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
This is a book about cells and tissues. Its primary objective is to build a series of visual three-dimensional images of the cells and tissues that make up the human body. This particular chapter, headed by the all- encompassing title "cells," is intended to prepare you to recognize and understand the images of cells and tissues, photographed by light and electron microscopy, that are presented in this atlas.
The number and variety of cells within every person is tremendous. Fortunately, the Herculean task of visualizing the complexity of the very cells of which we are made is greatly simplified when you realize that many cells, despite their dramatic differences in structure and function, are really more alike than not: they are variations on a theme. And what is that theme? Simply this: cells are designed to generate order out of chaos. We are surrounded by chaos. Biologists are wont to refer to chaos as Entropy - the concept, described in the second law of thermodynamics, that everything tends toward disorder. Entropy pervades our lives. Clean clothes, for example, don't happen on their own. The act of wearing and using clothes soils them. Does the pile of dirty clothes that results from a week's hard use suddenly appear in pristine form, washed, folded, and stacked in neat piles in your dresser drawers come Monday morning? Of course not. You painstakingly gather the heap of soiled clothes at the end of the week, put them in the washer, dry them, press them, fold them, carry them to the dresser, sort them, and put them neatly away. All of which takes energy. It takes energy to generate order out of chaos.
In the example given above, we are dealing with several pairs of pants or skirts, blouses or shirts, a few assorted undergarments, and some socks - maybe 50 items in a busy week. A "typical" mammalian cell has about ten billion protein molecules to look after. Since everything tends toward disorder, and numbers compound the problem exponentially, the entropic possibilities faced by a cell during the course of its daily life are bewildering. How do cells deal with Entropy? How do they generate such exquisite order in the face of potential molecular chaos? In microanatomic terms, cells accomplish order by means of beautifully bioengineered components that, at the expense of considerable amounts of energy, see that the right molecules get in the right places at the right times. All of which is a formidable logistic problem. How do cells do it?
That is, in large part, what this book is all about. A look at the microscopic anatomy of the cell can help us to understand how cells generate order from chaos and, by so doing, achieve that most precious quality - life. When you look at a cell with the light microscope, the first thing you are likely to see is the Nucleus - alarge, round, dark-staining body that contains the genetic material. In many cells, the Nucleus appears to be suspended in a small sea of Cytoplasm, a pale-staining Matrix that often contains small, blurry objects visible only when stained. The Cytoplasm is surrounded by an outer limiting membrane called the Cell Membrane (also plasmalemma or Plasma membrane). Unfortunately, the Cell Membrane, which measures only 80 â ‰ ˆ wide, is much too small to be seen with the light microscope (whose limit of resolution is 2000 â ‰ ˆ). This inability is a source of confusion to beginning students of microanatomy; if you can't see a cell's boundaries, you can't see where one cell ends and another begins. What you often see under the light microscope, then, is a gaggle of purple nuclei in a field of amorphous material. What can that tell about the organization of cells and tissues? How does that provide any insight into the ways in which cells generate order from chaos?
Enter the electron microscope. The electron microscope, which has the ability to resolve very small (2.5-A) objects, has allowed us to see that the Cytoplasm is not a field of amorphous material at all, but rather a highly organized system of organelles and inclusions. Once you see a number of electron images of a particular cell, you will rapidly recognize that kind of cell, much as you learn to recognize a particular make of automobile among a spate of others on a busy highway. This kind of sight-recognition not only allows you to distinguish specific cells in electron images, it allows you to mentally superimpose what you've seen in the electron microscope upon similar cells when you look at them with the light microscope. In the following chapters, you will look at a variety of cells as seen with the light microscope and with the electron microscope at similar magnifications. By carefully comparing the light and electron images of the very same cells, you will develop a kind of "x-ray vision" that will allow you to skillfully interpret light-microscopic images that once looked like little more than a group of nuclei in a fuzzy field.
It is first helpful to look at the kinds of structures you're likely to see within cells. In order to build up a "visual vocabulary" of cell components, this overview contains two illustrations: a drawing of a "typical" mammalian cell (Figure 1-1) and a low-magnification electron micrograph of a cell from the monkey pancreas (Figure 1-2). By referring to the image of specific structures in both the drawing and the electron micrograph as they are discussed, you can develop a good mental picture that will provide a basis for recognition of these same structures as they are encountered throughout the atlas.
