LESSON 30 EYE.pdf

_____________ LESSON 30 _____________ SENSE ORGANS I. THE EYE The eye is a complex organ that has all its constituents adapted for image capture. This function is performed by stimulation of a specialized nerve membrane, called retina. The structures that make up the ocular architecture are arrang...

_____________ LESSON 30 _____________ SENSE ORGANS I. THE EYE The eye is a complex organ that has all its constituents adapted for image capture. This function is performed by stimulation of a specialized nerve membrane, called retina. The structures that make up the ocular architecture are arranged in two dark compartments filled with a refractive medium in a gel state, together with muscular structures (muscles of the eye) and fibroconnective structures like tunics, which provide the eye with both its form as its function. The eyeball is a vesicle with a tendency to sphericity, but in the case of equidae and bovids it has a small anteroposterior compression. The wall of the eyeball is made up of three layers associated with each other. The outermost is a fibrous ocular membrane or tunic, which is subdivided into two: the sclera and the cornea, which determine the anatomical shape of the eye. The middle layer is a vascular tunic, called the uvea, which in turn is subdivided into the choroid, the ciliary body and the iris. And the innermost layer, in contact with the posterior chamber of the eye, is a neuroepithelial tunic, which in the posterior part has a visual portion or retina, and in the anterior part a blind area lined by a simple epithelium, which projects to the surface of the ciliary body and the iris. In it the optic nerve branches out. The eyeball is divided into two compartments: the anterior and posterior compartments. The anterior compartment is located between the cornea and the vitreous body, contains the aqueous humor, and is divided into two chambers, the anterior chamber, between the cornea and iris, and the posterior chamber, between the iris and the vitreous body, and it contains the lens. The posterior compartment is filled by the vitreous body. Lens The lens is a biconvex disk of elastic consistency. However, its shape varies with the animal species and with age. It is located behind the iris and is 1 related to the ciliary body through the zonular fibers. It is made up of three parts: the capsule, the subcapsular epithelium and the fibers of the lens. 1.- The capsule is a homogeneous and refractile layer, which, in addition to enveloping the lens, favours the refraction of light. It is made up of type IV collagen fibers and glycoproteins (laminin, entactin and fibronectin, and proteoglycans such as heparan sulfate). In the anterior part, the capsule, like a basement membrane, lines the subcapsular epithelium of the lens. 2.- The subcapsular epithelium of the lens represents its growth zone due to its ability to form new fibers. These cells are located below the anterior capsule of the lens and are arranged as a layer of cuboidal cells, with their basal area in contact with the capsule while their apical portion is related to the fibers of the lens. 3.- The fibers of the lens are highly differentiated cells and are organized as a dense cell palisade, being joined together by the so-called fissure junctions, which are formed by a complex of small interdigitations with a narrow base and a very wide body that serves as anchors. These modified cells have lost all their organoids and have undergone a hyalinization of the matrix. Figure 1. Schematic representation of the structures of the eye. 2 I.1. Fibrous tunic: sclera and cornea The sclera is a pearly white fibrous membrane consisting mainly of bundles of collagen fibers that provide the eyeball with strong resistance. The cornea is the anterior continuation of the sclera and it is transparent and avascular. It occupies approximately 1/6 of the total of the outer tunic. In a cross section, 5 regions are distinguished: a) The anterior or corneal epithelium is squamous stratified and has six to eight cell layers. The most important characteristic of this epithelium is the presence of free nerve endings. The purpose of these nerve endings is to protect the eye against possible trauma, favouring the automatic closure of the organ. b) Basal membrane, highly developed, PAS +, it is made up of mucopolysaccharides. c) The corneal stroma or Bowman’s body is made up of different thick layers of collagen fibers parallel to each other and perpendicular to the contiguous layer. This distribution of the collagen fiber bundles is responsible for the transparency of the cornea. d) The Descemet´s membrane corresponds to a thickening of the basement membrane of the posterior epithelium, which is very refracting. e) The posterior epithelium is a simple squamous epithelium. I.2. Vascular tunic or Uvea: choroid, ciliary body and iris The uvea is the vascular tunic is made up of the choroid, the ciliary body, the iris and the iridocorneal angle. The choroid is a layer rich in blood vessels and the presence of pigmented cells containing melanin is frequent, which participates in the darkening inside the eye that prevents light from reflecting behind the vitreous body and projecting onto the retina. The ciliary body is an anterior thickening of the choroid made up of connective tissue and muscle tissue (ciliary muscle). It presents rostral projections called ciliary processes that participate in the elaboration of the aqueous humor. The iris is the most rostral expansion of the ciliary body. It is arranged in front of the lens and, together with it, separates the anterior chamber of the anterior compartment from the posterior chamber. It is composed of a highly vascularized loose connective tissue with pigmented cells and is arranged like a membrane with a central opening or pupil, through which light enters the eye. The iridocorneal angle has the function of drainage of aqueous humor. I.3. Nervous Tunic: Retina The retina is the neuroepithelial and photoreceptor membrane of the eyeball. 