Overview – Bone
Plate 6-1. Compact Bone: The Osteon
Plate 6-2. Bone: Remodeling And The Osteoclast
Plate 6-3. The Osteocyte
Plate 6-4. Bone Growth: The Osteon And Periosteum
Bone is marvelous material. There are few substances, natural or man-made, that can match its durable combination of strength and flexibility. The femur of an adult athlete, for example, is so strong it can bear a vertical load of nearly 2000 lbs. Other bones, such as those in the middle ear, are so delicate they can transmit infinitesimally small sound vibrations so accurately that you can hear the exquisite overtones that issue forth from a Guarneri violin.
When tested against well-known man-made materials, bone fares extremely well. It is as strong as cast iron – it has the tensile strength of cast iron and can handle compressive loads as well as cast iron can – and yet is some 20 times more flexible; you can bend a bone 20 times more than a cast iron Rod of similar size and shape before it will break. Although some new space-age materials – notably the carbonfiber derivatives such as kevlar and graphite can equal bone’s strength and flexibility per unit of mass, none of these materials can repair itself as bone can.
Bone has the properties that suit it to be our mainframe in part because of the characteristics of the Matrix. The unmineralized, organic bone Matrix, called Osteoid, consists mostly of Collagenous Fibers (95%) and associated amorphous Ground Substance (5%). The Collagenous Fibers of the Osteoid seem to “seed” crystals of calcium salts, called Hydroxyapatite crystals, which align themselves preferentially along the Collagen fibrils. Hence, mature bone consists of a framework of organic bone Matrix that becomes heavily mineralized with carefully oriented crystals of Hydroxyapatite.
Whereas Hydroxyapatite gives hardness to bone, Collagen is responsible for bone’s flexibility. A whole bone, for example, immersed for some time in a decalcifying solution, will become completely decalcified; it will retain its shape and become extremely flexible.
Just as the materials of which bone is made are important, so also is the arrangement and orientation of those materials within the bone. Most long bones, for example, are basically hollow tubes. Hollow tubes are extremely strong, almost as strong as solid rods of the same material, and much lighter, because it is the wall of the tube (or Rod) that plays the key role in resistance to strain, twist and compression. The walls of hollow long bones consist of Compact Bone – bone that contains thousands of longitudinally oriented tiny tubes called osteons within it. Figure 6-1 is a drawing of a long bone, part of a femur that has been sawn in half along its length. The figure shows that a long bone has a dense wall of Compact Bone and a hollow core of Spongy Bone. The hollow core is traversed by bony plates (called Trabeculae) and spikes (called Spicules). These bony plates and Spicules, made of Spongy Bone, are often aligned precisely along the lines of stress that pass through the bone with normal use. With disuse, these Trabeculaeand Spicules become disorganized.
The structural changes of Spongy Bone that accompany use and disuse point out one extremely important feature: bone remodels itself constantly. It has ben calculated that every calcium ion in the skeleton is replaced at least once every 20 years. That bone is conatantly being remodeled is beneficial to fracture repair. After detecting a break in its structure, bone immediately sets to work bridging the gap with new bone. This new bone, called woven bone or callus, is subsequently replaced with mature bone as remodeling continues. Bone’s lability makes it suitable as a reservoir for minerals in general and for calcium in particular. Calcium is an important ion in the biochemistry of cells, and the critical level of calcium ion in the blood that bathes those cells is maintained with the aid of the skeleton. In times of low blood calcium, bone gives up calcium to the boood. Conversely, in times of high bood calcium, bone may take up calcium and deposit it into its mineralized Matrix.
Like all connective tissue, of which bone is a derivative, bone consists of a careful combination of cells and Extracellular Matrix. The Extracellular Matrix, as has been mentioned, consists of the organic component, or Osteoid, and the mineral component, the hydrosyapatite crystals. The orgainc Matrix is secreted by special cells called osteoblasts. Osteoblasts are akin to fibroblasts in that they secrete Collagen and an associated amorphous Ground Substance. Unlike fibroblasts, however, they also secrete a substance that permits the Collagen fibrils to become encrusted with crystals of Hydroxyapatite. Once the osteoblasts are encased in mineralized Matrix, they stop making Osteoid and become known as osteocytes. Although osteocytes do not produce Matrix, their presence is necessary to maintain bone in its living condition.
Just as some cells make bone, some cells destroy bone. These cells, called osteoclasts, are large, Multinucleate, phagocytic cells that literally eat their way through bone and release its elements to the blood for further use elsewhere in the body. Working in concert, the osteoclasts, which chisel away at bone, and the osteoblasts, which lay it down, remodel bone during growth and adult life to maintain its most efficient size and shape.
