A good place to begin a study of the skeletal system is with the overall functions of its organs, the bones and ligaments. Ligaments are fibrous bands that help hold the various bones together into an organized skeleton. Bones are rigid, mineralized structures that help perform five major roles in the body. Each is important for maintaining overall stability of the body’s internal environment, or homeostasis.

Structurally, a simple way to categorize the 206 or more bones of the skeleton is by shape. Usually they are divided into five categories: long bones, short bones, flat bones, irregular bones, and sesamoid bones. Figure 8-2 shows an example of each type. The size, shape, and appearance of bones vary according to the role of each bone in the skeleton. Some bones must bear great weight; others serve a protective function or serve as delicate support structures, such as for the fingers and toes.

Bones differ in size and shape and also in the amount and proportion of the two different types of bone tissue that compose them. Compact bone is dense and “solid” in appearance. Cancellous bone, on the other hand, is characterized by open space with a network of thin, branched crossbeams. Recall from Chapter 6 that cancellous bone is also called spongy bone or trabecular bone. Both compact and cancellous bone types are discussed when the microscopic structure of bone is described later in the chapter.

All five shape categories of bone discussed below have varying amounts of cancellous and compact bone in their structure.

Long bones are easily identified by their roughly cylindrical shape that is longer than it is wide. They also have enlarged and often uniquely shaped ends that articulate with other bones. The femur of the thigh and humerus of the arm are examples. Other examples include the radius, ulna, tibia, fibula, metacarpal bones, metatarsal bones, and phalanges.

Short bones are often described as cube- or box-shaped structures that are about as broad as they are long. Examples include each of the wrist (carpal) and ankle (tarsal) bones.

Flat bones are generally broad and thin with a flattened and often curved surface. Certain bones of the skull, the shoulder blades (scapulae), ribs, and breastbone (sternum) are typical flat bones.

Irregular bones are often clustered in groups and come in various sizes and shapes. The vertebral bones that form the spine and the facial bones of the skull are good examples.

Sesamoid bones, which are sometimes grouped with the irregular bones, often appear singly rather than in groups. The name comes from “sesame seed” because these bones often resemble sesame seeds in size and shape. The number of sesamoid bones in the body can be many or only a few—the number and size vary from one person to the next. Sesamoid bones usually develop in the tendons close to the joints. The patella (kneecap) is the largest sesamoid bone—one of the few that consistently appear in the human skeleton.

Parts of a Long Bone

A long bone consists of the following structures visible to the naked eye: diaphysis, epiphyses, articular cartilage, periosteum, medullary (marrow) cavity, and endosteum. Identify each of these structures in the tibia shown in Figure 8-3, A. The tibia is the longer, stronger, and more medially located of the two leg bones.

1. Diaphysis (dye-AF-i-sis)—main shaftlike portion. Its hollow, cylindrical shape and the thick compact bone that composes it adapt the diaphysis well to its function of providing strong support without adding cumbersome weight.

2. Epiphyses (eh-PIF-i-seez)—the proximal and distal ends of a long bone. Epiphyses have a bulbous shape that provides generous space near joints for muscle attachments and also gives stability to joints. Look at Figure 8-3 to note the innumerable small spaces in the bone of the epiphysis. They make this kind of bone look a little like a sponge—hence its name spongy, or cancellous, bone. A soft connective tissue called red marrow fills the spaces within this spongy bone. Early in development, epiphyses are separated from the diaphysis by a layer of cartilage, the epiphyseal plate. The cartilage layer is eventually replaced by bone, forming an epiphyseal line. The region between the epiphyses and diaphysis (in a mature bone) or the epiphyseal plate region (in a growing bone) is called the metaphysis (meh-TAF-i-sis).

3. Articular cartilage—thin layer of hyaline cartilage that covers the articular or joint surfaces of epiphyses. The resiliency of this material cushions jolts and blows.

4. Periosteum (pair-ee-OS-tee-um)—dense, white fibrous membrane that covers bone except at joint surfaces, where articular cartilage forms the covering. Many of the periosteum fibers penetrate the underlying bone and weld these two structures to each other. In addition, muscle tendon fibers interlace with periosteal fibers, thereby anchoring muscles firmly to bone. The periosteum is a critically important membrane that, depending on its location, also contains bone-forming and bone-destroying cells and blood vessels that become incorporated into bones during their initial growth or subsequent remodeling and repair. This important membrane is essential for bone cell survival and for bone formation, a process that continues throughout life.

5. Medullary cavity—a tubelike hollow space in the diaphysis of a long bone, also called a marrow cavity. In the adult the medullary cavity is filled with connective tissue rich in fat—a substance called yellow marrow.

6. Endosteum (en-DOS-tee-um)—a thin fibrous membrane that lines the medullary cavity of long bones. The endosteum lines the spaces of spongy bone as well. Like the periosteum, the endosteum has various types of bone cells and the stem cells that produce them.

