EYE AND EAR
Objectives:
You should be able to recognize:
| Eye | Ear | |||
| vitreous body |
auditory canal
| |||
 |
Blue Histology |
Slides
| D-174 | Eye, rabbit (H&E) | |
| D-175 | Eye (Masson) | |
| D-176 | External ear canal (H&E) | |
| D-177 | Internal ear, Guinea pit (H&E) |
Optional slides
| D-7 | External ear (elastic stain) | |
| D-170 | Lacrimal glands (H&E) | |
| D-171 | Eye lid (H&E) | |
| D-181 | Optic nerve, transverse section (H&E) |
Slide descriptions
D-175, Human eye (Masson)
Examine the slide against a light background without a microscope. The bulging of the prominent blue band identifies the cornea at the front of the eye. The tough outer covering of the posterior five-sixths of the eye which is continuous with the cornea, is the sclera. Masson stain should render collagenous areas all green but for some reason patches of the sclera and cornea came out yellow. Ignore this quirk of staining. The bundle of pink axons enclosed by a blue sheath emerging at the back of the eye is the optic nerve. The nerve's coat of dura mater is continuous with the sclera. Lining the internal surface of the wall of the eye is the retina. It has become artifactually detached over much of its surface. When this happens in life it is bad news. Here it is just an inconvenience.
The retina ends at a line about six millimeters behind the junction of cornea and sclera. This line, is called the ora serrata. Anteriorly, the retina is transformed into the epithelium of the ciliary body. The oval structure which appears to be drawn with dotted lines is what remains of the lens. The iris is seen as two grayish lines which protrude from the ciliary body, and lie anterior to the lens. The space between the iris and the cornea is named the anterior chamber. The posterior chamber is the small space between the front of the lens and the back of the iris. In life, these two chambers contain aqueous humor. The large vitreous space behind the lens is filled with the gelatinous vitreous humor.
Examine the structure of the back of the eye under the microscope. It is best to choose a place where the retina is still attached to the outer layers, (illustration). The green-stained outermost layer of dense connective tissue is the sclera. Internal to the sclera is a pigmented layer containing many blood vessels: this is the choroid. Observe that the innermost part of the choroid is a single layer of capillaries. Further out in the choroid the vessels are larger, (illustration). This organization reflects the main function of this layer, to supply nutrients for the high metabolic activity of the retina. Internal to the choroid is a single layer of densely pigmented cells, the pigment epithelium, which separates the neuroretina from the choroidal vessels, and contributes to the blood-brain barrier, (illustration). Both it and the retina develop from the neuro-tube. How does this occur? On which side of the pigment epithelium is its basal lamina located? This is essential to understand.
Classically the retina is described as containing ten layers, all of which are clearly visible in your slides, (illustration). With the aid of the figures in your textbook, identify these layers, starting with the pigment epithelium. The retinal neurons located closest to the pigment epithelium are the photoreceptors (rods and cones). You can easily distinguish them in places where the retina has detached from the pigment epithelium. Rods are characterized by inner and outer segments which are long and slender, while those of cones are shorter and fatter. In your slides, the cones are easily identified by their wider, oval-shaped inner segments. Above this layer of inner segments is the layer of photoreceptor nuclei, which is about five to six nuclei deep. This is called the outer nuclear layer. Between the inner segments of the photoreceptors and the outer nuclear layer is the external limiting membrane, which appears as a thin red line. The photoreceptors make synaptic contacts with bipolar cell processes and a class of interneurons called horizontal cells in the whitish layer below the nuclei of the photoreceptors. This layer is called the outer plexiform layer. The underlying inner nuclear layer contains the cell bodies of bipolar cells, horizontal cells, other interneurons called amacrine cells and glial (Müller) cells. It is not possible to classify the cell types within the inner nuclear layer of this preparation. Beneath the inner nuclear layer is another region of synapses, called the inner plexiform layer.
