Bones are far more than rigid supports; they are dynamic, living organs that continuously adapt and renew throughout a person’s life. Beyond providing structural support and protection for internal organs, bones also enable movement by serving as attachment points for muscles. They play a significant role in mineral storage, particularly for calcium and phosphate, and house the body’s blood-producing factories.
Types of Bone Tissue
The interior of a bone features two distinct types of tissue: compact bone and spongy bone. Compact bone, also known as cortical bone, forms the dense, hard outer shell of most bones, making up about 80% of the human skeleton. It provides substantial strength and rigidity, particularly in long bones like those in the arms and legs.
Compact bone is organized into microscopic cylindrical units called osteons, or Haversian systems, which run parallel to the bone’s long axis. Each osteon consists of concentric layers of calcified matrix, known as lamellae, surrounding a central Haversian canal. These canals house blood vessels and nerve fibers.
Spongy bone, also referred to as cancellous or trabecular bone, lies inside the compact bone. It has a porous, lattice-like structure composed of thin, interconnected bony spicules called trabeculae. This network makes spongy bone lighter than compact bone, while still providing structural support and helping to distribute forces across joints. The open spaces within this meshwork are often filled with bone marrow and blood vessels. Spongy bone is commonly found at the ends of long bones, as well as in flat bones like the ribs, skull, and vertebrae, where it helps absorb shock.
Bone Marrow
Within the porous network of spongy bone, a soft, gelatinous tissue known as bone marrow resides. This tissue is broadly categorized into two types: red bone marrow and yellow bone marrow.
Red bone marrow is responsible for hematopoiesis, the process of producing all types of blood cells. This includes red blood cells, which transport oxygen; white blood cells, which fight infection; and platelets, which are essential for blood clotting. In newborns and young children, most bones contain red bone marrow, but as a person ages, much of it is gradually replaced by yellow marrow. In adults, red bone marrow is primarily found in the flat bones of the axial skeleton, such as the pelvis, ribs, sternum, vertebrae, and the ends of long bones like the femur and humerus.
Yellow bone marrow, in contrast, consists mainly of fat cells (adipocytes) and serves as a storehouse for fats. It is predominantly found in the central cavities (medullary cavities) of long bones. Yellow marrow also contains mesenchymal stem cells, which can differentiate into cartilage, bone, or fat cells. Under certain conditions, such as severe blood loss or anemia, yellow bone marrow can convert back to red marrow to increase blood cell production.
Nourishing the Bone
Bones are living tissues that require a continuous supply of oxygen and nutrients and efficient removal of waste products. This nourishment is delivered through an intricate network of blood vessels. Major nutrient arteries enter the bone, branching into smaller vessels that supply the bone marrow and the inner regions of compact bone.
Within compact bone, blood vessels and nerve fibers travel through Haversian canals, which run parallel to the bone’s long axis. These canals are interconnected by Volkmann’s canals, which run perpendicularly, connecting different Haversian systems. This interconnected vascular system ensures that bone cells (osteocytes) receive the necessary blood supply through tiny channels called canaliculi.
Nerves also innervate bone tissue, playing roles beyond just pain sensation. Both sensory and sympathetic nerve fibers are present within the bone, contacting various bone cells, including osteoblasts and osteoclasts, as well as blood vessels. These nerves can influence processes like bone development, remodeling, and blood flow regulation within the bone.
Bone Remodeling
Bone remodeling is a process where old bone tissue is constantly broken down and replaced with new bone. This ongoing cycle helps maintain skeletal strength, repairs microscopic damage, and regulates calcium levels in the body. The process involves a coordinated effort between two primary cell types: osteoclasts and osteoblasts.
Osteoclasts are specialized cells responsible for bone resorption, breaking down and removing old or damaged bone tissue. They release enzymes that dissolve the bone matrix, creating small cavities. Osteoblasts, which are bone-forming cells, then move into these areas.
Osteoblasts then lay down new bone matrix, a mix of proteins like collagen, which becomes mineralized with calcium, phosphate, and other minerals. This new tissue fills the spaces created by osteoclasts, rebuilding and strengthening the bone. This balanced process of resorption and formation ensures that the skeleton adapts to mechanical stresses and can repair micro-damage. The entire cycle of bone remodeling is influenced by various factors, including hormones and mechanical loading.