Parts of Flat Bones and Other Bones

The structure of the flat bone is very similar to that of a long bone, but simpler. As you can see in Figure 8-4, a flat bone of the cranium has outer and inner walls made of compact bone. These hard walls are called the internal table and external table. Between is a region called the diploe, which is made up of cancellous bone. Other flat bones, such as the ribs and sternum, have a similar overall structure. Like long bones, flat bones are covered in a periosteum and the inner spaces are lined with endosteum.

Red marrow fills the spaces of the cancellous bone inside many flat bones. The sternum, which contains red marrow even in adulthood, is one example. To help in the diagnosis of leukemia and certain other diseases, a physician may decide to perform a needle puncture of one of these bones. In this type of diagnostic procedure, a needle is inserted through the skin and compact bone into the red marrow, and a small amount of the marrow is then aspirated and examined under the microscope for evidence of normal or abnormal blood cells. The procedure, called aspiration biopsy cytology (ABC), is discussed on p. 619.

Short bones, irregular bones, and sesamoid bones all have features similar to those of flat bones.

The basic structural components and cell types of bone were described briefly in Chapter 6. In the paragraphs that follow, additional information about bone structure and cell types will serve as a basis for learning the functional characteristics of this important tissue. Understanding how a bone forms and grows, how it repairs itself after injury, and how it interacts with other tissues and organs in maintaining various important homeostatic mechanisms is based on a knowledge of its basic structure—a structure as unique as its chemical composition.

Compact Bone

Compact bone constitutes about 80% of the total bone mass in the adult human body. It contains many cylinder-shaped structural units called osteons (AHS-tee-onz), or Haversian systems. Note in Figure 8-5 that each osteon surrounds a central canal that runs lengthwise through the bone. Living bone cells in these units are literally cemented together to constitute the structural framework of compact bone. The unique structure of the osteon permits delivery of nutrients and removal of waste products from metabolically active, but imprisoned, bone cells.

Several simple structures make up each osteon: lamellae, lacunae, canaliculi, and a central canal. As you read the following descriptions, identify each structure in Figure 8-5.

Lamellae (lah-MEL-ee). Concentric lamellae are cylinder-shaped layers of calcified matrix in the osteon. Lamellae (layers) of hard bone matrix are also present outside the osteon. Interstitial lamellae are layers of calcified matrix between osteons. They are the remnants of older osteons that have been altered by bone growth or remodeling. A few layers of bone matrix also run around the outer boundary of compact bone, encircling all the osteons. These layers that run along the inner circumference (along the endosteum) and outer circumference (along the periosteum) of a bone are called circumferential lamellae.

Lacunae (lah-KOO-nay). These are small spaces in bone matrix that contain tissue fluid and in which bone cells lie imprisoned between the hard layers of the lamellae.

Canaliculi (kan-ah-LIK-yoo-lye). These ultra-small canals radiate in all directions from the lacunae and connect them to each other and to a larger canal, the central canal.

Central canal. Also called an osteonal canal or Haversian canal, the central canal extends lengthwise through the center of each osteon. The central canal is lined with endosteum and contains blood vessels, lymphatic vessels, and nerves. Nutrients and oxygen move from the central canal through canaliculi to the lacunae and their bone cells—a short distance of about 0.1 mm or less.

Parallel central canals are connected to each other by transverse canals (Volkmann canals). These communicating canals contain nerves and vessels that carry blood and lymph from the exterior surface of the bone to the osteons.

Cancellous Bone

Cancellous, or spongy, bone constitutes about 20% of the total bone mass and differs in microscopic structure from compact bone. As you recall, the structural unit of compact bone is the highly organized osteon. There are no osteons in cancellous bone. Instead, it consists of crisscrossing bony branches called trabeculae. Bone cells are found within the trabeculae. Nutrients are delivered to the cells and waste products are removed by diffusion through tiny canaliculi that extend to the surface of the very thin bony branches. Cancellous bone is often called trabecular bone.

Note that the cancellous bone shown in Figure 8-6 lies between two layers of compact bone, much like the filling in a sandwich. Recall that the middle layer of spongy bone is called the diploe. This layered organization is typical of flat bones such as those found in the skull. Cancellous bone is also found inside short and irregular bones, as well as inside the epi-physes (see Figure 8-3) and lining the medullary cavities of long bones (see Figure 8-5, A).

The placement of trabeculae in spongy bone is not as random and unorganized as it might first appear. They are fractal in nature, meaning that they appear random, but there is really an underlying, complex organization to them. The bony branches are actually arranged along lines of stress, and their size and orientation will therefore differ between individual bones according to the nature and magnitude of the applied load (Figure 8-7). This feature greatly enhances a bone’s strength and is yet another example of the relationship between structure and function.