# Bone 🦴 --- Our **bones** are the most obvious part of the [[musculoskeletal system]], and are made out of rigid yet flexible [[connective tissue|supportive connective tissue]]. Their function is to provide support for your body and protection for the internal organs. Bones are the body's main storage of [[calcium]], which can be released into the bloodstream by [[bone cells|osteoclasts]]. Human adults have roughly 206 individual bones and 306 [[joint|joints]]. As a connective tissue, bone has a large amount of [[extracellular matrix|ECM]] which is very rigid, yet flexible enough to withstand a lot of stress. Bone ECM has a lot of [[collagen]] fibers in it, which makes it so flexible. They can take a lot of force, and if they do break they can [[bone repair|repair themselves]] quite readily. The process of making bones is called [[ossification]]. Once the bone has been ossified, the bone itself can continue to grow either longways ([[interstitial growth]]) or in girth ([[appositional growth]]). [[Vitamin D]] is essential for bone growth because it helps [[calcium]] absorption, which is necessary for the the minerals associated with calcification. [[Vitamin C]] is essential for bone growth because it is required for [[collagen]] production. Bones are also constantly undergoing. There are several types of [[bone cells]], such as osteoclasts, which break down bone material, osteoblasts that build up that material, and osteocytes which maintain it from within their own pockets of bone called **lacunae**. ## Compact & Spongy Bone Bones generally come in two forms, **compact bone** and **spongy bone**. Every bone in the skeleton has at least some component of compact bone, while certain ones have an additional segment of spongy bone in the center. ![[compact and spongy bone.png]] ### Compact Bone **Compact bone** also called **cortical** or **dense bone**, is a the hard, dense type of bone tissue that you really think of when you think of "bones". It comprises about 80% of our skeletons, and has a slower rate of turnover than spongy bone, meaning it is much less sensitive to hormonal or dietary changes. ### Spongy Bone **Spongy bone** , also called **trabecular bone** or **diploë** is a much less dense version of bone tissue that has a pocked, sponge-like appearance to the naked eye (hence the name). The main, noodle-like formations of the spongy bone are called **trabeculae**, which are somewhat like miniature, solid [[bone|osteons]] in compact bone. [[bone cells|Osteocytes]] live in their lacuna along the lamellae, and can reach their nutrients by the canaliculi that open to the outside. Because of this, spongy bone is lighter, more resistant to stress, and can act like a scaffolding to the compact bone. It's found in the [[long bone#Epiphysis & Diaphysis|epiphyses and diaphysis]] of long bones, in vertebral bones, the sternum, ribs, skull and pelvis. >[!tip]- The medullary cavity is not filled with spongy bone > When first learning about the features of bones, I somehow came to the understanding that the [[long bone#Medullary Cavity|medullary cavity]] of long bones were filled with spongey bone and the bone marrow "filled in the gaps" around the trabeculae. This just isn't true. It's just bone marrow all the way down. ## Periosteum & Endosteum The **periosteum** is a fibrous membrane that covers the whole outside of the bone (except at the joints, which is covered with a thin layer of [[cartilage|articular cartilage]]. The **endosteum** is similar to the periosteum, but it covers the inside of the [[long bone#Medullary Cavity|medullary cavity]] and the [[#spongy bone|trabeculae]] of spongy bone. They contain [[blood vessels|blood]] and [[lymphatic vessels]] and [[nerves]]. These are where are where [[bone remodeling]] and [[appositional growth]] take place, and where [[tendons]] and [[ligaments]] attach to the bone. ## Hydroxyapatites The combination of rigidity and flexibility is the vital quality of the bone [[extracellular matrix|ECM]]—It must be rigid enough to provide support, yet flexible enough to withstand a lot of stress. The rigidity comes from tiny mineral crystals called **hydroxyapatites**. These cluster around [[collagen]] fibers, which grants the bone it's flexibility. Bones are about 65% mineral (mostly [[calcium]] and [[phosphate|phosphorus]]) and 35% organic substances. ![[bone molecular structure.png]] ## Classifications Bones come in a variety of shapes and sizes, but there are some features that can be used to describe all bones. There are five different shapes that bones can be categorized by: **long bones**, **short bones**, **flat bones**, **sesamoid bones** and **irregular bones**. ![[bone type.png]] - [[long bone|Long bones]] are the most "bone" looking bones. They are cylindrical and longer than they are wide, which function as levers and work with muscles to move around. - **Short bones** are pretty cube shaped, with roughly equal dimensions. These are only found in the wrists and ankles, and function to provide stability and support. - **Flat bones** are flat in that they are pretty thin, but they are not straight flat, they are usually curved. These bones usually function as attachment for muscles and protection for internal organs. They have three layers, two layers of compact bone on either side of a layer of spongy bone. - **Sesamoid bones** are bones that are sesame seed shaped. They protect tendons from compressive forces by acting like pulleys. Not everyone has the same number of and placement of sesamoid bones, but they are usually found in the hands, feet, knees and neck. - **Irregular bones** are all the other types of bones that can't be classified otherwise. They are generally complex shapes like vertebrae or the small bones in the ears. ___