1. Classifications of sensory receptors. List the receptors that fit into each class.
2. Description of general features of the sense organs.
3. The major steps in the embryonic development of the eye.
4. 3 compartments in the eye, give the boundaries of each.
5. 3 basic tunics that make up the globe of the eye. List the major components of each, in order from anterior to posterior.
6. Description of sclera in terms of its predominant tissue type, its vascularity and the proportion of the eye it covers.
7. 5 layers of the cornea and description of composition of each.
8. Comparison of the rods and cones in terms of their visual acuity in bright and low light.
9. General characteristic features of the audiovestibular organ: the external, middle and internal ear.
10. Ultrastructure and functions of the tympanic membrane.
11. Bone and membranous labyrinthes of the internal ear.
12. Vestibular portion of the membranouth labyrinth – vestibular organ.
13. Ampullary crests, spots of the utricle and saccule: disposition, ultrastructure and functions.
14. The hearing organ. Membranous labyrinth of the cochlea.
15. Spiral Corti’s organ: disposition and functions.
16. Cells components of the Corti’s organ. Supporting cells: types,structure and disposition.
17. .Receptory cells of the Corti’s organ: types and ultrastructure.
18. Audiovestibular organ histophysiology.
The central nervous system receives some information from outside and the inner organs of the organism with the aid of senses. The sensations resulted from this information reflect something existing regardless of our consciousness – the objective reality of everything around us. Needless to say, without our sense ograns we would be completely helpless and unable to survive.
A vital function of the nervous system is the gathering of sensory information, which is derived from variety of specialized sensory nerve endings.
These include:
-sensory endings in the skin to detect touch (fine touch, pressure) pain and temperature;
-tendon endings and muscle spindles to detect movement and position of the limbs;
-chemoreceptive organs such as carotid body;
-sensory endings on the tongue to detect taste;
-sensory endings in the olfactory mucosa to detect smell.
In addition, information is obtained by the specialized sensory organs, the eye and the ear; the ear and the vestibular system detect sound, acceleration and position, and the eye perceives light.
The term sense organ means special organ, which can recognize some exact irritation from outside. Due to the origin and structure sense organs (or sensory structures) are classified in the next way:
1. Primary sensory (neurosensory) organs
a) smell organ;
b)visual organ
2. Secondary sensory (sensoepithelial) organs
a) taste buds;
b) audiovestibular organ
3. Sensory endings.
Analizator is common term, which means the neurophysiologic system, which consists of three components: sensory, connective and central. The first portion is present by sensory organ or ending, the last one – brain cortex of granular (sensory) type. They are connected by nerves, which resemble the intermediate part of analizator. Due to the type of sensation there such principal analizators in human body: visual, audiovestibular, smell, taste, touch, pressure, pain and so on.
Smell organ
Analizator of smell includes olfactory mucosa, olfactory nerve and field of cortex near hippocamp on the inner surface of hemispheres.
Olfactory mucosa lies in the roof of nasal cavity (superior concha and adjacent part of septum) and has yellowish brown color in contrast with pink color of the respiratory mucosa.
The olfactory epithelium is tall, pseudostratified columnar epithelium about
Sustentacular cells are most numerous, 50- to 60 mm-tall columnar cells whose apical aspects have a striated border composed of microvilli. Their oval nuclei lie in the apical part of the cell. There are secretory granules here housing a yellow pigment chatacteristic of the color of mucosa. Electron micrographs of sustentacular cells demonstrate their junctional complexes with the olfactory vesicle region of sensory cells as well as with contiguous sustentacular cells. These cells are believed to provide the physical support, nourishment and electrical insulation for the olfactory cells.
Olfactory cells are bipolar neurons whose apical aspect, the distal terminus of its slender dendrite, is modified to form bulb, the olfactory vesicle, which projects above the surface of the sustentacular cells. Their nuclei are spherical and lie basally. Six to eight long nonmotile olfactory cilia extend from the olfactory vesicle on the free surface of the epithelium. Axon arises from the basal part of olfactory cell, penetrates the basal lamina and joins similar axons to form bundles of nerve fibers. Each axon, although unmyelinated, has a sheath of Schwann cells. The nerve fibers pass through the cribrifirm plate in the roof of nasal cavity to synapse with secondary neurons in the olfactory bulb.
Basal cells are short, basophilic, pyramid-shaped cells, which lie near the basement membrane. Their nuclei are centrally located. These cells have considerable proliferative capacity and can replace sustentacular cells. Life span of last ones is less then a year in healthy person.
Few brush cells may be found in the olfactory mucosa with thick short microvilli and apparently contacting nerve fibers originating in the trigeminal (V) nerve. They probably receive some proper sensation from olfactory mucosa. 
The lamina propria of the olfactory epithelium contains lymph and venous plexuses, the former communicating with the suprachoroid space via capillaries running with the fila olfactoria. Branched tubuloalveolar serous glands (the glands of Bowman) lie in the lamina. They secrete a watery fluid, carried to the surface by slender ducts. This fluid acts as a solvent for odoriferous substances. Olfactory cells are frequently damaged owing to repeated exposure to infection and other trauma, and some cells may be loosed.
Visual organ
Visual organ – eye – is peripheral part of visual analizator.
The last one includes: sensory part (retina), connective n. opticus and occipital lobe cortex.
