Factors Affecting Bone Growth o Genetics o Nutrition Vitamins D3 and C o Hormones Human Growth Hormones hGH Produced by anterior pituitary gland Stimulates liver to produce IGF both directly stimulating growth of cartilage in epiphyseal plate Thyroid Hormones T3 T4 Influences basal metabolic rate of all cells Helps regulate normal activity at the epiphyseal plates Sex steroid hormones estrogens and androgens large amounts secreted at puberty dramatically accelerate bone growth Increase rate of cartilage growth and bone formation in epiphyseal plate leading to an increase in bone length Mineralization rate zone 5 greater than cartilage growth zone 2 leads to progressive sealing of growth plates Growth plate closes faster in response to estrogen Synthesis of Vitamin D3 Exposure to sunlight is an important natural component of Vitamin D3 production However too much UV exposure also increases one s chances of skin cancer See Jablonski and Chaplin article for more details on the evolution of skin color and its implications on physiology Synthesis of Biologically Active Vitamin D3 UV Light Dietary Intake skin Liver Kidneys 7 dehydrocholesterol Cholecalciferol Inactive form of vitamin D3 Calcidiol 25 hydroxycholecalciferol 1 25 dihydroxycholecalciferol Biologically active form Calcitriol of vit D3 Vitamin D Receptors VDR Polymorphisms in the VDR gene have been linked to several diseases including osteoporosis diabetes cancer and cardiovascular disease etc Sakharkar et al Journal of Basic and Clinical Pharmacy 2017 Modified from https media springernature com original springer static image prt 3A978 3 319 67199 4 2F23 MediaObjects 978 3 319 67199 4 23 Part Fig2 287 HTML gif Pathways in Calcium Homeostasis The hormones calcitonin thyroid gland and parathyroid hormone parathyroid glands help regulating plasma calcium phosphate levels through the regulation of osteoblast osteoclast activity and through the elimination retention of calcium in the kidneys High plasma Ca Calcitonin released Low plasma Ca PTH released Relationship Between Age and Bone Mass Bone density peaks at about 30 years of age Women lose bone mass more rapidly than men as the synthesis of sex steroids decreases more dramatically in aging females than it does in aging males Related article Estrogen and the Skeleton Evolution of our skeleton All chordates phylum chordata share the presence of a notochord a dorsal nerve cord pharyngeal slits and a postanal tail at least at some stage in their development Primates arose 85 55 million years ago first from small terrestrial mammals which adapted to living in the trees of tropical forests many primate characteristics represent adaptations to life in this challenging environment including large brains visual acuity color vision a shoulder girdle allowing a large degree of movement in the shoulder joint and dextrous hands See the Scientific American article Lucy s Baby for information regarding the evolution of bipedalism and life on land as opposed to life in trees Take home message bone structure closely relates to function Racial and sexual polymorphisms Do they exist Are they reliable Are they meaningful See the Expanding the Evolutionary Explanation for Sex Differences in the Human Skeleton article for more info on sexual dimorphism in the human skeleton