Welcome to the wonderful world of histology. Histology is how we study the human body on the cellular level. Tissue can be classified into four different types: Epithelial, connective, nerve, and muscle tissue. This week we are looking at connective tissue. However, we are getting specific. Connective tissue can be further divided into hard and soft tissue. Included in hard tissue are bone and cartilage. Today we are looking at the bone.
The bone itself comes in two types: compact and trabecular bone. Compact or cortical bone is the hard dense outside shell of bones, while the trabecular or cancellous bone is the honeycomb strut-like bone that fills the inside of the bone. Trabecular bone lightens the bone overall while maintaining strength creating room for bone marrow that produces blood cells (red marrow) or stores fat (yellow marrow). While we think of bone as a hard and solid object it is alive. 65% of it is mineral calcium and phosphate makes up the majority but there is also magnesium, sodium, and potassium in the mix. Collagen is the next most abundant material it runs through bone like a scaffolding that the mineral content is deposited on. Without collagen, bone would be incredibly brittle. Collagen allows it to flex slightly as the forces run through it.
So, let’s look at bone on a cellular level. I must say bone is one of the most beautiful structures to look at underneath the microscope. It is organised into osteons as seen as the circles below. It is said that we get an entirely new skeleton every 10 years this is because our bone is constantly remodelling. First, there are the osteoblasts that secrete the bone matrix. They then become trapped in the matrix and transform into osteocytes. Osteocytes can be found in the spider-like black dots in the diagram, these dots are called lacuna. These cells are still alive and in need of a blood supply so at the centre of the osteon is a blood vessel running in the Haversian canal. The little spider-like legs are called canaliculi they contain osteocyte processes as they reach for the blood supply.
Finally, we have osteoclasts. These are the cells that break down osteons so that they can be remodelled and built again. Osteoclasts are a little weird; they are relatively large cells that are related to macrophages and have multiple nuclei, but they do serve a purpose. Our bones need to be strong with each step we take. There are around 6,000 newtons of force transmitted through our hips. Constant remodelling of bone allows us to keep our bone at its strongest. So we can carry on through daily life. So there you go the microscopic world of bone is a busy one you might not have heard about.
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