Starting with the outside of the cell, the first structure is the Cell Membrane. Called by a number of names, including the Plasma membrane, plasmalemma, and outer limiting membrane, the Cell Membrane is crucial to a cell's function because it is the interface between the outside world and the inside of the cell; the Cell Membrane lies between the order within the cell and the potential disorder without.
The composition of the membrane surrounding a particular cell can vary dramatically from region to region. In addition, the cell membranes surrounding different kinds of cells can be different from one another. Referring to "the Cell Membrane" as a unit can be misleading because doing so implies that the Cell Membrane is a single entity. This erroneous notion of the homogeneity of the Cell Membrane is unfortunately reinforced by images generated by transmission electron microscopy of sectioned material. The electron images of a variety of cellular membranes look quite similar.
A "typical" Cell Membrane, for example, is shown in the electron micrograph in Figure 1-2. This illustration is a relatively low magnification electron micrograph of a cell from the pancreas of the squirrel monkey. If you look at the region indicated by the arrow, you will see a pair of dark lines where two cells are adjacent. These dark lines represent the cell membranes of two neighboring cells. At higher magnification, as shown in the inset, each Cell Membrane looks like a set of railroad tracks: that is, each Cell Membrane looks like two electron-dense lines separated by a clear interspace. This appearance led to the name the Unit Membrane, which refers to the electron image presented by the Cell Membrane and the membranes that surround the cytoplasmic organelles when viewed in cross-section by conventional transmission electron microscopy. The uniform appearance of the membranes surrounding the cell and its organelles is misleading. Mebranes vary tremendously in structure and function. To be sure, membranes share many similarities in fundamental organization; they are bimolecular lipid leaflets that contain proteins. But the Lipids and the proteins can vary considerably in composition, assume a variety of configurations, and perform a variety of functions. The membranes in the myelin sheaths surrounding the axons of nerves, for example, are effective electrical insulators, whereas the cell membranes of proximal tubule cells of the kidney are highly efficient ion pumps.
Looking back at the diagram (Figure 1-1) and electron micrograph (Figure 1-2), the cell is seen to contain a number of organelles that are surrounded by membranes. These organelles include Mitochondria (M), the rough Endoplasmic Reticulum (RER), the smooth Endoplasmic Reticulum (SER), the Golgi Apparatus (G), Secretory Granules (S), lysosomes (L), and the Nucleus (N). Why are so many organelles surrounded by membranes? Different parts of the cell must perform different functions, and membranes provide a superb means for compartmentalization within the cell. Membrane limited organelles may be thought of as compartments that can move about from one region of the cell to another. In addition, each cell contains on the order of ten billion protein molecules. Many of these proteins are enzymes that catalyze biochemical reactions, which depend upon surface contact between the participants in the reaction. Membranes not only provide a tremendous amplification of surface area within the cell, they contain specific enzymes. The specificity of molecular interactions that occur in enzymatically catalyzed biochemical reactions, then, can be greatly enhanced by the presence of membranes within cells.
The mitochondrion provides an excellent example of an Organelle that uses membranes to perform exquisite biochemical maneuvers.
Often - and quite appropriately - referred to as "the power plant of the cell," the mitochondrion contains two sets of membranes: an outer membrane, which defines the outer limits of the Organelle, and an inner membrane, which is folded into little baffles called Cristae. Mitochondria produce ATP, the chemical energy "currency" of the cell, in large quantities. Cells with high energy requirements usually have many Mitochondria. Cells with very high energy requirements usually have Mitochondria that contain many Cristae. The membranes of the Cristae contain arrays of enzymes associated with oxidative phosphorylation, one of the essential phases of ATP production. Increasing the number of mitochondrial Cristae vastly amplifies the amount of membrane surface available for the enzymes involved in the process of oxidative phosphorylation.
The Endoplasmic Reticulum comes in two morphologically distinct varieties: rough and smooth. The rough-surfaced Endoplasmic Reticulum, usually called the rough Endoplasmic Reticulum or the rough ER, consists of a series of interconnected, flattened, membrane-limited sacs (called Cisternae) in which the membranes are encrusted with ribosomes. Ribosomes, which have the electron-microscopic appearance of small dense dots, are the sites of protein assembly in cells. Consequently, the rough ER, being a system of membranes and attached ribosomes, participates in the synthesis and concentration of proteins.
