Bone Formation, Growth,

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• Bone Formation, Growth, Maintenance

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• 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

• Ossification - the process of bone formation
• Begins 2 months fetal age
• Ends in late puberty age 20 (slightly earlier
  in females)

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Bone Formation

• 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.

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• Long Bone Formation and Growth

Figure 5.4a
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• 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

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• X-Ray of Newborn

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Bone Formation

• 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.

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• 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.

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• 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.

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• Long Bone Formation and Growth

Figure 5.4b
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• 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.

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• 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.

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• Bone Density

• AGE
• Young bone is made faster than its destroyed
• Maturity there is a balance
• Old age less bone is deposited

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• 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.

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• Bone Density

• Applied Stress
• Weight bearing exercise thicker, stronger
  bones
• Lightly stressed bones are thinner weaker

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• Bone Density

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Hormones the Skeleton

• 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

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Hormones the Skeleton

• 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.

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Hormones Vitamin D

• 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)

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• End