Histology Text Atlas Book

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CHAPTER 20: The Senses

Overview

Human beings - indeed, all animals - must interact favorably with the environment so that they can survive and reproduce. They must be aware of their surroundings so they can react to circumstances to better their chances for survival. Awareness has been set at a very high priority by natural selection. Consequently, during the course of many millions of years, special cells that are capable of detecting various classes of environmental stimuli have evolved.

Sources of energy from the environment that can serve as effective stimuli take many forms. Among those most commonly detected by different animals are light (the sense of sight), compression waves in air and water (the sense of sound), and specific molecular configurations (the chemical senses - taste and smell). The organs of special sense, the eye, ear, nose, and tongue, are tremendously complex. Each organ would take a book in itself to describe. This chapter concentrates on the special regions of the eye, ear, tongue, and nose that contain the receptor cells active in the detection and sensory transduction of stimuli supplied by the environment.

Sensory transduction is the process by which an environmental stimulus is detected, received, and transduced into bioelectric signals that are carried to the brain in the form of nerve impulses. In the eye, the retina contains the photosensitive cells, called rods and cones, that respond to light. Plate 20-1 shows how the retina is incorporated into the structure of the eyeball; Plates 20-2 and 20-3 illustrate the cellular elements of the monkey retina that receive incoming rays of light, transduce their energy into neuronal excitation, and transmit nerve impulses to the brain along the optic nerve.

In the ear, the cochlea, a tiny spiral of bone and soft tissue, contains exquisitely sensitive mechanoreceptors, the hair cells, that respond to vibrational stimuli. Plate 20-4 presents a matched pair of light and electron micrographs that illustrate the organ of Corti - the part of the inner ear that contains the hair cells - in its normal position within the cochlea.

In the nose, the olfactory epithelium contains chemoreceptors - the olfactory receptors, a series of bipolar neurons that are excited by chemical stimulation. Plates 20-5 and 20-6 illustrate the ultrastructure of human olfactory epithelium and its sensory receptors. In the oral cavity, the tongue contains several types of projections, the papillae, that bear taste buds. Each taste bud is sensitive to sweet, sour, salty, or bitter stimuli. Plate 20-6 presents a low-magnification electron image of a single human taste bud from a circumvallate papilla. Many other types of sensory receptors exist in addition to those mentioned above. In this chapter we focus on the sensory regions of the eye, ear, nose, and tongue - areas of tissue that will be of greatest interest to students of microanatomy.

 

The Eye: Sclera, Choroid, And Retina

In order to understand the microanatomy of the retina and its photoreceptors, it is important to see how the retina is incorporated into the structure of the eye itself. This structure is clearly shown in the plate at right, a light micrograph of a cross section through the posterior wall of the eyeball of a large monkey, the pig-tailed macaque. The retina of the macaque is quite similar to that of man, and the specimen at right was preserved unusually well by intravascular perfusion.

In this photomicrograph, the outside of the eyeball is at the top of the figure, and the inside of the eye, filled with vitreous humor (V), is at the bottom. Working from the outside in, the outermost part of the posterior wall of the eyeball is protected by pads of fat, here represented by clusters of fat cells (F). Close to these protective fat pads are numerous skeletal muscle fibers (M) - muscles associated with voluntary movements of the eye. These muscles attach to the sclera (S), here seen as a thick, uniform layer of dense regular connective tissue. It is the sclera that gives strength to the wall of the eyeball, and in so doing it does double duty, for not only does it confer strength and shape to the spherical eyeball, but it acts as a tendon that links muscle to eye.

Beneath the sclera lies the choroid, (Ch), a highly vascular mass of loose connective tissue that is interposed between the sclera and the retina (R). The choroid is filled with small blood vessels (BV), which, taken together, are called the choriocapillaris. The choriocapillaris supplies the oxygen-hungry photoreceptors of the retina with blood.

