Bone and cartilage (comparative anatomy)

Bone is the hard substance that forms the supportive framework of the bodies of all vertebrate organisms. This framework, the skeleton, is composed of hundreds of separate parts called bones. The bones support the bodies of vertebrates and protect their delicate internal organs, such as the brain, lungs, and liver, from injury. In addition, the muscles are attached to the bones, which act as levers to enable their function in actions as diverse as walking or swallowing. Furthermore, bone provides the calcium needs of the body, serves as the main repository for calcium storage, and contains the sites where red blood cells are made.

Much of the bone in adult vertebrates derives from cartilage, elastic, fibrous connective tissue, which is the main component of fetal vertebrate skeletons. Such bone, for example, that of the long bones, is called cartilage bone. Cartilage is an extracellular matrix made by body cells called chondrocytes. It is surrounded by a membrane, the periosteum, and much of its firmness and elasticity arises from plentiful fibrils of the protein collagen that it contains. These fibrils and their many interconnections provide mechanical stability and very high tensile strength, while allowing nutrients to diffuse into the chondrocytes to keep them alive. The blood vessels which surround the cartilage in the periosteum provide all the needed nutrients and remove the cellular waste materials produced by life processes.

The cartilage-containing skeletons of newborn vertebrates become cartilage bone by ossification, a process that includes calcification, chondrocyte destruction, and replacement by bone cells, which lay down more bone. This cartilage, called hyaline cartilage, remains at the articular sites of bones. In young vertebrates, cartilage is the site for the continued growth and calcification that produces the bone lengthening required for the attainment of adult size and stature. In addition to the cartilage bone, the so-called membrane bone occurs exclusively in the top portion of the skull.

Bone is thought to have developed over a half billion years ago, as shown by its presence in the fossils of fishlike carnivores of that time. In those creatures, it seems to have been formed into interconnected external plates covering their bodies as sheaths that strengthened and protected their bodies. The existence of bone only at the surfaces of these fossils has led many scientists to suppose that the first function of bone was protection rather than body support. Be that as it may, bone has both functions in modern organisms. It is interesting to note that many of these early organisms lacked bone in their heads. It seems possible that this lack may have led to the development of separate mechanisms for the formation of membrane bone and cartilage bone, which have different means to the same end.

Physical Characteristics of Bone

To best serve their biofunctions, bones must be very hard, strong, and rigid but remain supple enough to stay unbroken under normal conditions. These characteristics are provided by the collagen fibrils and insoluble calcium phosphate which make up the bones. The bones must also be light enough to allow vertebrates to move easily and remain erect.

Overly, heavy bones are prevented by the occurrence of two general types of bone tissue. The first of these is compact bone, the portion most familiar because it makes up the hard exterior of many bones, except at their very ends. The second bone type, cancellous bone, which appears spongy, is found at the ends of long bones and inside them. It serves to lighten the bones, acting in the same fashion as the air-filled sinuses of the skull, which diminish the overall weight of the skull without weakening it. Bones are covered on their outsides by the important fibrous membrane called the periosteum, along with cartilage. Their insides are lined by an endosteum membrane, very similar to the inner layer of the periosteum.

It is also useful to think of bones in terms of woven, lamellar, and osteonic forms. These terms indicate the relative number of cells in a bone matrix region and the arrangement of collagen fibers in the region. Collagen fibers of woven bone crisscross within the bone matrix and its bone cells are distributed randomly. In lamellar bone, the collagen fibrils are more ordered, and fewer bone cells are present. Osteonic bone is also well-organized. However, its cells are found in concentric rings with narrow channels (Haversian canals) inside them. A blood vessel passes through each canal and feeds the concentric cell rings formed around it. The bone layers form from the outside in the internal bone cavity. This narrows its diameter more and more. A Haversian canal and its rings develop when cancellous bone is converted into compact bone.

