Cell and Tissue Biology

Nervous Tissue & Special Senses

Terms List

The key objects to concentrate on during this lab are listed below. You need to learn 1) how to recognize each object, 2) understand it's primary functions, and 3) understand how structure is related to function.

  • Spinal ganglion: axon, myelin sheath, schwann cell, node of ranvier, Schmidt-lantermann incisures, dorsal root ganglion, peripheral nerve branch, proximal nerve branch, sensory neurons, nissl substance
  • Spinal cord: gray matter, white matter, dendrites, central canal, ependymal cells, pia mater, subarachnoid space, motor neuron, dorsal horn, ventral horn
  • Cerebellum: cerebellar folium, white matter, cerebellar cortex, granular cell layer, molecular cell layer, purkinje cells, astrocytes, oligodendrocytes, molecular cell layer neurons
  • Pre and post central cortex: cerebral motor cortex, cerebral sensory cortex, giant betz cells, oligodendrocytes, molecular layer, external granular layer, external pyramidal layer, pia mater
  • Brain cortex (golgi stain): Pyramidal cells, stellate cells, astrocyte
  • Third ventricle: pia mater, glial cell, pericyte, astrocyte, ependymal cells, neuron, oligodendrocyte, hypothalamic neurons, median eminence, pars tuberalis
  • Choroid plexus: choroid plexus epithelium
  • Optic chiasm: astrocyte, oligodendrocyte, optic nerve fibers, pericyte
  • Artery, nerve, vein slide: perineurium, nerve fiber bundle, endoneurium, schwann cell, myelinated axons
  • Jejunum: enteric ganglion, submucousal plexus, myenteric plexus
  • Olfactory epithelium: Bowman's gland, olfactory epithelium, olfactory knobs, olfactory cell nuclei, supporting cell nucleus, basal cell nuclei, myelinated nerve, unmyelinated nerve
  • Eye: cornea, lens, sclera, retina, extraocular muscle, ciliary body, descemets membrane, iris, anterior lens capsule, anterior lens epithelium, corneal epithelium, corneal stroma, sphincter pupillae, ganglion cell layer, inner plexiform layer, bipolar cell layer, outer plexiform layer, nuclei of rods, photoreceptors, pigmented epithelium, choroidea
  • Lens: anterior capsule, lens bow cell nuclei, lens fibers
  • Iris: posterior surface, sphincter pupillae, posterior pigmented epithelium, dilator pupillae, ciliary body
  • Eyelid: hairy skin, orbicularis oculi, keratinized epithelium, sebaceous glands, hair follicles, meibomian gland, palpebral conjunctiva, conjunctival epithelium
  • Cochlea: ampulla, bony labyrinth, modiolus, organ of corti, temporal bone, scala media, scala tympani, scala vestibule, basilar membrane, limbus spiralis, spiral ligament, stria vascularis, vestibular membrane, tectorial membrane, inner hair cells, outer hair cells, nerve fibers, outer pilar cell, supporting deiter cells, supporting Hansen cells, lamina spiralis ossea, outer sulcal cells, modiolar bone, acoustic nerve, facial nerve, stapedius muscle, vestibular nerve, macula, stapes, ampulla cristae, cupula, type 1 hair cell

Nervous Tissue & Special Senses

The objectives of this laboratory are learn to recognize the following structures and their parts.

Nervous tissue: Size and shape of neurons and neuroglia as they appear in distinct parts of the central and peripheral nervous systems, and the relationship of these cells to surrounding tissues.

Special senses: Recognize the parts, together with associated functions, of the ear, eye, and olfactory epithelium.

Nervous tissue is composed of nerve cells (or neurons), neuroglial cells, and blood vessels. The nervous tissue organizes and coordinates (often with endocrines) most of the functions of the body.

The estimated number of neurons in the brain varies from 10 11 to 1014. The cerebral cortex alone is said to have 15 billion neurons. Remember that we also have the enteric nervous system which may have as many neurons as the brain.

Similar to most other cells neurons can receive and respond to chemical or physical signals. They differ from other cells in having the ability to convert signals into electrical impulses using special ion channels. These impulses are conducted over long distances and transmitted to the next cell be it another nerve cell, muscle, or gland.

Neuroglia, or the "other brain cells," constitute half of the weight of the brain, but, being much smaller than the neurons, they outnumber the neurons more than 10 times. During embryonic development glial cells appear to guide the migrations of neurons and the growth of axons and dendrites. Neuroglia retain, under certain conditions, the capability to divide mitotically. In fact, uncontrolled proliferation of glial cells is the most common cause of brain tumors. Neuroglia offer structural and metabolic support for neurons, functioning as the connective tissue of the nervous system. Neuroglia probably play other, yet largely unknown, roles.

Because the routine stains such as ours reveal only nuclei of neuroglial cells we have added the following descriptive information in order to help you distinguish among the types of neuroglial cells.

Neuroglia are divided into four groups: (1) astrocytes, (2) oligodendrocytes, (3) ependymal cells, and (4) microglia (not real glial cells).