The retina lies in the lower third of the figure. Many students of histology, when confronted with the task of learning retinal microanatomy, set out to devise mnemonic devices to memorize names of the layers, which at first seem to make little sense. This step is unnecessary, for with an understanding of the cellular basis for retinal organization (described in detail in the next plate), the histologic organization of the retina seems obvious and the reasons for the names become apparent.

In the low-magnification light micrograph in Plate 20-1, the outermost layer of the retina - the layer closest to the outside of the eyeball - is the pigment epithelium (PE). The pigment epithelium is closely associated with the layer of rods and cones (LRC), the extensions of the photoreceptors active in sensory transduction. Beneath the layer of rods and cones lies the outer nuclear layer (ONL); beneath the outer nuclear layer is the outer plexiform layer (OPL). Still working toward the center of the eye, the next layer is the inner nuclear layer (INL), which lies atop the inner plexiform layer (IPL). The inner plexiform layer is closely apposed to the underlying layer of ganglion cells (LGC), from which emanate axons of the nerve fiber layer. The nerve fiber layer (NFL), the innermost layer of the retina, is close to the vitreous humor that fills the eyeball and maintains its turgor. The arrow indicates the path of light through the eye; note that light must pass through all of the layers of the retina before it reaches the photoreceptors.

Plate 20-1

Plate 20-1, Light micrograph of a cross section taken through the posterior wall of the eyeball of the pig-tailed macaque. BV, blood vessel in choroids; Ch, choroid; F, fat cell; INL, inner nuclear layer; IPL, inner plexiform layer; LGC, layer of ganglion cells; LRC, layer of rods and cones; M, muscle; NFL, nerve fiber layer; ONL, outer nuclear layer; OPL, outer plexiform layer; PE, pigment epithelium; R, retina; S, sclera; V, vitreous humor; arrow, direction of light path through eye. 325 X

 

The Eye: The Retina

The cellular basis of retinal organization is clearly shown in the figures at right. Figure B is a light micrograph of a cross section through the monkey retina; Figure C is an electron micrograph of a serial section through the same retina photographed at the same magnification; and Figure A is a schematic drawing of the major types of cells whose relative vertical arrangement generates the histologically distinct layers of the retina. As in the preceding plate, the outside of the retina is toward the top of the micrographs; the inside, toward the bottom. The arrow indicates the direction of the light path through the retina.

In Figure A, the uppermost cell is a cone photoreceptor. The outer segment of the cone (OS) is arranged in parallel with the outer segments of thousands of other rods and cones to form the layer of rods and cones (LRC) evident in Figures B and C. The outer segments are membranous extensions of the photoreceptor cell bodies such as that of the cone drawn in Figure A. The apical pole of the cell body is a dense region called the ellipsoid (E), which is packed with dark-staining mitochondria. The lower part of the cell body contains the nucleus (N). Taken together, the nuclei of the cell bodies of the rod and cone photoreceptors constitute the outer nuclear layer (ONL, Figures B and C) of the retina. The base of the cone gives rise to an axon (A), which travels inward to meet the dendrite of the next cell in the series - the bipolar cell. The synaptic contacts between photoreceptors and bipolar cells are, for the most part, made in the outer plexiform layer (OPL, Figures B and C). The dendrites of the bipolar cell (DB) extend from the cell bodies of the bipolar cells, which contain centrally located nuclei (NB). The nuclei of the bipolar cells lie in the inner nuclear layer. From there, the axons of the bipolar cells (AB) travel inward to meet the dendrites of the next cell in the series, the ganglion cell. The axons of the bipolar cells and the dendrites of the ganglion cells make synaptic contacts in the inner plexiform layer (IPL, Figures B and C). The cell bodies of the ganglion cells, from which the dendrites grow, are located in the layer of ganglion cells (LGC, Figures B and C). Each ganglion cell gives rise to an axon (AG). The axons of the ganglion cells pass through the nerve fiber layer (NFL, Figures B and C) and coalesce to form the optic nerve. The optic nerve travels from the retina to the brain and carries visual information to higher centers within the central nervous system.