The eye receives a variety of visual stimuli that can have significant effect on human life and behavior. It is elegantly constructed transducer that converts light into electrical impulses and transmits these electrical impulses to the brain for processing. The eye also have a mechanism that form images. The eye is a complex and highly developed photosensitive organ that permits an accurate analysis of the form, light intensity, and color reflected from objects.
Internal structures of the human eye
The eye contains three concentric layers: an external layer – fibrous tunic (sclera and cornea); a middle layer – vascular or uveal tract, consisting of choroids, ciliary body and iris; and an inner layer of nerve tissue, the retina. The photosensitive retina proper communicates with the cerebrum through the optic nerve and extends forward to the ora serrata.
Diagram of the right eye, seen from above, showing the structure of the eye, retina, fovea, and ciliary body. An enlarged diagram of the fovea is shown at lower right: (1) axons of ganglion cells; (2) bipolar cells; (3) rods; (4) cones. Enlarged diagrams of the ciliary body (upper right) and retina (lower left) are also shown.
All structures of the eye may be organized in three systems or apparatuses due to their functions:
1. Dioptric (cornea, lens, vitreous body).
2. Visual (retina)
3. Accommodative (ciliary body)
Tunica fibrosa
The eye’s outermost tunic has two main components. The anterior surface forms the transparent cornea; the posterior – opaque (white) sclera. The junction between the cornea and sclera is the limbus. This tunic proves tough, fibroelastic support for the eye.
Cornea is transparent clear avascular disk bulging from the front of the eye, which has a smooth not uniformly curved surface. It belongs to the dioptric media of an eye.
Anatomically the cornea consists of the cornea proper and the limbus, a transition zone about
A cross section of the cornea shows five layers:
1. Anterior epithelium.
2. Anterior limiting membrane (Bowman’s)
3. Substantia propria.
4. Posterior limiting membrane (Descemet’s)
5. Endothelium.
Anterior epithelium is stratified squamous nonkeratinized epithelium
Bowman’s membrane is cell-free
Substantia propria, the stroma, forms 90 % of the cornea thickness. It is composed of lamellae of type I collagen fibers arranged in many layers, the lamellae at different angles. Collagen fibers within each lamella are arranged parallel one to another. Ground substance between lamellae contains chondroitin sulfate and keratin sulfate. Stellate flattened fibroblasts are the main type of cells here.
Posterior limiting membrane (Descemet’s) is 5 to
The corneal endothelium (posterior epithelium) is a simple cuboidal epithelium lining the internal surface of cornea. It is responsible for synthesis of proteins that are necessary for secreting and maintaining Descemet’s membrane. Excess fluid within the stroma is resorbed by this epithelium.
The cornea is avascular and its nutrition (and hydration) depends on diffusion from blood vessels in the limbus and from aqueous humor, through the endothelium.
Sclera is opaque white connective tissue, which covers the eye’e posterior five sixths. This is dense fibrous tissue, about 1mm thick. It is composed of flat bundles of collagen fibers, which lie mainly parallel to the surface.
The sclera has three layers:
1. episclera.
2 substantia propria
3. lamina fusca.
The sclera consists of dense fibro-elastic connective tissue, the fibres of which are arranged in bundles parallel to the surface. This layer contains little ground substance and few fibroblasts. The sclera varies in thickness, being thickest posteriorly and thinnest at the coronal equator of the globe.
Tunica vasculosa (uvea)
The middle tunic of the eye has three components: the choroids (posterior pert), ciliary body (middle) and iris (anterior).
The choroid is a layer of loose, highly vascular connective tissue lying between the sclera externally and the retina internally. The choroid and retina are separated by a thin membrane known as Bruch’s membrane which probably represents the basement membrane of the pigmented epithelium. The choroid contains numerous large, heavily pigmented melanocytes which confer the dense pigmentation characteristic of the choroid. The pigment absorbs light rays passing through the retina and prevents interference due to light reflection.
The coroid consists of loose connective tissue, which houses a dense network of blood vessels. Connective tissue cells and melanocytes are numerous. The latter give the choroid its dark colour. Small blood vessels are especially frequent in the innermost part of the choroid, which is called the choriocapillary layer. This layer supplies the retina with nutrients. Bruch’s membrane is located between the choroid and the retina. It consists of two layers of collagen fibres and a network of elastic fibres between them.
Choroids layers.
1. Suprachoroid.
2. Vascular lamina.
3.Choroidcapillaris.
4. Basal complex (membrane of Bruch)
Section of choroid and sclera. The choroid is a highly vascular layer (arrowheads) of connective tissue containing melanocytes that prevent the reflection of incident light. Many of the nutrients for the retina come from choroid blood vessels. The sclera is a dense layer of connective tissue rich in fibers of collagen type I, arranged in parallel bundles. Pararosaniline–toluidine blue (PT) stain. Medium magnification.
