The bone structure consists of a number of layers. These include the periosteum, compact bone, spongy bone, and an inner core of bone marrow. Here we explain the anatomy of the bone and the function of each part.
It is important for bones to be strong to support our body weight. In some cases, their main function is to provide protection such as the skull and ribs. However, they must also be light enough to make movement possible.
Long bone structure
The structure of a long bone consists of several sections:
- Diaphysis: This is the long central shaft.
- Epiphysis: Forms the larger rounded ends of long bones.
- Metaphysis: Area between the diaphysis and epiphysis at both ends of the bone.
- Epiphyseal Plates: Plates of cartilage, also known as growth plates allow the long bones to grow during childhood. Once we stop growing (between 18 and 25 years of age) the cartilage plates stop producing cartilage cells. Over time, they are gradually replaced by bone.
Covering the ends of bones, where they form a joint with another bone, is a layer of hyaline cartilage. This is also known as articular cartilage. It is a firm but elastic type of cartilage, which provides shock absorption to the joint. The hyaline cartilage, and has no neural (nerve) or vascular (blood) supply.
If you cut a cross-section through a bone, you would first come across a thin layer of dense connective tissue. This is known as the Periosteum. It consists of two layers; an outer ‘fibrous layer’ containing mainly fibroblasts, and an inner ‘cambium layer’ containing progenitor cells. The progenitor cells develop into osteoblasts (the cells responsible for bone formation). The periosteum provides a good blood supply to the bone and a point for muscular attachment.
Compact bone structure
Under the periosteum is a thin layer of compact bone (often called cortical bone). This provides the bones strength and consists of tightly stacked layers of bone that appear to form a solid section. However, they do contain osteons, which are like canals, providing passageways through the hard bone matrix.
On the inside of the compact bone structure, you would find a different kind of bone, known as spongy or cancellous bone. This is a more porous, lightweight type of bone with an irregular arrangement of tissue that allows maximum strength. In a long bone, this is normally found at either end of the bone, in flat or irregular bones it is a thin layer found just inside the compact bone. Interestingly, compact bone constitutes up to 80% of the bone’s weight, with spongy bone making up the additional 20%, despite its much larger surface area.
Medullary cavity & bone marrow
The centre of the bone shaft is hollow and known as the Medullary Cavity. This contains both red and yellow bone marrow. Yellow bone marrow is mainly fatty tissue, while red bone marrow is where the majority of blood cells are produced. This is found in higher proportions in the flat and irregular bones.