This landscape of blue, red, and gold is a cross section through a developing human bone. Human bones have a hard outer layer made of condensed bone and a softer inner core of spongy bone. This ...

Medically reviewed by Gagandeep Brar, MD Key Takeaways Bone marrow fills the cavities in your bones and produces blood cells.Red blood cells carry oxygen, white blood cells fight germs, and platelets ...