ciliary body
The ciliary body extends forward from the choroids as a ringlike triangular thickening at the level of the lens. It has same layers as the choroids but without choriocapillaris. Its primary structural components are the ciliary processes and the ciliary muscles. The ciliary processes are irregular epithelium covered connective tissue outgrowth of the ciliary body that extends toward the lens. They serve as origins for the fibers of the circular ligament of Zinn (zonule), which insert in edge of the lens to anchor it. The two layers of pigmented epithelium cover these processes derive from the layers of the optic cup. The inner ciliary epithelium borders the internal cavity of the eye. Its cells have the basolateral plasma membrane infoldings with ion- and water-transporting cells. They secrete aqueous humor, which flows through the pupil to the anterior chamber. From here, the fluid penetrates the nearest tissue to reach Schlemm’s canal. The deeper, simple columnar epithelial layer derives from the optic cup’s outer layer. The ciliary muscles comprise three smooth muscle bundles near the junction of the ciliary body and sclera. The contraction of all of these muscles pulls the ciliary body and choroids forward, releasing tension on the zonule and allowing the lens to become rounded for near vision. The relaxation of all groups increases tension on zonule, flattening the lens to allow focus on distant objects. The adjustment of individual muscles enables the eye to focus on intermediate distances.
The ciliary body is an inward extension of the choroidea at the level of the lens. Ciliary processes are short extensions of the ciliary body towards the lens. A small amount of loose connective tissue similar to that of the choroid is located between smooth muscle cells which form the bulk of the ciliary body. They form three bundles, the ciliary muscle.
The inner surface of the ciliary body and its processes are lined by two layers of columnar cells which belong to the retina – the ciliary epithelium formed by the pars ciliaris of the retina. The outer cell layer is pigmented, whereas the inner cell layer, i.e. the layer that faces the posterior chamber of the eye, is nonpigmented.
The ciliary processes contain a dense network of capillaries. The cells of the inner layer of the ciliary epithelium generate the aqueous humor of the eye. , i.e. they transport the plasma filtrate generated by the capillaries in the ciliary processes into the posterior chamber of the eye. Thight junctions between the cells form the blood – aqueous humor barrier.
Fibers, which consist of fibrillin, extend from the ciliary processes towards the lens and form the suspensory ligament of the lens. These fibres are also called zonule fibres. Two of the bundles of the ciliary muscles attach to the sclera and strech the ciliary body when they contract, thereby regulating the tension of the zonule fibres. The reduced tension will result in a thickening of the lens which focusses the lens on close objects – a process called accomodation.
Anterior view of the ciliary processes showing the zonules attaching to the lens. Zonule fibers are bundles of microfilaments (oxytalan fibers) from the elastic fiber system. The zonules form columns (A) on either side of the ciliary processes (B), which meet on a single site (C) as they attach to the lens.
Section of ciliary processes showing their double layer of pigmented and nonpigmented epithelial cells. Note also the core of connective tissue.
Section of a ciliary process.
Note the dark granules of melanin located in the cytoplasm of the inner epithelial cells. The outer epithelium is devoid of melanin. PT stain. High magnification.
iris
This structure controls the amount of light that reaches the retina and gives the eye its color. In front the lens, it projects as a flat ring from the ciliary body, leaving a circular opening – the pupil- at its center. The iris includes the most anterior extensions of the tunica vasculosa and tunica interna, forming the border between the anterior and posterior chambers. The anterior chamber lies between the cornea and iris; the posterior chamber is located between the iris and the lens-zonule complex.
Iris has next layers.
1. Anterior epithelium.
2. Anterior limiting membrane.
3. Vascular layer.
4. Posterior limiting membrane.
5. Posterior epithelium
The posterior surface of the iris is covered by the retina. The inner layer of the retina, i.e. the layer facing the posterior chamber, is called the posterior epithelium of the iris. Both layers of the retina are pigmented, but pigmentation is heavier in the inner layer. In the region of the central opening of the iris, the pupil, the retina extends for a very short distance onto the anterior surface of the iris. The iridial stroma consists of a vascularized loose connective tissue rich in melanocytes in addition to macrophages and fibrocytes, which are all surrounded by a loose meshwork of fine collagen fibers. The anterior surface of the iris is not covered by an epithelium – instead of we find a condensation of fibrocytes and melanocytes, the anterior border layer of the iris.
The iris forms the aperture of the eye. Myoepithelial cells in the outer (or anterior) layer of the retina, i.e. the layer adjacent to the stroma of the iris, have radially oriented muscular extensions. These extensions form a flat sheet immediately beneath the anterior layer of the retina, the dilator pupillae muscle. Embedded in the central portion of the iridial stroma are smooth muscle cells which form the annular sphincter pupillae muscle. In humans, this muscle surrounds the pupil as a less than
The pigmentation of cells in the stroma and anterior border layer of the iris determines to color of the eyes. If cells are heavily pigmented the eyes appear brown. If pigmentation is low the eyes appear blue. Intermediate levels create shades of green and grey.
Section of iris, a structure consisting of a core of connective tissue highly vascularized in certain regions (arrowheads). The outer covering layer is heavily pigmented to protect the eye’s interior from an excess of light. In contrast, the inner layer contains no pigmented cells. Dilator and constrictor (sphincter) pupillary muscles control the diameter of the pupil. PT stain. Medium magnification.
The anterior surface of the iris is rough and contains pigment cells and fibroblasts. Its stroma is a poorly vascularized connective tissue with fibroblasts and melanocytes. The vascular layer has a lot of blood vessels. The posterior surface is smooth, heavily pigmented, and continuous with the double-layered epithelium covering the ciliary processes.
