Dr. Sassia Lecture Bone Histology Part II June 29 2024.pdf

University of Tripoli Faculty of Medicine Histology & Genetics Department HS141 Histology of Bone (Chapter 8) Part II Instructor: Dr. Sassia Omar Regeai  Osteogenesis or Bone Development ❖ Osteogenesis (ossification) the process of bone ti...

University of Tripoli Faculty of Medicine Histology & Genetics Department HS141 Histology of Bone (Chapter 8) Part II Instructor: Dr. Sassia Omar Regeai  Osteogenesis or Bone Development ❖ Osteogenesis (ossification) the process of bone tissue formation. ❖ In embryos this leads to formation of the skeletal system. ❖ In children and young adults, ossification occurs as part of bone growth. ❖ In adults, it occurs as part of bone remodeling and repair.  Osteogenesis or Bone Development By the sixth or seventh week of embryonic life, the actual process of bone development, ossification (osteogenesis), begins. There are two types of ossification process: 1) Intramembranous ossification (fibrous membrane model) 2) Endochondral ossification (hyaline cartilage model)  Osteogenesis or Bone Development They differ in the way new bone is formed, but the histological structure of the bone is the same regardless of the type of ossification processes. In both processes, the bone tissue that appears first is primary or woven. Primary bone is a temporary and is soon replaced by the definitive secondary lamellar bone. During bone growth, areas of primary bone, areas of resorption, and areas of secondary bone all appear side by side.  1) Intramembranous Ossification Intramembranous ossification is the direct conversion of embryonic mesenchymal tissue to bone. The process begins when mesenchymal cells differentiate into osteoblasts, which begin to synthesize osteoid that will eventually mineralize into bone. The flat bones of the face (jaws: mandible and maxilla), most of the skull bones (frontal, parietal, part of occipital and temporal) and the clavicles are formed via intramembranous ossification. 1) Intramembranous (between membranes) Ossification Intramembranous ossification follows four steps: Development of ossification Center, formation of matrix, Formation of Trabeculae, Development of Periosteum and Formation of Compact Bone During intramembranous ossification, compact and spongy bone develops directly from sheets of mesenchymal (undifferentiated) connective tissue.  Intramembranous Ossification Intramembranous ossification begins in utero during fetal development and continues on into adolescence. At birth, the skull and clavicles are not fully ossified nor are the junctions between the skull bone (sutures) closed. This allows the skull and shoulders to deform during passage through the birth canal. The last bones to ossify via intramembranous ossification are the flat bones of the face, which reach their adult size at the end of the adolescent growth spurt.  Endochondral Ossification Endochondral ossification is the process of bone development from hyaline cartilage model. Cartilage does not become bone. Instead, cartilage serves as a template to be completely replaced by new bone. Endochondral ossification involves the replacement of a hyaline cartilage model with bone beginning during fetal development and ending in late adolescence Endochondral ossification takes much longer than intramembranous ossification. All of the bones of the body (examples are long bones of the limbs, basal bones of the skull, pelvis, vertebral column and ribs) are formed through endochondral ossification. Endochondral ossification follows six steps during embryonic bone development: 1) Development of the cartilage model 2) Growth of the cartilage model 3) Development of the primary ossification center (POC) 4) Development of marrow cavity 5) Development of the secondary ossification center (SOC) 6) Formation of the articular cartilage and epiphyseal plate  Stages of Endochondral Ossification (1) Formation of a bone collar around the middle of the cartilage model and degeneration of the underlying cartilage (2) Followed by invasion of the resulting ossification center by capillaries and osteoprogenitor cells from the periosteum (3) Osteoid deposition by the new osteoblasts, calcification of woven bone, and its remodeling as compact bone (4) This primary ossification (POC) center develops in the diaphysis, along the middle of each developing bone. ❖ Secondary ossification centers (SOC) develop somewhat later by a similar process in the epiphyses. ❖ The primary and secondary ossification centers are separated by the epiphyseal plate (5) Epiphyseal plate provides for continued bone elongation. ❖ The two ossification centers do not merge until the epiphyseal plate disappears and becomes the epiphyseal line when full stature is achieved. Stages of Endochondral Ossification  Epiphyseal Plate Zones The plate of epiphyseal cartilage is divided into five zones, starting from the epiphyseal side of cartilage: 1. The resting zone consists of hyaline cartilage with typical chondrocytes. 