Evaluation of Bonesmashing: Bone Anatomy & Histology

[Driler]

Intro​

Hello everybody welcome back to another Driler classic. This thread is intended to serve as complementary material that shall give you an intermediate level understanding of bone anatomy and physiology. You could skip this one and then google something when you don’t understand it but I still recommend you go through this.​

Bone Shapes​

Bones can be classified by their shape and structure into one of the following categories: long, flat, sutural, irregular, short and sesamoid bones. I will only briefly explain flat and long ones. Because the rest holds little importance for our interests.​

Long
Firstly long bones are characterized by their rod like shape. They are hollow inside and filled with marrow. They are distinct in that they can be split into the diaphysis, hollow shaft composed mostly of dense cortical bone, metaphyses and epiphyses which are largely spongy bone surrounded by a thin cortical shell. In terms of looksmaxing, they will be interesting in the discussion on height protocols as the femur and tibia determine your height to a large extent.​

Flat
But this videos topic is bonesmashing, thus we will be much more interested in flat bones. Flat bones consist majorly of the craniofacial skeleton as well as the scapulae, sternum and ribs. Importantly, they do not contain diaphyses, metaphyses or epiphyses. Instead they are of a sandwich-like build with a layer of spongy bone in-between two layers of compact bone .​

Macroscopic Structure​

Regarding how the mineral deposits are structured, bones are divided into compact or spongy bone.

Compact
Firstly compact or cortical bone is a well organised structure composing of circumferential lamellae surrounding osteons. Osteons consist of between 4 and 20 concentric lamellar sheets of bone with a defined boundary between one osteon and the next, called a cement line, a thin layer of organic matrix. Generally in biology a lamella refers to a thin layer, membrane or plate of tissue. These concentric lamellae surround the haversian canal through witch blood and nervous supply runs. Adult skeleton consists of 80 percent cortical bone by weight with. Due to low porosity of 5 to 10 percent, cortical bone provides maximum resistance to torsion and bending and gives bone its compressive strength.

Spongy
Cancellous or spongy bone also contains lamellae that are arranged in at-least on first glance randomly formed trabeculae. In actuality these trabeculae are built in such a way to absorb most commonly occurring forces in an optimal manner. Trabeculae do not contain Haversian canals and instead their blood supply runs around the trabeculae. With a greater porosity of 50 to 90 percent, spongy makes up only 20 percent of the adult human skeleton and has significantly greater surface area per unit volume. It also possesses a higher rate of metabolic activity and remodeling. Because the primary bone cells lie on the surface and are in closer proximity to circulating growth factors and cytokines it responds more rapidly to mechanical stimuli and regulation of calcium homeostasis. The primary function of cancellous bone is allowance of deformation and absorption of loads.

Microscopic Structure​

Lamellar
In terms of microscopic level structural distinction. Why is this lamellar structure even used? Due to complicated biomechanical reasons the type of structure that offers the optimal amount of strength for an amount of bone is exactly this lamellar structure. The pattern lamellae are shaped and positioned in allows it to absorb and distribute loads very efficiently. There are four general forms of lamellar bone: osteons, trabecular, circumferential, and interstitial.

Woven
On the other hand woven bone forms the embryonic skeleton after birth and slowly gets replaced by lammellar throughout development or when your body needs to create a quick bone formation and doesn’t have time to organise it into lamellar structures, woven bone is made. You can sort of think of it as a SOS mechanism. After its formation your body will start the process of organising that anarchic structure into normal lamellar bone. Witch means that in any moment you don’t have much woven bone inside you. It mostly occurs when you break something or maybe bone-smash idk hahaha

Periosteum and Endosteum​

Periosteum
Basically, periosteum is the outer layer that covers all parts of bone except of enclosed joint capsules. Its functions are to isolate bone from surrounding tissues but it also a primary participant in bone modeling drifts thus it is involved in bone growth and repair. That is because its surface is covered by osteoblasts and osteoprogentor or stem cells that can differentiate and enact bone formation that way.

Endosteum
The endosteum is a layer that lines the marrow cavity, trabeculae of spongy bone and central canals inside osteons. Similarly to the periosteum it contains osteoblasts, osteoprogenitor cells and osteoclasts thus it is very active in bone growth and repair.