The smooth Endoplasmic Reticulum, which lacks ribosomes, is quite different. It is organized into a system of interconnected tubules and is associated with a variety of functions such as Glycogen metabolism, Steroid synthesis, and enzymatic detoxification of noxious substances. Ultimately, the rough and smooth ER are physically interconnected and should be thought of as different manifestations of a common system of intercellular membranes.
Fig. 1-2. Electron micrograph of a thin section taken through an exocrine cell of the monkey pancreas. G, Golgi Apparatus; L, lumen of Acinus; M, mitochondrion; N, Nucleus, Nu, Nucleolus; RER, rough Endoplasmic Reticulum; S, secretory granule; S', secretory granule pouring its contents into lumen of Acinus; arrow, pair of Plasma membranes of two adjacent cells. 12,000 X. Inset: high-magnification electron micrograph of region indicated by arrow in which two Plasma membranes, running parallel to one another, are cut in cross section; micrograph shows the trilaminar appearance of each of the two Plasma membranes. 129,000 X
The Golgi Apparatus
The Golgi Apparatus, named after a turn-of-the-century Italian anatomist who had a tremendous impact on biology, is a complex system of membrane-limited sacs and vesicles that is concerned with the modification and packaging of proteins and protein-Polysaccharide complexes. Often working in concert with the rough ER, the Golgi Apparatus receives material elaborated by the rough ER, chemically modifies it with enzymes in the Golgi membranes, and concentrates and packages the new product within membrane-limited vesicles called Secretory Granules. In addition, the Golgi can package proteins into membrane-limited vesicles, such as lysosomes, for use within the cell itself.
Lysosomes are membrane-limited organelles that contain a broad spectrum of vicious hydrolytic enzymes capable of breaking down everything from nucleic acids to proteins to fats. Originally called "suicide bags" because early cell biologists surmised the cell could open its lysosomes, release their contents, and rapidly dissolve itself when "its number was up," lysosomes serve a variety of essential functions. For one thing, cells use lysosomes to dispose of worn-out organelles. In addition, specialized cells such as macrophages use lysosomes in the intracellular destruction of ingested foreign materials such as bacteria. Other cells, such as endocrine cells of the pituitary gland, use lysosomes to digest excess product synthesized by the cell that is not needed at the time.
The Nucleus, which contains the genetic material, is surrounded by a double membrane continuous with the Endoplasmic Reticulum. Consequently, the membranes surrounding the Nucleus, called the Nuclear Envelope, represent a perinuclear cisterna of the Endoplasmic Reticulum. The Nuclear Envelope is perforated by nuclear pores, small openings that permit the vital exchange of materials between Nucleus and Cytoplasm. The Nucleus contains the chromosomes - discrete units of DNA, the genetic material, complexed with protein-visible only when the cell is in the midst of Mitosis, or cell division. At other times, the chromosomes are less condensed, and their strands are woven into an indecipherable tangle within the nucleoplasm called Chromatin. When the Chromatin is somewhat condensed, meaning that the genetic material is not unwound and thus is not available for "translation" of the genetic code into messenger RNA (which later dictates the sequence of amino acids that are strung together to make protein), the Chromatin stains darkly. This clumped, nontranscriptionally active Chromatin is called Heterochromatin. Transcriptionally active Chromatin, which takes little stain and thus looks pale, is called Euchromatin. A glance at the Nucleus, then, can determine whether a given cell is likely to be active in the Transcription of messenger RNA. If it is pale and has a great deal of Euchromatin, it probably is active; if it stains darkly and has a great deal of Heterochromatin, it probably is not.
The membrane-limited organelles briefly described above, and others as well, will be encountered frequently in the atlas. A number of other cytoplasmic components that are not enveloped by membranes should be mentioned in this overview.
Microtubules, as the name suggests, are tiny tubules that have an electron-dense wall and a clear center. Measuring only 240 â ‰ ˆ in diameter, microtubules have a number of important functions. They form the spindle fibers of the mitotic spindle. In addition, they are present in the Axoneme found in the shaft of the Cilium and Flagellum. Furthermore, they perform a number of supporting, or cytoskeletal, functions.
