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Bone Formation, Growth, & Maintenance

Bone Formation, Growth, & Maintenance. Types of Bone Cells. Osteocytes Mature bone cells Osteoblasts Bone-forming cells Osteoclasts Bone-destroying cells Break down bone matrix for remodeling and release of calcium Bone remodeling is a process by both osteoblasts and osteoclasts.

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Bone Formation, Growth, & Maintenance

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  1. Bone Formation, Growth, & Maintenance

  2. Types of Bone Cells • Osteocytes • Mature bone cells • Osteoblasts • Bone-forming cells • Osteoclasts • Bone-destroying cells • Break down bone matrix for remodeling and release of calcium • Bone remodeling is a process by both osteoblasts and osteoclasts

  3. Ossification - the process of bone formation Begins ~ 2 months fetal age Ends in late puberty ~ age 20 (slightly earlier in females) Bone Formation

  4. Long, short, and irregular bones develop from hyaline cartilage Flat bones develop from fibrous membranes - bone develops within sheets of connective tissue Bones ossify from the middle of the bone outward. All new tissue is spongy at first but then is broken down and rearranged into compact bone. Bone Formation

  5. Long Bone Formation and Growth Figure 5.4a

  6. Changes in the Human Skeleton • In embryos, the skeleton is primarily hyaline cartilage • During development, much of this cartilage is replaced by bone • Cartilage remains in isolated areas • Bridge of the nose • Parts of ribs • Joints

  7. X-Ray of Newborn

  8. Osteoblasts (bone-forming cells) cover the cartilage with bone matrix. Cartilage cells die as nutrition is cut off because the hard matrix does not allow for diffusion. Cartilage membranes (perichondrium) convert to periosteum and send osteocytes (bone cells) into lacunae (hollow spaces within the matrix where cells reside). {The cartilage cells are killed off and bone cells take over their “home”} The bone cells are able to survive within a solid matrix because they have the Haversian system to ensure a blood supply. Bone Formation

  9. Bone Growth - Length • Growth in length occurs at the ends (epiphyses). New cartilage is formed and gradually transformed into bone. The epiphyseal plate is a band of cartilage located in the epiphysis. As long as that plate is present, bone growth can continue. Cartilage is replaced by bone but new cartilage is produced in the epiphyseal plate, allowing bone growth to continue.

  10. Bone Growth - Length • Growth of long bones is controlled by hormones (growth hormones & sex hormones, which are produced at sexual maturity). The sex hormones cause the osteoblasts (bone forming cells) to divide more rapidly, causing a growth spurt. However, these hormones cause bone cells to replace cartilage faster than cartilage can form so the epiphyseal plates disappear and bone growth stops.

  11. Long Bone Formation and Growth Figure 5.4b

  12. Bone Growth - Diameter • Growth in diameter occurs as osteoblasts add bone tissue to the outer diaphysis and osteoclasts (bone-destroying cells) break down tissue in the middle, hollowing out a larger marrow cavity. This allows bones to grow without adding too much weight.

  13. Bone Maintenance • 98% of all bone growth is complete by age 20 (slightly earlier in females) • After cartilage cells have died, the bone-forming cells are mainly involved in repair and replacement of cells • The balance between bone growth and bone destruction is achieved at midlife. After ~50 years, bone mass gradually declines, but at a faster rate in women.

  14. Bone Density: • AGE Young: bone is made faster than it’s destroyed Maturity: there is a balance Old age: less bone is deposited

  15. Bone Density: • Hormones - influence the rate of mineral deposition in bone; absorption and secretion of calcium and and phospate. • Genetic & Environmental Factors - family history, diet (calcium & Vitamin D), etc.

  16. Bone Density: • Applied Stress Weight bearing exercise = thicker, stronger bones Lightly stressed bones are thinner & weaker

  17. Bone Density

  18. Thyroid Gland: Secretes calcitonin, which maintains the homeostatic level of calcium in the blood. It acts to lower blood calcium levels by inhibiting the reabsorption of calcium from bones Hormones & the Skeleton

  19. Parathyroid gland: Secretes parathormone (PTH), which regulates calcium and phosphate levels in the blood. It acts to increase levels of calcium and phosphate ions in the blood by promoting absorption of calcium from the intestines and kidney tubules and also by stimulating osteoclasts to break down bone. Hormones & the Skeleton

  20. The sun is the normal source of Vitamin D but you can also get it in your diet. If Vitamin D is present in adequate amounts, bone structure can be properly maintained. If Vitamin D is not present in adequate amounts, bones do not develop in children (Rickets) and adult bones are demineralized and become soft and pliable (Osteomalacia) Hormones & Vitamin D

  21. End

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