What is going on everybody? It's Medicosis Perfectionellus where medicine makes perfect sense. In the last video we had an introduction about the skeletal system. We talked about the structures of bones. Today let's dig deeper and talk about the microscopic structure of bones.
In order for this video to make sense to you, please watch the previous video first. You'll find it in my biology playlist on YouTube. What are the main cells of the bones?
Osteocytes. What are the cells that build up bone? osteoblasts and what are the cells that cut down bone osteoclast the main cells are here osteocytes and then we have osteoblasts versus osteoclasts the origin is mesodermal osteoblasts are building up osteoclasts are cutting down pause and review bone formation versus bone destruction is how homeostasis is kept in balance and as we have discussed in the last video bone is made of matrix and minerals. What's the main part of the matrix? Type 1 collagen.
What's the main mineral? Calcium. We call these hydroxyapatite crystals. We lay the minerals on top of the matrix to strengthen that collagen, to make your bones strong.
As you recall, type 1 collagen is to be found in bones, but type 2 is to be found in cart... cartilage or cartilage. Here is collagen.
It's a complex protein structure, therefore rough endoplasmic reticulum. It starts as pre-pro-collagen, then becomes pro-collagen. As it matures, it becomes collagen, and after translation, which is protein synthesis, we have post-translational modification of collagen.
As you see, we will end up with strong collagen fibers. Disruption of any of these steps will give you diseases. Hashtag pathology.
Again, collagen is in the matrix. Calcium and phosphate are the minerals. Let's talk about the aversion system, which is the main topic of today's video.
Here is a section in your bone, all right? What do we have? Layers upon layers upon layers of bone.
Can I call them layered or lamellae? Absolutely. Between those layers, we see bone cells, osteocytes, living in their cocoons, lacunae. That sounds good, Medicosus. But let's zoom out.
Do you remember matrix and minerals? Of course, I remember that. Matrix was the main part, and it can organize into osteones, and the osteones are the haversian systems. Each osteone is made of concentric.
lamellae as you see here. So here is an osteone, here's an osteone, another osteone, a third osteone, a fourth osteone, etc. Now let's cut them and traverse them vertically and horizontally. If you want to traverse them vertically, this is the Havergen canal, longitudinal canal.
How about traversing them horizontally? This is the Voegmans canal. Let's do it again. Lunges tunnel is the Havergian canal.
Out of this Havergian canal there is a horizontal Volkmann's canal. Why on earth do you need these canals? I'll tell you.
Because they contain blood vessels to nourish the bones. Because unlike cartilages which were avascular, bones are actually vascular. Not just blood vessels.
Lymph vessels too. And nerves. Nourishment and nerve supply. Amazing.
Moreover, Around each blood vessel, there is a canaliculus. The plural is canaliculi. Why do we need them?
Because they connect the lacunae together. How can this lacunae communicate with this lacunae, with this lacunae? Canaliculi, which are small canals.
This is a big canal, a virgin canal. This is a big canal, Volkmann's canal. But this is a small, tiny canal, canaliculus. So canaliculus is for connection and communication between the lacunae.
And this is the outer shell of your bone, and it's going to be covered by periosteum, as you know. This is super important, so please pause and review. What are the main cells of the bones? Osteocytes. What are the cells that build up bone?
Osteoblasts. What are the cells that cut down bone? Osteoclasts.
They cut down bone by eating it. Just like the macrophage. Oh, you will not believe it when I say to you that macrophages and osteoclasts have the same embryological origin, mesodermal origin. mesenchymal cells which actually makes sense because I am a phagocytic cell I eat foreign invaders I am also a phagocytic cells I eat bone why is this important for remodeling your osteoclasts follow the motto quote in order to make an omelet you'll have to crack some eggs close quote let's talk about vitamin D I have a separate video on this topic today we'll review it very quickly In order for you to have the active form of vitamin D, you need the following.
You need sunlight. You need your liver. You need your gut. You need your skin to accept the sunlight.
You need your liver again and your kidney again. Why do I need my gut? Because if I eat a source of vitamin D, I need my gut to digest it and absorb it.
Why do you need the liver? It provides me with cholesterol, which is the backbone of vitamin D. 7-dehydrocholesterol to be specific. Then, The skin will make it call a calciferol. So far, we have only one hydroxyl group.
The liver will add another hydroxyl group at carbon number 25. Congratulations, now we have 25 hydroxy vitamin D. How many hydroxyl groups do we have here? One from the liver, one from the skin. This has two.
You can call it calcify diol, two hydroxyl groups. The kidney will add a third one at carbon number one. And congratulations, you have a new product known as 1 and 25 dihydroxyvitamin D. How many hydroxyl groups so far?
Three. That's why we called calcitriol the most active form of vitamin D. What's the purpose of vitamin D in life?
Calcium homeostasis, calcium regulation. If you lack calcium, vitamin D will get you more calcium in your blood. This enzyme is very important to 1-alpha-hydroxylase because the kidney added a hydroxyl group at carbon 1-alpha. This enzyme is stimulated by a hormone known as parathyroid hormone, which comes from your parathyroid gland. Here's the parathyroid hormone.
It has one purpose in life, to raise calcium level in your blood. And it does this by three mechanisms. Absorption, resorption, reabsorption.
Absorption of calcium from your gut. Resorption of bone, which means destruction of bones to release calcium into the blood. And if calcium is about to fall into the urine, I'm gonna save it. Reabsorption of calcium in the kidney.
From all of these, calcium in your blood will go up. PTH increases calcium in the blood, but it trashes the phosphate. It's literally a phosphate trashing hormone.
Ta-da! PTH increases calcium but decreases phosphate. PTH comes from the parathyroid gland. Close to the parathyroid gland is the thyroid gland, which secretes calcitonin. Even though these two are neighbors, anatomically speaking, parathyroid versus thyroid, their functions couldn't have been more stark in difference.
PTH raises calcium and phosphate, calcitonin lowers calcium and phosphate. Have you noticed something? Both of them are trashing phosphate. That's dangerous because you need phosphate for ATP energy.
And that's why vitamin D is gonna come to save the day, i.e. to save the phosphate. Vitamin D is grateful to the PTH because the PTH is the reason for vitamin D's existence by stimulating this enzyme. So like father, like son, vitamin D is gonna say, I like you, daddy.
I'm grateful for you. Your job is to raise calcium in the blood. I will raise calcium in the blood as well. However, unlike you, daddy, I learned from the mistakes of the previous generations. The previous generations trashed the phosphate.
I am going to raise the phosphate and save it. So vitamin D increases calcium and increases phosphate in the blood. How does it do this? Again, absorption, resorption, reabsorption. So PTH raises calcium, lowers phosphate.
Calcitonin hates everybody. Vitamin D3 is the new generation. Like father, like son, I'm gonna raise the calcium. Unlike father, I learned from the mistakes of previous generations, and I'm gonna save the phosphate too.
Both of them are good for your bones. That's why vitamin D is paramount for your bone health. Pause and review. Bone remodeling. Let's break down old bone and rebuild new bone.
Remodeling is an ongoing active process. Say thank you to your osteoclasts and your osteoblasts, respectively. There are many factors that affect bone remodeling. Some are local, some are general, including hormones, including calcium and phosphate homeostasis.
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