Histology Text Atlas Book
Contents (Click on desired chapter)
Contents | Introduction | Cells | Epithelia | Connective Tissue | Blood | Cartilage | Bone | Muscle | Nerves | Skin | Circulatory System | Respiratory System | Oral Cavity | Alimentary Canal | Pancreas Liver And Gallbladder | Urinary System | Immune System | Male Reproductive System | Female Reproductive System | Endocrine System | The Senses |Appendix | Glossary
CHAPTER 6: Bone
Overview
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 Trabeculae and 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.
Plate 6-1
Compact Bone: The Osteon
As described in the Overview, there are two major classes of bone - Compact Bone and 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 Osteon not 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.
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
Plate 6-2
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. Osteoid consists 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.
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
Plate 6-3
The Osteocyte
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 *).
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
Plate 6-4
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.
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
Figure B. Electron micrograph of Periosteum of mouse toe. B, bone; CT, connective tissue; MB, mineralized bone Matrix; O, Osteoblast; OC, Osteocyte; P, Periosteum. 2,800 X
Plate 1-4
The Ovarian Endocrine Cell: A Steroid Factory
The cells we have examined so far-the pancreatic acinar cell, the Paneth cell, and the Goblet Cell-all actively synthesize proteins, protein-carbohydrate complexes, or both. The subject of Plate 1-4, an endocrine cell from the Ovary, is quite different: this cell synthesizes and secretes Steroid hormones. Steroid hormones, constructed from the Cholesterol molecule, are more akin to fats than to proteins or polysaccharides. Possessed of a 17-carbon, 4-ring system, Steroid hormones-of which there are many kinds in the body-are made by cells using Cholesterol as starting material.
Since the ovarian endocrine cell makes a product different biochemically from the products of the cells examined thus far, one would correctly predict that the ovarian endocrine cell's microanatomy would be different as well. In Plate 1-4, this cell, shaped like a long, slender football, is found in the Theca interna of a growing Follicle in the Ovary, where it secretes a Steroid hormone that is a precursor of the female sex hormone, Estrogen. The cell is not filled with stacks of rough ER; instead, its Cytoplasm contains an abundance of the smooth Endoplasmic Reticulum (SER). As described in the overview of this chapter, the smooth ER consists of a series of branched, interconnected, membrane-limited tubules. The smooth ER, so named because its membranes are Ribosome free (hence "smooth"), contains many enzymes necessary for Cholesterol biosynthesis. Consequently, cells that make Cholesterol, steroids or both usually have a well-developed smooth ER. The Cytoplasm of the cell in Figure A contains many large, spherical, electron-lucent lipid droplets (L). These lipid droplets are filled with Cholesterol, the precursor of the Steroid hormones made by the cell. In addition, the cell has many large, strange-looking Mitochondria (M). These Mitochondria, like those of most Steroid-secreting cells, have vesicular or tubular Cristae. These Mitochondria possess an Enzyme that participates in the conversion of Cholesterol to Steroid hormones. The relationship between the special arrangement of the mitochondrial inner membranes and steroidogenesis, however, is unknown.
The ovarian endocrine cell, then, has three ultrastructural features that are pronounced in and characteristic of Steroid-secreting cells: a well-developed smooth Endoplasmic Reticulum, large Mitochondria with tubular Cristae, and an abundance of lipid droplets in the Cytoplasm.
The Nucleus (N) of the ovarian endocrine cell looks different from the nuclei we have seen thus far because the Nucleus at right has been caught in tangential section; that is, the knife grazed the edge of the Nucleus instead of passing through its center. (A Nucleus cut in cross section [N'] is evident in another cell at the bottom of the plate). The tangential section through the Nucleus reveals the structure of nuclear pores (arrowhead). The Nucleus, as you recall from the overview, is surrounded by the Nuclear Envelope, which consists of two sets of membranes that are continuous with the Endoplasmic Reticulum. At intervals around its perimeter, the Nuclear Envelope is perforated by openings, the nuclear pores, that facilitate exchange of materials between the Nucleus and the Cytoplasm. A cross sectioned Nuclear Pore is encircled in the Nucleus (N') in the cell at the bottom of the figure.
Several red blood cells, or erythrocytes (E), are evident in the capillary that runs right next to the ovarian endocrine cells. Endocrine organs, which release their product into the bloodstream, are almost invariably endowed with a rich supply of capillaries.