Involuntary muscles. The sphincter pupillae is a ring of smooth muscle in the papillary margin that contracts under parasympathetic control to partly close the pupil. The dilator pupillae fibers, which extend like spokes between the ciliary body and the papillary margin, contract under sympathetic control to open pupil.
vitreous body
This transparent, gel-like body (mostly water and hyaluronan) fills the large vitreous space between the lens and the retina. Some peripheral fibris form its capsule. It contains a few macrophages and hyalocytes – stellate cells with oval nuclei that produce the fibrils and hyaluronan. During development, the central aretery extends from optic disk through the vitreous to the lens as hyaloid artery. It subsequently degenerates, leaving the narrow hyaloid canal.
lens
Lens is transparent, elastic, biconvex structure of the epithelial origin. It has no blood and nerve supply, beeourished by aqueous humor. It is suspended by the zonule of the ciliary body behind the pupil. Ciliary muscle contraction changes the curvature of the lens to enable focus on objects near or far, a process called accommodation. The lens has three main components.
1. The lens capsule is an elastic and transparent basal lamina that covers the entire lens and prevents wandering cells from penetrating it. It consists mainly offine type-III and type-IV collagen fibrils embedded in a glycoprotein- and glucosaminoglycan-rich matrix.
2. Subcapsular epithelium. The height of this low cuboidal epithelium beneath the capsule on the anterior lens surface increases to columnar near the lens equator, where cell division occurs. Its cells contain few organelles and form the lens fibers.
3. Lens fibers are long, narrow, hexagonal, specialized epithelial cells that make up most of lens. During differentiation, they loose their nuclei, fill with proteins crystalline (heat-schock), and develop a variety of specialized plasma membrane components, including junctional complexes and ridgelike processes.
The lens consists of a lens capsule, the subcapsular epithelium and lens fibres. It does not contain blood vessels or nerves.
The lens capsule is generated by the cells of the subcapsular epithelium and corresponds to a thick, elastic basal lamina. The zonule fibres insert into the lens capsule.
Cells of the subcapsular epithelium (or anterior lens cells) are mitotically active. In adult individuals they only cover the anterior “hemisphere” of the lens. As they divide, cells gradually move towards the equator of the lens where they tranform into lens fibres. The apical part of the gradually elongating cell extends between the subcapsular epithelium and adjacent lens fibres towards the anterior pole of the lens. The basal part extends towards the posterior pole. The nucleus remains close to the equatorial plane of the lens.
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The mature lens fibres, i.e. very long (up to
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The optical properties of the lens change from periphery to central parts, because of differences in the amounts of crystallins contained in lens fibres. These difference correct for distortions of colours and shapes (called spherical and chromatic aberrations) which commonly occur at the margins of glass lenses. These aberrations are easy to observe when you look through a loupe – or even iot-so-good microscopes at the margins of the field of view, where they are easy to detect when to slide is moved.
Section of the anterior portion of the lens. The subcapsular epithelium secretes the lens capsule, which appears stained in red. The lens capsule is a thick basement membrane containing collagen type IV and laminin. Below the subcapsular epithelium, note the lens fibers, which are cells that have lost their nuclei and organelles, becoming thin, elongated, transparent structures. Picrosirius-hematoxylin. Medium magnification.
Retina
Tunica interna (retina) is the innermost layer of eyeball. It comprises an anterior, nonsensitive portion, which lies over the ciliary body, and a posterior functional, or optical, portion, the photoreceptor organ. The optical (neural) retina lines the choroid from the papilla of the optic nerve posteriorly to the ora serata anteriorly.
Retina has 10 layers.
1. Pigment cell layer
2. Layer of rods and cones
3. External limiting membrane
4. External nuclear layer
5. External plexiform layer
6. Inner nuclear layer
7. Inner plexiform layer
8. Ganglionic layer
9. Layer of optic nerve fibers
10. Internal limiting membrane
Section of retina showing most of its components. PT stain. Low magnification.
The outermost layer (1) consists of the pigmented epithelial cells forming a single layer resting on Bruch’s membrane which separates them from the choroid superficially. Rod and cone processes of the photoreceptor cells comprise the next layer (2) with a thin eosinophilic structure known as the outer limiting membrane (3) separating them from a layer of densely packed nuclei described as the outer nuclear layer (4). The outer nuclear layer contains the cell bodies of the rod and cone photoreceptors. The almost featureless layer deep to this is known as the outer plexiform layer (5) and contains synaptic connections between the short axons of the photoreceptor cells and integrating neurones, the cell bodies of which lie in the inner nuclear layer (6). In the inner plexiform layer (7), the integrating neurones make synaptic connections with dendrites of neurones whose axons form the optic tract. The cell bodies of the optic tract neurones (sometimes called ganglion cells) comprise the ganglion cell layer (8). Deep to this is the layer of afferent fibres (9) passing towards the optic disc to form the optic nerve. Finally, the inner limiting membrane (10) demarcates the innermost aspect of the retina from the vitreous body.
The Neural Tunic: Retina
Similar to the retinal lining of the iris and ciliary body, the outer layer of the light sensitive retina forms a single layer of cuboidal cells – the pigment epithelium. The inner layer of the retina contains the photoreceptors, the first neurones which process the sensory information, and the neurones which convey the pre-processed sensory information to the central nervous system. Receptors, neurones, supporting cells and their processes are segregated into nine layers:
1. The layer of rods and cones contains the outer, rod- or cone-shaped light sensitive segements of the photoreceptive cells. The lights sensitive part and the perikayon of the rods and cones are connected by a narrowed bridge of cytoplasm. At the level of this connection the rods and cones are surrounded by the processes of a specialised type of glial cells, Müller cells, which form the
2. outer limiting membrane.