Astrocytes cover the neuron "all over," except in the synaptic clefts. They are large, star-shaped cells with numerous processes. The end feet of these processes form the glia limitans on the pial surface, and they also surround capillaries. There is evidence that astrocytes induce capillaries to become the blood-brain-barrier type.

One of the main functions of astrocytes is to remove K+ ions and neurotransmitters from the extracellular space. Astrocytes communicate with each other via gap junctions. This creates a large interconnected network and a huge K+ sink.

Astrocytes produce neurotrophic (growth) factors and interleukin-1. When there is neural damage, astrocytes may become phagocytic to remove debris. Unfortunately they form scars.

Oligodendrocytes have fewer cell processes than astrocytes. They have abundant RER and microtubules in the cytoplasm. They are responsible for myelin formation in the CNS. One oligodendrocyte may provide several different axons with a myelin sheath. Oligodendrocytes also produce nerve growth factors.

The ependyma lines the ventricles of the brain and the central canal of the spinal cord. At places the ependyma specializes to form the choroid plexus which produces cerebrospinal fluid (CSF).

Microglia are macrophages originating from blood monocytes. Multipotential pericytes may also develop into macrophages.

Schwann cells that originate from the neural crest serve the glial role in the peripheral (PNS), autonomic (ANS), and enteric nervous systems..

w84 Spinal ganglion, rat

The spinal ganglion is also called the dorsal root ganglion. Start with a higher-power view by following links [2,1,1]. The cell body (perikaryon, soma) of these neurons is large. Compare the size of the neurons to the surrounding satellite cells. The diameters of neuron cell bodies vary from 3 microns in the hypothalamus to 135 microns in the motor cortex. The ganglion cell (neuron) nucleus (identify) is pale-staining (euchromatic) with a prominent nucleolus. Neurons are busy cells actively transcribing and translating genetic information. The DNA of these cells is uncoiled, leading to the euchromacy. The prominent RNA (Nissl substance, identify) in the cytoplasm is further evidence of active information processing. In sensory cells such as these the RNA tends to be dispersed. This dispersed appearance is more evident in the cell to the right. The cell body together with the nucleus is the maintenance center for nerve cells, which typically have very long processes.

The Schwann (satellite) cells surround the neuron almost like a capsule. The Schwann cells are also responsible for producing the myelin sheath (identify) in the PNS.

Zoom out once and follow link [2]. The sensory neurons of the spinal ganglion are pseudounipolar: the soma gives rise to one process only, which, soon after emerging, curls and then bifurcates to give central and peripheral processes both of which resemble axons in structure. Identify the c.s. of the pseudounipolar process. Remember that it is a curled structure giving many cross-sectional views.

Although there have been recent challenges to this dogma, it is thought that mature neurons do not divide. The neuroglial cells, however, retain their capability to divide mitotically. Mitotic figures (identify) therefore are sometimes seen in neuroglia. In fact, uncontrolled proliferation of neuroglial cells is the cause of most brain tumors.

Note the difference in the degree of heterochromacy in the Schwann cell nuclei. Some have condensed chromatin because they are less active biosynthetically. Note the dispersity of the Nissl substance (RNA) in the neuron cell bodies, again typical of sensory neurons.

Zoom out once and note that the perikarya of the sensory neurons also varies enormously in size.

Go to the thumbnail and follow [1,1,1]. The peripheral processes of the sensory neurons resemble an axon even though they, strictly speaking, are dendrites. In the CNS only axons are myelinated. In the PNS dendrites may also be myelinated. For convenience we term all of these processes axons. These axons (identify) are surrounded by the myelin sheath (identify). Identify the Schwann cell nucleus. These cells produce myelin sheaths in the PNS.

Go to the thumbnail and follow [1,2]. Identify the node of Ranvier, an area devoid of myelin sheath. This is an area where there is a clustering of sodium channels and from which the action potential is regenerated. The Schmidt-Lantermann incisure (identify) represents an area where the myelin sheath contains some cytoplasm of the Schwann cell. The incisure in this view is artificially large.

Practice with thie unlabeled virtual slide: w84 Spinal Ganglion, Rat. This slide is the same as in the atlas. It is very high resolution. For good contrast staining see "186 Spinal Cord Ganglia" (below). Find the following: Spinal ganglion, axon, myelin sheath, Schwann cell nucleus, node of Ranvier, Schmidt-Lantermann incisure, Satellite cell, sensory neurons, Neuron nucleus and nucleolus, nissl substance (bodies).

Practice with thie unlabeled virtual slide: 186 Spinal cord ganglia. This shows several sections of spinal cord. Going left to right: top left is a very good sensory, spinal PNS ganglion with myelinated nerve tracts; next is a cross section of the spinal cord, then two longitudinal sections through the spinal cord. The second longitudinal section is better than the first due to sectioning artifacts. Ganglion: Find -- Nerve cell bodies, nucleus and nucleolus, Nissl bodies, satellite cells (the white space between the nerve cell body and many satellite cells is an artifact of fixation due to shrinkage of the tissues during processing), Schwann cells, myelinated nerve fibers, axon, myelin, connective tissue sheath. Spinal cord sections: Find -- Connective tissue and dura mater, white and gray matter (center & somewhat butterfly), nerve cell body and organelles, oligodendrocytes, astrocytes, microglial cells, capillaries, endothelial cell, Pericyte, myelinated fibers, axons, myelin. No central canal is seen in this section.