Longitudinal section through a Steroid-secreting endocrine cell in the Theca interna of a growing Follicle in the Ovary. E, Erythrocyte; L, lipid droplet,- M, mitochondrion; N, tangentially sectioned Nucleus; N', Nucleus cut in cross section; SER, smooth Endoplasmic Reticulum; arrow, free ribosomes in clusters (polysomes); arrowhead, tangentially sectioned Nuclear Pore; circle, cross sectioned nuclear pore. 13,200 X
Plate 1-5
The Osteocyte: A Quiescent Cell
Thus far, we have investigated the ultrastructure of cells that are engaged in the large-scale production of macromolecules. Although somewhat different from one another in fine structure, all of these cells share several common characteristics; they are large cells possessing an extensive Cytoplasm equipped with a wide array of organelles related to the biosynthesis and storage of secretory products. The Osteocyte illustrated at right, however, is strikingly different from the cells depicted previously in this chapter. Seen here as a small cell in a large field of bone (B), the Osteocyte (0), which sits in the center of the field, looks unremarkable. The most conspicuous feature is the round Nucleus (N), which consists mostly of Heterochromatin (*).
Heterochromatin, which represents dense aggregates of DNA and protein that stain darkly, is made up of portions of chromosomes that are coiled and not transcriptionally active; that is, they are not engaged in the Transcription of messenger RNA from the genetic material, DNA. Cells with large amounts of Heterochromatin are usually relatively inactive in terms of protein synthesis, and this Osteocyte is no exception. In the metabolic heyday of this Osteocyte, when it was young and vigorous, the cell had an extensive Cytoplasm, packed with Ribosome-studded Cisternae of the rough encloplasmic reticulum and replete with stacks of Golgi membranes. At that time, the cell, then called an Osteoblast, produced prodigious amounts of Collagen, a fibrous protein that makes up the bulk of the connective tissue in the body, and provides the framework for the mineralized Matrix of bone. The Osteoblast laid down Collagen until it painted itself into a corner and encased itself in the very product of its own secretory activity. Once imprisoned in calcified bone Matrix (B), the cell disassembled most of its organelles, resorbed the better part of its own Cytoplasm, and went into retirement, assuming the shrunken form of the Osteocyte in Plate 1-5. The Osteocyte, now but a shade of its former self, lies within a Lacuna (L). The entire Lacuna was once filled with the turgid Cytoplasm of the active Osteoblast. Now, the Lacuna is crossed by only a few strands of Cytoplasm. These strands of Cytoplasm (arrows), called osteocytic processes, pass through tiny channels in the bone Matrix called Canaliculi (C). The Osteocyte processes provide the living link between neighboring osteocytes and nearby capillaries that permit the osteocytes to receive the few nutrients they need to carry on their quiescent life, during which they serve to maintain bone.
Mature osteocytes, although dormant, are necessary to keep bone alive. In addition, they stand at the ready to be recalled into active service should the need arise. In times of low blood calcium, they can generate an active Cytoplasm and resorb needed calcium from bone. In addition, they can serve to rebuild bone lost from injury or disease. The Osteocyte at right, however, is doing no such thing; it is a dormant cell, and its structure betrays its biosynthetic quiescence.
Cross section taken from Compact Bone in the femur of the squirrel monkey showing an Osteocyte sitting in a field of bone. B, calcified bone Matrix; C, canaliculus; L, Lacuna; N, Nucleus of Osteocyte; O, Cytoplasm of Osteocyte; arrow, cytoplasmic extension of Osteocyte (Osteocyte process); *, Heterochromatin. 13,200 X
Plate 1-6
The Cytoplasm As Cell Product:Blood And Muscle Cells
Thus far in this chapter, the cells that we have examined have been "factories" that make a variety of substances-largely for export out of the cell - ranging from pancreatic enzymes to bone Matrix. This plate shows two kinds of cells that are radically different. They make proteins, to be sure, and synthesize them in very large amounts. But the proteins they make, instead of being exported outside the cell for use elsewhere, are retained inside the Cytoplasm for use by the cell itself. The Erythrocyte, or red blood cell, is a classic example of a cell in which the Cytoplasm is the cell product. A red blood cell is targeted toward one central function - the binding and release of molecular oxygen. So single in purpose is the Erythrocyte that, in the early stages of its development, its Cytoplasm is dedicated almost entirely to the synthesis of the oxygen-binding pigment Hemoglobin. The red blood cell is so good at Hemoglobin synthesis that, in the mature Erythrocyte, Hemoglobin comes to replace all of its organelles - even the Nucleus. The fully formed red blood cell is, in a manner of speaking, a crystal of Hemoglobin surrounded by a Plasma membrane.
Figure A at right illustrates this phenomenon quite clearly. Here we see a capillary (C) in the Myocardium of the heart passing between two cardiac muscle cells (MC) (also called cardiac muscle fibers). Within the capillary, a number of erythrocytes (E) are present. Each Erythrocyte is shaped like a biconcave disk. When cut in different planes of section, erythrocytes can display a variety of shapes, ranging from figure eights to doughnuts. Close examination of each Erythrocyte in Figure A will reveal that its Cytoplasm, devoid of detectable organelles, is filled with a dense, homogeneous Matrix, Hemoglobin. The Hemoglobin is wrapped in a Cell Membrane (too thin to be seen at this magnification), which is absolutely vital to the viability of the red cell. The Cell Membrane contains, among other things, a number of enzymatic pumps that act to prevent the highly hypertonic cell from literally exploding in the bloodstream.
