Bone Formation, Growth, and Remodeling Bone Formation is known as ossification

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Bone Formation, Growth, and Remodeling
Bone Formation is known as OSSIFICATION.
Describe the development of long bones:

  • What tissue begins the stages of long bone development?

Hyaline Cartilage

  • How many phases occur in the development of long bones?

    • First… Hyaline cartilage “seeds” begin the long bone formation early during fetal development. New bone is laid down by bone builders called osteoblasts.

    • Second… Osteoclasts digest the inner portion of the hyaline matrix as it is strengthened from the outside to create a marrow cavity along the inner portion of the bone.

  • What two bone cell types are responsible for the deposition and removal of bone as bone forms during embryonic, fetal, and childhood?

Osteoclasts are bone “eaters” that consume bone and cartilage as it is developing

Osteoblasts are bone “builders” that deposit bone as it is developing.

These two bone cells work together to build and remodel bone throughout your lifetime.

  • What is appositional growth? What hormones control this type of long bone growth? When does it end?

Appositional growth is the development of bone during growth spurts. Sex hormones (estrogen or testosterone) and growth hormone must be released in order for significant long bone growth to occur. Appositional growth ends when puberty ends.
Describe bone remodeling. Why do bones remodel/change their appearance? What factors will affect the shape and/or size of bone? Bone remodeling occurs throughout your lifetime in response to changing circumstances. Biological sex, nutrition, gravity, activity changes, disease states all play a role in the changing of the skeleton over time.

  • Controlled by 2 hormones:

1. Parathyroid Hormone PTH is released during HYPOCALCEMIA, when blood calcium levels are lower than normal. PTH activates osteoclasts to digest the bone and release calcium into the bloodstream to elevate blood calcium levels back to normal.

2. Calcitonin is released during HYPERCALCEMIA, when blood calcium levels are higher than normal. Calcitonin activates osteoblasts to deposit calcium into the bones and lower blood calcium levels.

-When blood calcium levels DROP below homeostatic conditions: SEE ABOVE

-When blood calcium levels RISE above homeostatic conditions: SEE ABOVE

Bone Fractures and Healing
Describe the events of bone fracture repair.

The Skeleton and Calcium

While it's relatively common knowledge that calcium is an important mineral in the human diet, and plays an important part in maintaining the health of the skeletal system, the exact role of calcium in skeletal organization is not as commonly understood. In fact, calcium is not simply an ingredient in bone, but bones store calcium for other functions, as well.


The skeletal system is essential to human function for a variety of reasons. First, muscles attach to bones, and while the muscles produce movement, the skeletal system is their foundation for doing so. Without a strong skeletal system, the body couldn't move. Additionally, strong bones provide protection for delicate internal organs such as the brain, lungs, heart, and reproductive organs. A strong skeleton is crucial to life and health.


The role of calcium in the skeletal system is twofold. Humans need to maintain strong bones in order to maintain a strong and functional skeletal system, and calcium is an important part of the bone matrix. Explains Dr. Lauralee Sherwood in her book "Human Physiology," the human skeleton is made up of about one third cartilaginous tissue called chondroitin sulfate and about two thirds bony salts called hydroxylapatite. The bony salts have the chemical formula Ca5(PO4)3OH.


The skeletal system isn't the only of the body's organ systems that uses calcium, however. The muscles, including the heart, rely upon calcium signals to initiate and maintain contraction. As such, calcium in the bloodstream is important to both muscular and cardiovascular function, and the body works hard to maintain a constant level of calcium in the blood. Notes Dr. Gary Thibodeau in his book "Anatomy and Physiology," the skeleton acts as a "warehouse" for calcium.

Calcium Storage

In order to maintain constant levels of calcium in the blood, body hormones can either add calcium to or remove it from bone matrix, as needed. Parathyroid hormone from the parathyroid gland in the neck pulls calcium from the bones to increase blood calcium, notes Dr. Thibodeau, while calcitonin from the thyroid gland adds calcium to bones to decrease blood calcium. Though calcium is crucial to skeletal integrity as well as to muscle function, the body prioritizes blood calcium above skeletal calcium.

Expert Insight

Certain factors can decrease the amount of available calcium in the bloodstream, and may cause the body to pull large amounts of calcium from the bones. In extreme cases and over time, this can lead to weakening of the bone matrix known as osteoporosis. Explains Dr. Sherwood, females are more susceptible to osteoporosis than males, since they have less bone density to begin with. Aging individuals are also more susceptible, since calcium consumption and absorption from the gut tend to decrease with age.


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