2. The proliferative zone, chondrocytes begin to divide rapidly and form columns of stacked cells parallel to the long axis of the bone. 3. The hypertrophic cartilage zone Chondrocytes in this zone stop dividing and undergo growth in size whose cytoplasm has accumulated glycogen. 4. The calcified cartilage zone, Chondrocytes are dead by apoptosis. The extracellular matrix become calcified. 5. The ossification zone osteoclasts digest the calcified cartilage , and osteoblasts replace it with bone tissue.  Bone Growth Bone growth occurs by 2 ways: 2) Appositional bone growth 1) longitudinal bone growth (bone grows in width) (bone grows in length)  Longitudinal Bone Growth Longitudinal growth of a bone occurs by cell proliferation (chondrocytes) in the epiphyseal plate to form new cartilage which is then replaced by bone. The epiphyseal plate is responsible for longitudinal bone growth. Long bones continue to grow in length until early adulthood. The rate of growth is controlled by hormones. Long bones stop growing at around the age of 18 in females and the age of 21 in males in a process called epiphyseal plate closure. During this process, cartilage cells stop dividing and all of the cartilage is replaced by bone All that remains of the epiphyseal plate is the visible epiphyseal line.  Appositional Bone Growth Appositional growth is the increase in the diameter (circumference) of bones by the addition of bony tissue at the surface of bones, does not involve endochondral ossification. Osteoblasts in the cellular layer of the periosteum produce new bone tissue beneath the periosteum. The osteoblasts differentiate into osteocytes. At the same time, osteoclasts on the interior surface resorb bone and widen the marrow cavity. The final result is an increase in both the diameter and marrow cavity of the bone. Bones can grow in thickness throughout life, but after age 25, ossification functions primarily in bone remodeling and repair.  Bone Remodeling Bone remodeling is the replacement of old bone tissue by new bone tissue. It involves the processes of bone deposition by osteoblasts and bone resorption by osteoclasts. The relative thickness of compact bone is maintained Remodeling of bone primarily takes place during a bone’s growth Also, injury, exercise, and other activities lead to remodeling of bone. about 5 to 10 percent of the skeleton is remodeled annually just by destroying old bone and renewing it with fresh bone Bone Remodeling  Bone Fracture & Repair A fracture is a broken bone caused by physical stress. Fractures are repaired in 4 steps:  METABOLIC ROLE OF BONE The concentration of calcium in the blood (9-10 mg/dL) and tissues is generally quite stable because of a continuous interchange between blood calcium and bone calcium. The skeleton serves as the calcium reservoir, containing 99% of the body’s total calcium in hydroxyapatite crystals. Two hormones for calcium homeostasis: 1) Calcitonin 2) Parathyroid hormone (PTH) Calcium Homeostasis  Osteoclastogenesis Osteoblasts synthesize the receptor for the activation of nuclear factor kappa Β ligand (RANKL), macrophage colony-stimulating factor (M-CSF), Osteoprotegerin (OPG) and parathyroid hormone receptors (PTH1R) When PTH binds to these receptors, it stimulates osteoblasts to secrete RANKL, a factor that induces the differentiation of preosteoclasts into osteoclasts The precursor of the osteoclast originates in the bone marrow. Osteoclasts have receptors for osteoclast stimulating factor, colony-stimulating factor-1, OPG, receptor for activation of nuclear factor kappa B (RANK), and calcitonin Osteoclastogenesis Bone-resorbing osteoclasts originate from hemopoietic cells of the monocyte–macrophage lineage under the control of bone-forming osteoblasts. Illustration includes RANKL, the receptor activator of NF-κB ligand; M-CSF, macrophage colony-stimulating factor; and OPG, osteoprotegerin Osteoclastogenesis Osteoblasts produce a decoy receptor, osteoprotegerin (OPG), that binds to excess RANKL  Joints A joint is defined as a connection between two bones in the skeletal system. Joints can be classified by the type of the tissue present (fibrous, cartilaginous or synovial), or by the degree of movement permitted (synarthrosis, amphiarthrosis or diarthrosis). Cartilaginous joints Synovial joint