w80a Spinal cord, monkey

Follow link [2,3,2] for a high-power view. This is a view of neurons, glial cell nuclei, and capillaries against a background of so-called neuropil. The neuropil is composed of axonal, dendritic and glial cell processes. Identify the labeled motor neuron. It is a multipolar neuron, the most common type in the mammalian nervous system. Multipolar neurons usually have many dendrites and a single axon. Most of the dendrites and the axon are not visible in this view because the section is thin and cuts through only a fraction of the cell body and dendritic tree. Neurons in general have a large pale-staining (euchromatic) nucleus with a very prominent nucleolus. (Stare at the neuron adjacent to the labeled cell to memorize and confirm this.) In sensory cells the RNA (Nissl substance) is disperse. In motor neurons it appears clumped (identify). Note that the RNA extends to a dendrite. Zoom out once and follow [1] for another view of the same structures.

Click on the thumbnail and follow [1,1] to get a view of a motor neuron with a greater number of dendrites visible even in this thin section.

Click on the thumbnail and then link [1]. In the spinal cord the neuron cell bodies (gray matter, identify) are located in the center and form a "butterfly" figure. The fiber tracts (white matter, identify) are located peripherally. This arrangement is opposite to that in the brain.

Click on the thumbnail and follow [2]. Identify the white and gray matter and the central canal. Click link [1] for a higher-power view of the central canal which is filled with CSF. The central canal is lined by glial cells called ependyma.

Zoom out once and follow [2]. Identify the branches of the anterior vertebrate artery and the accompanying veins. The pia mater (identify) is the innermost of the meninges and provides a continuous protective covering for the CNS. (This discontinuity here is an artifact.) The subarachnoid space (identify) is located between the pia mater and the dura mater and houses blood vessels and cerebrospinal fluid.

Click the thumbnail and follow [2,4]. Appreciate the abundance of blood vessels in both the white and gray matter (identify ). The capillaries in the CNS have a so-called blood-brain-barrier. The barrier is thought to be composed of the endothelial cells (which have very few endocytotic vesicles) and their basal lamina. The b-b-b is thought to fill a protective function for the brain, but it also impedes entry of some useful therapeutic substances.

Practice with thie unlabeled virtual slide: 80a Spinal cord - Monkey. Same as in Atlas. This is a very good slide with high resolution of cell types, because the tissue was fixed by perfusion of the vasculature (so no RBCs in the capillaries). However, because this is stained with Toluidine blue, there is very little contrast. Find the following: Connective tissue and dura mater, white and gray matter (center & somewhat butterfly), nerve cell body and organelles, axon versus dendrite, oligodendrocytes, astrocytes, microglial cells, capillaries, endothelial cell, Pericyte, myelinated fibers, axons, myelin, central canal, ependymal cells. Not good for spinal ganglion.

w80c Spinal cord, rat

This is a low power view of a silver stained specimen. Identify white matter and gray matter. The butterfly configuration of the gray matter is more apparent than in the previous slide. Follow [1,2]. Even though the preservation in this specimen is not optimal, you can clearly see that the fiber tracts of the white matter appear in cross-section. The myelin sheath has dissolved in preparation, leaving white rings around the dark-staining axons. Click the thumbnail and follow [2,1] for a similar view. The silver stain tends to highlight the cytoskeletal elements of nerve cells; hence the fibrillar appearance of the gray matter.

Practice: Try finding the objects we've just gone over using this unlabeled virtual slide: w80c Spinal cord, rat. This is excellent becasue they used a silver-impregnation technique so that the axon and cell bodies are the only things labeled. You can see the axons in perfect cross section.

w79 Cerebellum, chinchilla

This is a section of the cerebellar cortex only. (There is really much more to the cerebellum.)

In the brain the gray matter is peripheral (outside) and the white matter (identify) is central (inside). This is opposite to the case in the spinal cord. The cerebellar cortex is organized into folia (identify).

Click link [1] and identify the structures.

Follow links [1,2]. The cerebellar cortex is composed of three layers. The granular cell layer (identify) is densely packed with neurons. The molecular cell layer (identify) is the outermost and has relatively fewer neurons. The Purkinje cell (identify) layer is located between the other two layers and has dendrites extending into the molecular layer. In this thin section, only a few branches of the dendritic tree are visible. Note also the astrocyte nuclei.

Zoom out once and reidentify the labeled structures.

w79c Cerebellum, rat (Golgi stain)

Follow link [1] and identify the folium. Then click on [1] and identify the Purkinje cells. You should be aware that these sections are 50X thicker than the sections you have previously seen. The tradeoff is well worthwhile however, because the Golgi stain reveals the big picture.

w79b Cerebellum, rat (Golgi stain)

Follow links [1,1,1]. The Golgi stain, although unpredictable, highlights the cell body and most of the dendritic tree of the Purkinje cell. Identify the Purkinje cell body and appreciate the elaborate structure of its dendritic tree. Purkinje cells are the output cells of the cerebellar cortex. Their axons carry inhibitory impulses to deep cerebellar nuclei and lateral vestibular nuclei. Notice the numerous dendritic spines. These represent postsynaptic specializations. You can barely identify the initial segment of the Purkinje cell axon emerging from the opposite end of the dendritic tree.

