Bone Function, Development, and Growth

Bone Tissue Structure

Bones contain an outer, or compact, region and an inner, or spongy, region. Marrow is found within the cavities of long bones; red marrow produces blood cells, and yellow marrow primarily functions as energy storage.

Bones provide support and structure for the body. To do so, bones are made of two distinct types of bone tissue: compact bone and spongy bone. Compact bone is the hard bone tissue that provides rigid strength for the bones. The other type, spongy bone, is a type of bone composed of a collection of trabeculae bound together. A trabecula (plural, trabeculae) is an open lattice of rod-like connective tissue that forms spongy bone. Another name for spongy bone is trabecular bone. Human long bones have a characteristic shape. They have a knob-like structure at each end, called an epiphysis, and a long shaft between each epiphysis, called a diaphysis.

Because each diaphysis (plural: diaphyses) is found in the center of a long bone and needs to produce rigid strength, it is composed of compact bone tissue. In the very center of this bone part is a central space, called the medullary cavity, that stores bone marrow. In the case of human adults, this marrow is yellow, which means it is a source of fat. Children do not have as large a supply of yellow marrow within their bones. Instead, it develops as the children get older.

The epiphysis (plural: epiphyses) found at each end of a long bone is made mostly of spongy bone surrounded by a thin layer of compact bone. This compact bone provides some protection for the interior spaces found between the trabeculae of the bone tissue. These spaces are filled with red bone marrow, which is used by the body during the process of formation of red blood cells in the marrow of certain bones, called hematopoiesis.

One of the primary factors that contributes to a child growing taller is an increase in the length of the bones. The epiphyseal plate is the location where bone growth occurs, found between the epiphysis and the diaphysis. An epiphysis is considered distal when it is farthest from the center of the body. The proximal epiphysis is nearer to the center of the body. The epiphyseal plate is also called the "growth plate" because it is in this location where a layer of specialized cartilage increases in length, making the child taller.

To offer additional protection to the bone, a protective layer called the periosteum surrounds the outer surface of a bone. The periosteum is a thick double membrane of connective tissue that contains cells that both build and destroy the bone tissue. It is connected to the actual bone tissue through special structures called perforating fibers. These fibers anchor the periosteum to the bone itself. The periosteum also has a number of nerves and blood vessels passing through it, which maintain a constant internal environment within the bone. Ligaments and tendons also anchor to the periosteum.

Endosteum is a layer of connective tissue that covers the internal cavities and canals of bone. This tissue lines the medullary canals of the diaphysis and the trabeculae of the epiphysis. It serves to offer protection to the inner parts of the bones and is comprised of the same types of cells found in the periosteum.

Structure of Bone Tissues

Human long bones have unique internal and external structures that serve a variety functions, including bone growth, repair, points of muscle and ligament attachment, and red blood cell formation. The diaphyses are the central shafts of long bones where yellow bone marrow is stored. The epiphyses are the ends of the long bones and are the sites of red bone marrow formation.