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Bones–Structure, Stages, Process of Resorption | Gurugrah



The rigid organs known as bones are a component of the endoskeleton of vertebrates. They produce red and white blood cells, store minerals, and help the various organs move, support, and protect the body. Dense connective tissue includes bone tissue. In addition to carrying out a plethora of other functions, bones are lightweight yet robust and hard due to their complex internal and external structure and variety of shapes. The mineralized osseous tissue, also known as bone tissue, is one type of tissue that gives bone its rigidity and three-dimensional internal structure, similar to a honeycomb. Marrow, endosteum, and periosteum, as well as nerves, blood vessels, and cartilage, are additional types of tissue found in bones. There are 206 bones in an adult human body and 270 in an infant.

Their main functions are as follows:

Mechanical Protection:

Bones can protect internal organs, such as the heart and lungs, and the skull, which protects the brain. Shape Protection: Bones provide a support structure for the body.

Movement: The muscles, tendons, ligaments, bones, and joints of the skeleton work together to generate and transfer forces, allowing the body as a whole or individual part to be moved in three-dimensional space. Biomechanics is the study of how muscle and bone interact with one another.

Bones play a crucial role in the mechanical aspect of overshadowed hearing—sound transduction.

Production of blood: In a process known as hematopoiesis, the marrow, which is found within the medullary cavity of long bones and the interstices of cancellous bones, produces blood cells.

Growth factor storage:

The mineralized bone matrix stores important growth factors like insulin-like growth factors, transforming growth factors, bone morphogenetic proteins, and others.

Synthetic metabolic mineral storage:

Bones serve as reserves of minerals that are essential to the body, particularly calcium and phosphorus. Growth factor storage:

Fat Storage:

The yellow bone marrow stores fatty acids in a reserve.

Balance of acids and bases:

Bone absorbs or releases alkaline salts, which protect the blood from excessive changes in pH.


In addition to removing heavy metals and other foreign substances from the blood and minimizing their effects on other tissues, bone tissues can store these substances. These can be released gradually for excretion later.

Bone releases fibroblast growth factor 23 (FGF-23), which acts on the kidneys to reduce phosphate re-absorption, to regulate phosphate metabolism as an endocrine organ.


The osseous tissue, which is the primary tissue of bones and gives them their rigidity, is a lightweight composite material that is relatively hard. It is mostly made of calcium phosphate in a chemical arrangement called calcium hydroxylapatite. It has a tensile strength of 104-121 MPa, meaning it resists pulling forces well but not pushing forces, but it has high compressive strength. Although bone is essentially brittle, collagen plays a major role in its significant degree of elasticity. The osseous tissue is made up of both living and dead cells that are embedded in the mineralized organic matrix.

Structure of each individual bone

Bone is not a uniformly solid material; rather, there are gaps between its hard parts.

Compact bone or (Cortical bone)

The hard outer layer of bones is made up of compact bone tissue, also known as cortical bone. It gets its name from the fact that there are very few spaces and gaps in it. This tissue gives bones their smooth, white, and solid appearance and is responsible for 80% of an adult skeleton's total bone mass. Dense bone is another name for compact bone.

Trabecular bone

The trabecular bone tissue, also known as cancellous or spongy bone, is an open-cell porous network that fills the interior of the bone. It is made up of a network of rod- and plate-like elements that make the organ lighter and make room for blood vessels and marrow. The remaining 20% of total bone mass is trabecular bone, which has nearly ten times the surface area of compact bone. The trabeculae will be rearranged if the strain to which the cancellous is subjected changes for any reason. There is no microscopic distinction between cancellous and compact adult bone, even though they both exist.

Cellular structure

The bone is made up of several different kinds of cells, which have a cellular structure of

Osteoblasts are mononucleated cells that come from osteoprogenitor cells and form new bones. They produce osteoid, a protein mixture that mineralizes into the bone and is found on the surface of osteoid seams. On the surface of a bone, the osteoid seam is a small area of the newly formed organic matrix that has not yet mineralized. Type I collagen makes up most of the osteoid. Additionally, osteoblasts produce hormones that affect the bone itself, such as prostaglandins. Alkaline phosphatase, an enzyme involved in bone mineralization, and numerous matrix proteins are robustly produced by them. Osteoblasts are the undeveloped cells in bone.

Osteoblasts are essentially inactive cells in the lining of bones. They cover the entire surface of the bone and act as a barrier for some ions.

