Chapter 6 Bone Tissue

Bone Development and Growth

Bone Development

• Flat and long bones have different formation processes.
• Long bones stop growing in length during childhood but can repair themselves throughout life.
• Adult and children’s bones can be identified by anatomical differences.

Two Types of Ossification

• Intramembranous Ossification: Forms flat bones.
  • Starts with a sheet of dense irregular connective tissue in the shape of a flat bone.
• Endochondral Ossification: Forms long bones.
  • Starts with a piece of hyaline cartilage connective tissue in the shape of a long bone.
• Both processes result in bone formation with different shapes (flat or long).
• Irregular bones are formed by both processes.

Clinical Significance

• Understanding bone formation is important in clinical contexts, such as in cases of achondroplastic dwarfism, where bone growth is abnormal.

Intramembranous Ossification of Flat Bones

• Process of forming flat bones like the sternum or skull bones.

Steps of Intramembranous Ossification

1. Mesenchymal Stem Cells:
   • Stem cells in the developing flat bone create embryonic tissue similar to dense irregular connective tissue.
2. Osteogenic Cells and Osteoblasts:
   • Stem cells differentiate into osteogenic cells, which then form osteoblasts.
3. Osteoid Secretion:
   • Osteoblasts secrete osteoid, a mix of gel and collagen fibers.
4. Calcification:
   • Calcium phosphate from the blood attaches to the collagen fibers, forming spongy bone.
5. Osteocyte Formation:
   • Osteoblasts become trapped in the hardening bone matrix, forming lacunae and transforming into osteocytes.
6. Vascularization:
   • Blood vessels bring more calcium phosphate and nutrients to the developing bone.
7. Periosteum Formation:
   • Nearby connective tissue differentiates into the periosteum.
8. Compact Bone Formation:
   • Osteoblasts in the periosteum form compact bone superficially to the spongy bone.
9. Red Marrow Development:
   • Red marrow appears as blood cells are produced from stem cells in the marrow cavities.
10. Bone Structure:
    • The bone consists of a periosteum cover, compact osseous connective tissue, and spongy osseous connective tissue with marrow cavities from outside to inside.

Clinical Note: Fontanelles

• The flat skull bones of children take months to fully enlarge and close the gaps between the bones.
• These gaps are called "fontanelles" or little fountains, where a pulse can be seen from the blood vessels.

Endochondral Ossification

Formation of Long Bones

• Bones start small during development and grow longer in childhood.
• Longitudinal growth stops at the end of childhood, but bones can still grow wider and denser throughout life.
• Early development features cartilage structures that are gradually replaced by bone.
• Thyroid hormone and growth hormone stimulate chondrocyte cell division, leading to bone lengthening during development and childhood.

Cartilage and Bone

• Cartilage tissue does not turn into bone tissue; instead, cartilage dies and is replaced by bone tissue.
  *New cartilage grows, lengthening the bone.
• The epiphyseal plate (growth plate) is where cartilage cells continue to multiply.

Puberty and Bone Growth

• During puberty, gonads release estrogen and testosterone, accelerating chondrocyte cell division.
• This results in faster growth compared to childhood.
• Older cartilage is replaced by bone, strengthening the bone.

Cessation of Vertical Growth

• Bone lengthening continues until hormone levels rise significantly in the late teens.
• By around age 21, increased estrogen and testosterone levels cause chondrocytes in the growth plate to die.
• Osteoblasts then fill the area with compact bone, making further vertical growth impossible.
• The area where the epiphyseal plate was located is called the epiphyseal line.

Stages of Endochondral Ossification

1. Hyaline Cartilage Model:
   • A hyaline cartilage "model" grows in length and width through chondrocyte cell division.
2. Compact Bone Formation:
   • Osteoblasts in the periosteum form compact bone around the cartilage.
3. Cartilage Death:
   • The central cartilage area dies.
4. Vascularization and Endosteum Formation:
   • Blood vessels enter the shaft, bringing minerals, and the endosteum forms.
5. Spongy Bone Formation in Diaphysis:
   • Osteoblasts from the endosteum form spongy bone inside the diaphysis temporarily.
6. Medullary Cavity Formation:
   • Osteoclasts destroy the new spongy bone in the diaphysis, forming a marrow cavity.
7. Spongy Bone Formation in Epiphyses:
   • In the epiphyses, cartilage dies, and spongy bone forms, remaining for life.
8. Articular Cartilage:
   • Hyaline cartilage remains as articular cartilage, protecting the ends of the bone.
9. Growth Plate Activity:
   • Hyaline cartilage connective tissue cells in the growth plate multiply and grow, lengthening the bone until hormones halt this process at the end of puberty.
10. Bone Maintenance:
    • Osteoblasts add collagen throughout life to repair, increase bone density, and add width to bones.

Epiphyseal Plate vs. Epiphyseal Line

• Epiphyseal Plate: Made of hyaline cartilage connective tissue; present during childhood for vertical growth.
• Epiphyseal Line: Made of compact osseous connective tissue; appears in adulthood when vertical growth is no longer possible.

Bone Growth in Length During Childhood

1. Cartilage Cell Multiplication:
   • Cartilage cells in the epiphyseal plate multiply, stacking up and lengthening the bone.
2. Cartilage Replacement:
   • Cartilage cells die and are quickly replaced by bone tissue.
3. Hormonal Influence:
   • Thyroid hormone and growth hormone stimulate chondrocyte cell division throughout childhood.
4. Pubertal Acceleration:
   • Estrogen and testosterone surges during puberty accelerate bone lengthening.
5. Growth Cessation:
   • High estrogen and testosterone levels between ages 16-21 kill the cartilage cells at the growth plate, stopping bone lengthening.

Zones of the Growth Plate

• Resting Zone: Connects the epiphyseal plate to the epiphysis.
• Proliferation Zone: Cartilage cells multiply, stacking up and lengthening the bone.
• Hypertrophic Zone: Older cartilage cells enlarge.
• Calcification Zone: Oldest cartilage cells die, making way for blood vessels; calcium and phosphate arrive in the blood.
• Ossification Zone: Osteoblasts make collagen to which calcium and phosphate attach.

Bone Growth Over Lifespan

• Childhood to Age 21: Chondrocytes in the growth plate multiply and increase bone length.
• Throughout Life (Birth-100): Osteoblasts secrete collagen, increasing bone width and density, and repairing bone breaks.

X-Ray Differences: Child vs. Adult

• Child: Epiphysis and diaphysis are separated by the growth plate.
• Adult: Growth plate is gone, and the diaphysis and epiphysis are fused.
• Child: Bones have lower density (darkness) due to less calcium.
• Adult: Bones have higher density (brightness) due to more calcium deposition over time.