Practice with thie unlabeled virtual slide: Cerebellum - human. This slide shows the fold of the cerebellum layers very well. It is very good for all cell types, layers and Purkinje cell bodies. Find the following: Cerebellar folium, white matter, cerebellar cortex, molecular cell layer, Purkinje cell layer, granular layer, Granule cell neurons, Golgi type II cells, astrocytes, oligodendrocytes, Basket cells.

w78a Pre- and post-central cortex, monkey

This slide represents the cerebral cortex around the central sulcus. Identify the motor and sensory cortical areas.

Follow links [1,2]. Several layers parallel to the surface can be identified in the cerebral cortex. A few have been labeled here (identify).

Zoom out once and follow [1.1]. The giant, multipolar Betz cells (motor) are found in the motor cortex. These are much larger than surrounding cells (compare the sizes) because some send axons all the way to the lumbar spinal cord. Note again the large, pale staining nucleus, prominent nucleolus, and clumped RNA. In contrast, the oligodendrocyte nuclei (identify) are heterochromatic and small. Although they are not labeled, you should appreciate the abundance of capillaries.

Practice with thie unlabeled virtual slide: 184 Cerebrum - human. This slide is good for all cell types, gray and white matter (although the white matter is stained reddish), nerve cell bodies, Nissl bodies, axons, neuroglial cells, capillaries and adjacent astrocytes, large and small neurons.

w78f Brain cortex, rat (Golgi stain)

This is a poor specimen. It is labeled, however, so you could look at it. We will substitute a better picture next year.

w78d Third ventricle, monkey

Follow links [3,1,2]. By this time you should be able to recognize the neurons by their prominent nucleoli and pale-staining nuclei. Appreciate the abundance of blood vessels. Note the pericyte nucleus just outside the capillary. These cells are pluripotential and are probably one source for the microglia, the macrophages of the CNS. (Monocytes from the blood represent the other source.) Oligodendrocytes (identify) are known to produce the myelin sheaths in the CNS, and probably have additional, poorly known, functions. Note that their nuclei are heterochromatic as compared with the neuron nuclei. The oligodendrocytes are often called satellite cells when they are located next to neurons. Remember that the surrounding background is known as the neuropil. It is composed of axonal, dendritic, and glial cell processes.

Click the thumbnail and identify the third ventricle. It is filled with CSF.

Follow links [2,2,1]. Identify the ciliated ependymal cells. Appreciate the abundance of capillaries. Click link [1] to see a higher-power view of cilia. These cilia are thought to help propel fluid toward the fourth ventricle.

Click the thumbnail and follow [1,1]. The pia mater (identify) follows the larger blood vessels into the brain parenchyma. Click link [1] for a higher-power view and identify the structures. In the brain it is difficult to distinguish between arteries and veins by the thickness of the walls. Because of the protective covering of the calvarium both arteries and veins may have thin walls.

Click the thumbnail and follow links [2,1,1,1,]. Identify all the labeled structures. Note that the pericyte is located outside the capillary wall.

Practice: Try finding the objects we've just gone over using this unlabeled virtual slide: w78d Third ventricle, monkey. Be sure to see the ependymal cells.

w80d Choroid plexus, dog

Choroid plexi (identify) are located in the roof of the ventricles. They are composed of specialized ependymal cells which have been invaded by pia. The choroid plexi are the sites of CSF production.

Click link [1,1]. The choroid plexi are composed of finger-like processes lined by cuboidal epithelium (identify) and housing blood vessels (identify) that are outside the blood-brain barrier. Follow link [1]. The zonulae occludentes, not visible in the LM, occur between the apical plasma membranes of the neighboring epithelial cells and form the blood-CSF barrier.

w78e Optic chiasm, monkey

Follow links [1,2,2]. The optic chiasm is a good place to identify neuroglial cells because there are no neurons present. Identify the astrocytes. With routine staining the astrocyte cell processes do not usually show. Astrocyte nuclei can be distinguished from other neuroglial cells by their less densely staining chromatin. Oligodendrocyte nuclei (identify), on the other hand, are usually more densely stained. Again their cell processes are not visible. Pericytes (identify) can be distinguished by their location just outside the capillary wall. Note that the endothelial cell nucleus is "inside" the capillary wall. Click link [1] for a higher-power view of an astrocyte with one cell process visible.

Click the thumbnail and follow [1,1,1,1] for a good comparison between astrocyte and oligodendrocyte nuclei.

Click the thumbnail and follow [1,2,1]. Identify the pericytes, the astrocyte nuclei and the myelinated nerve fibers. (This is the optic chiasm.)

w54a Artery, vein, nerve, dog

Follow links [1,1,1]. This is a cross section of a peripheral nerve fiber bundle (identify). The nerve bundle is surrounded by connective tissue called the perineurium (identify). Notice that the perineurium also encloses capillaries (identify).