Finally, the nuclei of ganglion cells are arranged in another layer, called the ganglion cell layer. The axons of the ganglion cells form the nerve fiber layer, lying closest to the vitreous. These axons pass out of the eye as the optic nerve. Light must pass through all these layers of neurons in order to reach the light sensitive photoreceptor outer segments, where it is transduced into a chemical and then a neural signal.
Now observe the anterior portion of the eye. At the ora serrata the neural retina shrinks to a simple cuboidal epithelium. It and the extension of the pigment epithelium extend forward as a double layer of two simple cuboidal epithelia with their apical surfaces touching each other. This double layer covers the ciliary body and the back of the iris, as shown in the diagram in this syllabus. Over the ciliary body the outer, non-pigmented layer (the one at the free surface) secretes aqueous humor, while the underlying cells are heavily pigmented to absorb stray light, (illustration). Over the back of the iris it is the cells on the surface that are pigmented and the underlying layer is modified into myoepithelial cells, (illustration). These latter are eosinophilic because of the presence of contractile proteins. What is the function of these cells? The pupillary sphincter muscle which contracts the pupil is made up of ordinary smooth muscle cells. Where would you expect these cells to be located? The pupil, of course, is the hole through which light passes into the eye. By contracting or expanding, and thus changing the diameter of the pupil, the iris controls the amount of light which enters the eye.
A second set of smooth muscle cells occurs in the ciliary body. Locate it. This ciliary muscle is unusual for smooth muscle in that it is under voluntary control. You can willingly contract this muscle to change the focal distance of your eye, a process called accommodation.
The lens does not show up well on your slide because it is a very tough, brittle structure and a challenge to section. However on some of your slides it is possible to see that the lens was covered by a green-staining, acellular capsule which is thickest on the anterior surface (hold the slide up to the light). Beneath the anterior capsule is a single layer of cuboidal epithelial cells, the lens epithelium. These cells divide throughout life, giving rise to the lens fibers, remnants of which are stained deep red. Very fine fibers, called zonule fibers attach the lens to the ciliary processes. They are anchored in the lens capsule. If you look carefully you can see them on your slide. (Obviously what you will see will be segments of individual fibers since it is too much to expect one fiber to be caught in the plane of section all the way from a ciliary process to the lens.)
Look at the limbus region where the sclera changes into
cornea. Can you see the epithelial layer of red-staining conjunctiva fusing with
the cornea here? (illustration)
The conjunctiva extends over part of the white of the eye and then turns back
under the eyelid. Compare the appearance of the cornea with that of the sclera.
Observe three structural specializations that increase the transparency of the
cornea. What are
they?
|
1. _______________________________________________________ 2. _______________________________________________________ 3.
_______________________________________________________ |
The cornea is covered on its anterior surface by a stratified squamous epithelium called the corneal epithelium and on its posterior surface by the corneal endothelium.(a simple cuboidal epithelium). From which germ layer does each come? Finally, locate the canal of Schlemm. This "canal" lies in the connective tissue just anterior to the angle formed by the iris and the posterior part of the cornea. It is lined by red-staining epithelial cells. Aqueous humor, formed by the ciliary body, enters the canal through a series of channels called the trabecular network. Schlemm's canal drains into venules which you will not see.
D-174 Eye, rabbit (H&E)
This extra section of the eye is included to show the lens. There is little reason for you to view the other parts which are less well preserved than in your specimen of human eye. (The iris has not been cut through the pupil and so it extends all of the way across the eye. Do not let this confuse you.)
The lens has several zones of color representing different ages of tissue. The oldest tissue, the nucleus, is extremely dense and takes up stain poorly. The younger cells (fibers) towards the periphery stain better. Scan along the anterior surface to where the single layer of low epithelial cells, covered by an acellular capsule, is still attached to the underlying fibers. The line between the epithelium and the underlying fibers originally was a lumen when the lens budded off the ectoderm as a hollow ball. It is important to understand this geometry in order to make sense of why the anterior surface of the lens is covered with epithelium but the back of the lens is not. Observe that the capsule does extend around the back of the lens.