Osteocytes are made from osteoblasts that have migrated into and become entrapped and surrounded by their own bone matrix. Lacunae are the locations they occupy. Osteocytes communicate with osteoblasts and other osteocytes probably through a number of processes that reach out to them. To varying degrees, they serve the following roles: maintenance of the matrix, bone formation, and calcium homeostasis They have also been shown to regulate the bone's response to stress and mechanical load as mechano-sensory receptors. They are mature cells from the bone.

The cells that cause bone resorption (remodeling of bone to reduce its volume) are called osteoclasts. Osteoclasts are large, multinucleated cells that reside in resorption pits, or Howship's lacunae, on the surface of bones. After the bone surface is broken down, these lacunae, or resorption pits, remain behind. Osteoclasts have mechanisms that are similar to those of circulating macrophages and are derived from the monocyte stem cell lineage. Osteoclasts reach maturity on distinct bone surfaces or migrate there. Active enzymes against the mineral substrate, like tartrate-resistant acid phosphatase, are secreted upon arrival.

Structure at the molecular level


The bone matrix makes up the majority of bone. There are organic and inorganic components. The hardening of this matrix that holds the cells in place is what makes bone. Osteocytes are formed when these cells become trapped within osteoblasts.


Calcium, which is found in the form of hydroxyapatite, and crystalline mineral salts make up the majority of the inorganic. The matrix is initially laid down as osteoid that has not been mineralized and is made by osteoblasts. Osteoblasts secrete alkaline phosphatase-containing vesicles during mineralization. The foci for calcium and phosphate deposition are created as a result of this cleaving of the phosphate groups. After that, the vesicles break open, forming a base for the growth of crystals.


Type I collagen makes up the majority of the organic portion of the matrix. This is synthesized as tropocollagen inside the cell and then exported, resulting in fibrils. There are also a number of growth factors in the organic part, whose functions are not completely understood. Glycosaminoglycans, osteocalcin, osteonectin, bone sialoprotein, osteopontin, and Cell Attachment Factor are some of the factors that are present. The hardness of the bone matrix is one of the main features that set it apart from that other cells.

Woven or lamellar:

Two types of bone can be identified microscopically according to the pattern of collagen forming the osteoid (collagenous support tissue of type I collagen embedded in glycosaminoglycan gel

1) woven bone characterized by a haphazard organization of collagen fibers and is mechanically weak, and

2) lamellar bone which has a regular parallel alignment of collagen into sheets (lamellae) and is mechanically strong.

When osteoblasts rapidly produce osteoid, which initially occurs in all fetal bones but is later replaced by more resilient lamellar bone, woven bone is produced. Adults with Paget's disease or fractures develop woven bone. Despite having fewer randomly oriented collagen fibers and being weaker, woven bone forms quickly; The term "woven" refers to the way the fibrous matrix appears on the bone. Lamellar bone, which is highly organized in concentric sheets and has a much lower ratio of osteocytes to surrounding tissue, takes its place quickly. Lamellar bone, which first appears in the third trimester of the fetus[3], is more robust and contains numerous columns of collagen fibers that run parallel to other fibers in the same layer. These columns are referred to as osteons. Similar to plywood, the cross-sectional fibers run in opposite directions in alternating layers, enhancing the bone's resistance to torsion forces. In the early stages of a fracture, woven bone forms before being gradually replaced by lamellar bone in a process that is referred to as "bony substitution."


The human body has five types of bones: Sesamoid, long, short, flat, and irregular

The diaphysis, or shaft, of long bones, is significantly longer than it is wide. Spongy bone and compact bone make up the majority of them, with less marrow, which is found in the medullary cavity. The fingers and toes, as well as the majority of the limb bones, are long bones. The kneecap, ankle, and wrist are the only exceptions.

A thin layer of compact bone surrounds a spongy interior in short bones, which are roughly cube-shaped. The sesamoid bones, as well as the bones of the wrist and ankle, are short bones.

Two parallel layers of compact bones are sandwiched between a layer of spongy bone in flat bones, which are typically curved and thin. The sternum and the majority of the skull's bones are flat.

The aforementioned categories do not apply to irregular bones. They have a spongy interior surrounded by thin, compact bone layers. Their namesake describes their complicated, irregular shapes. The spine bones are irregular bones, and the sesamoid bones are embedded bones in tendons. The tendon's angle increases as a result of them holding the tendon further away from the joint. This increases the muscle's leverage. The patella and the pisiform are two examples of sesamoid bones. Lamellar bone formation occurs more slowly than woven bone. The osteoid formation rate is limited to about 1 to 2 micrometers per day due to the orderly deposition of collagen fibers.