3. The outer nuclear layer contains the nuclei and perikarya of the rods and cones. Their processes form part of the
4. outer plexiform layer, where they form synapses with the processes of neurones whose cell bodies are located in the
5. inner nuclear layer. The cells of the inner nuclear layer are concerned with the initial processing of the sensory input. The three major neurone types are horizontal, bipolar and amacrine cells. The inner nuclear layer also houses the perikarya of the Müller cells.
6. The inner plexiform layer contains the processes of the inner nuclear layer neurones which convey the sensory input to the
7. ganglion cell layer. Ganglion cells are not evenly distributed. There are few of them towards the periphery of the retina. Close to the fovea, ganglion cells form a densely packed layer. Both ganglion cells and the cell bodies located in the inner nuclear layer which contact the rods and cones of the fovea are displaced towards the margins of the fovea.
8. Layer of optic nerve fibres. The axons of the ganglion cells travel in this layer towards the optic disc. Towards the optic disc, the thickness of this layer increases as more and more axons are added to it.
The inner limiting membrane corresponds to a basal lamina formed by the Müller cells.
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The three layers of retinal neurons. The arrows indicate the direction of the light path. The stimulation generated by the incident light on rods and cones proceeds in the opposite direction.
Functions of a retinal pigment epithelial cell. Note that the apical portion has abundant cell processes that fill the spaces between the outer segments of the photosensitive cells, and the membrane of the basal region has invaginations into the cytoplasm. This is a cell type with several functions, including the synthesis of melanin granules (by a process described in Chapter 18) that absorb stray light in the eye chamber. This is depicted on the right side of the figure, which shows the organelles that participate in melanin synthesis. On the left side of the figure, lysosomes containing enzymes synthesized in the rough endoplasmic reticulum (RER) coalesce with the phagocytized apical parts of the photoreceptor, digesting them. In addition to these activities, pigment cells are probably active in ion transport, since they maintain an electrical potential between the two surfaces of the epithelium membrane. The relatively well-developed smooth endoplasmic reticulum (SER) participates in the processes of vitamin A esterification.
The close association of Müller cells with neural elements in the sensory retina. Müller cells (dark fibrous cells) appear to be structurally and functionally equivalent to the astrocytes of the central nervous system, in that they envelop and support the neurons and nerve processes of the retina.
Photoreceptors. Both rods and cones are modified bipolar neurons, each with inner and outer segments lying outside the external limiting membrane, an outer conducting fiber (a dendrite) passing to the cell body in outer nuclear layer, and inner conducting fiber (an axon) extending into the outer plexiform layer. Human visual organ is invertile – light must traverse all the layer of retina to reach pigmental layer and then refract to photoreceptors). Rods have light-sensitive photopigment rhodopsin, composed of transmembrane protein orsin bound to cis retinal, the aldehide form of vitamin A. Cones contain photopigment iodopsin. There are three types of cones, each containing a different variety of pigment. Rodes are responsible for the black-white vision and cones – for color.
Ultrastructure of rods (at right) and cones (at left). The rectangular outlined region is shown in the electron micrograph in the next figure
Electron micrograph of a section of the retina. In the upper part of the figure are the inner segments. This photosensitive region consists of parallel membranous flat disks. Accumulation of mitochondria takes place in the inner segment (see Figure 24–13). In the middle of the figure is a basal body giving rise to a cilium that is further modified into an outer segment.
Electron micrograph of the interface between the photosensitive and pigmented layers in the retina. In the lower portion are parts of 2 pigment epithelium cells, revealing specialized junctions (J) between their lateral plasmalemmas. Above the pigment cells are the tips of several outer segments of rod cells that interdigitate with apical processes of the pigment epithelium (P). The large vacuoles containing flattened membranes (arrows) have been shed from the tips of the rods. L, lysosomal vesicles.
Accessory structures of the eye
The conjunctiva, eyelids and lacrimal glands are the accessory structures of the eye.
Structure of the eyelid.
Photomicrograph of a section of a lacrimal gland. Note the sections of the tubuloalveolar secretory portions, excretory ducts, and blood vessels. The secretory cells contain only small amounts of RNA (basophilia). They produce secretory material poor in proteins. H&E stain. Medium magnification.
Photomicrograph of a section of a lacrimal gland. Three small excretory ducts, 2 blood vessels, and numerous tubuloalveolar secretory units can be seen. PT stain. Medium magnification.
Optic Nerves
Because of the origin of the retinae and optic nerves from the developing forebrain, the optic nerves (cranial nerves II) corresponds to fibre tracts connecting parts of the CNS – in this case the ganglion cells of the retina with neurones in the lateral geniculate nucleus of the thalamus and neurones in the superior colliculus and pretectum of the midbrain.