Click link [1] and re-identify the perineurium. The individual nerve fibers are surrounded by a delicate connective tissue sheet called the endoneurium (identify). A few myelinated axons are labeled. Identify them. Note the nuclei of the Schwann cells. Remember that these cells produce the myelin sheath in the PNS. Mast cells (identify) are found everywhere in the body.

Practice: Try finding the objects we've just gone over using this unlabeled virtual slide: w54a Artery, vein, nerve, dog.

w41bpl Jejunum, dog (enteric neurons)

The enteric nervous system is located all along the intestine. The total number of neurons in the enteric nervous system may equal that in the brain. The enteric nervous system is thought to function independently from the CNS although its activity can be modulated by the ANS.

Click link [1]. The enteric neurons are located in the submucosal (Meissner's) and myenteric (Auerbach's) plexi.

Follow links [3,1]. Identify the enteric ganglion. Although not evident in this picture, these cells are multipolar neurons. As is typical of other neurons, they have pale-staining nuclei and prominent nucleoli (identify).

Click the thumbnail and follow [1,1,1] for another view. Although they are not labeled here, notice the small nuclei of the Schwann cells.

Practice: Try finding the objects we've just gone over using this unlabeled virtual slide: w41b Jejunum, dog.

w81 Olfactory epithelium, rat

This is a slide you (should) have seen before.

Follow links [1,1,1] and identify the olfactory epithelium and the unmyelinated nerve bundles. The nerve bundles are the axons of the olfactory bipolar cells.

Follow [1,1]. Identify the olfactory knobs and the matted cilia. These represent the dendrites of the bipolar olfactory cells.

Click on the thumbnail and follow [1,1,1,2]. Identify all the labeled structures. Click link [1] and identify the olfactory knobs and the nuclei of the bipolar olfactory cells. Bipolar neurons are relatively uncommon in humans, occuring in the eye and ear in addition to the olfactory epithelium.

Practice: Try finding the objects we've just gone over using this unlabeled virtual slide: w81 Olfactory epithelium, rat.

w83a Eye, rat

The human eye is approximately a sphere, 2.5 cm in diameter. (Note: In the eye "inner" always refers to the center of the ball. Remembering this will help you understand some of the associated nomenclature.) The eye can be thought to be composed of three layers, (1) corneosclera (outermost), (2) uvea, which is composed of the iris, ciliary body, and choroid, and (3) retina. Inside the eyeball are the lens, vitreous body, and intraocular fluid.

Identify cornea, sclera, and the lens inside. The eye is very difficult to fix because the fixatives do not penetrate the lens. The lens in rodents is huge and almost always cracks in preparation, displacing other structures from their normal relationships.

Return to w83a Eye, rat and follow links [3,1]. Although the cornea is transparent, it doesn't look that way in histological slides. The anterior surface is covered with stratified squamous epithelium (identify) about five cells thick. Even the outermost cells retain their nuclei. In primates there is a so-called Bowman's membrane underlying the epithelium. It probably functions to keep the cornea even. The stroma is the thickest portion of the cornea and is composed of collagen and occasional fibroblasts (keratocytes). The collagen fibers are organized in laminae parallel to the anterior surface, but the direction of the fibers in adjacent laminae is disposed at approximately a 90 degree angle to give a criss-cross pattern. This arrangement of collagen, the lack of blood vessels, and the relative state of dehydration of the cornea may explain its transparency.

The dorsal surface of the cornea is lined by a simple squamous epithelium anterior to which lies the Descemet's membrane (identify). The Descemet's membrane provides even tension and prevents deformation of the cornea. The dorsal surface epithelium, although not clearly seen here, plays an important role in pumping out water from the cornea, but also providing nutrients and oxygen to the cornea from the intraocular fluid. The cornea also receives oxygen from the outside air. Some contact lenses are known to impede this direct oxygen supply.

The lens is surrounded by a capsule (identify) which is rather thick anteriorly. Identify the sphincter pupillae muscle at the edge of the iris.

Click link [1]. The anterior surface of the lens (just underneath the capsule) is lined by cuboidal epithelium. It is the source of the lens fibers. The lens capsule is actually the basal lamina of these epithelial cells. The capsule is impermeable to cells, e.g., macrophages and lymphocytes. It prevents lens proteins from getting into contact with other tissues. Were that to happen lens protein would be treated as a foreign antigen.

Zoom out once and click link [2] for a higher-power view of the sphincter pupillae muscle. Notice the heavily pigmented cells in the stroma and on the dorsal surface of the iris.

Click on the thumbnail and follow link [4]. The cornea is continuous with the sclera (identify) and they are indistinguishable histologically. The sclera is not transparent because the collagen fibers are more randomly oriented. Note the extraocular muscle attaching to the sclera. Identify the choroidea and retina attached to the inside of the sclera.

Click link [2]. The ciliary body (identify) is located inside the eye at the level of the corneoscleral junction (limbus). (The limbus represents the beginning of the "white" of the eye.) The ciliary body forms a ring in the eye surrounding the cornea. It produces the zonulae (of Zinni) that anchor the lens. The rat and other rodents lack the ciliary muscle used by humans and other species for accommodation. Note that the retina begins where the ciliary body ends. This, and the papilla, are the only places where the retina is anatomically attached to the uvea.

