Transcription
Today is a very special day for Jeffrey the skeleton because we're going to talk about one of the most incredible tissues in the human body: bone tissue. Even though we associate bones and skeletons with death and holidays like Halloween, bone tissue is a living, dynamic tissue constantly changing and remodeling. Your bones are constantly building up new bone tissue and breaking down old bone tissue. But why and how does this happen? And how much can you influence your bone health with diet, exercise, and lifestyle choices? Today, we'll answer these questions by exploring bones, showing you what makes them strong and hard, and how blood and nutrients are delivered through this seemingly solid structure. Again, we'll explore how you can influence bone health and reduce your risk of conditions like osteoporosis. Jeffrey the skeleton believes this will be one of the most anatomically awesome videos of all time, but he's obviously biased. Let's do this!
Let's start with how bone may not be as solid as we think. If we look at one of Jeffrey's bones, the outer portion is called compact bone. Compact bone is the dense outer portion of all bones, but it can be relatively thin at bone ends. As you can see, it didn't take much to chip away some of Jeffrey's bone ends to expose the other type of bone tissue underneath, which we'll discuss shortly. On the shafts of long bones, compact bone is much thicker, like on the tibia we cut into. To the naked eye, compact bone looks dense and solid. However, microscopically, it's quite porous. If we cut through Jeffrey's femur (shown on the left), and zoom in (bottom right), you'll see an intricate system of interconnected canals containing blood vessels. You'll also see repeating circular structural units called osteons.
Take a moment to appreciate how awesome it is that compact bone is organized into these osteons. They're aligned parallel to the bone's length, giving it considerable strength. Zooming into an osteon, you see multiple concentric rings, or circular plates, of solid, hard bone tissue. Embedded in each ring are osteocytes (osteo = bone, cyte = cell). These cells maintain bone tissue by exchanging nutrients and waste with the blood. But these cells are stuck in hard bone tissue—how are nutrients distributed? The osteon's center has a central canal with blood vessels. Before the bone hardened, osteocytes sent out cytoplasmic extensions (cellular arms) to reach neighboring osteocytes, literally connecting. An osteocyte connects with others in its ring and sends extensions to connect with osteocytes in the ring behind it, and so on. I'm sorry if I'm getting excited, but bone is pretty anatomically awesome!
All the hard bone tissue making up the circular plates is the extracellular matrix. Its composition gives bone its hardness and flexibility, and helps us understand bone health and conditions. Zooming in further, you see that bone is made of collagen and a hard, crystalline substance called hydroxyapatite. Collagen is the most abundant protein in the human body, and we'll discuss its importance for bone shortly. Hydroxyapatite, made of calcium phosphate and calcium hydroxide, gives bone its hardness and strength, resisting compression. Calcium is crucial for bone health because it's needed to make hydroxyapatite. Vitamin D is also important for calcium absorption.
Collagen is a protein fiber with tremendous tensile strength, helping bone resist being pulled apart. Remember, hydroxyapatite gives bone compressive strength. A healthy bone is about 30% collagen and 55% hydroxyapatite. This ratio is important; imbalances can lead to diseases. Osteomalacia ("soft bone") and rickets are caused by vitamin D deficiency, preventing proper calcium absorption and hydroxyapatite formation. This results in a higher proportion of collagen, making bones softer and deformable. Contrast this with osteogenesis imperfecta, a problem synthesizing collagen, making bones brittle and easily broken (as depicted in the *Unbreakable* movie trilogy). This shows the importance of the hydroxyapatite and collagen ratio for optimal bone health.
This highlights the importance of calcium and vitamin D for bone health. We'll also discuss exercise, but first, we need to talk about the other type of bone tissue deep to compact bone: spongy bone. Looking at the picture, you can see why it's called spongy bone. It's an intricate network of tiny bone beams called trabeculae (hence, trabecular bone). Spaces between trabeculae allow blood vessels to weave in and out, getting close to the cells making up the bone beams. These beams still contain collagen and hydroxyapatite.
Suspended in the spaces of spongy bone is red bone marrow, which produces red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Blood vessels move throughout compact and spongy bone to supply nutrients and pass through red bone marrow, picking up blood cells to distribute throughout the body. Red bone marrow is found in specific places in the adult skeleton—mostly in the axial skeleton (skull, spine, sternum, rib cage, pelvic bones) and some in the proximal humerus and femur. Although spongy bone's trabeculae may look random, they're organized to handle the bone's stresses, illustrating marvelous biological architecture.
Let's talk about exercise and its effect on bone health. Remember, bone is a dynamic tissue constantly remodeling. To understand this, we need to mention osteoclasts (which resorb or break down old bone tissue) and osteoblasts (which build up new bone tissue). Osteoblasts deposit the extracellular matrix around themselves, creating new bone tissue. The bone calcifies, and they get stuck, becoming osteocytes. If osteoclasts and osteoblasts are paced equally, bone density stays the same.
Astronauts in space experienced bone density loss (15-20%) due to osteoclasts outpacing osteoblasts because of the lack of gravitational stress on bones. Bone is a "use it or lose it" tissue. Exercise, such as running and resistance training, stimulates osteoblasts to outpace osteoclasts, increasing bone density. This is why exercise is crucial for long-term bone health. Bone density peaks in our 30s and gradually declines with age. We can slow this decline with impact activities, resistance training, and sufficient calcium and vitamin D.
Women are eight times more likely to develop osteoporosis than men, especially after menopause. Estrogen has an osteoprotective function, inhibiting osteoclasts. After menopause, estrogen levels drop dramatically, inhibiting osteoclasts less, leading to increased bone resorption and osteoporosis. Men can develop osteoporosis, but it's less likely, as testosterone also protects bones, though it declines less dramatically than estrogen.
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