Bone matrix, which includes both organic and inorganic components, makes up bones. Osteoblasts deposit collagen, also known as osteoid, as the bone matrix. Through the process of mineralization, which uses inorganic salts like calcium and phosphate as well as chemicals released by the osteoblasts, osteoid becomes harder.
An osteon (or Haversian system) is the fundamental microscopic unit of bone. Osteons are essentially cylindrical structures that can be up to 0 millimeters long. 2 mm in diameter.
A central canal (Haversian canal) is surrounded by lamellae of compact bone tissue that make up each osteon. The Haversian canal contains the bone’s blood supplies. The boundary of an osteon is called the cement line. Osteons can be arranged into woven bone or lamellar bone.
A photograph taken under a microscope of an osteon provides a detailed view of the anatomy of compact bone.
The collagen fibers in woven bone are arranged irregularly, and it has a poor mechanical strength.
On the developing ends of an immature skeleton or, in adults, at the site of a healing fracture, woven bone can be seen. Although it is mechanically weak and has an irregular collagen fiber organization, woven bone forms quickly.
The fibrous matrix is called woven because of its crisscross appearance. Its mechanical weakness is caused by a high ratio of osteocytes to hard inorganic salts.
Woven bone is replaced by lamellar bone during development. Lamellar bone is highly organized in concentric sheets, in contrast to woven bone, and has a much lower proportion of osteocytes to surrounding tissue. The mechanical strength of lamellar bone is due to the collagen’s regular parallel alignment into sheets, or lamellae.
Trabecular bone is visible in the center of the femur’s head in this cross-section, which also shows lamellar bone on the edges.
The compact or cortical bone in the skeleton is made up of lamellar bone, which includes the long bones of the arms and legs. Similar to plywood, lamellar bone’s fibers can be seen running in opposite directions in alternating layers in a cross-section, helping the bone resist torsion forces.
Trabecular bone is a term for the same lamellar bone when it is loosely arranged. The name “trabecular bone” refers to the spongy pattern that it exhibits on x-rays. Active bone marrow fills the trabecular bone’s cavities. After a fracture, woven bone initially forms, but over time, a process known as bony substitution causes it to be gradually replaced by lamellar bone.
BONE AS AN ORGAN: MACROSCOPIC ORGANIZATION
The skeleton contains two different types of bones: flat bones (such as the skull, scapula, mandible, and ileum) and long bones (such as the tibia, femur, and humerus). ). These are created through two different types of development, intramembranous and endochondral, though long bones actually go through both cellular processes during development and growth. The main distinction between endochondral and intramembranous bone formation is the latter’s presence of a cartilaginous model, or anlage.
Long bones have two wider extremities (the epiphyses), a hollow cylindrical midshaft (the diaphysis), and a transition region (the metaphysis) between them. During the period of development and growth, a layer of cartilage called the epiphyseal cartilage—which also functions as the growth plate—separates the epiphysis from the metaphysis and midshaft, which each develop from separate ossification centers. The longitudinal growth of bones is mediated by this layer of proliferating cells and expanding cartilage matrix; it gradually mineralizes and is subsequently remodeled and replaced by bone tissue by the end of the growth period (see section on Skeletal Development). The cortex (compact bone), a thick and dense layer of calcified tissue that encloses the medullary cavity where the hematopoietic bone marrow is housed in the diaphysis, forms the exterior of the bones. The cortex gradually becomes thinner as it approaches the metaphysis and epiphysis, and the internal space is filled with a network of thin, calcified trabeculae that make up the cancellous or trabecular bone. These tiny trabecular spaces are continuous with the diaphyseal medullary cavity and contain hematopoietic bone marrow as well. A non-calcifying layer of articular cartilage covers the outer cortical bone surfaces at the epiphyses.
As a result, bone comes into contact with soft tissues on both its internal and external surfaces, known as the endosteal surface and periosteal surface, respectively. The periosteum and endosteum of these surfaces, respectively, are lined with osteogenic cells.
The cells and matrix components in cortical and trabecular bone are the same, but they differ structurally and functionally. The primary structural difference is quantitative: 80% to 90% of the volume of compact bone is calcified, whereas only 15% to 25% of the trabecular volume is calcified (the remainder being occupied by bone marrow, blood vessels, and connective tissue) The result is that 70% to 85% of the interface with soft tissues is at the endosteal bone surface, including all trabecular surfaces, leading to the functional difference: the cortical bone fulfills mainly a mechanical and protective function and the trabecular bone mainly a metabolic function, albeit trabeculae definitively participate in the biomechanical functi
More focus has recently been placed on cortical bone structure because of how closely cortical porosity is related to both the remodeling process and bone strength. Cortical porosity does, in fact, correlate with a rise in fragility fractures (3).
