Skeletal System: Day ThreeBone Formation, Growth, and Remodeling, Bone Diseases and Disorders

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Bone Development and Growth

Bones form by replacing connective tissues in the fetus.

Some form within sheetlike layers of connective tissue (intramembranous bones), while others replace masses of cartilage (endochondral bones).

Ossification is another word for bone building.

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Intramembranous Bones

The flat bones of the skull form as intramembranous bones that develop from layers of connective tissue.

Osteoblasts deposit bony tissue around themselves. Once osteoblasts deposit bone and are located in

lacunae, they are called osteocytes. Cells of the membranous connective tissue that lie

outside the developing bone give rise to the periosteum.

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Intramembranous Bone

Endochondral Bone Growth

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Endochondral Bones

Most of the bones of the skeleton fall into this category.

1. They first develop as hyaline cartilage models and are then replaced with bone.

2. Cartilage is broken down in the diaphysis and progressively replaced with bone while the periosteum develops on the outside.

3. Cartilage tissue is invaded by blood vessels and osteoblasts that first form spongy bone at the primary ossification center in the diaphysis.

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5. Osteoblasts beneath the periosteum lay down compact bone outside the spongy bone.

6. Secondary ossification centers appear later in the epiphyses.

7. A band of hyaline cartilage, the epiphyseal plate, forms between the two ossification centers.

8. Layers of cartilage cells undergoing mitosis make up the epiphyseal plate.

9. Osteoclasts break down the calcified matrix and are replaced with bone-building osteoblasts that deposit bone in place of calcified cartilage.

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10. Epiphyseal plates are responsible for lengthening bones while increases in thickness are due to intramembranous ossification underneath the periosteum.

11. A medullary cavity forms in the region of the diaphysis due to the activity of osteoclasts.

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Bone Formation: Ossification

By birth, most hyaline cartilage have been converted to bone except for two regions:

1. Articular cartilages• Cover long bone ends (epiphysis)• Persist for lifetime• Reduce friction at the joint surfaces

2. Epiphyseal Plate• Allows for longitudinal growth of long bones during

childhood• Will eventually disappear leaving only the epiphyseal

line

Bone Growth

New cartilage is continuously formed

Old cartilage is broken down and replaced by bony matrix

Process of long-bone growth controlled by hormones: growth hormone and sex

hormone Ends during adolescence when epiphyseal plate is converted

to bone

New cartilage added on the external part of the articular cartilage and on the epiphyseal plate surface away from medullary cavity

Old cartilage in broken down and replaced by bone on the internal part of the articular cartilage and on the epiphyseal plate near the medullary cavity

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Homeostasis of Bone Tissue

Osteoclasts tear down and osteoblasts build bone throughout the lifespan with the processes of resorption and deposition, with an average of 3% to 5% of bone calcium exchanged annually.

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Bone Remodeling

Bone is very active tissue

Bones are remodeled continuously in response to two factors:

1. Calcium levels in the blood2. Pull of gravity and muscles on the skeleton

Bone Remodeling:Calcium levels in blood

Blood calcium low Parathyroid glands release parathyroid hormone (PTH) PTH activate osteoclasts (destroy bone cells) Osteoclasts break down bone and release calcium into the

blood

Blood calcium high Thyroid gland releases calcitonin Calcitonin stimulates Calcium to be deposited from the

blood into the bone

Bone Remodeling: Pull of gravity and muscles on the skeleton

Bones become thicker to increase strength where bulky muscles attached

Bones of physically inactive people tend to lose mass

Rickets

Disease of children where bones fail to calcify

Bones soften and a bowing of the weight-bearing bones of the legs occur

Due to lack of calcium in the diet or a lack of Vitamin D (necessary to absorb calcium into the blood)

Not seen much in U.S., but is a problem in other parts of the world.

Osteoporosis A loss in bone mass leading

to thin, fragile bones

Common consequence of aging, esp. in women

Estrogen helps to maintain skeleton in women

Lack of estrogen during menopause in women as well as a diet poor in calcium and protein, lack of Vitamin D, smoking, insufficient weight-bearing exercise can contribute to osteoporosis.