The widest part of the lens is called the equator. This is where growth occurs. Cells of the epithelium just anterior to it divide and push their neighbors to the equator. Here the cells turn under the epithelium and elongate, (illustration). You can see the row of cells that have been pushed under and are progressively elongating. This is one of the great sights in histology. (Hey, beauty is in the eye of the beholder). Follow the line of nuclei to where it finally becomes irregular. This is at the point where the nuclei are dismantled and the fibers become featureless, transparent prisms of highly concentrated crystallin proteins.
D-177 Internal ear, cochlea (H&E)
The internal ear is a small, complex structure embedded in dense bone. This makes sectioning difficult, and each slide is unique in its orientation and in what it includes. All of the slides show the cochlea: the small, shell-shaped receptor for sound. Many sections also have parts of the vestibular apparatus, the utricle, saccule and semicircular canals (example). The entire inner ear lies in an irregular hollowed out cavity in the bone called the osseous labyrinth. This bony cavity is lined by endosteum, as you would expect, and is filled with a fluid called perilymph. Loosely suspended in the osseous labyrinth is a closed epithelial bag, also with a complex shape. It is called the membranous labyrinth and is filled with endolymph. For the most part the membranous labyrinth is suspended by wisps of tissue away from the walls of the osseous labyrinth. However is some particular areas the membrane is directly attached to the bone. You can see these relationships if your section has cuts through a vestibular structure such as the utricle (illustration). Semicircular canals will show up as oblique or round cross-sections of membranous labyrinth within the osseous labyrinth. They expand to form the ampullae, just before opening into the utricle. If none of these structures is present, check out the figures in your text book. In very lucky cases part of a vestibular receptor organ (called a macula in the utricle and the saccule, or a crista in the ampullae) might be visible, (illustration). If so, examine it at high power (illustration). If your slide does not show a macula or crista turn, again, to the pictures in your atlas or textbook. Understand the basic structure of these sense organs and the way that the two types are individually specialized in structure and function.
| (Note: | your letter grade in this course, A,B,C,or F depends on how much of the vestibular system is present on your slide -- Oh yes, we knew who you were when we passed out the slide sets!) |
Hold your slide up to the light and, using your text as a guide, decide which structures are present on it. Show your slide to your teammates and see if they agree with you. Now concentrate on the cochlear duct. It is embedded in reddish or purple-staining bone. This coiled duct is sectioned several times to give a set of round profiles through it. (For a few of you the section will be so far off the midline that it cuts longitudinally along the coils. If so, see me and I will plead with Phil to give you another slide.)
Pick
one turn of the cochlea.
Locate the scala vestibuli, scala media and scala tympani. The scala media (also
called the cochlear duct) is the membranous labyrinth with endolymph in it. Figure
out that by being tacked down to the endosteum along two sides, the cochlear duct
divides the perilymphatic chamber into two parts, the scalae vestibuli and tympani.
Go up to higher power and identify each of the structures associated with the
scale media, the:
| 1. | organ of Corti with three rows of outer hair cells and one of inner hair cells. | |
| 2. | basilar
membrane. It is stretched between the spiral ligament and the osseous spiral lamina. | |
| 3. | the lesser important vestibular membrane. | |
| 4. | tectorial membrane which the cilia of the hair cells are embedded in. This gelatinous membrane may have shrunk and pulled away from the hair cells. | |
| 5. | stria vascularis, with capillaries running in the epithelium. |
Look up the function of each of these structures. Then, find the spiral ganglion as it coils around the inner edge of the cochlear duct. Observe the ganglion cells. Yep, it is a sensory ganglion instead of sympathetic. Look how closely packed together the cell bodies are. You can also find the acoustic nerve fibers extending from the ganglion cells out to the hair cells in the organ of Corti if you look carefully enough.