Compact Bone: The Osteon
As described in the Overview, there are two major classes of bone – Compact Boneand Spongy Bone. Compact Bone, which is very strong and dense, forms the tough outer cortex of long bones. Spongy Bone, as the name suggests, is loosely organized and consists of thin Trabeculae that fill the marrow spaces in the hollow cores of long bones. This plate illustrates Compact Bone and its unit of structure, the Osteon.
The Osteon, also called the Haversian System, is an efficient structure. The Osteonnot only provides a model of the interrelationship between structure and function, but also demonstrates clearly how physical limitations of biological systems can govern the shape of the structures that evolve within those physical limits. One of the major limits that has imposed a certain geometry on Compact Bone involves diffusion. As mentioned in the chapter on cartilage, the cartilage cells, or chondrocytes, receive their nutrients by diffusion through the Matrix. Bone cells, or osteocytes, cannot do that, simply because mineralized bone Matrix is an effective diffusion barrier. Hence, whereas cartilage can be avascular, bone cannot.
The vascular component of bone, around which the Osteon is organized, is shown in Figure A at right, a thick cross section through human Compact Bone that has been ground wafer-thin on an abrasive disk. The soft tissues are gone, leaving the mineralized bone Matrix and the cavities and canals that, in life, contained the soft tissues. These cavities have been filled with India ink to provide contrast. The mineralized bone Matrix appears grayish, and the cavities in which cells and soft tissues were situated are black. The center of the field is occupied by an Osteon, whose outer limits are here traced by a dotted line.
Through the center of the Osteon runs the Haversian canal (HC), a cylindric channel that contains one or more blood vessels. Because osteocytes must exchange metabolites with the general circulation to live and since mineralized bone Matrix blocks diffusion, each Osteocyte must exist close to a blood vessel. Consequently, osteocytes take station equidistant from the blood supply in concentric rings around the Haversian canal, as shown in Figure A. Here, the holes in which osteocytes sit, called lacunae (L), are filled with India ink and appear black. Close inspection of the light micrograph will reveal that each Lacuna gives rise to a number of spiderweb-like projections called Canaliculi (arrows). In living tissue, osteocytes send out thin cytoplasmic processes that pass through the Canaliculi and touch one another. The osteoblastic processes from the innermost ring of osteocytes run centrally and contact the capillary in the Haversian canal. Consequently, metabolites can be exchanged between osteocytes and the bloodstream. Not only does this concentric arrangement of osteocytes around a central canal favor metabolite exchange, it also promotes an internal architecture of great strength. Tubes and cylinders are strong structures; it is no accident that engineers use them often in situations that call for structural strength combined with flexibility. The concentrically arranged osteocytes secrete cylindrical lamellae (layers) of bone around themselves. The lamellae, like the osteocytes themselves, are arranged in concentric layers. Furthermore, the lamellae contain organized arrays of Collagen fibrils and mineralized Matrix that vary in orientation from Lamella to Lamella, as do the overlapping, cross-grained layers in a sheet of plywood.
Although the structure of the Osteon-filled Compact Bone appears rigid and permanent, it is not. Bone is constantly being remodeled during life; osteons are built, resorbed, and replaced. A mature Osteon is outlined by the dotted line in the center of the field. Just to its left lie some interstitial lamellae (IL) – the remnants of an Osteon that was resorbed to make way for a new one.
Plate 6-1, Ground cross section of human Compact Bone stained with India ink. HC, Haversian canal; IL, interstitial lamellae; L, Lacuna; arrows, canaliculus; dotted line, perimeter of Osteon. 700 X
Bone: Remodeling And The Osteoclast
The Osteon, illustrated by light microscopy in Plate 6-1, is the fundamental unit of structure of the Compact Bone found in the cortex of large long bones. Part of an Osteon is shown at low magnification by electron microscopy in Figure A. Whereas all of the cellular elements of the Osteon are absent from the section of ground bone shown in Plate 6-1, they are present and visible in Figure A at right. Here, the Haversian canal (HC) in the center of the Osteon is lined by the thin endothelial cell of a single large capillary (C). Just outside the capillary but inside the bone (B) that surrounds the Haversian canal lie several mesenchymal cells (M), embryonic connective tissue cells that can develop into osteoblasts. One of the Mesenchyme cells in the field has recently differentiated into an Osteoblast (OB) that is actively secreting new bone Matrix (*). During the growth of bone, osteoblasts – large cuboidal cells that contain many Cisternae of the rough Endoplasmic Reticulum elaborate and secrete large quantities of Osteoid. Osteoidconsists mostly of Collagen fibers associated with a small amount of amorphous Ground Substance. Shortly after Osteoid is laid down by the Osteoblast, the Collagen fibrils within the Osteoid “seed” crystals of calcium salts that mineralize the bone Matrix and give it its characteristic hardness. Hydroxyapatite crystals appear in a mineralization front in the Osteoid near the Osteoblast. The mineralization front, evident in electron micrographs of growing bone, is indicated by an arrowhead in Figure A. In time, the mineralization front moves through the Osteoid and surrounds the Osteoblast. When the Osteoblast is encased by mineralized Matrix, it stops making Osteoid, shrinks to a shade of its former self, and becomes an Osteocyte. The Osteocyte (0), one of which is shown sitting in its Lacuna in Figure A, is connected with the blood supply (and with other osteocytes) by long, thin cytoplasmic extensions called osteocytic processes that pass through tiny channels, or Canaliculi (arrows), in the mineralized bone Matrix.
