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2 molecules that circulate in the blood as well as regulators that are produced locally in the bone. There are a number of bone diseases that are the subject of vigorous research. Composition and structure There are two types of bone in the skeleton: the flat bones (for example, the bones of the skull and ribs) and the long bones [for example, the femur (Fig. 1) and the bones of the hands and feet]. The growth and development of the two bone types are different and, unlike flat bones, long bones have a central cavity. However, both bone types are characterized by an outer layer of dense bone, known as cortical or compact bone, and an inner spongy bone material made up of thin trabeculae, known as cancellous or spongy bone. Trabeculae are thin, lattice-like structures or bony spicules that make up the porous network within cancellous bone (Fig. 2). Cortical bone consists of osteons, which are columns formed by layers of bone (lamellae) in an orderly concentric cylindrical arrangement around tiny canals called a central or Haversian canal (Fig. 3). These interconnecting canals carry the blood vessels, lymph vessels, and nerves through the bone and com- municate with the periosteum and the central cavity, filled with yellow bone marrow. The periosteum is a thin membrane cov- ering the outer surface of bone and consisting of layers of cells that participate in the remodeling and repair of bone. Osteons exist beside each other, connected by incomplete lamellae to form a dense and resilient structure (Fig. 4). Bone marrow is a crucial and multifaceted component of the skeletal system, primarily found within the cavities of bones. There are two main types of bone marrow: red marrow and yellow marrow. Red marrow is found in contact with the tra- beculae within cancellous bone in both flat and long bones, whereas yellow marrow is found in the central cavity of long bones. Red marrow is responsible for the production of blood cells through hematopoiesis, including red blood cells (erythro- cytes), white blood cells (leukocytes), and platelets (thrombo- cytes). Additionally, red marrow plays a pivotal role in maintain- ing the body's oxygen-carrying capacity, immune response, and blood clotting mechanisms. Yellow marrow, on the other hand, consists of fat cells and functions primarily as an energy reserve. In times of increased energy demand, yellow marrow can convert back into red marrow to support hematopoiesis. Together, these types of bone marrow ensure the essential functions of blood production and energy storage, making them vital components of the skeletal system and the overall homeostasis of the human body. Bone is formed by the laying down of osteoid (the young hya- line matrix of true bone) by osteoblasts and the mineralization of the osteoid by the development and deposition of crystals of calcium phosphate (in the form of hydroxyapatite) within it. These minerals, organized in a regular pattern on a collagen scaffold, provide bone with stiffness. Osteoid contains largely fibers of type I collagen (a member of a large family of collagen proteins) and lesser amounts of numerous noncollagenous proteins. These proteins include osteonectin, osteopontin, bone sialoprotein, biglycan, decorin, and osteocalcin. Type I col- lagen is the most abundant protein in bone tissue. It forms the Bone (continued) + ward ' s science Fig. 2: Trabeculae are thin, lattice-like structures or bony spicules that make up the porous network within cancellous bone. (Credit: iStock Photo) Fig. 3: The osteon is the structural unit of compact bone. (Created with BioRender.com)