There are other types of cells in the retina in addition to those mentioned above. There are horizontal elements called amacrine cells and horizontal cells; there are glial cells called Muller cells. To understand these cells, however, it is advantageous to first learn about the relative positions of the rod and cone photoreceptors, the bipolar cells, and the ganglion cells. Given an understanding of their positions, as drawn in Figure A, one can envision all the classic histologic layers of the retina. With that understanding, a close comparison of the drawing with the light and electron micrographs shown in Figures B and C will not only help in understanding the organization of the retina, but is good preparation for further investigation of the fine structure of retinal photoreceptors as described in Plate 20-3.

Plate 20-2

Plate 20-2, Figure A. Drawing of a cone photoreceptor, a bipolar cell, and a ganglion cell showing their relative positions in the retina, which generate the "layers" of the retina shown in Figures B and C. From the top down, structures are: OS, cone outer segment; E, ellipsoid; N, nucleus of cone photoreceptor; A, axon of cone; S1, synapse between cone and bipolar cell; DB, dendrite of bipolar cell; NB, nucleus of bipolar cell; AB, axon of bipolar cell; S2, synapse between bipolar and ganglion cell; NG, nucleus of ganglion cell; AG, axon of ganglion cell. 1,000 X

Figures B and C. Matched pair of light and electron micrographs of the retina of the macaque. A, axon of cone; Ch, choroid; E, ellipsoid; INL, inner nuclear layer; IPL, inner plexiform layer; LGC, layer of ganglion cells; LRC, layer of rods and cones; N, nucleus of cone; NFL, nerve fiber layer; ONL, outer nuclear layer; OPL, outer plexiform layer; PE, pigment epithelium; arrow, light path through retina. 1,000 X

 

The Eye: Photoreceptors Of The Retina

The photoreceptors of the retina - the rods (R) and cones (C) - are illustrated by electron microscopy in Figure A. Rods "see" only black and white and come into play in dim light. Cones are color sensitive and are used for vision in bright light. Although rods and cones differ somewhat in structure and function, they share many cytoarchitectural features.

Each rod or cone is a specialized sensory cell, a bipolar neuron that is composed of two major structurally distinct regions: the inner segment (IS) and the outer segment (OS). These regions are evident in the longitudinally sectioned cone in Figure A. The inner segment, which contains the nucleus (N), consists of the cell body of the bipolar neuron and its apical pole. Whereas the basal pole of the cell tapers sharply to form an axon (A), the apical pole tapers gradually to form the ellipsoid (E). The ellipsoid of the cone is a football-shaped body, filled with many long mitochondria, that provide ATP for the bioenergetically demanding process of photoelectric sensory transduction that occurs in the outer segment.

The cone's outer segment, like that of adjacent rod photoreceptors (R), consists of a series of stacked cell membranes that are rich in photosensitive pigments. In living tissue, the outer segments, which in these preparations are artificially bent, are oriented parallel to the rays of incoming light (arrow). The membranous lamellae of the photoreceptors are oriented at right angles to the long axis of the outer segment. Consequently, the membranes and their contained photopigments are optimally positioned to be struck and stimulated by photons focused on the retina by the eye's dioptric apparatus.

The distal tips of the rod outer segments are intimately associated with the outermost layer of the retina, the pigment epithelium (PE, Figure B). (When looking at these micrographs, remember that each outer segment is a continuous, linear structure. In Figures A and B, the outer segments were distorted during specimen preparation, and they weave in and out of the plane of section, giving them the misleading appearance of discontinuity and making them look like "islands" of tissue, which they are not). The pigment epithelium serves a vital function in the normal metabolism of rod photoreceptors. The rod outer segments, it seems, are in a continuous state of flux; new stacks of membrane are added at the base of the outer segment, and old, worn-out stacks of membrane are shed from its distal tip. When the worn-out lamellae are ready to be shed from the tips of the outer segments, they are phagocytosed by cells of the pigment epithelium. The shed membranes, taken up by cells of the pigment epithelium, and engulfed by lysosomes, are evident as residual bodies (arrowhead, Figure B) in the cytoplasm of the epithelial cells themselves.

