Bone (Leaving Cert Biology): Revision Notes

Bone

• Bone is dense connective tissue that forms the major part of the skeleton.
• Long bones, like the femur, are enclosed by a membrane called the periosteum. This membrane contains blood vessels and nerves.
• The long shaft of the bone is called the diaphysis.
• The head of the bone is called the epiphysis.

We will study:

1. Compact bone
2. Spongy bone
3. Medullary cavity
4. Cartilage
5. Bone growth
6. Bone renewal

Compact Bone

• Compact bone is composed of bone cells called osteoblasts, enclosed in a matrix.
• The matrix is composed of;
  1. 70% inorganic salts (calcium, carbonate and phosphates).
  2. 30% organic material (protein-collagen).
• The inorganic material gives strength to the bone.
• The organic material gives the bone its flexibility.
• Compact bone also contains blood vessels and nerve fibres.

Spongy Bone

• Found mainly in the epiphysis of long bones.
• It looks like a sponge under a microscope.
• Its function is to provide strength and rigidity without adding excessive weight.

Medullary Cavity (Bone Marrow)

• The medullary cavity is filled with bone marrow, a soft material.
  • Red bone marrow: active in making blood cells.
  • Yellow bone marrow: inactive and contains numerous fat-storage cells. It can convert to active red marrow if needed.

Cartilage

• Cartilage is a flexible material made of protein fibres embedded in a rubbery matrix.
• Cartilage protects the ends of long bones and acts as a shock absorber.
• It lacks blood vessels and nerves.

Bone Growth

• Before week 8 of embryonic development, there is no bone, only cartilage.
• From week 8 onward, cartilage is replaced by bone in a process called ossification.
• Osteoblasts: Bone-forming cells that replace cartilage with bone.
• Osteoclasts: Cells that digest and remove old bone.
• Growth Plate: A section of cartilage between the medullary cavity and spongy bone that allows the bone to lengthen.
  • In the growth plate, osteoblasts form new bone to increase the bone length.
  • Growth plates stop functioning at approximately age 16 in females and 18 in males.

Bone Renewal

• Bone is constantly being renewed throughout life. There are two types of bone cells involved in this process: 3. Catabolic cells (osteoclasts):
  • These bone-digesting cells remove calcium from the inside of the medullary cavity. They deposit this calcium in blood vessels.
  • This prevents bones from becoming too heavy.
  4. Anabolic cells (osteoblasts):
  • These bone-forming cells create new bone.
  • The activity of these cells is influenced by physical activity, hormones, and diet.

Bone renewal is influence by:

• Physical activity
  • Stimulates osteoblasts, resulting in stronger bones.
• Hormones
  • Human Growth Hormone (HGH) and sex hormones (testosterone, oestrogen, progesterone) increase bone size and strength.
  • Growth spurts during puberty are due to higher hormone levels.
  • After menopause, lower oestrogen levels may reduce bone strength, increasing the risk of conditions like osteoporosis.
• Diet
  • Calcium is crucial in the diet because bones are continually being broken down and rebuilt.