Bone Structure and Function Quiz

Types of Bones

• Cortical Bone: Also known as compact or lamellar bone; dense and forms the outer layer of bone.
• Trabecular Bone: Also referred to as medullary, cancellous, or spongy bone; less dense, found within the interior of bones.

Bone Matrix Composition

• Composed of Organic (30-35%) materials:
  • Osteoid, primarily made of Type I collagen.
  • Glycosaminoglycans (GAGs) such as chondroitin sulfate, keratin sulfate, and hyaluronic acid.
  • Non-collagen proteins including osteocalcin, osteonectin, and osteopontin.
• Composed of Inorganic (65-70%) materials:
  • Hydroxyapatite crystals (Ca10(PO4)6(OH)2) provide bone rigidity and strength.
  • Includes additional minerals that enhance bone strength.

Major Bone Cell Types

• Osteoblasts: Responsible for bone formation by secreting osteoid proteins.
• Osteoclasts: Involved in bone resorption, breaking down bone tissue.
• Osteocytes: Function as mechanosensors and facilitate mineral transport.

Osteoblasts Function

• Secrete proteins, forming osteoid as a scaffold for mineral deposition.
• Alkaline phosphatase (ALP) promotes bone mineralization by breaking down pyrophosphate (PPi).
• RANK ligand (RANKL) produced by osteoblasts activates osteoclasts; osteoprotegerin (OPG) inhibits this process.
• Parathyroid hormone (PTH) partially stimulates osteoclast activity through osteoblasts, while calcitriol enhances osteoblast differentiation and OPG release.

Osteoclasts Function

• Stimulated by RANKL and inhibited by calcitonin temporarily.
• Create Howship's lacunae by secreting acid and proteolytic enzymes to dissolve bone matrix components.

Bone Remodeling Process

• Bone remodeling is a balanced cycle of formation and resorption regulated by RANK and OPG, along with hormone influences (PTH, vitamin D, calcitonin).
• Essential for maintaining structural integrity of the skeleton and calcium homeostasis.

Osteocytes Role

• Reside in a fluid-rich environment that aids calcium and phosphate transport.
• Form gap junctions with other osteocytes and osteoblasts for communication and signaling.
• Sense mechanical loading through the movement of bone fluid and exhibit piezoelectric properties.

Calcium Regulation in the Body

• Major calcium reservoir is in bone, primarily as hydroxyapatite.
• Intracellular calcium levels are low; tightly regulated due to its physiological importance.
• Extracellular calcium concentration is significantly higher than intracellular, with approximately 50% in ionized form.

Hormonal Regulation of Calcium

• Parathyroid Hormone (PTH): Increases serum calcium by promoting bone resorption and reabsorption in kidneys; initiates vitamin D activation.
• Vitamin D (Calcitriol): Increases intestinal calcium absorption and renal reabsorption. Stimulates both osteoblastic and osteoclastic activities.
• Calcitonin (CT): Reduces serum calcium by inhibiting osteoclastic activity and renal reabsorption.

Phosphate Homeostasis

• Closely linked to calcium regulation; hormones affecting calcium also impact phosphate levels.
• PTH raises serum phosphate initially by stimulating bone resorption but ultimately decreases overall phosphate due to renal excretion.
• Calcitriol enhances intestinal phosphate absorption while promoting bone turnover.
• Calcitonin generally results in a reduction of phosphate levels due to increased renal excretion.

Test your knowledge on the different types of bones, their matrix components, and the major cell types involved in bone tissue. This quiz covers the functions of cortical and trabecular bone and the roles of osteoblasts, osteoclasts, and osteocytes in maintaining bone health.