The skeletal system forms the body’s internal framework. It is composed of several distinct, living components that qualify as organs, contributing to structure, movement, and critical metabolic processes. The major organs include the bones themselves, specialized connective tissues like cartilage and ligaments, and the highly active tissue known as bone marrow.
Bones: The Main Structural Component
Each individual bone is a complex, living organ. The adult human body contains 206 bones, which provide the primary framework and attachment sites for muscles, enabling movement. Bone tissue (osseous tissue) is a dense connective tissue that is constantly undergoing remodeling and repair.
Bone is primarily composed of two distinct tissue types: compact and spongy bone. Compact bone, also called cortical bone, forms the solid, hard outer layer of all bones, accounting for about 80% of total bone mass. Its structure is characterized by tightly packed cylindrical units called osteons, which run parallel to the long axis of the bone, providing strength for weight-bearing and protection.
Deep to this dense exterior is spongy bone, also known as cancellous or trabecular bone, which has a honeycomb-like structure. This tissue is made up of thin, interconnecting plates called trabeculae, oriented along lines of stress to provide strength without excessive weight. Spongy bone receives nutrients through the spaces between the trabeculae, which are often filled with bone marrow. The living cells within both types of bone, primarily osteocytes, reside in small spaces called lacunae and maintain the bone matrix, consisting of organic collagen fibers hardened by inorganic calcium phosphate crystals.
Connective Tissues: Cartilage and Ligaments
The joints rely on two specialized connective tissues considered functional organs of the skeletal system: cartilage and ligaments. These components ensure smooth motion and stability where bones meet. Cartilage is a semi-rigid tissue that provides cushioning and flexibility throughout the skeleton.
Articular cartilage at the ends of bones offers a smooth, low-friction surface, allowing for movement and absorbing shock. Cartilage also supports soft tissues, such as the nose and ears, and plays a role in the growth of long bones.
Ligaments are strong bands of dense connective tissue made primarily of collagen fibers. Their function is to connect one bone to another, stabilizing joints and preventing excessive movement. This stabilization maintains proper alignment during physical activities, protecting the joint from injury.
Bone Marrow: The Internal Organ
Bone marrow is a distinct, soft, spongy organ housed within the internal cavities of bones. It is a highly vascular tissue that constitutes approximately four percent of the total body mass in adults. It exists in two main forms: red marrow and yellow marrow.
Red bone marrow is the site of hematopoiesis, the process responsible for the production of all blood cells. This tissue contains hematopoietic stem cells that generate billions of new red blood cells, white blood cells, and platelets daily. This constant production replenishes the blood supply, as each type of blood cell has a finite lifespan.
Yellow bone marrow largely replaces red marrow as a person ages and consists mainly of fat cells, giving it its characteristic color. Its primary function is to store fat for energy reserves. However, in situations of severe blood loss or hypoxia, yellow marrow can convert back into red marrow to increase the body’s capacity for blood cell production.
Metabolic and Protective Roles
The skeletal system’s influence extends beyond its structural capacity, encompassing metabolic and protective functions that affect the entire body. Bones serve as the body’s main reservoir for minerals, storing and regulating calcium and phosphate. This mineral homeostasis is a continuous process where calcium is released from the bone matrix into the bloodstream or deposited back into the bone to maintain levels necessary for nerve function and muscle contraction.
The bones protect delicate internal organs. The cranium protects the brain, the rib cage shields the heart and lungs, and the vertebral column encases the spinal cord.
The bones work with muscles as mechanical levers to facilitate movement. The skeletal system provides the rigid attachments necessary for muscles to contract against, translating force into motion. Bone tissue has also been recognized as an endocrine organ, producing hormones like osteocalcin, which helps regulate blood sugar and fat deposition.