Zoom out once and click link [1]. Identify the layers of the retina. Click link [2]. Identify the sclera and choroidea. Note the capillaries inside the choroidea and the innermost pigment epithelium layer of the choroidea. The cell processes of the pigment epithelium interdigitate with the outer segments of the rods (and cones). However, there is NO anatomical connection between these two layers. (Some textbooks erroneously give the pigment epithelium as part of the retina.)

Identify rod inner- and outer- segments and their nuclei. (Remember that rats are nocturnal and have only rods.)

Zoom out once and follow [1]. Identify the rest of the layers of the retina. In humans the bipolar cell layer is much thicker.

Zoom out once and reidentify the layers. The ganglion cell layer is so poorly preserved that we left it unlabeled, but it is the right-most layer on the screen. There are capillaries in this layer, but most of the nutrients and oxygen for the outer layers of the retina come from the choroid layer. That is why it is so important to treat retinal detachments promptly.

Click the thumbnail and follow [2,1]. The ciliary body produces the intraocular fluid and the lens-suspending zonulae. The intraocular fluid provides nutrients and oxygen to the lens and cornea which lack vasculature. It also is responsible for maintaining intraocular pressure. Since this eye is deformed, the pathway for exit of intraocular fluid is not visible, except for the final portion, the episcleral vein (identify). Note also that the iris is artificially squeezed against the cornea.

For the following review of interocular fluid circulation, you may want to return to Fig. 24-8 of your Junqueira textbook. The intraocular fluid produced by the ciliary body circulates through the posterior chamber and travels between the lens and iris into the anterior chamber. The intraocular fluid is then absorbed into the trabecular meshwork (located at the junction between the iris and cornea). From the trabecular meshwork the fluid flows into the canal of Schlemm, a circular tube located at the corneoscleral junction. The canal of Schlemm, in turn, empties into the episcleral veins.

Glaucoma (increased intraocular pressure) if not promptly treated can permanently damage the retina and lead to blindness. Glaucoma may be caused by overproduction of intraocular fluid or by inflammatory adhesions between the iris and lens. The most common cause of glaucoma, however, is a narrowing of the angle between the iris and the cornea, resulting in physical blockage of the trabecular meshwork.

Accommodation: The lens is suspended from the ciliary body by the zonulae (suspensory fibers). When the ciliary muscle inside the ciliary body is relaxed, the opening formed by it is larger, the zonulae are tight, and the lens is flattened to facilitate far vision. When the ciliary muscle contracts (accommodates) the circular opening formed by it becomes smaller, the zonulae become relaxed, and the lens, due to its elastic nature (especially the capsule), becomes more curved for near vision. This elasticity is usually lost with age.

w83d Retina, cow

Follow links [1,1] and identify the layers of the retina. Click link [2] and again identify the layers. Click link [1]. Even though the preservation is rather poor, you can identify the ribbon-like outer segments of the rods. Zoom out once and click link [2]. The outer segments of the cones are not preserved, but their inner segments, with their abundant mitochondria, are clearly visible. The outer limiting membrane (identify) is not really a membrane. Instead it is formed by the elaborate junctional complexes between the photoreceptors and Mueller cells (neuroglia of retina). It looks like a membrane in the LM, however.

The rods are the source of an interphotoreceptor retinoid-binding protein. This protein is exceptionally antigenic. A penetration wound to one eye can cause a severe bilateral inflammation of the uveal tract (sympathetic ophthalmia), and can lead to blindness or loss of both eyes.

Click on the thumbnail, follow [1,1,1], and identify the layers.

Practice: Try finding the objects we've just gone over using this unlabeled virtual slide: w83d Retina, cow.

w83f Lens, cow

This is a portion of the cow lens, but it also contains parts of the ciliary body and sclera. Identify these structures.

Follow [1,1] and identify the lens bow, where the anterior epithelial cells elongate and migrate into the lens stroma. This process occurs throughout life. Identify also the lens capsule.

Click link [1]. Eventually the nuclei of the lens fibers disappear and the fibers are closely stacked together in hexagonal arrays (identify). They lose all organelles except for junctional complexes between neighboring cells. The epithelium is present only on the anterior surface of the lens. The lens capsule is thinner at the dorsal surface. The zonulae from the ciliary body are attached to the lens capsule at the "edge" (equator) of the lens.

Practice: Try finding the objects we've just gone over using this unlabeled virtual slide: w83f Lens, cow.

w83g Iris, cow

The iris limits the amount of light that enters the eye.

Orient yourself with this low-power picture. Then click link [1] and identify the sphincter pupillae muscle. Click link [1] for a higher magnification view. Notice that the muscle extends all the way to the pupillary edge of the iris.

Click on the thumbnail and follow [1,2]. There is no epithelium on the anterior surface of the iris, but the posterior surface is composed of two, pigmented epithelial layers. Identify the stroma and the sphincter pupillae muscle. The color of the iris is determined by the amount of pigmented cells in the stroma. (This cow must therefore have had blue eyes.)

Click on the thumbnail and follow [2,1,1]. The dorsal-most epithelium (identify) of the iris is pigmented. The anterior-most of the two pigmented epithelia is also the dilator pupillary muscle (identify) with its prominent actin filaments. Both the iris muscles are of neuroectodermal origin.

