Chapter 7 (2)

• Histology of compact bone reveals osteons (haversian systems). Concentric lamellae surround a central (haversian) canal running longitudinally. Perforating (Volkmann) canals—transverse or diagonal passages. Circumferential lamellae fill the outer region of dense bone. Interstitial lamellae fill irregular regions between osteons.
• Spongy bone is made up of a network of bones covered with endosteum. These thin slices of bone are called spicules, while thin plates of bone are called trabeculae. The spaces inside spongy bone contain red bone marrow. Unlike compact bone, spongy bone has few osteons and no central canals. All osteocytes are located close to the bone marrow. Trabeculae develop along the bone's lines of stress, providing strength with minimal weight.
• Bone marrow is a soft tissue that occupies the marrow cavities of long bones and small spaces of spongy bone. Red bone marrow is responsible for producing blood cells and is found in nearly every bone in a child. In adults, it is mainly located in the skull, vertebrae, ribs, sternum, part of the pelvic girdle, and the proximal heads of the humerus and femur. The red marrow is a type of myeloid tissue that contains hemopoietic tissue.
• Yellow bone marrow is found in adults. It is the fatty marrow that does not produce blood. It can transform back to red marrow in the event of chronic anemia.
• Ossification or osteogenesis is the formation of bone. In the human fetus and infant, bone develops by two methods: Intramembranous ossification (Produces flat bones of skull and clavicle in a fetus and thickens long bones throughout life) and Endochondral ossification.
• During infancy and childhood, the epiphyses fill with spongy bone. The cartilage is limited to the articular cartilage covering each joint surface and to the epiphyseal plate. A thin wall of cartilage separates the primary and secondary marrow cavities. The epiphyseal plate persists through childhood and adolescence. It serves as a growth zone for bone elongation.
• By the late teens to early 20s, all remaining cartilage in the epiphyseal plate is generally consumed. The gap between epiphyses and diaphysis closes, primary and secondary marrow cavities unite into a single cavity, and bone can no longer grow in length.
• Ossification continues throughout life with the growth and remodeling of bones. Bones grow in two directions: Length and Width.
• With bone elongation, the epiphyseal plate is where cartilage transitions to bone. It functions as the growth zone where bone elongates and has typical hyaline cartilage in the middle with transition zones on each side where cartilage is replaced by bone. Metaphysis is the zone of transition facing the marrow cavity.
• Interstitial growth is growth from within. Bone elongation is a result of cartilage growth within the epiphyseal plate. The epiphyses close when the cartilage is gone. The epiphyseal line of spongy bone marks the site of the former epiphyseal plate. Lengthwise growth is finished and it occurs at different ages in different bones.
• There are two types of dwarfism: Achondroplastic dwarfism and Pituitary dwarfism.
• With Achondroplastic dwarfism, the long bones stop growing in the person's childhood so they have a normal torso and short limbs. This dwarfism is formed as a failure of cartilage growth in metaphysics and spontaneous mutation produces a mutant dominant allele. The people with this condition are called dwarfs.
• Pituitary dwarfism is formed as a lack of growth hormone so they have normal proportions with short stature. Some folk call them midgets I call them fairies/pixies.
• Appositional growth occurs at the bone surface. It continually grows in diameter and thickness and has intramembranous ossification. Osteoblasts of the inner periosteum deposit osteoid tissue and become trapped as tissue calcified. They lay down the matrix in layers parallel to the surface and form circumferential lamellae. The osteoclasts of the endosteum then enlarge the marrow cavity.
• Bone remodeling (absorption and deposition) occurs throughout life to and to about 10% of the skeleton per year. It repairs microfracture, releases minerals into the blood, and reshapes bones in response to use and disuse.
• Wolff’s law of bone says that the architecture of bone is determined by the mechanical stresses placed on it. Remodeling is a collaborative and precise action of osteoblasts and osteoclasts. Bony processes grow larger and stronger in response to mechanical stress.
• A mature bone remains a metabolically active organ as it is involved in its own maintenance of growth and remodeling. It exerts a profound influence over the rest of the body by exchanging minerals with tissue fluid. The disturbance of calcium homeostasis in the skeleton disrupts the function of other organ systems, especially the nervous and muscular organ systems.
• Mineral deposition (mineralization) is the process in which calcium, phosphate, and other ions are taken from blood and deposited in the bone.
• Osteoblasts produce collagen fibers that spiral the length of the osteon. These fibers become encrusted with minerals; i.e. Hydroxyapatite crystals (which form at solubility product(which is the critical level of calcium times phosphate concentration)). The first few crystals act as seed crystals that attract more calcium and phosphate from solution.
• Abnormal calcification (ectopic ossification) is the formation of a calculus (calcified mass) in an otherwise soft organ such as a lung, the brain, eye, kidney(s), muscle, tendon, or artery (arteriosclerosis).