16.0.3 POJA-L7203 LOCOMOTOR SYSTEM: BONE
INTRODUCTION-3: FORMATION OF BONE
Membranous Osteogenesis or intramembranous ossification.
INTRODUCTION-3: FORMATION OF BONE
Membranous Osteogenesis or intramembranous ossification.
1. This process is a direct osteogenesis process. i.e. spots of mesenchyme cells in the embryonic connective tissue differentiate into osteoblasts that subsequently synthesize collagenous fibres type 1 embedded in various bone matrix proteins BMP’s. The intramembranous bone formation is characterised by forming bone tissue without a preformed cartilage model.
Bony islets develop and increase in size. Subsequently via the processes of remodelling a bony network composed of trabeculae is created. The bony spongy-like structure with meshes will harbour vascularised mesenchyme and primary bone marrow.
Spindle-like mesenchymal cells accumulate around the definite bone as a kind of primitive periosteum.
During the further development the initial spongy bone is replaced by compact bone i.e. erosion tunnels and cavities in the bone are created by the osteoclasts. Osteoblasts line the walls of the erosion tunnels and lay down concentric layers of osteoid and bone. They eventually becomes enclosed by the matrix and turn into osteocytes. This results in osteons with central Haversian canals. Through these processes the primary spongy bone is gradually converted into compact bone.
Bony islets develop and increase in size. Subsequently via the processes of remodelling a bony network composed of trabeculae is created. The bony spongy-like structure with meshes will harbour vascularised mesenchyme and primary bone marrow.
Spindle-like mesenchymal cells accumulate around the definite bone as a kind of primitive periosteum.
During the further development the initial spongy bone is replaced by compact bone i.e. erosion tunnels and cavities in the bone are created by the osteoclasts. Osteoblasts line the walls of the erosion tunnels and lay down concentric layers of osteoid and bone. They eventually becomes enclosed by the matrix and turn into osteocytes. This results in osteons with central Haversian canals. Through these processes the primary spongy bone is gradually converted into compact bone.
2. In compact and spongy bone, the periosteum is a layer of fibrous connective tissue that covers the external surface of these bones. Periosteal cells (osteoprogenitor cells and bone-lining cells) form the inner layer on the bone surface. The osteoprogenitor cells with slender nuclei and acidophilic cytoplasm are flattened or squamous. The further differentiation into osteoblasts is regulated by the transcription factor RUNX2, as well as by IGF-1/2.
Bone cavities and spaces between the spiculae and canals are lined by a similar connective tissue layer that is now called the endosteum. These endosteal cells are the osteoprogenitor cells and produce directly bone tissue.
The collagenous fibres are intertwined giving rise to the microscopic woven bone. Most flat bones are formed by intramembranous formation.
Bone cavities and spaces between the spiculae and canals are lined by a similar connective tissue layer that is now called the endosteum. These endosteal cells are the osteoprogenitor cells and produce directly bone tissue.
The collagenous fibres are intertwined giving rise to the microscopic woven bone. Most flat bones are formed by intramembranous formation.
3. In later development remodelling changes the primary spongy bone into secondary compact bone if such bone is required. The primary spongy bone that consists of woven bone is replaced by secondary or lamellar spongy bone also known as cancellous bone or trabecular bone.
4. In compact bone the collagenous fibres are orderly arranged in the lamellae and course in alternate directions from one lamella to the other. The results are parallel lamellated bone and/or concentric lamellated bone with osteons and typical Haversian systems. The osteogenic or cylindrical unit is the so-called osteon or Haversian system. It consists of concentric lamellae of bone matrix separated by cement lines that function as boundaries of osteons. The vascular and nerve supply of the osteon run in this Haversian canal. Perforating (Volkmann’s) canals connect osteonal canals to one another. They run at a right angle to the previous mentioned Haversian vessels. These channels surrounded by lamellar bone contain blood vessels and nerves that travel from the periosteum as well as from the endosteum toward the osteonal canal. In contrast to the Haversian canals Volkmann’s canals are not enveloped by concentric lamellae!
In the diaphysis and epiphysis areas numerous nutrient foramina or openings are present. They are open passages in the bone for blood vessels to and from the marrow cavities.
Another vascular network is discovered the so-called transcortical vessels (TCV) that can be either arterioles or venules traversing perpendicularly across the entire length of the cortical bone to form a direct connection between the endosteal and periosteal circulations. Bone marrow immune cells use these vessels to reach the bloodstream in case of an inflammatory disease.
In the diaphysis and epiphysis areas numerous nutrient foramina or openings are present. They are open passages in the bone for blood vessels to and from the marrow cavities.
Another vascular network is discovered the so-called transcortical vessels (TCV) that can be either arterioles or venules traversing perpendicularly across the entire length of the cortical bone to form a direct connection between the endosteal and periosteal circulations. Bone marrow immune cells use these vessels to reach the bloodstream in case of an inflammatory disease.
5. The process of osteocytic remodelling includes the reversible remodelling of the microenvironment surrounding the osteocyte cell body (perilacunar bone matrix) and its long cytoplasmic processes (pericanalicular bone matrix). It is the ability of an osteocyte to respond to a reduced mechanical stress by secreting matrix metalloproteinases (MMPs) that degrades the bone matrix leaving an empty space around its own cytoplasm. The osteolytic role is also involved in calcium/phosphate ion homeostasis.