Algonquin College

Anatomy and Physiology

Chapter 5 .Skeletal System | Section 5.4 (of 5.7) Structures of Long Bones

Structures of Long Bones

Although they may seem lifeless and simple, bones are in fact complex structures filled with live cells and tissues. Bones interact with one another at joints, and different bones and joints are specialized for different types of movements.

Long bone structure

Long bones are the most common bones found in the mammalian body. They are composed mostly of compact bone, and are roughly cylindrical in shape with enlarged ends filled with spongy bone.

At each end of the bone, the proximal and distal epiphyses are where long bones make contact with other bones at joints. Each epiphysis (also called the head of the long bone) is covered in articular cartilage to allow bones to slide past one another more easily. The interior of epiphyses is filled with spongy bone, and is differentiated from the interior of the diaphysis by the epiphyseal line (in adults) or plate (in juveniles). Some epiphyses are also sites of red blood cell formation in adults.

The diaphysis or shaft of a long bone makes up most of the length of the bone. It is roughly cylindrical in shape, and is separated from the epiphyses by the epiphyseal line/plate. The diaphysis is composed of a thick layer of compact bone surrounding the medullary cavity.

In adults, the medullary cavity can also be called the yellow marrow cavity, but in infants it is called the red marrow cavity, as it is filled with newly forming red blood cells. The medullary cavity is lined with a thin layer of connective tissue called the endosteum.

The diaphysis is covered by the periosteum, a protective layer of fibrous connective tissue. The periosteum is anchored to the underlying bone by perforating or Sharpey’s fibers.

Labelled diagram of the structure of long bone
Figure 5.4 - Long Bone Structure

Microscopic structure of compact bone

Trapped within dense compact bone, mature osteocytes (bone forming cells) are found within small cavities called lacunae. The lacunae are arranged in layers of concentric circles surrounding central (Haversian) canals, which run lengthwise through the diaphysis. Each cylindrical section of layers of lacunae (called lamellae) together with the central canal they surround is called an osteon or Haversian system.

All lacunae are connected to the central canals by canaliculi, which extend outward from the central canals. The canaliculi allow the osteocytes access to nutrients and other materials that would not otherwise be able to reach them through the hard compact bone matrix. Also serving to supply the bone with nutrients, the perforating (Volkmann’s) canals run transversely from the outside to the inside of the bone and create passageways for blood vessels to enter the bone.