Paradoxically, as bone is being laid down in one place, it is often being removed from another. This phenomenon, essential to the continuous remodeling of bone, is shown clearly in Figure A. Here, an Osteoblast is actively secreting Osteoid that is becoming mineralized (arrowhead). While bone is being made near the center of the Osteon, however, it is being destroyed at the periphery of the same Osteon by a large phagocytic cell called an Osteoclast (OC).
The Osteoclast at the upper left corner of Figure A is shown at higher magnification in Figure B, in which part of the Haversian canal (HC) is at the bottom of the micrograph, surrounded by concentric lamellae of mineralized bone Matrix (B). At the top of the micrograph, much of the Osteon‘s bone is gone, and the region formerly occupied by bone is occupied by a very large Osteoclast (OC). The Osteoclast, which sits in a depression (arrowhead) called a Howship’s Lacuna, is a large, Multinucleate cell; three of its nuclei (N1, N2, and N3) are visible in this thin section. Osteoclasts secrete hydrolytic enzymes that dissolve bone; the elements dissolved from the mineralized Matrix are taken into the Osteoclast‘s Cytoplasm and released into the bloodstream. It is interesting to note that Parathyroid Hormone mobilizes osteoclasts. Consequently, bone resorption not only contributes to the remodeling of bone, but also serves to elevate blood calcium concentrations.
Plate 6-2, Figure A. Electron micrograph of an Osteon in the femur of the squirrel monkey. B bone; C, capillary; HC, Haversian canal; L, Lamella; M, mesenchymal cell; O, Osteocyte in Lacuna; OB, Osteoblast; OC, Osteoclast; *, Osteoid made by Osteoblast; arrows, Canaliculi containing osteocytic process; arrowhead, mineralization front. 3,500 X Figure B. Enlarged photograph of Osteoclast present in Figure A. B, bone; HC, Haversian canal; N1, N2, and N3, nuclei of Osteoclast; OC, Osteoclast; arrowhead, Howship’s Lacuna in which Osteoclast sits. 5,200 X
When osteoblasts stop secreting Osteoid and become completely surrounded by mineralized bone Matrix, they are known as osteocytes. Although osteocytes do not produce Matrix, they seem to be essential for the maintenance of bone, and stand at the ready to act as osteoblasts once again should the need arise.
Figure A, an electron micrograph of a monkey femur, shows an Osteocyte (O) sitting in its Lacuna within a mass of mineralized bone Matrix (B). The collagenous nature of bone is apparent at the perimeter of the Lacuna (LC), where the characteristic cross-banded pattern of Collagen (*) is visible in Collagen fibrils oriented parallel to the plane of section. The ultrastructure of this Osteocyte, like that of most osteocytes, suggests that it is an inactive, resting cell. Its centrally located Nucleus (N) is filled with densely stained Heterochromatin, indicating that most of the genetic material is supercoiled and not active in Transcription of messenger RNA. The scanty Cytoplasm is devoid of rough-surfaced endoplasmic reticulum, so prominent in the Osteoblast, and the Osteocyte has adopted a stellate shape. The cell body sends out many arms called osteocytic processes (OP) that tunnel through tiny Canaliculi (arrows) in the mineralized bone Matrix. It is through these osteocytic processes that the Osteocyte takes in nutrients and sends out wastes. The processes from the osteocytes in the innermost lamellae of a given Osteon extend into the Haversian canal and are available to exchange metabolites directly with the capillary (or capillaries) that course through the canal. The osteocytes at the periphery of the Osteon, located some distance from the blood supply of the Haversian canal, send out osteocytic processes that make direct contact with osteocytic processes from other, more centrally located osteocytes. Presumably, metabolites are then transferred along the chain of osteocytes in a “bucket-brigade” fashion.