BONE AS A TISSUE: BONE MATRIX AND MINERAL
Bone matrix consists mainly of type I collagen fibers (approximately 90%) and non-collagenous proteins For maximum bone strength, the fibers in lamellar bone are forming arches. This fiber arrangement enables the highest collagen density per unit volume of tissue. Trabecular bone and periosteum are examples of surfaces where the lamellae can be parallel to one another, while the cortical bone Haversian system is an example of a surface where they can be concentric. On the collagen fibers, inside of them, and in the matrix surrounding them, hydroxyapatite [3Ca 3 (PO 4) 2 (OH) 2] crystals have a spindle- or plate-shaped shape. They frequently align themselves with the collagen fibers in the same direction.
There is no preferential organization of the collagen fibers when bone forms quickly during development and fracture healing, or in tumors and some metabolic bone diseases. This type of bone is known as woven bone as opposed to lamellar bone because they are then less closely packed and found in somewhat randomly oriented bundles. Large and numerous osteocytes, irregular collagen fiber bundles, and delayed, disorderly calcification that takes place in patches with irregular distributions are all characteristics of woven bone. During the remodeling process that comes after normal development or healing, mature lamellar bone gradually replaces woven bone (see below).
Bone matrix contains a variety of non-collagenous proteins that have been purified and sequenced, but their functions have only been partially understood () (4). However, not all of the non-collagenous proteins in the bone matrix are produced by osteoblasts; about 25% of them are plasma proteins that are preferentially absorbed by the bone matrix, like a 2-HS-glycoprotein that is produced in the liver. The major non-collagenous protein produced is osteocalcin, which makes up 1% of the matrix, and may play a role in calcium binding and stabilization of hydroxyapatite in the matrix and/or regulation of bone formation, as suggested by increased bone mass in osteocalcin knockout mice A knockout mouse model has shown that the matrix protein matrix gla appears to inhibit premature or inappropriate mineralization as another negative regulator of bone formation. Biglycan, a proteoglycan that is expressed in the bone matrix and positively regulates bone formation, on the other hand, positively regulates bone mass and bone formation, as shown by decreased bone mass and bone formation in biglycan knockout mice. Recently, it was discovered that osteocalcin has a significant endocrine function that affects the pancreatic beta cell. Its hormonally active form, undercarboxylated osteocalcin, stimulates the release of insulin and improves insulin sensitivity in adipose tissues and muscle, which improves the utilization of glucose in peripheral tissues (2).
| PROTEIN | MW | ROLE |
|---|---|---|
| Osteonectin (SPARC) | 32K | Calcium, apatite and matrix protein binding Modulates cell attachment |
| α-2-HS-Glycoprotein | 46-67K | Chemotactic for monocytes Mineralization via matrix vesicles |
| Osteocalcin (Bone GLA protein) | 6K | Involved in stabilization of hydroxyapatite Binding of calcium Chemotactic for monocytes Regulation of bone formation |
| Matrix-GLA-protein | 9K | Inhibits matrix mineralization |
| Osteopontin (Bone Sialoprotein I) | 50K | Cell attachment (via RGD sequence) Calcium binding |
| Bone Sialoprotein II | 75K | Cell attachment (via RGD sequence) Calcium binding |
| 24K Phosphoprotein (α-1(I) procollagen N-propeptide) | 24K | Residue from collagen processing |
| Biglycan (Proteoglycan I) | 45K core | Regulation of collagen fiber growth Mineralization and bone formation Growth factor binding |
| Decorin (Proteoglycan II) | 36K core + side chains | Collagen fibrillogenesis Growth factor binding |
| Thrombospondin & Fibronectin | Cell attachment (via RGD sequence) Growth factor binding Hydroxyapatite formation | |
| Others (including proteolipids | Mineralization | |
| Growth Factors IGFI & IGFII TGFβ Bone morphogenetic proteins (BMPs) | Differentiation, proliferation and activity of osteoblasts Induction of bone and cartilage in osteogenesis and fracture repair |
A secondary bone called lamellar bone is produced when woven bone is modified. Lamellar bone is mechanically robust and has collagen sheets aligned in a regular, parallel pattern called lamellae. It is very well organized into concentric sheets, and the ratio of osteocytes to surrounding tissue is much lower.
When the body must produce bone more quickly, the woven bone is created. As a result, the structure becomes disorganized because the speed prevents the collagen fibrils from being deposited in an orderly manner.
Lamellar bone represents main bone in mature skeleton. Collagen fibers are arranged in layers or lamellae, which is what gives lamellar bone its name. Compared to the haphazard arrangement of the woven bone, this gives it more stiffness.
Calcium and crystalline mineral salts make up the majority of the bone’s inorganic component, while Type I collagen makes up the majority of the organic component of the matrix.
Based on their microscopic arrangements, the types of bones woven bone and lamellar bone are distinguished from one another. These two bone structures have different functions. Lamellar bone is a more structured bone compared to woven bone, which is more haphazard.