One very important feature of photoreceptor fine structure, not apparent in the micrographs at right, is the joining of the inner and outer segments of each rod and cone photoreceptor by a slender stalk, the connecting cilium. This modified cilium, which has a "9 + 0" axoneme and arises from a basal body at the tip of the inner segment, gives rise to the outer segment itself. In a sense, then, the outer segment may be regarded as a highly modified ciliary derivative. The presence of the basal body from which the cilium arises is essential for regeneration of rod outer segments that have degenerated in response to adverse conditions such as vitamin A deficiency.

Plate 20-3

Plate 20-3, Figure A. Electron micrograph of longitudinal section through photoreceptors of the monkey retina. A, axon of cone; C, cone photoreceptor; E, ellipsoid; IS, inner segment of cone; N, nucleus of cone; OPL, outer plexiform layer; OS, outer segment of cone; R, rod photoreceptor; RN, nucleus of rod; ROS, rod outer segment; arrow, direction of light passing through retina. 2,700 X

Figure B. Electron micrograph of choroid, pigment epithelium, and distal portion of photoreceptors of the monkey retina. Ca, capillary in choroid; M, melanocyte in choroid; PE, pigment epithelium; ROS, rod outer segment,, arrow, direction of light passing through retina; arrowhead, residual body in pigment epithelium containing remains of phagocytosed tip of rod outer segment. 5,700 X

 

The Ear: The Organ Of Corti

The human ear is tremendously complex; its structure is beyond the scope of this atlas. This plate describes one specific component of the ear, that part crucial to the sensory transduction of sound waves into nerve impulses-the organ of Corti.

Buried deep within the bony recesses of the cochlear spiral, the organ of Corti, the epithelial organ of hearing, is a spirally wound epithelial sheet endowed with exquisitely mechanosensitive vibration detectors called hair cells. These hair cells are excited by extremely small displacements of their stereocilia, which are modified microvilli that project from the hair cell surface and contact an overlying structure called the tectorial membrane.

In order to understand the way in which the hair cells are stimulated during sound detection, it is necessary to become familiar with the overall organization of the organ of Corti. Figures A and B at right are a matched pair of light and electron micrographs of serial sections taken through the organ of Corti. The organ is anchored to a bony shelf, the limbus spiralis (LS), which projects from the central bony shaft of the cochlea, the modiolus, in the same way as does the spirally wound drill blade of a carpenter's wood-boring bit. An epithelium-lined sheet of connective tissue called the basilar membrane (BM) projects from the base of the limbus spiralis and supports the organ of Corti.

The tectorial membrane (TM) projects from the top of the limbus spiralis and, in living tissue, covers the tip of the stereocilia (arrows) that project from the free surface of the hair cells. During the process of hearing, sound waves ultimately cause the basilar membrane to vibrate, which in turn causes the hair cells and their stereocilia to vibrate as well. However, the tectorial membrane, into which the stereocilia insert, is motionless. Consequently, sound waves cause mechanical stimulation of the hair cells by creating shear forces between the stereocilia (which vibrate) and the tectorial membrane (which does not). The organ of Corti, then, is an elaborate structure whose complex microanatomic architecture is dedicated to the achievement of one major goal: mechanical stimulation of hair cells by sound waves.