Types of Ossification Process​

Endochondral
The hallmark of endochondral ossification is bone formation through a cartilaginous intermediate. This is the basis of longitudinal growth of long bones at the physeal plate thus the predominant process involved in long bone formation. (It is initiated by the proliferation of chondrocytes into a hyaline cartilage framework with four distinct zones. The resting zone is furthest from the site of bone formation and contains normal chondrocytes. As the chondrocytes proliferate (proliferative zone), they hypertrophy (zone 3) and release alkaline phosphatase, resulting in chondrocyte apoptosis and release of angiogenic factors including VEGF. The zone of dead chondrocytes (calcification zone) creates a barren matrix that promotes capillary ingrowth and migration of osteoprogenitors. This relates to height protocols.)

Intramembranous
Flat bones of the face and clavicles are predominantly created through a biological process called intramembranous ossification. In contradistinction to endochondral ossification, the hallmark of intramembranous ossification is spontaneous bone formation without a cartilaginous intermediate. It is a process in witch your bones quote on quote invade neighbouring tissue, typically the dermis, and ossify it by depositing mineral and collagen structure. That type of ossification is responsible for distraction osteogenesis and theoretically it would also be responsible for the adaptive response that arises from bonesmashing or mewing, if there is one, thus it is presumably a key physiologic phenomenon for looksmaxing.

Matrix​

This presents a question of what is the composition of bone after these formation processes. So lets briefly analyse that. 90% of bone is the matrix while only 10% are alive and metabolically active cells. Of the matrix two thirds are made of minerals (hydroxyapatite) to be precious. Other third is collagen tissue, primarily type I collagen, and it provides elasticity and bending strength. Although bone consists of just 10% actual cell tissue, it would be false to assume it is a inert dead tissue that just provides our body structural support.

Bone Cells​

That is because mineral and collagen structures are constantly going through a process of degrading and constructing back themselves via modeling and remodilng. So lets begin by differentiating between 4 main types of bone cells and describe their distinct physiologic roles.

Osteoblast
As I mentioned when I talked about the periosteum osteoblasts line the surfaces of bone, in normal conditions the coverage is around 5%. Their role is that of bone forming cells due to the ability to synthesize and secrete organic bone matrix. Osteoblastic activity is marked by alkaline phosphatase release. Primary mediators of osteoblast differentiation and activation include bone morphogenic proteins and other members of the TGF signalling family. After their activation they have three fates: remain quiescent osteoblasts lining bone, become osteocytes or return to their original form as osteoprogenitor cell line.

Osteoprogenitor
Now notably even if all osteoblast lining bone surface activated they don't posses much bone formation power. Due to that, there exists osteoprogenitor cells that act as stem cells. Osteoprogenitor cells remain dormant and undifferentiated until receiving signals to migrate to a site, proliferate, and differentiate into osteoblasts. They reside in the bone canals, endosteum, periosteum, and marrow.

Osteoclast
Osteoclasts are multinucleated cells who's role is that of bone-resorptive cells. They attach to bone matrix and solubilize it via acidification, allowing matrix phagocytosis. There are various factors and cytokines that regulate osteoclast formation and activity. These include some inflammation related signalling pathways like NfkB and interleukins 1 and 6 but also calcium homeostasis regulators like PTH, vitamin D, calcitonin and osteoprotegerin.

Osteocyte
Their role is mainly detection of mechanical stress that they interpret and signal out instructions to osteoblast and osteoclast. Signal then acts as an adaptive response to physical conditions that the body finds itself in. They account for 90 percent of all bone cells. Osteoblasts that have surrounded themselves with organic matrix to live within vacuoles called lacunae become ostecyte. Once surrounded by matrix, they develop cytoplastic projections that traverse the bone and allow direct communication with other bone cells. Osteocytes can also be prompted to directly resorb bone by parathyroid hormone and might be involved in hunger signalling.

Outro​

This pretty much sums up everything you need to know about Bone Anatomy & Histology so you can fully understand the ensuing content. There is but a one anatomic concept I left out witch is the BMU. That is done in the episode on bone modeling and remodeling and as I said this one exists to help you better understand all other content. So please interact with this thread and goodbye.