Ganglions cell axons run towards the optic disc where they turn towards the sclera. Numerous bundles (or fascicles) of axons pass through the choroid and openings in the sclera, the lamina cribosa. The axons become myelinated in this region. Collectively, the bundles form the optic nerve. Like other parts of the CNS, the optic nerve is surrounded by the three meninges – the outer dura mater, the middle archnoid and the inner pia mater, which are separated from each other by subdural and subarachnoid spaces. At the eyeball, the dura fuses with the sclera while the arachnoid and pia mater merge with the choroid. Connective tissue septa, which arise from the pia mater, separate the fibre bundles in the optic nerve. The axons in the optic nerve are supported by astrocytes and oligodendrocytes. Microglia is also present.
Eyelid
The posterior (facing the eyeball) and anterior (facing the world) surfaces of the eyelid are also called conjunctival and cutaneous parts. The cutaneous part is covered by skin and contains sweat glands, sebaceous glands and, along the margins of the lid, 3-4 rows of hairs – the eyelashes. Modified apokrine sweat glands, the glands of Moll, empty into the follicles of the eyelashes. The eyelashes lack arrector pili muscles.
The inner, conjunctival part of the lids is lined by conjunctiva. Beneath the conjunctiva, large sebaceous glands are embedded in a plate of dense connective tisssue containing many elastic fibres. The plate and the sebaceous glands within it are called the tarsal plate the tarsal glands (also Meibomian glands). Extensive skeletal muscle bundles between the tarsal plate and the skin belong to the orbicularis oculi muscle.
Conjunctiva
The margins of the cornea merge with the conjunctiva. The conjunctiva extends over the the ‘white of the eye’, which corresponds to the anterior part of the sclera, folds back and continues over the posterior part of the eyelid. At the opening formed by the eyelids, the conjunctive merges with the skin which covers the anterior surface of the eyelids.
The epithelium of the conjunctiva varies from stratified squamous (most of it) to stratified columnar (at the reflection from the sclera to the eyelid). It contains goblet cells. The conjunctival epithelium rest on the loose connective tissue of its lamina propria.
Taste bud
Taste is a sensation perceived by taste buds, receptors located principallyon the tongue (there are about 10.000 taste buds on the human tongue) and in smaller numbers on the soft palate and laryngeal surface of the epiglottis. Lingual taste buds are embedded within the stratified epithelium of the circumvallate, foliate, and fungiform papillae. Chemicals enter the taste pore, a small aperture providing access to receptor cell.
Taste buds are composed of at least four types of cells, which can be distinguished with electron microscope. Type I and type II cells are tall, with microvilli at their surface. Type I cells are dark and contain apical secretory granules; type II cells have no granules. Although their function is unknown, they may support the activity of type III cells. The last ones (sensory cells) are also tall and are characterized by the presence of numerous vesicles in the basal part that resemble synaptic vesicles. The close proximity of dendritic processes of sensory nerves to these accumulations of synaptic vesicles is the basis for assigning taste reception to type III cells. Basal cells are relatively undifferentiated young cells, which proliferate and give rise to the other cells, are the fourth cell type
THE EAR
Audiovestibular organ is the perypheral portion of the audio and vestibular analizators, which posesses the second place after visual system by the information percieved from outside. The receptor cells of these organs are secondary sensory, because at first special sensoepithelial cells percept the irritation and then transmit them to the bipolar neurons.
Audiovestibular organ includes the external, middle and internal ear.
The external ear consists of auricle, external auditory meatus and tympanic membrane. The auricle (pinna) is funnel-like plate of elastic cartilage covered with skin, which collects and focuses sound waves toward the meatus. Auricle has typical shape due to a plate of elastic cartilage, 0,5 to 1mm thick, covered by e perichondrium with high content of elastic fibers. The surface of auricle is covered by the skin with poorly developed hairs and sweat glands. Modified apocrine sweat glands (ceruminous glands) secrete a waxy cerumen. Sounds gathered by the auricle are carried inward by the meatus to vibrate the tympanic membrane (eardum) covering its internal orifice. This membrane has three layers: the outer epithelium (thin skin), middle dense connective tissue (external radial and internal circular fibers), and inner cuboidal epithelium of the mucosa. Malleus is attached to the center of the internal surface of membrane. The upper portion of tympanic membranelacks collagenous fibers and is called the flaccid party (Shrapnell’s membrane) Notice: tympanic membrane is freely fixed to the wall of meatus, it has no its own rate of oscillation, which allows to transform sound wave into mechanical oscillation without changes.
The middle ear is cleft-like cavity in the temporal bone, the tympanic cavity, with a canal or duct, the auditory (Eustahian) tube, that connects it with nasopharynx.
Tympanic cavity is flat, box-like air space, whose wall is covered with simple squamous or low cuboidal epithelium resting on the lamina propria over periosteum. The cavity contains three ossicles (compact bones): malleus, incus and stapes suspended by tiny ligaments. Two small muscles lie here too: tensor tympani and stapedius muscle. The oval window in the medial wall is occupied by the basal plate of stapes, which separates the tympanic cavity from the scala vestibuli of the cochlea; the latter is filled with perilymph.
The inner ear lies in the temporal bone and consists of irregular bony cavities (bony labyrinth) with membrane-like structures inside – membranous labyrinth. The last one contains endolymph and is surrounded with perilymph.
Bony labyrinth consists of vestibule, three semicircular canals and cochlea.
Membranous labyrinth compounds are the next: the utricle and saccule in the vestibulum, three semicircular ducts and cochlear duct.
