Bones are broadly categorized into two main types: compact bone and spongy bone. These distinct tissues work together to provide bones with strength, flexibility, and various biological functions.
Understanding Compact Bone
Compact bone, also known as cortical bone, represents the dense and solid outer layer of all bones. It provides protection and contributes significantly to the strength of bones. Comprising approximately 80% of the adult human skeleton, compact bone is characterized by its organized microscopic structure.
The fundamental unit of compact bone is the osteon, or Haversian system. Each osteon consists of concentric rings of bone matrix called lamellae, which surround a central Haversian canal. These canals contain blood vessels and nerve fibers, supplying nutrients. Within the lamellae are small spaces called lacunae, each housing a mature bone cell known as an osteocyte. Tiny channels called canaliculi radiate from the lacunae, connecting osteocytes to each other and to the central canal, facilitating nutrient and waste exchange.
Compact Bone Versus Spongy Bone
Compact bone and spongy bone, also referred to as cancellous or trabecular bone, exhibit differences in their structure and function. Compact bone is dense and solid, forming the hard exterior of bones, while spongy bone has a more open, porous, and lattice-like structure. Compact bone is smooth and solid, whereas spongy bone has a honeycomb-like or trabecular arrangement.
Compact bone is primarily responsible for providing strength, rigidity, and protection. Its dense packing of osteons allows it to resist significant bending and compressive forces. In contrast, spongy bone, with its network of thin plates called trabeculae, is lighter and less dense, contributing to shock absorption and reducing the overall weight of the bone. Spongy bone also houses red bone marrow, which is involved in blood cell production. While compact bone forms the outer layer of all bones, spongy bone occupies the interior, particularly at the ends of long bones.
Anatomical Structures Dominated by Compact Bone
The diaphysis, or shaft, of long bones is primarily composed of compact bone. This tubular midsection provides substantial strength and support for bearing body weight and resisting mechanical stresses. The thick walls of compact bone in the diaphysis enable these long bones to act as rigid levers for movement.
Beyond the shafts of long bones, compact bone forms the hard outer layer, or cortex, of all bones in the human skeleton. This includes flat bones, such as those found in the skull and the sternum, as well as irregular bones like the vertebrae. For instance, the skull bones, which protect the brain, are made of two layers of compact bone with a layer of spongy bone (diplöe) sandwiched between them, providing both protection and some shock absorption. The sternum, a flat bone in the chest, has a compact outer cortex, enclosing a cancellous interior. Even irregular bones like vertebrae, while having cancellous bone in their body, are covered by a thin coating of cortical (compact) bone, with thicker coverings on the vertebral arch and processes.
The Roles of Compact Bone
Compact bone contributes significantly to the body’s structural integrity and physiological processes. Its dense and rigid nature provides the framework for the entire skeletal system, supporting the body against gravity and external forces. This structural support allows for upright posture and overall body shape.
The presence of compact bone as the outer shell of bones also enables movement. Muscles attach to bones, and the strong, unyielding nature of compact bone allows it to act as a system of levers, translating muscle contractions into a wide range of motions. Compact bone plays a protective role for delicate internal organs. For example, the compact bone of the skull safeguards the brain, and the sternum and ribs shield the heart and lungs from external impact. In addition to these mechanical functions, compact bone serves as a reservoir for essential minerals, particularly calcium and phosphorus, which are released into the bloodstream as needed to maintain mineral balance in the body.