Centrioles are small cylinders, consisting of nine radially disposed "triplets" of microtubules and associated dense material, that are often found in the region called the cell center.
Centrioles migrate to the poles of the mitotic spindle in animal cells. In addition, they are found at the base of the Cilium, where they are called basal bodies.
Microfilaments, like microtubules, are extremely important parts of the cytoskeletal apparatus of cells. Microfilaments contain the protein Actin and are often associated with cytoplasmic movements and regional shortening of the cell. Microfilaments, being quite small (around 50 â ‰ ˆ in diameter), often associate in groups, or bundles, in parallel array.
Cilia and Microvilli
Cilia and Microvilli are both specializations of the cell surface. Cilia are extremely interesting in that they are motile. The ciliary shaft is specialized in such a way that the Cilium can bend in an organized fashion. Groups of cilia can coordinate their beats in a wavelike manner and move material along the surface of the cell. Microvilli, also extensions of the cell surface, are quite different from cilia. Each microvillus is a fingerlike extension of the Cell Membrane that is supported by a core of Actin filaments. Microvilli increase the area of the cell surface available for absorption. Microvilli are plentiful in regions such as the intestine or the kidney where mass transport of material in and out of the cell is required.
Many structures in the Cytoplasm that have not been emphasized in this overview will be encountered in the chapters of this atlas. By now, however, you will be familiar enough with the major "cast of characters" found within the typical cell to be able to begin study of cells and tissues.
Plate 1 - 1
The Pancreatic Acinar Cell: A Protein Factory
In our study of cells, we begin with a truly remarkable cell, the pancreatic acinar cell - a cell that is highly specialized for the assembly and packaging of proteins for export. Proteins are macromolecules made of amino acids. Pancreatic acinar cells take up amino acids from the blood and assemble them into enzymes, which are proteins. Among the enzymes manufactured by pancreatic acinar cells are Amylase (which digests carbohydrates), Trypsin (which digests proteins), and Lipase (which digests fats).
The subcellular components used by the pancreatic acinar cell to synthesize, package, and release proteins for export are illustrated by electron microscopy in the plate at right. Each pyramid-shaped acinar cell, like all cells, is surrounded by a Plasma membrane (arrows). A large, round Nucleus (N) with a prominent Nucleolus (Nu) is found near the base of the cell. Much of the Cytoplasm of the cell is filled with rough Endoplasmic Reticulum (RER). The rough ER consists of an extensive series of flattened, membrane-limited sacs, or Cisternae. The outer surface of these Cisternae is encrusted with thousands of ribosomes, tiny Ribonucleoprotein particles that serve as sites of protein assembly. Numerous Mitochondria (M) are evident in the Cytoplasm as well. These Mitochondria provide chemical energy, in the form of ATP, necessary for the biosynthesis of macromolecules that takes place in the pancreatic acinar cell.
A glance at the electron micrograph at right will reveal that the apex of the cell looks different from the base of the cell. The apical pole of the cell is crammed with membrane-limited, electron-dense inclusions called zymogen granules (Z). Zymogen granules contain the protein to be exported from the cell; that is, they contain the pancreatic enzymes that will be poured out of the cell and into the lumen (L) of the Acinus. (The Acinus is a ball-shaped group of acinar cells clustered about a central hole, or lumen, that leads to a duct that will convey the secretions out of the pancreas and into the duodenum of the small intestine, where the secretions assist in the digestion of food). Among the zymogen granules is the Golgi Apparatus (G), which consists of a series of flattened, membrane-limited sacs and vesicles. No ribosomes are found on Golgi membranes.