Click on the thumbnail and follow [2]. Like the rat, the cow ciliary body seems to lack the ciliary muscle used by humans for accommodation.

Click on link [2]. The pars plicata of the ciliary body are sausage-like structures, and appear here as finger-like processes. Follow link [1]. The intraocular fluid is an ultrafiltrate of blood. Notice the capillaries present in this view. The epithelium is composed of two layers, the outer pigmented, and the inner non-pigmented. (Remember the naming convention for the eye.) The non-pigmented cells are thought to produce the zonulae. Even though it is not visible in the LM, there is a blood-ocular fluid barrier composed of junctional complexes between the abutting apices of the two epithelia.

Practice: Try finding the objects we've just gone over using this unlabeled virtual slide: w83g Iris, cow.

Practice with thie unlabeled virtual slide: 154 Eye. This is a very rare slide of the eye as it is preserved very well for nearly all structures. Cornea, pupil, lens anterior chamber, posterior chamber, sclera, choroid, retina, optic nerve, Vitreous body, extraocular muscle. Find the following: Corneal epithelium, Bowman's membrane, corneal stroma, Descemet’s membrane, corneal endothelium, anterior lens capsule, anterior lens epithelium, subcapsular epithelium, choroid, uvea, Limbus, ora serrate, ciliary body, inner nonpigmented epithelium, outer pigmented epithelium, zonula fibers, iris, pigmented epithelium, sphincter constrictor muscle (sphincter pupillae), dilator muscle, iridocorneal angle, Hyalocytes, trabecular meshwork, Canal of Schlemm Retina, ganglion cell layer, inner plexiform layer, bipolar cell layer (inner nuclear layer), outer plexiform layer, outer nuclear layer (nuclei of rods and cones), photoreceptors segments, pigmented epithelium, choroid.

w83h Eyelid, rat

Follow link [2]. The outside of the eyelid is covered by thin skin (identify). The inside of the eyelid is covered by non-keratinized epithelium called the palpebral conjunctiva (identify). (This is continuous with the conjunctiva covering the sclera.) The Meibomian gland (identify) is a large, modified sebaceous gland. Click link [3] for a higher power view of these structures. Then follow link [2] for a higher-power view of the conjunctival epithelium.

w82a Cochlea, chincilla

The inner ear, located inside the temporal bone (identify), is composed of tortuous canals and cavities: the bony labyrinth (identify). This extremely complex three-dimensional structure is impossible to visualize in this thin section. Therefore you are strongly urged to study the model that Martha Sweeney has, and Figures 24-24 and 24-28 in your Junquiera text.

This specimen is from the chinchilla whose temporal bone (identify) is much thinner than in humans. The chinchilla cochlea is longer than in humans, and it also "sits" differently in the head. The bony cochlea turns around its axis, which is spongy bone called the modiolus (identify). In humans this spiral makes 2.75 turns. Because it is a spiral, one sees several cross-sections in one single longitudinal section.

The bony labyrinth houses the membranous labyrinth where the sensory receptor cells are located. The membranous labyrinth, in turn, can be divided into two parts: the vestibular labyrinth housing the organs of equilibrium, and the cochlea housing the organ of hearing, the organ of Corti. In this low power view you can vaguely identify the organ of Corti and the ampulla of the vestibular system.

Click link [1]. The membranous labyrinth of the cochlea (scala media, identify) is filled with endolymph which resembles intracellular fluid. (The volume of the cochlear endolymph is only 2 microliters. This should give you an idea of the smallness of the inner ear.)

The membranous labyrinth divides the bony labyrinth into two compartments, scali vestibuli (identify) and tympani (identify) which are filled with perilymph that is similar to extracellular fluid elsewhere.

Follow link [1]. When seen in a cross-section, the scala media is roughly triangular with the following boundaries: The outer wall is composed of the stria vascularis (identify), a unique epithelium because it contains capillaries. It produces the endolymph. Between the stria and bone is the spiral ligament (identify, not a real ligament) which serves as an attachment of the membranous labyrinth to bone. The barrier toward the scala vestibuli is provided by the Reissner's (vestibular) membrane (identify) which is only two squamous cell layers thick. The base of the triangle (the partition toward the scala tympani) is composed of the basilar membrane (identify) and the lower ridge of the lamina spiralis ossea, which you will shortly see in a higher-power view. The organ of Corti (identify) rests on the basilar membrane.

Follow link [2] and identify the lamina spiralis ossea, which is an extension of the modiolus. Note the nerve fibers inside the lamina. It is important to note that the limbus spiralis (identify) sits on the bony lamina. The limbus spiralis is responsible for secreting and anchoring the tectorial membrane.

Zoom out once and click {1]. The jelly-like tectorial membrane (identify) lies over the sensory hair cells (identify).

Go to the thumbnail and click [1,1]. Sound produces a vibration in the basilar membrane (re-identify) and an unsynchronized movement of the tectorial membane because the origin of the tectorial membrane sits on the bony ridge.. The sensory cells have coherent bundles of giant microvilli at their apical surfaces. The tectorial membrane sits directly on these microvilli and the unsynchronized movement causes bending of the bundles, the initial event in the sensation of sound.

