The central canal of an osteon contains

This is a high power photo of a single Haversian system. Can you identify the concentric lamellae, central canal and the lacunae. Because of the way the bone is prepared for sectioning, you cannot see the osteocytes in the lacunae, only the spaces left behind. The alternating bright and dark concentric rings (lamellae) are due to an alternating arrangement of collagen fibres in the bone matrix. The collagen fibres in each layer are parallel to each other, but at right angles to the fibres in the alternating layers on either side. (Think about how this will affect the bones ability to resist tensile stress).

• Compact bone is the hard material that makes up the shaft of long bones and the outside surfaces of other bones. Compact bone consists of cylindrical units called osteons. Each osteon contains concentric lamellae (layers) of hard, calcified matrix with osteocytes (bone cells) lodged in lacunae (spaces) between the lamellae. Smaller canals, or canaliculi, radiate outward from a central canal, which contains blood vessels and nerve fibers. Osteocytes within an osteon are connected to each other and to the central canal by fine cellular extensions. Through these cellular extensions, nutrients and waste are exchanged between the osteocytes and the blood vessels. Perforating canals provide channels that allow the blood vessels that run through the central canals to connect to the blood vessels in the periosteum that surrounds the bone.

• Spongy bone consists of thin, irregularly shaped plates called trabeculae, arranged in a latticework network. Trabeculae are similar to osteons in that both have osteocytes in lacunae that lie between calcified lamellae. As in osteons, canaliculi present in trabeculae provide connections between osteocytes. However, since each trabecula is only a few cell layers thick, each osteocyte is able to exchange nutrients with nearby blood vessels. Thus, no central canal is necessary.
  

Figure 1.Main features of a long bone.

Here are the main features of a long bone (refer to Figure 1):

• The diaphysis, or shaft, is the long tubular portion of long bones. It is composed of compact bone tissue.

• The epiphysis (plural, epiphyses) is the expanded end of a long bone. It is in the epiphyses where red blood cells are formed.

• The metaphysis is the area where the diaphysis meets the epiphysis. It includes the epiphyseal line, a remnant of cartilage from growing bones.

• The medullary cavity, or marrow cavity, is the open area within the diaphysis. The adipose tissue inside the cavity stores lipids and forms the yellow marrow.

• Articular cartilage covers the epiphysis where joints occur.

• The periosteum is the membrane covering the outside of the diaphysis (and epiphyses where articular cartilage is absent). It contains osteoblasts (bone‐forming cells), osteoclasts (bone‐destroying cells), nerve fibers, and blood and lymphatic vessels. Ligaments and tendons attach to the periosteum.

• The endosteum is the membrane that lines the marrow cavity.

Here are the main features of short, flat, and irregular bones:

• In short and irregular bones, spongy bone tissue is encircled by a thin layer of compact bone tissue.

• In flat bones, the spongy bone tissue is sandwiched between two layers of compact bone tissue. The spongy bone tissue is called the diploë.

• The periosteum covers the outside layer of compact bone tissue.

• The endosteum covers the trabeculae that fill the inside of the bone.

• In certain bones (ribs, vertebrae, hip bones, sternum), the spaces between the trabeculae contain red marrow, which is active in hematopoiesis.

Bones are considered organs because they contain various types of tissue, such as blood, connective tissue, nerves, and bone tissue. Osteocytes, the living cells of bone tissue, form the mineral matrix of bones. There are two types of bone tissue: compact and spongy.

Compact Bone Tissue

Compact bone (or cortical bone) forms the hard external layer of all bones and surrounds the medullary cavity, or bone marrow. It provides protection and strength to bones. Compact bone tissue consists of units called osteons or Haversian systems. Osteons are cylindrical structures that contain a mineral matrix and living osteocytes connected by canaliculi, which transport blood. They are aligned parallel to the long axis of the bone. Each osteon consists of lamellae, which are layers of compact matrix that surround a central canal called the Haversian canal. The Haversian canal (osteonic canal) contains the bone’s blood vessels and nerve fibers (Figure). Osteons in compact bone tissue are aligned in the same direction along lines of stress and help the bone resist bending or fracturing. Therefore, compact bone tissue is prominent in areas of bone at which stresses are applied in only a few directions.

Art Connection

Which of the following statements about bone tissue is false?

1. Compact bone tissue is made of cylindrical osteons that are aligned such that they travel the length of the bone.
2. Haversian canals contain blood vessels only.
3. Haversian canals contain blood vessels and nerve fibers.
4. Spongy tissue is found on the interior of the bone, and compact bone tissue is found on the exterior.

Spongy Bone Tissue

Whereas compact bone tissue forms the outer layer of all bones, spongy bone or cancellous bone forms the inner layer of all bones. Spongy bone tissue does not contain osteons that constitute compact bone tissue. Instead, it consists of trabeculae, which are lamellae that are arranged as rods or plates. Red bone marrow is found between the trabuculae. Blood vessels within this tissue deliver nutrients to osteocytes and remove waste. The red bone marrow of the femur and the interior of other large bones, such as the ileum, forms blood cells.

Spongy bone reduces the density of bone and allows the ends of long bones to compress as the result of stresses applied to the bone. Spongy bone is prominent in areas of bones that are not heavily stressed or where stresses arrive from many directions. The epiphyses of bones, such as the neck of the femur, are subject to stress from many directions. Imagine laying a heavy framed picture flat on the floor. You could hold up one side of the picture with a toothpick if the toothpick was perpendicular to the floor and the picture. Now drill a hole and stick the toothpick into the wall to hang up the picture. In this case, the function of the toothpick is to transmit the downward pressure of the picture to the wall. The force on the picture is straight down to the floor, but the force on the toothpick is both the picture wire pulling down and the bottom of the hole in the wall pushing up. The toothpick will break off right at the wall.

The neck of the femur is horizontal like the toothpick in the wall. The weight of the body pushes it down near the joint, but the vertical diaphysis of the femur pushes it up at the other end. The neck of the femur must be strong enough to transfer the downward force of the body weight horizontally to the vertical shaft of the femur (Figure).

Does the central canal of an osteon contain bone marrow?

What is found within the central canal of an osteon quizlet?

What is the central opening of an osteon?