The hair cells are arranged into two groups - the inner hair cells (IHC) and the outer hair cells (OHC). Both groups of hair cells are held in place by an elaborately sculpted set of supporting cells, including the inner and outer pillar cells (IP, OP) and the inner and outer phalangeal cells (IPC, OPC). These supporting cells have thin processes that arch out and around to hold the apical poles of the tall, slender hair cells firmly in place in a manner similar to the flying buttresses that support the walls of a tall, slender Gothic cathedral. These features of supporting cell microanatomy, not readily apparent in sectioned material, have recently been revealed in detail by scanning electron microscopy.

The hair cells are innervated by neurons that travel to the organ of Corti via the cochlear nerve. Myelinated fibers of the cochlear nerve (CN), evident below the limbus spiralis (LS) in Figures A and B, constitute the cochlear branch of VIII, the acoustic nerve, which conveys auditory information to the brain, wherein it is processed to generate our perception of sound.

Plate 20-4

Plate 20-4, Figures A and B. Matched pair of light and electron micrographs of sections taken through the organ of Corti. BM, basilar membrane; CD, cochlear duct; CN, cochlear nerve; IHC, inner hair cell; IP, inner pillar cell; IPC, inner phalangeal cell; LS, limbus spiralis; OHC, outer hair cell; OP, outer pillar cell; OPC, outer phalangeal cell; TM, tectorial membrane; arrows, stereocilia atop hair cells. Figure A, 540 X; Figure B, 970 X

 

The Nose: Respiratory And Olfactory Epithelia

The human nose, an exquisitely sensitive odor detector and discriminator, is equipped with over a million olfactory receptors. Each olfactory receptor is a primary sense cell, a bipolar neuron that sends its dendrite to the site of stimulus reception and its axon to the olfactory bulb in the brain. Because the site of stimulus reception is the free surface of the olfactory epithelium in the nasal cavity, the olfactory receptors are vulnerable - they are truly "naked neurons," exposed to the outside world on one side and the brain on the other. This exposure has interesting clinical implications.

In humans, the olfactory receptors are situated in a small region of olfactory epithelium that is confined to an area of approximately 2 cm². The rest of the nasal cavity, it seems, is devoid of olfactory receptors. Respiratory and olfactory epithelia are markedly different in structure and function, as illustrated in the photomicrographs at right.

Figure A is an electron micrograph of the human respiratory epithelium in which there are no olfactory receptors; Figure B is an electron micrograph of the olfactory epithelium itself. In Figure A, it is evident that the respiratory epithelium is lined by a ciliated pseudostratified columnar epithelium that contains three major cell types: ciliated cells (C), goblet cells (G), and basal cells (B). The surface of the ciliated cells bears both cilia and slender microvilli that project well into the mucous layer that lines the nasal cavity (NC). The goblet cells, located in between the ciliated cells, are packed with mucous droplets (M) that extend from the level of the nucleus to the cell surface. The basal cells lie on top of the basement membrane, which, in turn, is supported by the connective tissues of the lamina propria.

The olfactory mucosa, shown in Figure B, is located in the superior region of the nasal cavity. Much thicker than the respiratory epithelium, it consists of a pseudostratified columnar epithelium and the underlying highly cellular lamina propria (LP) Figure B, a low-power electron micrograph through the human olfactory epithelium, shows the four major cell types: ciliated olfactory receptors (O), microvillar cells (M), supporting (or sustentacular) cells (S), and basal cells (B). In addition, degenerating olfactory receptors (D) are present. All cells, with the exception of basal cells, reach the free surface of the epithelium. In living tissue, the epithelial surface is covered with a blanket of mucus. In Figure B, the mucus was washed away during tissue preparation, exposing the surface specializations of the underlying cells. Here, we find cilia that extend from the olfactory vesicles (arrows) of the ciliated olfactory receptors, and microvilli extend from the microvillar cells (M). When the olfactory epithelium is seen in longitudinal section, as in Figure B, the uppermost nuclei are those of the microvillar cells. Next are the somewhat flattened, heterochromatic nuclei of the supporting cells (N'). Beneath these are the round, largely euchromatic nuclei of the ciliated olfactory receptors (N), seen to be distributed in a broad band that covers half the thickness of the epithelium. Deep to the nuclei of the ciliated olfactory receptors, lying above the basement membrane (BM), are the nuclei of the basal cells (B). The basal cells are capable of undergoing mitotic division and replacing lost ciliated olfactory receptors. Olfactory receptors, unique among human nerve cells, degenerate and are replaced in the normal course of life. Olfactory receptors are the only neurons known that not only "turn over" during normal life, but are replaced following loss from illness or injury.