Go to the thumbnail and follow [1,1,1]. The organ of Corti is composed of auditory sensory receptors (hair cells) and several types of supporting cells. The endolymph bathes only the tops (where the giant microvilli are) of the sensory cells and supporting cells. These together form a limiting lamina. The bases of the sensory cells are located below this limiting lamina.

Click link [1]. The inner hair cells (identify), about 3,500 in number in humans, form a single row, are roundish, and are totally enclosed by supporting cells. At the present time it is thought that the inner hair cells are the main cells responsible for sound detection. It is also presently thought that, once damaged, these cells cannot be replaced. (Think about this the next time you attend a rock concert.) Some 95% of the afferent fibers from the cochlea originate at the bases of the inner hair cells.

Zoom out once and click [2]. The outer hair cells (identify) are cylindrical in shape and are supported by Deiter cells (identify) only at the base and at the top, and the major portion of the cell lies naked in the surrounding fluid (cortilymph?). The outer hair cells are arranged in three rows and their number is approximately 20,000. The outer hair cells have abundant actin and other cytoskeletal proteins in their cytoplasm, and are thought to be capable of shortening and lengthening. They are thought to move the basilar (or tectorial?) membrane and thus modify the tuning of inner hair cells. It is true that the inner ear emits sounds, and this is thought to be due to the outer hair cells shortening and lengthening. In fact these otoacoustic emissions can be used as hearing tests. This is particularly useful with newborns since they cannot cooperate.

Click on the thumbnail and follow [1,2,1]. The afferent fibers from the cochlea are actually the dendrites of the bipolar cells of the spiral ganglion. These bipolar neurons (identify) are the only neurons that have a myelin sheath (identify) surrounding the cell body. Axons (the acoustic nerve) from spiral ganglia terminate in the brainstem. The overwhelming majority of these axons are myelinated with 50 lamellae of myelin. This may be a structural basis for uniform conduction velocity.

The ear is not only for hearing. It also provides information on acceleration and the direction of gravity, and so is important for balance and coordination of movements.

Click the thumbnail and follow link [4]. Identify the crista ampullaris of a semicircular canal (most likely the posterior) and the macula utriculi inside the membranous vestibular labyrinth. It is also filled with endolymph.

Follow links [2,1] for a higher-power view of the crista ampullaris of a semicircular canal. The sensory epithelium contains two types of sensory cells, type I hair cells having been labeled. Type I cells may be the main sensory cells. The cupula, only part of which is shown here, is similar to the tectorial membrane in structure and function. It lags behind the endolymphatic movement inside the canal and bends the bundles of giant microvilli. Identify the afferent nerve fibers.

Zoom out twice and follow [1,1]. The otoconia (calcium carbonate crystals) lie directly over the giant microvilli of the sensory cells. Movement of the head causes the otoconia to bend the microvilli. Depending on the direction of the bending the sensory cells are either stimulated or inhibited.

Go to the thumbnail and click link [3]. Identify the acoustic nerve inside the modiolus. The stapedius muscle is also labeled. The vestibular and facial nerves (identify) run together in the internal acoustic meatus. The facial nerve in this specimen is small, possibly as a result of damage during dissection.

Take-home messages: The inner ear is well protected by the temporal bone, the hardest bone in our body. The inner ear is very small, the whole membranous labyrinth fitting onto a dime with space left over. The inner ear is filled with two kinds of fluid, the endolymph and the perilymph. The endolymph is similar to intracellular fluid in that it has very high concentrations of potassium. The cochlea is a spiral and is organized tonotopically. There are different kinds of sensory cells.

Practice: Try finding the objects we've just gone over using this unlabeled virtual slide: w82a Cochlea, chincilla. This slide is the same as in the Atlas and is very good for many structures of the cochlear and the vestibular apparatus; however, the next slide is better for the organ of Corti and the Maccula. This slide is good for the Crista ampullaris and the nerves. Do not study the organ of Corti in this slide. Find the following: Crista ampullaris, cupula (a little remains), type 1 hair cell, bony labyrinth, modiolus, temporal bone, scala vestibule, vestibular membrane, scala media, scala tympani, basilar membrane, stria vascularis, limbus spiralis, spiral ligament, nerve fibers, facial nerve, stapedius muscle, spiral ganglion, vestibulocochlear (acoustic) nerve.

Practice: Try finding the objects we've just gone over using this unlabeled virtual slide: Cochlea, inner ear. This slide is better for some structures and the H&E staining of cochlea cells, nerves and hair cells gives a good contrast. Some organs of Corti and the Macula are preserved very well. Find the following: Macula and otolith, hair cells, the bony labyrinth, modiolus, Helicotrema, the scala chambers, basilar membrane, limbus spiralis, spiral ligament, nerve fibers, spiral ganglion, vestibulocochlear (acoustic) nerve, organ of Corti, tectorial membrane, inner and outer hair cells, inner and outer pillar cell, support cells, Inner and Outer Phalangeal (Deiter cells), Cells of Hansen, Tunnel of Corti, Claudius cells, stria vascularis.