The structural polarization evident within the pancreatic acinar cell - that is, the marked difference between the apical and basal regions, or poles, of the cell - is an extremely important feature, for it underlies the functional polarization that makes this type of cell such an efficient protein factory. Raw materials in the form of amino acids in the circulating blood are delivered by capillaries to the base of the cell. These amino acids, the building blocks of protein, are transported across the Plasma membrane and into the cell itself. Once within the Cytoplasm, the amino acids make contact with the tremendous surface area of the rough Endoplasmic Reticulum. The ribosomes on the outer surface of the rough ER, in concert with appropriate messenger RNA and transfer RNA molecules, facilitate the assembly of amino acids into proteins. The newly synthesized protein molecules are then released within the Cisternae of the RER. Small Ribosome-free vesicles called Transfer Vesicles bud off from the RER in the region of the Golgi Apparatus. Transfer Vesicles fuse with the Golgi membranes and empty their proteinaceous contents into the Golgi. The Golgi Apparatus then modifies - and packages - the newly formed enzymes into membrane-limited vesicles that fuse to form the conspicuous zymogen granules. Under appropriate conditions of nervous or hormonal stimulation, the pancreatic acinar cell discharges its content of zymogen granules into the lumen of the Acinus, then prepares itself for another cycle of synthesis, storage, and release of digestive enzymes.
Electron micrograph of acinar cells from the pancreas of the squirrel monkey. G, Golgi Apparatus; L, lumen of Acinus; M, mitochondrion; N, Nucleus of acinar cell; Nu, Nucleolus; RER, rough Endoplasmic Reticulum; Z, zymogen granules; arrows, Plasma membranes of adjoining acinar cells. 11,000 X
The Paneth Cell: A Glycoprotein Factory
Paneth cells are large cells, situated within the recesses of the Intestinal Glands, that possess prominent gylcoprotein-containing Secretory Granules. Although they were discovered over a century ago, the precise function of Paneth Cells is not known. What is known, however, is that the large Secretory Granules found in the apical Cytoplasm of Paneth Cells contain the antibacterial Enzyme Lysozyme. It is also known that the Paneth Cells of rodents can phagocytose and degrade intestinal microorganisms with their lysosomal apparatus. Consequently, it is generally believed that Paneth Cells may contribute to the regulation of intestinal flora. Whatever their function in the intestine, their structure suggests that they are different from other intestinal cells. They are long-lived, are not known to undergo Mitosis, and are instantly recognizable with the light and electron microscope by virtue of their large, unique, Glycoprotein-packed Secretory Granules.
Figure A, a low-magnification electron micrograph, reveals the major ultrastructural features of a Paneth cell within the intestine. Like the pancreatic acinar cell depicted in Plate 1-1, the Paneth cell is pyramidal. The pyramid configuration is adopted by many secretory cells and serves them well. They need a large surface area at the base of the cell to take in raw materials, and a large volume of basal Cytoplasm to contain the mass of rough Endoplasmic Reticulum associated with the synthesis of proteins for export. Once the proteins are synthesized and condensed into tightly packed Secretory Granules, the cell product - now at the apical pole - requires little volume to house it and can be accommodated at the narrow vertex of the pyramid. Such pyramidal cells can then be conveniently arranged around a common lumen into which their secretions can be poured. In the Paneth cell in Figure A, for example, the Nucleus (N), with its prominent Nucleolus (Nu), sits at the broad base of the cell (arrow). The basal Cytoplasm is tightly packed with parallel stacks of flattened Cisternae of the rough Endoplasmic Reticulum (RER). A prominent Golgi Apparatus (G) is evident in the vicinity of the large Secretory Granules (S).
Figure B shows the striking structural and functional spatial relationship between the rough ER, Golgi Apparatus (G), and Secretory Granules (also called secretory vesicles) (S). Here, the Cisternae of the RER bud off periodically to give rise to Transfer Vesicles (T), which are tiny membrane-limited vesicles filled with secretory product generated by the rough Endoplasmic Reticulum that carry material to the Golgi for further processing and packaging into secretory vesicles (S'). The Transfer Vesicles enter the Golgi at its convex face, or forming face. Within the Golgi, complex sugars are added to the protein molecules by enzymes on the Golgi membranes. The resultant glycoproteins are released from the Golgi at its concave face, or secretory face. There, the Glycoprotein-laden vesicles, commonly called condensing vacuoles, have a coarse, dense, granular Matrix (arrowhead). The condensing vacuoles fuse to form Secretory Granules. In this image, an individual secretory granule (S') has been captured at an early stage in its formation. In addition, several mature Secretory Granules (S) are evident.