Bones are either “long” or “short” bones. Most long bones are in the arms and legs. They are divided into three parts: a shaft (the diaphysis), the long central part of the bone; a flared portion at each bone end (the metaphysis); and a rounded bone end (the epiphysis). The short bones, designed for flexibility, include those in the skull, spine, hands, and feet. The centers of bones—medullary cavities—are most often filled with either red or yellow bone marrow. The yellow marrow is mostly fat. Red marrow is a network of blood vessels, connective tissue, and blood-cell-making tissue. Red blood cells (erythrocytes) are made in this red marrow. Each bone has nerves that stimulate it and blood vessels that supply nutrients and take away wastes.

Bone Composition, Development, and Remodeling

Between 66 and 70 percent of bone is an inorganic mineral composite made of calcium phosphate and calcium carbonate, which is mostly hydroxyapatite. Much of the remainder of bone is the fibrous protein collagen. Together, these minerals and proteins are called the bone matrix. Within the bone matrix are the three types of specialized cells which ensure its formation, remodeling as needed, and continuity throughout life. The first cell type, the osteoblast, produces the bone matrix and surrounds itself with it, synthesizing collagen and stimulating mineral deposition. The second cell type, the osteocyte, is a branched cell that becomes embedded in the bone matrix is interconnected and acts in the control of the mineral balance of the body. Finally, the osteoclast cells destroy the bone matrix whenever it is remodeled during skeleton growth or the repair of bone breaks and bone fractures.

The stepwise conversion of cartilage into bone begins when the chondrocytes of hyaline cartilage enlarge and arrange themselves in rows. This is followed by the synthesis of collagen fibers and mineral deposition around them. Just below the inner surface of the periosteum a vascular membrane—the perichondrium—forms and supplies the osteoblasts needed for bone formation. Simultaneously, osteoclasts excavate layers through the bone layer and set the stage for the formation of additional bone.

All the bones in the bodies of vertebrates change their sizes and shapes as these organisms pass through their lives. The processes involved are collectively called remodeling. An example of such change is the growth of the long bones in circumference as the limbs grow from puberty to adulthood. During such bone growth, the periosteum provides the osteoblasts required to deposit bone matrix around the bone exterior and to calcify it. At the same time, the endosteum-derived osteoclasts often dissolve bone in the interior, thus enlarging the marrow cavity.

Remodeling in such cases occurs in response to biosignals, including those caused by increases in the need for bone to bear additional weight or to anchor increased muscle mass. Conversely, inactivity and the lack of exercise can result in remodeling, which produces diminished bone mass. The complex changes involved in bone remodeling are also controlled by vitamin D and hormones originating in the pituitary, thyroid, and parathyroid glands. Abnormalities in bone growth and remodeling are associated with a great many bone diseases, ranging from rickets to bone cancer.

In addition to bony endoskeletons, exoskeletons, cartilaginous endoskeletons, and hydrostatic skeletons are also common in the animal kingdom. Each animal’s structural makeup supports its specific locomotion, feeding, and behavioral needs. The bones in flying fish and birds have small air pockets to allow easier flight and better oxygen intake during flight. Similarly, the endoskeletons in sharks, rays, skates, and chimeras are mostly made of lightweight cartridges, allowing them to swim quickly without exerting themselves. Fossil records indicate that ancient shark species likely had heavier bones, but this evolutionary change helped them become apex predators. Other animals’ bones and cartilage protect them, such as turtles' ribs, which evolved over time to create a bony shell. Some bones serve reproductive roles. Some chameleons, moose, and elk grow bony casques or antlers on their heads to attract a mate.

Principal Terms

Articular: Pertaining to bone joints

Bone: The dense, semirigid, calcified connective tissue that is the main component of the skeletons of all adult vertebrates

Calcification: Calcium deposition, mostly as calcium carbonate, into the cartilage and other bone-forming tissue, which facilitates its conversion into bone

Cartilage: Elastic, fibrous connective tissue, which is the main component of fetal vertebrate skeletons, turns mostly to bone and remains attached to the articular bone surfaces.

Collagen: A fibrous protein very plentiful in bone, cartilage, and other connective tissue

Connective Tissue: Any fibrous tissue that connects or supports body organs

Osteoblast: A bone cell that makes collagen and causes calcium deposition

Periosteum: The fibrous membrane that covers all bones except at points of articulation, containing blood vessels and many connections to muscles

Bibliography

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