Another classic example of a cell that fills itself with its own product is the Skeletal Muscle cell (usually called a Skeletal Muscle Fiber). The cell synthesizes the proteins Actin and Myosin, often (and somewhat misleadingly) called contractile proteins. The Skeletal Muscle Fiber uses highly ordered arrays of Actin and Myosin to achieve its major function, muscle contraction - a powerful shortening of the cell along its long axis. It is no accident, then, that the Skeletal Muscle Fiber fills its Cytoplasm with ordered arrays of Actin and Myosin filaments. Consequently, when one views a microscopic image of a Skeletal Muscle Fiber, one sees little other than a mass of ordered myofilaments.
Figure B is an electron micrograph of a small part of a Skeletal Muscle Fiber taken from the quadriceps (thigh) muscle of a marathon runner. Here, the most conspicuous elements are the myofibrils (MF)-long, cylindric units filled with Actin and Myosin filaments. In many muscle fibers, few organelles are visible. In this section, however, many Mitochondria (M) are present. These organelles, as you know, provide the ATP essential for muscle contraction. In addition to Mitochondria, large accumulations of Glycogen (G) are present in the space beneath the Cell Membrane (arrow), between the myofibrils, and even between the myofilaments within the myofibrils. The tremendous amount of intracellular Glycogen in this particular section is unusual, resulting from the procedure of "Glycogen loading" commonly used by endurance athletes (such as the marathon runner who donated this tissue), who ingest large amounts of carbohydrates prior to an athletic event to provide high-energy fuel for thousands of cycles of muscular contraction and relaxation.
Figure A. Electron micrograph of a capillary in the monkey heart. C, capillary wall; E, Erythrocyte; MC, Cardiac Muscle Fiber. 5000 X Figure B. Electron micrograph of a longitudinal section through a Skeletal Muscle Fiber from the thigh muscle of a marathon runner. G, Glycogen; M, mitochondrion; ME, Myofibril. 35,000 X
Plate 1-7
Stages In The Life Of A Cell: The Lymphocyte And The Plasma Cell
The cells that we have observed up to this point have been mature cells. Although it is tempting to think that these cells, frozen in time by electron microscopy were always as they appear today, it simply is not so. Cells, like the very people they compose, have cycles of life and may adopt quite different structures to suit their stage of life at any given time.
The cells illustrated at right look quite different from one another. Figure A depicts a Lymphocyte in the circulating blood; Figure B displays a Plasma cell in the connective tissue.
The Lymphocyte (L) in Figure A is a small, round cell. Its Nucleus (N), which contains a prominent Nucleolus (Nu), occupies most of the space in the cell. The scanty Cytoplasm contains a few Mitochondria (M), a small Golgi stack (G), some free ribosomes, and a well-developed Centriole (arrow). The Centriole, which contains a well-organized array of three triplets of microtubules in its core, is shown at higher magnification in the inset. (Centrioles are found at the poles of the mitotic spindle of dividing animal cells, provide the basal bodies found at the bases of motile cilia, and often - as in the Lymphocyte shown here - occupy the cell center in interphase cells). The Lymphocyte, flanked on one side by an Erythrocyte (E), is in close contact with the capillary wall (C).
The Plasma cell (P) shown in Figure B is dramatically different from the Lymphocyte. The Plasma cell produces antibodies, immunoglobulins that combine with foreign antigens and, in so doing, provide the first line of defense in the immune response. This particular Plasma cell was found in the loose connective tissue of the Lamina Propria of the small intestine. In this electron image, it occupies a position between another Plasma cell (P') and a small nerve bundle (NB). The Cell Membrane of the Plasma cell is in close contact with connective tissue fibrils (CT). Its Cytoplasm contains an extremely well developed system of rough Endoplasmic Reticulum (RER). Numerous Mitochondria (M) are present. A prominent Golgi stack (G) sits atop the large Nucleus (N). All of these ultrastructural features underline the fact that the Plasma cell is active in the biosynthesis of proteins for export, proteins that take the form of antibodies.