Vestibular apparatus comprises the utricle, saccule and semicircular ducts. Cochlear duct organ of Corty is responsible for the hearing.
Sensory cells of vestibular apparatus lie in the ampular cristae of canals and maculae in the utricle and saccule. Suppoting (sustentacular) cells and two types of sensory cells are found here.
Type I hair cell is piriform or flask-shaped, with a nucleus in the globular base and a short neck.
Type II hair cell is cylindrical, with not exact placed nucleus.
The both types of cells have special stereocilia over the apical surface and one motile kinocilium, all of them are embedded in cuticular plate with otoliths. In cristae, hairs are covered by cupulae, composed of a gelatinous material similar to otolithic membrane but lacking otoliths.
Sustentacular cells are tall columnar cells, lying between and around hair cells, and have irregular shape.
Structure of maculae.
Crista ampullaris. Top: Structure of the crista ampullaris. Bottom: Movements of the cupula in a crista ampullaris during rotational acceleration. Arrows indicate the direction of fluid movement. (Redrawn and reproduced, with permission, from Wersall J: Studies of the structure and innervation of the sensory epithelium of the cristae ampullares in the guinea pig.
Functional summary
Vestibular apparatus allows to recognize movement and location of human body.
Linear movements of the head cause displacement of the endolymph which disturbs the positions of otoliths within the otolithic membrane and consequently, the membrane itself, thereby bending the stereocilia of the hair cells. Movements of stereocilia are transduced ind action potentials, which are conducted by synapses to the vestibular portion of the vestibulo-cochlear nerve for transmitting to the brain.
Circular movements of the head are sensed by receptor sites in the semicircular ducts housed within the semicircular canals. Stereocilia of the sensoepithelial hair cells of the ampular crests are embedded in the gelatinous cupula. Movements of the endolymph within ducts change the orientation of the cupula, which subsequently distors the stereocilia of the hair cells. This mechanical stimuli are transduced in a nerve impulses, which are transferred by synapses to branches of the vestibular portion of vestibulo-cochlear nerve.
Vestibular portion of the VIII cranial nerve supplies dendrites to the cristae ampullares and maculae of utricle and saccule. Vestibular ganglion with bipolar cells lies in the wall of internal auditory meatus, their axons comprise the vestibular portion of the abovementioned nerve.
cochlea
The cochlea is spiral tube which has 2,5 turns around modiolus. Bony spiral lamina lies on the middle wall of this tube and spiral ligament on the outer. The basilar membrane extends from the spiral lamina to the outer wall, the vestibular membrane (Reissner’s ) passes between the spiral lamina and the outer wall above this. Thus there are three cavities: the scala vestibuli above, the scala tympany below, and the cochlear duct between this two. Two scalae are filled with perilymph, they communicate at the apex of cochlea by narrow cavity helicotrema. The cochlear duct of membranous labyrinth contains endolymph.
Spiral ganglion lies around the modiolus in the wall of bony cochlear duct. Spiral lamina arise over it. Ganglion is formed by proper bipolar nerve cells. Their myelinated axons run together to form the acoustic nerve. Myelinated dendrites lose their sheaths as they perforate the bone and pass to the organ of Corti, terminating hair cells.
Structure of the cochlea. Cochlear duct is triangular shaped, its walls are: upper – vestibular membrane, lower – basilar membrane, outer – stria vascularis (columnar epithelium with capillaries).
At a high magnification one can see the Corti’s organ, which is disposed on the low wall of the membranous labyrinth (basilar lamina) and consists of two types of the cells: sensory (receptors) and supporting. Due to the topography spiral organ cells are divided into the inner and outer groups, boarded by the inner tunnel, which is made of pillar cells. Ciliary cells are the receptory apparatus of Corti’s organ; they rest on the supporting cells. The outer supporting cells include phalangeal cells of Deiters (3-5 rows), outer supporting Hensen cells lie lateraly and then boarder Claudius cells are disposed. There are inner phalangeal cells inside from the inner pillar cells. All the supporting cells are resting on the basilar membrane. Inner sensory cells are oval-shaped and lie in one row. Outer sensory cells are cilindrical-shaped and are organized into 3-5 rows. Tectorial membrane lie up to the spiral organ and it is connected with limbus.
Scanning electron micrograph of 3 rows of outer hair cells (A) and a single row of inner hair cells (B) in the middle turn of a cat cochlear duct. x2700.
functional summary
Sound waves picked up by the external ear are converted to vibrations by the tympanic membrane, and these vibrations then are transmitted through the chain of middle ear ossicles to the fenestra ovalis, and hence to the perilymph of the vestibule. This sets up pressure waves in the perilymph of the scala vestibuli, that pass, at first, through the vestibular membrane in its floor to endolymph, in the cochlear duct, and then across the basilar membrane to perilymph of the scala tympani. The waves finally are dissipated at the round window. The passage of sound waves from scala vestibuli to scala tympani across the cochlear duct causes the oscillation of basilar membrane (different regions at different frequencies). The oscillation causes shearing forces between the tectorial membrane and the stereocilia of hair cells, with depolarization of the cells and result ierve impulses in the cochlear nerve, which pass to the auditory cortex.
Students’ Practical Activities
Students must know and illustrate such histologic specimens:
Specimen 1. Cornea.