Plate 20-5

Plate 20-5, Figure A. Low-magnification electron micrograph of respiratory epithelium from the human nasal mucosa. B, basal cell; C, ciliated cell; G, goblet cell; Ly, lymphocyte; M, mucus droplets in goblet cell; NC, nasal cavity. 3,500 X

Figure B. Low-power electron micrograph of human olfactory epithelium. B, basal cell; BM, basement membrane; D, degenerating olfactory receptor; LP, lamina propria; M, microvillar cell; N, nucleus of olfactory receptors; N', nucleus of supporting cell; NC, nasal cavity; O, ciliated olfactory receptor; S, supporting cell; arrow, olfactory vesicle bearing olfactory cilia. 1,200 X

 

The Nose; Olfactory Receptors

Several of the olfactory receptors described in the previous plate are shown at higher magnification in Figure A at right. Here, one of the receptors has been cut along its length from cell body (O) to dendrite tip (arrow). The nucleus (N) lies at the center of the bipolar neuron. Above the nucleus are most of the cytoplasmic organelles typically associated with sensory neurons - elements of the rough endoplasmic reticulum, the Golgi apparatus, and a large population of free ribosomes. The basal pole of the bipolar nerve cell body gives rise to an extremely thin axon (0.1 µm in diameter) that travels through the basement membrane into the underlying lamina propria. Here, it joins with other similar axons and forms one of many tiny nerve bundles - the file olfactoria - that form the olfactory nerve that travels to the olfactory bulb in the brain. The apical pole of the bipolar neuron sends a slender dendrite (D) to the free surface of the olfactory epithelium. As the dendrite approaches the surface, it acquires many mitochondria. The dendrite terminal swells to form the olfactory vesicle, a structure that projects above the epithelial surface.

Figure B shows the olfactory vesicle (OV) in more detail. Here, it is evident that the olfactory vesicle is stabilized by a network of cytoplasmic microtubules (*) that enter from the shaft of the dendrite (D) below. Some 10 to 30 olfactory cilia, the probable sites of chemosensory transduction, project from basal bodies (B) embedded in the lateral and apical margins of the olfactory vesicle. These olfactory cilia, presumed to be immotile in man, display the typical "9 + 2" pattern of axonemal substructure at the ciliary base. Several micrometers distal to the base, however, the ciliary shaft tapers sharply to form a long, slender filament supported by two to four single microtubules.

In addition to the ciliated olfactory receptors, the human olfactory epithelium contains microvillar cells - cells of unknown function that occupy a superficial position in the olfactory epithelium. These cells, one of which is shown in Figure C, are characterized by a flask-shaped cell body and a round, euchromatic nucleus (N). Several short, stubby microvilli (arrow) project into the mucous layer that lines the nasal cavity (NC). The basal pole of the cell tapers sharply to form a slender cytoplasmic extension (E) that travels toward the basement membrane. Near the microvillar cell sits a degenerating cell (D), a reminder that all of the cells in the olfactory epithelium, including the olfactory receptors themselves, are in a constant state of cellular turnover.

Beneath the olfactory epithelium lies the highly cellular lamina propria, which contains large secretory glands known as Bowman's glands. A cross section through one of the secretory acini of a Bowman's gland is shown in Figure D. The large, pyramidal cells, filled with electron-dense secretory granules (G), suggest that the secretory product of these glands may add a serous component to the mucus that moistens and bathes the free surface of the olfactory epithelium.