Figure A. Low-magnification electron micrograph of a Paneth cell from the mouse. CT, connective tissue of the Lamina Propria; G, Golgi Apparatus; L, Lysosome; M, mitochondrion; N, Nucleus; Nu, Nucleolus; RER, rough Endoplasmic Reticulum; S, secretory granule; arrow, base of Paneth cell. 9,000 X Figure B. Electron micrograph of secretory region in the apical Cytoplasm of a Paneth cell. G, Golgi Apparatus; L, Lysosome; RER, rough Endoplasmic Reticulum; S, secretory granule; S', secretory granule forming from condensing vacuoles of the Golgi; T, transfer vesicle at forming face of Golgi; arrowhead, condensing vacuole at secretory face of Golgi. 25,000 X
The epithelium lining the intestine contains many goblet cells. The primary function of the Goblet Cell is to secrete Mucus, a slippery, viscous substance, rich in mucopolysaccharides, that serves to protect and lubricate the lining of the intestine. It comes as no great surprise that goblet cells were so named because they resemble goblets; they usually have a broad apex and a narrow base. As shown in Plates 1-1 and 1-2, most secretory cells engaged in the elaboration of proteins for export are shaped like pyramids: they have broad bases and narrow cell apexes. Given that the Goblet Cell is a secretory cell, one might wonder why it is organized in the opposite way, like an inverted pyramid with a broad apex and a narrow base.
The answer lies in the nature of its secretory product. The Goblet Cell makes enormous quantities of Mucus, and Mucus is highly hydrated. Consequently, the secretory product of the Goblet Cell occupies a much greater volume in the apical pole of the cell than, say, the zymogen granules occupy in the apical pole of the pancreatic acinar cell. A little Enzyme goes a long way; a little Mucus doesn't. Hence, while the secretory product of the pancreatic acinar cell, which is in concentrated form and has a low water content, can be packaged in a small space, the Mucus elaborated by the Goblet Cell cannot.
The plate at right is a low-magnification electron micrograph of a Goblet Cell from the ileum of the small intestine. The Goblet Cell, readily recognized by its large Complement of Mucus droplets (MD) is flanked on either side by intestinal absorptive cells (A) with a radically different structure. The absorptive cells have many Microvilli (mv)-tiny fingerlike projections of the Plasma membrane supported by cores of Actin filaments-projecting from the cell surface. The Microvilli at the left side of the figure are cut in longitudinal section; others, at the right side, are cut nearly in cross section (*). Goblet cells, too, normally have some Microvilli. When the cells begin to release Mucus droplets, as the cell at right is doing, the Microvilli are lost, and the membrane-limited Mucus droplets are released into the lumen of the intestine (L).
The Mucus droplets are elaborated by a system of intracellular organelles that are quite similar to those illustrated earlier within the pancreatic acinar cell and the Paneth cell. Protein synthesis occurs on the many ribosomes that adorn the surface of the rough Endoplasmic Reticulum (RER). The newly synthesized proteins, along with some attached sugars, are released into the lumen of the rough Endoplasmic Reticulum. From there, they are passed into the Golgi Apparatus, wherein the proteins are modified and more sugars are added. At the secretory face of the Golgi, condensing vacuoles containing the freshly made Mucus fuse (arrow) and form the large membrane-limited Mucus droplets. The Mucus droplets accumulate in the apical Cytoplasm, and are packed so tightly that the remainder of the Cytoplasm-and the Nucleus (N)-are shoved aside and forced to occupy a relatively small space at the base (and along the sides) of the cell. When viewed with the light microscope, the Mucus droplets of a Goblet Cell are usually conspicuous; the Cytoplasm, however, is not, and the Nucleus is often barely visible as a dense, flattened body at the base of the cell.
Electron micrograph of a longitudinal section through a Goblet Cell from the ilium of the small intestine. A, columnar absorptive cells that flank the Goblet Cell; G, Golgi Apparatus; L, lumen of intestine, MD, Mucus droplets; MV, Microvilli cut in longitudinal section; N, Nucleus of Goblet Cell; RER, rough Endoplasmic Reticulum; *, Microvilli cut in near cross section; arrow, condensing vacuoles fusing to form Mucus droplet. 15,000 X
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
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
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
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 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
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