Bones

Osteoporosis and bone fractures are common ailments in adults. Wikipedia says this about the human skeleton.

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue. Bones come in a variety of shapes and have a complex internal and external structure, are lightweight yet strong and hard, and serve multiple functions. One of the types of tissue that makes up bone is the mineralized osseous tissue, also called bone tissue, that gives it rigidity and a coral-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum, periosteum, nerves, blood vessels and cartilage. At birth, there are over 270 bones in an infant human's body, but many of these fuse together as the child grows, leaving a total of 206 separate bones in an adult. The largest bone in the human body is the femur and the smallest bones are auditory ossicles.

Scientists are investigating the human skeleton, and here are links to some of their research.

Depression, anxiety, and smoking are associated with lower bone mineral density (BMD) in adults, but these factors have not previously been studied during adolescence, when more than 50% of bone accrual occurs. This longitudinal preliminary study is the first to demonstrate that smoking and depressive symptoms in adolescent girls have a negative impact on adolescent bone accrual and may become a red flag for a future constrained by low bone mass or osteoporosis and higher fracture rates in postmenopausal years.