Based on the relationship between structure and function in cells, the Plasma cell appears to be a cell of high metabolic activity engaged in intense protein synthesis, whereas the Lymphocyte does not. These cells that look so different from one another, however, are actually different forms of the same cell. Certain kinds of lymphocytes, called B-lymphocytes, represent the immature form of Plasma cells. The B-Lymphocyte is, in a sense, the transport form of the Plasma cell. B-lymphocytes do not perform their functions in the bloodstream; they are not really blood cells at all, but connective tissue cells that simply use the bloodstream to get from their birthplace in the bone marrow or lymphoid tissues to their ultimate destination in the connective tissues. Under appropriate conditions, the B-Lymphocyte will leave the circulation, enter the connective tissues, and develop into a mature, full-blown, Antibody-producing Plasma cell such as the one shown in Figure B.
Figure A. Electron micrograph of a Lymphocyte in a capillary in the lung of the macaque. C, capillary wall; E, Erythrocyte; G, Golgi Apparatus; L, Lymphocyte; M, mitochondrion; N, Nucleus; Nu, Nucleolus; arrow, Centriole. 16,800 X (Inset of Centriole, 60,500 X) Figure B. Electron micrograph of a Plasma cell in the Lamina Propria of the intestine. CT, connective tissue fibrils; G, Golgi Apparatus; M, mitochondrion; N, Nucleus; NB, nerve bundle; P, Plasma cell; P', adjacent Plasma cell; RER, rough Endoplasmic Reticulum. 15,000 X
Plate 1-8
The Ciliated Cell: Specializations Of The Cell Surface
The cell surface, as described in the overview to this chapter, is an interface between the cell and its surroundings. In many ways, a cell is akin to a sessile organism; it is at the mercy of its environment. As a result, many cells have evolved intricate modifications of the cell surface that interacts with their immediate environment. Many of these modifications exist at the molecular level and cannot be seen. Other modifications of the cell surface, such as cilia and Microvilli, are complex, highly efficient structures that are not only visible by light and electron microscopy, but also present in sufficient numbers to be available for experimental investigation.
The nasal cavity is lined by tissue that is directly exposed to the atmosphere. It produces copious amounts of Mucus that serves to lubricate the surface of the nasal cavity, prevent it from drying out, and entrap foreign particles present in the air. To prevent chronic congestion, the Mucus must be removed as fast as it is produced, by either swallowing or expectorating. To facilitate removal, the conductive airways of the respiratory system are equipped with motile cilia, whose coordinated beating moves the blanket of Mucus upwards toward the mouth. The plate is an electron micrograph of a ciliated cell within the epithelium of the human nasal cavity. The tall, columnar cells have prominent nuclei (N) at the basal pole of the cell and a conspicuous Golgi Apparatus (G) in the center. Clusters of Mitochondria (M) fill the apical pole of the cell. These Mitochondria are well positioned to produce ATP as an energy source for the motile cilia nearby. Cilia (C) project upward from the cell surface into the nasal cavity (NC), in which they move the layer of Mucus (removed during tissue preparation) toward the oral cavity. Each Cilium, actually a mechanochemical engine fueled by ATP, inserts into a Basal Body (B), a Centriole-like structure that sits just beneath the cell surface. The ciliary shaft, a small structure measuring 0.2 Â µm in diameter, is stabilized by a somewhat stiff Axoneme.
When seen in longitudinal section at low magnification, as in Plate 1-8, the Axoneme appears as a set of dense lines that run parallel to the ciliary long axis. When cut in cross section and viewed on end at high magnification, as in the inset, the ciliary Axoneme presents a striking and distinct ultrastructure. The Plasma membrane, here displaying its typical trilaminar "railroad-track" image, covers the outside of the Cilium. Within the space enclosed by the Plasma membrane lies the Axoneme, which has the "9 + 2" arrangement of microtubules typical of motile cilia. Nine outer doublets of microtubules, arranged in a ring just inside the Plasma membrane, surround a central pair of microtubules in the core of the Axoneme. Close inspection of the image will reveal that several electron-dense "arms" extend from each outer doublet toward its neighbor in a clockwise direction. These structures, called dynein arms (arrow, inset), provide the force for ciliary movement. Movement of the arms causes adjacent outer doublets to slide past one another. This sliding is restrained by a set of Radial Spokes (arrowhead, inset), not clearly shown here, that extend from the central pair to the outer doublets. The restraining force of the Radial Spokes, when set against the active sliding of adjacent doublets generated by the dynein arms, transduces the sliding movement into a bending movement - the very bending movement associated with the active stroke of the motile Cilium. The coordinated beating of the cilia of the respiratory epithelium moves the blanket of Mucus along the surface of the nasal cavity at an astonishingly rapid rate.
Electron micrograph of a longitudinal section through the epithelium lining the human nasal cavity. B, Basal Body; C, Cilium; D, degenerating cell; G, Golgi Apparatus; M, mitochondrion; N, Nucleus; NC, nasal cavity. 11,500 X Inset: Cross section through motile Cilium from the same epithelium. Arrow, dynein arm; arrowhead, radial spoke. 131,000 X