Skeletal System: Bone Growth and Structure

• This text refers to the skeletal system.

Longitudinal Bone Growth

• Longitudinal growth defines how long bones grow in length
• Chondrocytes divide in the zone of proliferation
• Chondrocytes mature and stop dividing in the hypertrophy and maturation zone
• Chondrocytes die, and their matrix calcifies in the zone of calcification
• Calcified cartilage is replaced with bone in the zone of ossification.

Bone Cells

• Osteoblasts synthesize and secrete the bone matrix, including collagen, and aid in the mineralization process
• Osteocytes maintain bone health, derived from osteoblasts
• Osteoclasts break down damaged tissue through resorption

Compact Bone

• Compact bone is dense.
• It makes up the outer layer, providing bone strength
• It accounts for 80% of bone weight

Spongy Bone

• Spongy bone is lighter and less dense, with a honeycomb structure
• Very strong and help absorb shock
• It accounts for 20% of bone weight

Long Bone Structure

• The diaphysis is the shaft of the bone
• Epiphyses are the ends of the bone
• Articular cartilage, made of hyaline cartilage, protects bones during movement
• The marrow (medullary) cavity is a hole in the diaphysis
• Red bone marrow makes blood cells and is found in the pelvis, spine, and ends of long bones
• Yellow bone marrow stores fat for energy and is in the shafts of long bones
• Compact bone is the hard outer region
• Compact bone resists linear compression and twisting forces
• Spongy bone (cancellous bone) contains spaces that support bone and make it strong

Bone Types

• Long bones are longer than wide, examples; femur and humerus
• Long bones provide movement, support weight, and blood cell production
• Short bones are cube-like examples; carpals and tarsals
• Short bones offer stability, shock absorption, and limited movement
• Flat bones are thin and flat examples; skull, ribs, sternum
• Flat bones provide protection, muscle attachment, and blood cell production
• Irregular bones are complex and varied examples; vertebrae and pelvic bones
• Irregular bones functions are protection, support, and muscle attachment

Ossification Types

• Intramembranous ossification is where Bone forms directly from mesenchyme (soft tissue)
• It happens in flat bones like the skull
• Intramembranous produces direct bone growth without cartilage
• Endochondral ossification happens when Bone forms by replacing cartilage with bone.
• It happens in long bones, like the arms and legs.
• Endochondral ossification replaces cartilage with bone as the body grows

Hormones in Bone Growth

• Growth Hormone (GH)Source of GH: pituitary gland
• GH helps growth and strengthening of bones so kids need it
• Sex Hormones (Estrogen and Testosterone). Source of Estrogens is the ovaries
• Testosterone is produced in the testes.
• Sex hormones influence bone density and structure
• Estrogens close epiphyseal growth plates after puberty, ending bone lengthening and helps maintain bone density
• Testosterone promotes osteoblast activity, inhibits osteoclast activity, and is usually for boys Bones continue to grow from 18-21 years old

Thyroid Hormones

• Thyroid Hormones (Thyroxine and Triiodothyronine)source; thyroid gland
• thyroid hormones are important for bone health, growth, and maintenance
• Thyroid Hormones modulate osteoblast/osteoclast activity, maintaining bone density and preventing bone loss

Parathyroid Hormone (PTH)

• Source: parathyroid glands
• PTH regulates blood calcium levels by stimulating osteoclasts to resorb bone
• Excessive PTH can lead to bone loss
• Calcitonin, source; thyroid gland
• Calcitonin lowers blood calcium levels by inhibiting osteoclast activity
• Vitamin D (Calcitriol)source; sunlight and dietary sources
• Vitamin D enhances calcium absorption and supports bone mineralization
• Adequate vitamin D levels maintain bone strength

Bone Remodeling

• Bone Breakdown (Resorption)-Osteoclasts remove old/damaged bone, releasing calcium.
• Bone Formation (Deposition)-Osteoblasts build new bone, adding calcium/minerals.

Calcium Levels and Hormones

• Low calcium levels: Parathyroid Hormone (PTH) is released
  • PTH directs bones to release calcium.
  • PTH directs kidneys to retain calcium.
  • PTH enhances intestinal calcium absorption with Vitamin D.
• High calcium levels: Calcitonin is released.
  • Calcitonin directs bones to store calcium.
  • Calcitonin directs kidneys to eliminate extra calcium

Influences of Bone Remodeling

• Mechanical Stress: Weight-bearing activities and muscle contractions stimulate bone formation
• Hormones: PTH increases bone resorption, Calcitonin inhibits osteoclasts and growth hormone stimulates bone growth
  • Sex hormones (estrogen & testosterone) maintain bone mass and inhibit excessive bone resorption
• Calcium & Phosphate Levels: Adequate levels are necessary for bone mineralization
• Vitamin D: Enhances calcium absorption, promoting bone health
• Age: Bone remodeling slows with age, potentially leading to bone loss
• Genetics: Influences bone density and susceptibility to osteoporosis
• Inflammatory Factors & Cytokines: Chronic inflammation can accelerate bone loss
• Medications: Certain drugs, like glucocorticoids, can lead to bone thinning

Bone Repair

• Hematoma Formation: Initial blood clot to stop bleeding and protect area
• Fibroblast and Chondroblast Activity: Soft callus formation creates a bridge
• Osteoblast Activity: Bone callus (woven bone) provides initial hardness

Connective Tissues

• Specialized Connective tissue: Cartilage, bone and blood
• Loose connective tissue: Support and connect body parts
• Reticular tissue: Made of net-like-fibers, helping to trap harmful particles and support the immune system
• Adipose Tissue in the forms of fat tissue
• Cartilago: found in joints, nose, ears and parts of the airway