The cellular communication between Osteocyte and capillary is shown clearly in Figure B. A Haversian canal (HC), shown at intermediate magnification, has two capillaries – a large, central capillary (C1) and a small, peripheral one (C2). Because the specimen shown here was fixed by intravascular perfusion, the blood cells have been washed out of the vessels and are not evident in the image. If you look at the periphery of the Haversian canal where the soft tissue meets the bone (arrowhead), you will see several places where osteocytic processes (OP) enter the connective tissue lining the canal itself (arrows). At that site, the canaliculus flares out like the mouth of a funnel. The osteocytic process seems to branch and sends out lateral extensions that form a ring around the perimeter of the canal, thereby greatly increasing the surface area of the cell available for metabolite exchange (see area near *).
Plate 6-3, Figure A. Osteocyte within Osteon of monkey femur. B, mineralized bone Matrix; LC, Lacuna; N, Nucleus; O, Osteocyte; OP, osteocytic process; arrows, Canaliculi; *, cross-banded Collagen fibrils in bone Matrix. 17,300 X Figure B. Cross section through Haversian canal in monkey femur. C1, C2, capillaries; HC, Haversian canal; L, Lamella of Osteon; OP, osteocytic process; *, region where osteocytic process enters Haversian canal and branches; arrows, point of entry of osteocytic process into Haversian canal; arrowhead, perimeter of Haversian canal where soft tissue and Osteoid meet mineralization front of bone Matrix. 5,000 X
Bone Growth: The Osteon And Periosteum
Compact bone typically grows by apposition, which occurs by the secretion of material in successive layers, much as a mason lays a brick wall. Appositional growth occurs within osteons, which are the structural units located within thick regions of Compact Bone, and beneath the Periosteum, a layer of dense connective tissue that covers the outside of Compact Bone.
Some imagination is needed to understand the way in which appositional growth occurs within the Osteon. For example, in the Haversian System (or Osteon) depicted in Figure A, the Haversian canal (HC) is in the center surrounded by several concentric lamellae of mineralized bone Matrix, numbered L1, L2, L3, and L4. Of these, the outermost Lamella, L4, is the oldest; the innermost Lamella, L1, is the youngest. The innermost Lamella, L1, was being laid down when the specimen was fixed. Consequently, several osteoblasts shown here – O and O’ – were “caught in the act” while building a Lamella. On the outside, these osteoblasts are surrounded by mineralized bone Matrix (MB). On the inside, however, they are lined by Osteoid (OS), which the osteoblasts have just secreted. Even at this low magnification, the Collagenous Fibers, of which Osteoid is largely composed, are readily visible. These osteoblasts, then, have been photographed at a critical stage in their life cycles – a stage in which they are about to cease being osteoblasts, which actively secrete Osteoid, and become osteocytes, which do not. When these cells become surrounded on all sides by mineralized bone Matrix, they will be mature osteocytes – cells that maintain contact with soft tissue only by virtue of their osteocytic processes (arrows). Osteocytes become imprisoned in a penitentiary built of their own secretions.
One might wonder, knowing that mineralized bone Matrix is hard and solid, how the outermost Lamella, L4, got pushed out from the secretory osteoblasts of the Haversian canal. The answer is simple: it did not. When the Osteon was young, its Haversian canal was huge, the same diameter as the outermost Lamella. When the outermost Lamella was formed, much as L1 is now being formed in Figure A, its osteocytes became surrounded by a ring of bone, the Haversian canal became smaller in diameter, and new osteoblasts differentiated from mesenchymal cells and made a smaller ring of bone, L3, just inside the oldest, outermost Lamella. As successive layers of bone were laid down in this manner, one inside the other, the Haversian canal became smaller and smaller until it reached its present size as shown in Figure A. Whereas trees grow with the oldest annual rings in the center of the trunk, osteons grow in exactly the opposite way – their oldest lamellae are on the outside.
The appositional growth that occurs on the Periosteum is much simpler to understand. The outer surface of most long bone is covered by Periosteum, a tough membrane of dense connective tissue. The Periosteum is to compact bone as the Perichondrium is to cartilage. The Periosteum (P), shown by electron microscopy in Figure B, contains Collagenous Fibers and connective tissue cells (CT). In addition, the inside of the Periosteum – the side that abuts against the bone – contains osteoblasts (O). These osteoblasts secrete Osteoid that becomes mineralized to form mineralized bone Matrix (MB). As in the Haversian System, the osteoblasts, when encased within their own mineralized secretions, become osteocytes (OC). In this way, Compact Bone (B) increases in thickness at the Periosteum.
Plate 6-4, Figure A. Cross section through developing Osteon of monkey femur. B, bone; HC, Haversian canal; L1, L2, L3, and L4, lamellae of bone; MB, mineralized bone Matrix; O, O’, osteoblasts; OS, Osteoid; arrows, osteocytic process. 4,800 X