Plate 20-6

Plate 20-6, Figure A. Electron micrograph of a longitudinal section through several receptors in the human olfactory epithelium. D, dendrite of olfactory receptor; N, nucleus of olfactory receptor; NC, nasal cavity; O, ciliated olfactorv receptor; S, supporting cell; arrow, olfactory vesicle. 5,000 X

Figure B. Longitudinal section through an olfactory vesicle atop a ciliated olfactory receptor. B, basal body; C, olfactory cilium; D, dendrite; NC, nasal cavity; OV, olfactory vesicle; *, microtubule. 20,500 X

Figure C. Longitudinal section through a microvillar cell in the human olfactory epithelium. D, degenerating cell; E, cytoplasmic extension; N, nucleus of microvillar cell; NC, nasal cavity; arrow, microvilli. 5,100 X

Figure D. Cross section through secretory acinus of Bowman's gland in the lamina propria of the human olfactory epithelium. G, secretory granules; L, lumen; LP, lamina propria. 1,600 X

 

The Tongue: The Taste Bud

The microanatomy of the tongue itself was covered in Chapter 12, The Oral Cavity. That section of the atlas introduced the taste bud, the sense organ that houses the chemoreceptors responsible for the detection of dissolved sweet, bitter, sour, and salty substances that come into contact with the tongue.

Figure A is a longitudinal section through a single taste bud within the circumvallate papilla of the human tongue. At the top of the micrograph is the surface of the stratified squamous epithelium (E) that covers the papilla. This free epithelial surface is a mucous membrane that is in direct contact with the oral cavity (OC). A few stray bacteria (*) are evident near the taste pore, the entrance to the taste bud. The taste pore (P, inset) is a narrow channel that, in living tissue, permits fluids to pass from the oral cavity into the apical pole of the taste bud, wherein they make contact with microvilli (arrow, inset) that are extensions of the cell surface of the taste cells themselves. (In Figure A, the taste pore is sectioned slightly off-axis, giving the misleading impression of being covered by epithelium. The inset affords a better view of the taste pore.)

The cells within the taste bud are usually referred to as light cells (L) and dark cells (D). In Figure A, the origin of these names is evident: the light cells have a relatively electron-lucent cytoplasm, and the dark cells, a more dense cytoplasm. The light cells typically contain a large, round, euchromatic nucleus. The dark cells, on the other hand, have a more heterochromatic nucleus that is somewhat irregular in shape. Numerous profiles of nerve fibers (N) are evident as they course through the taste bud. At present, some controversy exists among specialists in the field as to whether the light and dark cells are indeed different "cell types," or simply represent different morphologic manifestations of a single type of cell caught at different stages of development. In any case, both light and dark cells are long, slender, fusiform cells that extend to the region of the taste pore. At the lumen of the taste pore (see inset), the apical poles of the light and dark cells are thrown into long, irregular folds that resemble thick microvilli (arrow, inset). Above the microvillar surface, the taste pore is often filled with dense, amorphous material that resembles the contents of the electron-dense, membrane-limited vesicles that crowd the apical cytoplasm of the dark cells.

Lateral to the taste bud lies the stratified squamous epithelium that lines the circumvallate papilla. Cells of the stratum spinosum, here pulled apart from one another by shrinkage during fixation, are held together by numerous desmosomes that give the epithelium its characteristic "spiny" appearance.

 

Taste Bud

Plate 20-7

Plate 20-7, Figure A. Low-magnification electron micrograph of a longitudinal section taken through a taste bud within a circumvallate papilla of the human tongue. D, dark cell; E, stratified squamous epithelium that covers the circumvallate papilla; L, light cell; N, nerve fiber; OC, oral cavity; *, bacteria in vicinity of taste pore. 4,400 X Inset. Longitudinal section through a region of the taste pore from the taste bud shown in Figure A. P, taste pore; arrow, microvilli of light and dark cells. 6,700 X

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