differ among bones of different embryological development (the origin and growth of the embryo), but more specifically they differ in two broader categories.
The process of general bone formation is known as ossification. The two types of ossification are called intramembranous and endochondral. To understand these categories, we must first discuss the four main types of bones. Most of our limb bones are categorized as long bones, which are longer than they are wide, as the name indicates. Short bones, such as the bone found in our ankles, are very cubic and shaped like blocks. The other two types of bone include flat bones (such as the shoulder blades), and irregularly shaped bone, like vertebrae and some of the bones in our skull that don’t fit into the other 3 types.
Intramembranous Ossification is the process behind the growth of more flat and irregularly shaped bones. It occurs when more bony tissue forms to replace previously connective membranous tissue that acts as a template for the bone formation. Osteoblasts, which are bone forming cells, make their way to these membranes and surround them with a calcified matrix, forming something with resemblance to a bone. The more common form of ossification, however, is Endochondral Ossification. Endochondral Ossification involves a specific form of cartilage called hyaline cartilage that forms the base for the bone’s growth. The cartilage cells involved in bone growth are known as chondrocytes. These osteoblasts are stimulated by blood vessels that enter the center of the bone shaft (the long part of the bone is the shaft, and is also called the diaphysis) to begin growing bone. The area where the artery kick starts the formation of bone is known as the primary ossification center. More arteries make their way into sites at the very ends of the long bone (these sites are then called the epiphyses) and form secondary ossification centers there. The bone continues to replace the cartilage until cartilage only remains as articular cartilage on joint surfaces, and as cartilage between the diaphysis and epiphysis as the growth plate. This growth plate, also sometimes called the epiphyseal plate, is what creates bone lengthening after a bony structure has been formed.
As previously mentioned, longitudinal bone growth is the result of cellular processes in the cartilage growth plate. In a zoom-in of the growth plate, we can see that the chondrocytes making up the cartilage are arranged in an almost columnar configuration. The cells show a fascinating cascade of cell differentiation as the bone grows. The chondrocytes differentiate (turn from chondrocytes to bone cells) in the direction of the diaphysis (the long part of the bone), so that the bone formation will result in the shaft lengthening. These chondrocytes are divided up into 5 primary zones of differentiation. The one closest to the epiphyseal end (where differentiation begins) is called the resting/reserve zone. This zone consists mainly of pre-formed hyaline cartilage with a few round chondrocytes, spread relatively further apart. Scientists hypothesize that this zone helps secure the growth plate and bony tissue in the epiphysis. The next zone is the proliferative zone, which contains columns of flatter chondrocytes that divide to produce more chondrocytes, allowing for the continuation of the cascade and the growth of the bone. The zone of hypertrophy occurs when proliferative cells enlarge (or become hypertrophic) and mature. After this, we have the zone of calcification where the now enlarged cells exhibit apoptotic behavior and die, while allowing the matrix tissue surrounding them to harden and calcify. Arteries and osteoblasts penetrate this zone to help lay down bone. This process of laying down bone finishes in what is called the zone of ossification, closest to the bone shaft, where new bone is ultimately formed as the end result. By the calcification and the proliferation within the growth plate, the bone on the diaphysis side of the growth plate is able to elongate, which is what we probably think of when long bones grow.
Surely, the process of formation of bone is fascinating at the cellular level (with chondrocyte differentiation and growth) as well as on a larger scale, as blood vessels enter the new growth and interconnect with the larger circulatory system. But why do we stop growing? Eventually, the epiphyseal growth plate at the end of the shaft closes off, as the rate of proliferation of chondrocytes decreases. This drop in proliferation occurs near 17-25 years old in humans. However, osteoblasts and osteoclasts (bone destroying cells) help to constantly remodel your bones to adapt to lifestyle changes and injuries. In younger years, as bones remodel the bone is built faster than it is broken down. That is why bone density is high and risk of diseases like osteoporosis (loss of bone mass) is lower. However, when we get older, this rate of bone production is harder to keep up, which is why bone health can easily decline. So what can you do to help out your bones? Exercise, specifically weight bearing exercises, are really helpful for maintaining bone density. By introducing microscopic fractures in your bones through these exercises, it can help to stimulate your remodeling system to build more bone. Also, including calcium in your diet will help, as bone creating depends on creating a calcified matrix. Hopefully you found reading about how bones form, grow, and remodel very interesting!
What Did You Learn?
1. How are long bones formed?
Long bones are formed through the process of endochondral ossification. This means that the bone is built from a hyaline cartilage template. The entry of blood vessels provides the nutrients needed to stimulate osteoblasts and begin ossification of the bone through the primary ossification center. This is a trend that continues in forming two secondary ossification centers at the epiphyses of the bone, in the end leaving only cartilage on the joint surface and in the growth plate. This then sets up the bone for the slow elongation growth that occurs after the bones have been created.
2. How can you help out your bone remodeling system in adulthood?
By adulthood, bone growth through the cellular chondrocyte columns of the growth plate has largely ended due to the closing off of the growth plate. However, bone remodeling through osteoblast creation and osteoclast resorption continues. Common diseases in late adulthood where bone creation is not as fast, involve weakening of the bone due to high porosity and low bone mass. To combat these, you need to aid your osteoblasts. Having a healthy intake of calcium can help to provide a good environment for the matrix calcification that is integral in bone growth. As well, exercises that exert force will cause bone remodeling to occur more.