The human skeleton

Counts for the adult skeleton vary slightly with the source. Cleveland Clinic gives 206 to 213 bones, while UC Davis gives 206.

A newborn has more bones than an adult. Cleveland Clinic says babies are usually born with 270 bones, and UC Davis says around 300.

The count falls because some bones join together as a child grows, forming larger bones in the adult skeleton.

The soft spots on a baby's skull, called fontanelles, allow for brain growth and flexibility during birth.

The axial skeleton, made up of the bones of the head, neck, back, chest and pelvis, accounts for 80 bones.

Bone mass peaks for most people during the third decade of life. Bone density declines after that peak.

The femur, or thigh bone, is the longest and strongest bone in the body. It runs from the hip to the knee.

Most adult femurs are about 18 inches (46 centimetres) long.

A femur can support up to 30 times the weight of the body.

The smallest bone is the stapes, in the middle ear. It is about 3 to 3.5 millimetres long and carries sound vibrations to the inner ear.

Bones make up about 12% to 15% of body weight.

Bone is renewed throughout life. Osteoclasts remove old bone, then osteoblasts lay down new matrix, and in a healthy adult the amount removed equals the amount formed.

Estimates of how fast the whole skeleton turns over differ. The University of Arkansas for Medical Sciences says every bone is completely reformed about every 10 years, while an NIH-hosted bone chapter estimates that about 30% of skeletal mass is renewed each year in a young adult.

One remodelling site takes about 87 days to complete: roughly 3 days of removal, 14 days of reversal and 70 days of new bone formation.

Type I collagen makes up about 90% of the bone matrix, and hydroxyapatite crystals bind to it.

Compact bone is 80% to 90% calcified, whereas spongy (trabecular) bone is only 15% to 25% calcified.

Marrow is a soft, fatty tissue at the centre of most bones. It makes red blood cells, white blood cells and platelets.

The body holds just under 6 pounds (about 2.5 kilograms) of red and yellow marrow combined.

All marrow is red until about age 7. After that, yellow marrow, which stores fat, gradually replaces some of it.

In orbit the spine straightens and lengthens, and NASA says crew height can increase by as much as 3%.

A paper in the journal Spine reports that long-duration microgravity results in approximately 5 centimetres of lengthening of body height.

In that study, MRI scans of six NASA crew members after a 6-month space station mission found lumbar disc heights not appreciably different at any time point, which runs against the idea that discs swell in weightlessness.

The same scans showed lean muscle falling from 86% to 72% of the total paraspinal muscle area immediately after the mission. Recovery of 68% of that loss occurred over the next 6 weeks, still leaving lean muscle below preflight values.

Bone density falls in orbit. NASA puts the loss at between 1% and 1.5% a month on missions of four to six months, and UC Davis says astronauts can lose up to 1% of bone mass a month.