The Science Behind Growing Tall
By William Huang
We are all familiar with growth. As our age grew, our height did as well. As such, the growth of the entire body is an extremely fascinating and complex process. In total, there are 206 bones in the adult human body and each one has a unique shape, structure, and function. 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: long, short, flat, and irregular. 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 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. The growth plate, known as the epiphyseal plate, is what creates bone lengthening after a bony structure has been formed.
Longitudinal bone growth is the result of cellular processes in the cartilage growth plate. 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: resting/reserve, proliferative, hypertrophy, calcification, and ossification. 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.
Citations:
https://training.seer.cancer.gov/anatomy/skeletal/growth.html
https://training.seer.cancer.gov/anatomy/skeletal/growth.html
https://www.betterhealth.vic.gov.au/health/ConditionsAndTreatments/bones
https://academic.oup.com/endo/article/143/5/1851/2989643
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