Lecture 6: Calcium Homeostasis & Control of Growth: Study Guide

Bone structure

• Understand the structure and metabolic differences of cortical bones and trabecular bones.

  • Trabecular bones have more active metabolism and remodeling than cortical bones.

• Bones contain 99% of the total body calcium

Bone remodeling

• Mainly active in trabecular bones

• The function of osteoclasts and osteoblast in bone remodeling.

  • Osteoclasts: old bone resorption.

  • Osteoblasts: new bone formation.

Vitamin D

• Obtained mainly from two sources: diet and de novo synthesis

• De novo synthesis initiates in the skin and requires UV light exposure, subsequent steps occur in liver and kidney.

• Vitamin D promotes calcium and phosphate absorption and promotes bone mineralization.

*De novo = starting fresh, built from the ground up.

Softening/weakness of bones caused by vitamin D deficiency

• Rickets in children (bowed legs)

• Osteomalacia in adults (prone to fracture)

Parathyroid hormone (PTH)

• Secretion from parathyroid glands is increased in response to lower blood calcium

• PTH helps increase blood calcium by the following mechanisms: increased kidney calcium reabsorption (prevent loss) and vitamin D3 synthesis in kidney; increased vitamin D3 promotes calcium absorption in the gut; PTH promotes bone resorption and subsequent release of calcium into blood (via activation of osteoclasts)

Mnemonic: “Pulls The calcium Higher”

Calcitonin

• Released by C cells in thyroid glands in response to elevated blood calcium

• Calcitonin helps reduce blood calcium by inhibiting osteoclast activity and kidney calcium re-absorption

Mnemonic: “Tones the calcium down”

Osteoporosis

Osteoporosis in women is due to the sudden loss of estrogen after menopause

Growth hormone (GH) and IGF-1

• Hypothalamus hormones and pituitary hormones regulating IGF-1 secretion: GHRH stimulates GH release while somatostatin inhibits GH release from pituitary. GH promotes IGF-1 secretion by liver and other cell types.

• GH secretion shows a diurnal rhythm and peaks at night

• GH decreases with age.

• GH disorders: Gigantism (overproduction of GH before closure of epiphyseal plates); acromegaly (overproduction of GH after closure of epiphyseal plates/after puberty)