Skeletal System II PDF

BIOL 116 Skeletal System II 1 Bone Marrow Occupies - cavities of long bones & small spaces within spongy bone Red marrow (hematopoietic tissue) Reticular tissue, capillaries and hematopoietic cells (give rise to RBC, WBC & platelets) Located in nearly all ch...

BIOL 116 Skeletal System II 1 Bone Marrow Occupies - cavities of long bones & small spaces within spongy bone Red marrow (hematopoietic tissue) Reticular tissue, capillaries and hematopoietic cells (give rise to RBC, WBC & platelets) Located in nearly all children bones Adults (skull, vertebrae, ribs, sternum, pelvic girdle, & proximal heads of humerus and femur) Yellow marrow found in adults Reticular tissue accumulates fat and transforms into adipose cells Fatty marrow that does not produce blood Can transform back to red marrow in the event of chronic anemia 2 Bone Development – Formation of Bone Ossification or Osteogenesis - formation of bone In the human fetus and infant, bone develops by two methods; Intramembranous ossification Produces flat bones of skull, mandible & clavicles Thickens long bones throughout life Endochondral ossification Bone grows from pre-existing model composed of hyaline cartilage Begins at around 6-8 weeks of fetal development and continues to 20s 3 Intramembranous ossification 4 Intramembranous ossification Step 1 Process begins in mesenchyme tissue Mesenchymal cells cluster around blood islands forming ossification center (site where bone develops) Mesenchymal cells → osteoprogenic cells → osteoblasts Osteoblasts secrete organic extracellular matrix called osteoid (uncalcified matrix) 5 Intramembranous ossification Step 2 Calcification begins - calcium & minerals crystalize on top of collagen fibers Osteoblast become trapped and become osteocytes (within lacunae) At the edge of growing bone - Osteoprogenic cells differentiate into new osteoblasts 6 Intramembranous ossification Step 3 Ossification continues and forms thin immature spongy branches (spicules) Spicules touch and grow around existing blood vessels – eventually forming trabeculae of spongy bone Mesenchymal cells differentiate and give rise to surrounding periosteum 7 Intramembranous ossification Step 4 Osteoblasts beneath periosteum fill in trabeculae & reorganize as osteons - layer of compact bone on each side Spongy bone matures and fills with red bone marrow Flat bones have a sandwich of Compact/Spongy/Compact 8 Endochondral Ossification 9 Endochondral ossification Replacement of Cartilage by Bone 10 Endochondral ossification Developing in Cartilaginous Model Mesenchymal cells differentiate into chondroblasts & fibroblasts Chondroblasts secrete extracellular matrix Once trapped chondroblasts → chondrocytes Fibroblasts creates the perichondrium. Bone model grows via appositional/interstitial growth 11 Endochondral ossification Step 1 1. Mesenchymal cells of the perichondrium further differentiate into osteoblasts (begin to calcify the matrix) Osteoblasts secrete osteoid bone collar cutting off nutrients from chondrocytes 12 Endochondral ossification Step 2 2. Chondrocytes die, leaving spaces (cavities) Matrix calcifies blocking nutrient supply to chondrocytes Deep chondrocytes hypertrophy (enlarge) and undergo apoptosis (die) Primary ossification center forms. Chondrocytes die, leaving space (cavity) Cavity weakens model but collar stabilizes it 13 Endochondral ossification Step 3 3. Periosteal bud Nutrient artery passes through perichondrium (via nutrient foramen) and enters the cavity Stimulates cells in perichondrium to differentiate into osteoblasts forming periosteum Osteoblasts move in and replace the calcified cartilage with osteoid forming spongy bone Osteoclasts remodel diaphysis to create medullary cavity Surrounding bone will be replaced by compact bone 14 Endochondral ossification Step 4 4. Bone elongates & medullary cavity enlarges as osteoclasts breakdown newly formed spongy bone in diaphysis Epiphysis are made of cartilage which allows the bones to elongate through the mitotic division of chondrocytes At birth most long bones have a bony diaphysis and two cartilaginous diaphysis 15 Endochondral ossification Step 5 5. Before/after birth branches of the epiphysial artery invade the epiphysis, giving rise to the secondary ossification center (no medullary cavity formed) Hyaline cartilage covering epiphysis develops into articular cartilage Hyaline cartilage between epiphysis & diaphysis remains as the epiphyseal growth plate (lengthwise growth) 16 Endochondral Ossification Bone elongation 17 18 Bone Remodeling Absorption & Deposition - occurs throughout life - 10% of skeleton per year Remodeling is a collaboration between action of osteoblasts and osteoclasts Repairs microfractures, releases minerals into blood, reshapes bones in response to use & disuse Wolff’s law of bone: architecture of bone is determined by the presence or absence of mechanical stresses placed on bone Bony processes grow larger in response to mechanical stress 19 Calcium Homeostasis 20