In order to lay the collagen fibers in parallel or concentric layers, lamellar bone needs a surface that is relatively level.


During the fetal stage of development, bone is formed through two processes: Ossification of the intramembranous space and the endochondral space.

Intramembranous ossification

Intramembranous ossification mainly occurs during the formation of the flat bones of the skull; the bone is formed from mesenchyme tissue.

The steps in intramembranous ossification are:

Development of an ossification center


Formation of trabeculae

Development of periosteum

Endochondral ossification

Endochondral ossification, on the other hand, occurs in long bones, such as limbs; the bone is formed from cartilage.

The steps in endochondral ossification are:

Development of cartilage model

Growth of cartilage model

Development of the primary ossification center

Development of the secondary ossification center

Formation of articular cartilage and epiphyseal plate

The formation of the articular cartilage and the epiphyseal plate Endochondral ossification begins at points in the cartilage that are referred to as "primary ossification centers." While some short bones begin their primary ossification after birth, the majority of them appear during fetal development. The diaphyses of long bones, short bones, and particular parts of irregular bones are formed by them. After birth, secondary ossification results in the formation of long bone epiphyses and irregular and flat bone extremities. A growing area of cartilage known as the epiphyseal plate separates the long bone's diaphysis and both epiphyses. The diaphysis and both epiphyses become fused together (epiphyseal closure) when the child reaches skeletal maturity, which is between the ages of 18 and 25. This occurs when all of the cartilage is replaced by bone.

Bone marrow

Bone marrow can be found in virtually any bone with cancellous tissue. All of these bones are exclusively filled with red marrow when a baby is born, but as a child gets older, yellow or fatty marrow takes its place. In adults, the femur, ribs, vertebrae, and pelvic bones contain the majority of red marrow.


The process of resorption, followed by the replacement of bone with little change in shape, known as remodeling or bone turnover, occurs throughout a person's life. Bone remodeling units are osteoblasts and osteoclasts that are joined by paracrine cell signaling.


The purpose of remodeling is to shape and sculpt the skeleton during growth, regulate calcium homeostasis, and repair tiny fractures caused by everyday stress.

Calcium balance

The osteoclasts' bone resorption process is a crucial step in regulating calcium balance because it releases calcium that has been stored into the systemic circulation. Resorption actively unfixes calcium, thereby raising circulating calcium levels, while bone formation actively removes calcium from the bloodstream in its mineral form. At specific sites, both of these processes take place simultaneously.


According to Wolff's law, repeated stress, such as weight-bearing exercise or bone healing, causes the bone to thicken at the stress points.

Paracrine cell signaling

The action of osteoblasts and osteoclasts is controlled by a number of chemical factors that either promote or inhibit the activity of the bone-remodeling cells, controlling the rate at which bone is made, destroyed, or changed in shape. It has been hypothesized that this is a result of bone's piezoelectric properties, which cause the bone to generate small electrical potentials under stress.[4] Additionally, paracrine signaling is used by the cells to regulate one another's activity.

Osteoblast stimulation

By inhibiting the ability of osteoclasts to break down osseous tissue and by increasing the secretion of osteoid, osteoblasts can be stimulated to increase bone mass.

The pituitary, thyroid, and sex hormones (estrogens and androgens) all secrete growth hormone, which stimulates bone formation through increased osteoid secretion. Osteoblasts can also be induced to secrete a number of cytokines that promote the reabsorption of bone by stimulating osteoclast activity and differentiation from progenitor cells.[5] These hormones also promote increased secretion of osteoprotegerin. Osteocyte stimulation, parathyroid hormone, and vitamin D cause osteoblasts to secrete more RANK-ligand and interleukin-6, which in turn causes osteoclasts to reabsorb more bone. These same substances also make osteoblasts secrete more macrophage colony-stimulating factor, which helps progenitor cells turn into osteoclasts, and make less osteoprotegerin.

Osteoclast inhibition

Calcitonin and osteoprotegerin slow down the rate at which osteoclasts resorb bone. Calcitonin, which is produced by parafollicular cells in the thyroid gland, has the ability to directly inhibit osteoclast activity by binding to receptors on osteoclasts. Osteoprotegerin, which osteoblasts secrete and can bind to RANK-L, prevents osteoclast stimulation.



By Chanchal Sailani | January 21, 2023, | Editor at Gurugrah_Blogs.



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