Stained with heamatoxylin and Eosin.
The cornea is an avascular structure consisting of five layers. The outer surface is lined by stratified squamous epithelium about five cells thick which is not normally keratinised. This epithelium is supported by a specialised basement membrane known as Bowman’s membrane which is particularly prominent in man. The bulk of the cornea, the substantia propria, consists of a highly regular form of dense collagenous connective tissue forming thin lamellae. Fibroblasts and occasional leucocytes are scattered in the ground substance between the lamellae. The inner surface of the cornea is lined by a layer of flattened endothelial cells which are supported by a very thick elastic basement membrane known as Descemet’s membrane. Illustrate and indicate: 1.Stratified squamous epithelium. 2.Bowman’s membrane. 3.Substantia propria. 4.Descement’s membrane. 5.Layer of flattened endothelial cells.
Specimen 2. Wall of the eye.
Heamatoxylin and Eosin.
The three layers of the wall of the eye are illustrated in this specimen. The inner photosensitive retina is a multi – layered structure, the outermost limit of which is defined by a layer of pigmented epithelial cells.
Retina is made up of three basic cell types, neurons, pigmented epithelial cells and neuronesupport cells. Histologically, the retina is traditionally divided into 10 distinct histological layers.
Illustrate and indicate:
I.Sclera.
II.Choroid.
III.Retina:
1.Single layer of pigmented epithelial cells. 2.Rod and cone photoreceptor layer. 3.Outer limiting membrane. 4.Outer nuclear layer. 5.Outer plexiform layer. 6.Inner nuclear layer. 7.Inner plexiform layer. 8.Ganglion cell layer. 9.Layer of afferent fibers. 10.Inner limiting membrane.
Specimen 3. Cochlear axial section.
Stained with haematoxylin and eosin.
At a low magnification find the section of cochlea. Cochlea is the bony labyrinth, which make up 2,5 turns around bony axis and looks like the cockle-shell. Spiral osseous lamina arises from the osseous axis inside into the space of the channel. Spiral ligament (triangular-shaped bulge of the periosteum) lies on the opposite side. Membranous cochlear labyrinth is disposed inside in the bony channel and divides it into two floors: upper – vestibular or scala vestibule and lower – scala tympani. At a cross section membranous labyrinth is triangular-shaped structure, whose walls are vestibular membrane (upper), basilar membrane (lower) and vascular stripe (stria vascularis). Spiral Corti‘s organ is disposed on the basilar lamina.
Illustrate and indicate: 1. Osseous spiral lamina. 2. Spiral ligament. 3. Scala vestibuli. 4. Scala media: a) vestibular membrane; b) basilar membrane; c) stria vascularis. 5. Scala tympani. 6. Organ of Corti.
Specimen 4. Organ of Corti.
Stained with haematoxylin and eosin.
At a high magnification one can see the Corti’s organ, which is disposed on the low wall of the membranous labyrinth (basilar lamina) and consists of two types of the cells: sensory (receptors) and supporting. Due to the topography spiral organ cells are divided into the inner and outer groups, boarded by the inner tunnel, which is made of pillar cells. Ciliary cells are the receptory apparatus of Corti’s organ; they rest on the supporting cells. The outer supporting cells include phalangeal cells of Deiters (3-5 rows), outer supporting Hensen cells lie lateraly and then boarder Claudius cells are disposed. There are inner phalangeal cells inside from the inner pillar cells. All the supporting cells are resting on the basilar membrane. Inner sensory cells are oval-shaped and lie in one row. Outer sensory cells are cilindrical-shaped and are organized into 3-5 rows. Tectorial membrane lie up to the spiral organ and it is connected with limbus.
Illustrate and indicate: 1. Basilar membrane.2. Pillar cells. 3. Tunnel of Corti. 4. Inner sensory cells. 5. Outer sensory cells. 6. Outer phalangeal cells. 7. Inner phalangeal cells. 8. Outer supporting Hensen cells. 9. Outer boarder Claudius cells. 10. Tectorial membrane. 11. Spiral limbus. 12. Spiral ganglion.
References:
a) basic
1. Practical classes materials.
3. Stevens A. Human Histology / A. Stevens, J. Lowe. – [second edition]. –Mosby, 2000. – P. 377-383.
4. Wheter’s Functional Histology : A Text and Colour Atlas / [Young B., Lowe J., Stevens A., Heath J.]. – Elsevier Limited, 2006. – P. 400-425.
5. Ross M. Histology : A Text and Atlas / M. Ross W.Pawlina. – [sixth edition]. – Lippincott Williams and Wilkins, 2011. – P. 896-949.
b) additional
6. Eroschenko V.P. Atlas of Histology with functional correlations / Eroschenko V.P. [tenth edition]. – Lippincott Williams and Wilkins, 2008. – P. P. 491-507.
7. Charts:
http://intranet.tdmu.edu.ua/index.php?dir_name=kafedra&file_name=tl_34.php#n15
8. Volkov K. S. Ultrastructure of cells and tissues / K. S. Volkov, N. V. Pasechko. – Ternopil : Ukrmedknyha, 1997. – P. 20-23.
http://en.wikipedia.org/wiki/Histology
http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.html
http://www.udel.edu/biology/Wags/histopage/histopage.htm
Methodical instruction has been worked out by: ass. Lytvynyuk S.O.
