The bones do not lay bare within the human body. Instead, there is a membrane that covers, or lines, most of the bones of the human body, called the periosteum. It lines every bone's outer portion, with one exception: it is absent at the joints of long bones of the body. These bones include the femur, tibula, humeri, and radii; the ulna, metacarpals, metatarsals, and phalanges are also considered long bones. The bones also have a lining inside, which is called the endosteum; the periosteum that covers the bones of the skull is called the pericranium.
This bone membrane is fibrous, dense, and composed of connective tissue. It is divided into two separate layers. The outer layer is called the fibrous layer. It contains cells that synthesize collagen and the extracellular part of body tissue. These cells are also important in the healing of wounds.
The inner layer of the periosteum is called the cambium layer. This layer contains cells, called progenitor cells, that can change into osteoblasts, which are cells responsible for the growth and formation of the bone. These cells can also be important in healing. For example, when a bone breaks, these cells change into osteoblasts and another type of cell called chondroblasts, which form cartilage cells. This cell differentiation is an important part of the healing process.
The periosteum has blood vessels and nerves. The blood vessels provide vital nourishment to the bone. The nerves give the bones sensation or feeling. For example, there are nociceptors in the periosteum. Nociceptors are sensory receptors that send signals to both the brain and the spine when there is a dangerous form of stimulation. Often, the transmission of these nerve signals leads to the perception of pain.
When picturing the periosteum, it may seem probable that this membrane just rests on the bone. However, this is not the case. It is actually attached to the bone by fibers that are referred to as Sharpey’s fibres. These fibers are actually a configuration of connective tissues that contain bundled up fibers of collagen.