* * * * *
Optional Slides
You have already looked at five other slides associated with the eye or ear. Review these tissues if you wish.
D-171 Eye lid.
This is a sagittal section through the eyelid. Find the outer surface lined by skin with its usual accessory structures. The inner surface, the conjunctiva, is lined with a wet unkeratinized stratified cuboidal epithelium. This delicate membrane both protects the eye and provides a smooth surface so the eyelid can glide over the cornea. Here, then, is a transition from skin to a mucosa. Eyelashes lie at the junction. One of these stout hairs may be present on your slide, or your section may have missed them.
Elaborately developed compound glands, called Meibomian glands, lie between the follicles of the eyelashes and produce a ring of oil along the rim of the eyelid to retain the tears. The tissue in which the Meibomian glands are embedded is named the tarsal plate. It has some characteristics of chondroid tissue which imparts a permanent shape and a great deal of elasticity to the eyelid. Voluntary dermal muscles open and shut the lids. You will have no trouble finding them in the middle of the eyelid. What orientation do they have in the eyelid?
D-170 Lacrimal gland
This compound branched gland is predominantly tubular. It consists of a number of lobules. The secretory cells are tall cuboidal with granules containing lysozyme, an antibacterial enzyme. They are all serous. The relatively few ducts are lined with low, cuboidal epithelium. You can find plasma cells between the tubules. The secretory cells transport the antibodies (IgA) from them into the tears.
Because the secretory units are elongate the proximal portions have to have a substantial sized lumen. Their cross sections look like lifesavers, allowing you to distinguish the lacrimal from salivary glands. Another obvious distinguishing characteristic is the absence of striated ducts. These structures found only in salivary glands. Look at the diagram comparing the structures of lacrimal and other compound serous glands in the gland section of your syllabus.
The lacrimal gland produces tears. The apparatus for capturing them from the eye and transferring them to the nasal cavity is described in your book. Read about it if you are interested, not because it will be on any exam in this course.
Click here for the previous description of this slide in the glands section (and use the back button to return to the eye-ear section.)
D-181 Optic nerve, cross-section (H&E)
The optic nerve is misnamed. It is not a true nerve at all but a drawn out extension of the brain. The retina is a genuine part of the CNS and so is the tract of axons which connects it with the brain. As such it is surrounded by the three meninges, as you observed before. (illustration). The pia is the only meningeal layer with unusual disposition here. It not only covers the surface of the nerve, but also invades from the surface to divide the axons into fascicles. The neurons are nonmyelinated on your slide (can you tell?) but do have a myelin sheath for most of their length. The very obvious nuclei among the axons are very obviously of glia (illustration). You probably looked at the optic nerve cut longitudinally when you examined slide D-175. If you return to that slide now you can appreciate that it is made up of fascicles of axons separated by sheets of pia.
D-7 External ear (elastic stain)
Perhaps you recall that the pinna of the ear is supported by elastic cartilage. If in doubt, briefly reexamine slide D-7 by holding it up to the light then placing it under the microscope or twiddle your own ear back and forth.
D-176, External ear canal (H&E)
The ear canal is very close to the TMJ, giving it a special relevance to you. Put your finger over the opening of your ear and press down while you open and close you open and close your mouth to see how close they are. This section was cut transversely to the external ear canal about a centimeter in from the surface. The presence of a portion of the parotid shows how close it also is to the TMJ.
You already looked at this slide during your skin lab. Now you can look more generally at the structure of the external ear canal. In particular, note the presence of cartilage plates. Why is this cartilage not the elastic variety... or is it?
The canal is lined by thin skin with a few small hairs and associated sebaceous glands, (illustration). The special structures here are the ceruminous glands. These coiled tubular glands are modifications of apocrine sweat glands, to produce earwax. Click here to go to the description of the ceruminous glands from your skin section (and use the back button to return to the eye and ear.
* * * * *