Bone Growth and Ossification

• Bone growth, remodeling, and repair are ongoing processes throughout life.

Bone Cells

• Osteoprogenitor cells are stem cells found in bone, also known as osteogenic cells.
• Osteoblasts are immature bone-forming cells involved in fetal bone formation, adult bone maintenance, and repair.
• Osteoblasts create the extracellular matrix, which consists of collagen fibers and ground substance, and become trapped in lacunae.
• Osteocytes are mature bone cells that maintain bone structure, they are mature osteoblasts located within lacunae.
• Osteoclasts are bone-absorbing cells rich in lysosomes.

Bone Development and Ossification

• Ossification is the process of bone formation within connective tissues during embryological development.
• Intramembranous ossification is bone development between fibrous connective tissue sheets, occurring in the skull.
• Intramembranous ossification is sometimes referred to as "1 step" bone formation because DICT undergoes ossification directly.
• Endochondral ossification is bone formation within cartilage.
• Endochondral ossification is a "2 step" bone formation process because DICT becomes cartilage, then the cartilage turns into bone.
• Chondrocytes lay down hyaline cartilage, shaping future bones during endochondral ossification.
• A bone collar comprised of compact bone matrix is secreted by osteoblasts.
• Osteoblasts from the blood form spongy bone in the diaphysis (shaft) of long bones, forming a primary ossification center.
• Secondary ossification sites create bone centers in the epiphyses.
• The epiphyseal plate, a cartilage band, functions as a growth plate, enabling bone lengthening until puberty.

Bone Growth

• Appositional growth allows bones to increase in width (girth).
• Osteoprogenitor cells in the periosteum differentiate into osteoblasts, which deposit new extracellular matrix (ECM).
• New outer rings of ECM are formed as outer circumferential lamellae.
• A blood vessel from the periosteum may be incorporated into new bone, eventually forming a central canal as an osteon develops.

Hormonal Effects

• Growth hormone (GH) stimulates overall bone growth
• GH stimulates the epiphyseal plates
• Sex hormones (estrogen and testosterone) increase growth during adolescence.
• Parathyroid hormone (PTH) stimulates osteoclasts when blood calcium levels are low.
• Calcitonin, produced by the thyroid gland, inhibits osteoclasts when blood calcium levels are high.
• Vitamin D is converted to a hormone that facilitates calcium absorption in the intestine, promoting bone production.

Bone Remodeling

• Bone remodeling renews bone at a rate of up to 18% per year, allowing bones to adapt to stress.
• Blood calcium levels are regulated by hormones that increase or decrease blood calcium concentrations.

Bone Repair

• Bone repair occurs after a fracture.
• A hematoma (blood clot) forms between broken bones within six to eight hours after the break.
• A fibrocartilaginous callus forms between broken bones at approximately three weeks.
• A bony callus replaces the cartilaginous callus with bone after about three months.