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

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

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.

Long Bone Formation and Growth Figure 5.4a

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

X-Ray of Newborn

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.

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.

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.

Long Bone Formation and Growth Figure 5.4b

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.

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.

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

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.

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

Bone Density

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

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.

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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