3 It is composed of a set of specialized neurons that are related to teloglial cells and whose nerve fibers are lined by oligodendrocytes, which is why this membrane is considered as one more component of central nervous tissue. The retina is made up of the retina proper, the macula lutea or fovea, the blind or ciliary retina and the blind spot or exit zone of the optic nerve. The optic retina is made up of photosensitive and intermediate neurons, which are arranged in 10 layers: the pigment epithelium forms the outermost layer, to which is associated the layer of rods and cones, which is related to bipolar neurons, and these, in turn, with ganglion neurons whose axons form the optic nerve. 1. Pigment epithelium 2. Layer of rods and cones 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. Optic nerve fiber layer 10. Inner limiting membrane The pigment epithelium is formed by cubical to columnar cells with numerous melanin granules, separated from the choroid by the Bruch or vitreous membrane. The epithelium has numerous anfractuosities in the basal area, induced by the capillaries of the choriocapillary layer, which project towards the retinal basal area. The epithelial cells show their apical border with numerous irregular microvilli, where the rods and cones are embedded. The pigment epithelium not only has the function of absorption of excess of light by melanin, but also carries out functions of transporting nutrients and metabolites between the capillaries of the choriocapillary layer and the layer of rods and cones. Likewise, this epithelium performs phagocytosis of degenerated parts of photosensitive neurons. The layer of rods and cones is formed by two types of photosensitive neurons, which although they have a similar constitution, differ morphologically, especially in their external segment. The rods have a nuclear zone, an inner segment, an outer segment and an axonal termination. The nucleus is the centre of the perikaryon, is large with abundant euchromatin and is located in the outer nuclear layer. 4 A B C Figure 2. Scheme and structure of the retina (A). Cones and rods (B). Outer segment (C). The inner segment, close to the perikaryon, is an elongated structure with numerous mitochondria and an apical portion where a centriole is located that serves as the basal body of the specialized cilium of the cell. In contact with the pigment epithelium, the outer segment is located, consisting of a lateral neck with microtubules similar to those of cilia. The axolemma dilates and forms a cylindrical or rod-like structure, inside which there are numerous flattened sacs or discs, stacked one on top of the other, with a very clear membrane and a regular lumen, which contain a visual pigment called rhodopsin. One of the details that defines this structure is the fact that the disks in the distal portions degenerate, detach and are phagocyted by the pigment epithelium; the discs are replaced in the basal area. These cells are related to night vision. The rods end in a coniform structure like an axon, which is related to the dendritic system of the bipolar cells of the upper layer. The axon-dendritic relationship of both cells is very complex and developed, forming part of the outer plexiform layer. The axon is very thin and ends in a thick dilation, in which there are a series of excavations that are occupied by the dendritic structures of the bipolar neurons. Microvesicles appear at the axonal termination that are involved in nerve transmission. In the membrane of the apical zone of the inner segment, these photosensitive cells are attached to the Müller cells by strong desmosomes and form the layer that is traditionally known as the outer limiting membrane. The cones have a very similar morphology to that of the rods, with a nuclear zone, the inner and outer segments and an axonal termination. These cells are also arranged as a palisade, are related by desmosomes to the Müller cells and participate in the formation of the outer limiting membrane. However, they have morphological and functional differences with the rods that are fundamentally located in the outer segment and determine the functional behaviour of both cells. Thus, the disks of the rods are regular and closed, while the disks located at the base of the cones may show unions with the membrane of the axolemma, maintaining a relationship with the external space. This fact 5 makes it possible to explain that the degradation of iodopsin (a visual pigment that is sensitive to different colours, in addition to regulating daytime vision) takes place within the outer segment of the cones, while the rhodopsin of rods is eliminated by detachment of the most apical discs that are phagocytosed by the cells of the pigment epithelium. Similarly, another difference between the two cells is that the axon and its synaptic bulb are highly developed in rods, while in cones have a triangular and sometimes branched shape. The distribution and number of rods and cones in the retina of different animals vary depending on the type of vision they have. Thus, in many nocturnal vertebrates only rods are differentiated and in diurnal birds, on the contrary, cones predominate. Bipolar cells contain two cytoplasmic processes, one corresponding to the axon and the other to the dendrite. Rod bipolar neurons are usually large, and the outer plexiform layer is part of their dendrites. They are related to several rods at the same time. Bipolar cells, which are related to cones, are very small, hence they are called dwarf bipolar cells, and only have a relationship with a cone. There are others, flat bipolar cells, that synapse with several cones at the same time. In general, bipolar neurons have synaptic contacts with ganglion and amacrine cells forming the inner plexiform layer. Horizontal cells have a large, polygonal perikaryon and emit long and thin cytoplasmic processes. Their axonal processes contact with the dendrites of bipolar cells and their dendrites with the axonal terminations of the rods and cones. The amacrine cells, described by Cajal, are characterized by the absence of axon. Their soma is small and oval. Their dendritic processes are long and interrelate with axonal endings, mainly of bipolar neurons. The soma of bipolar, horizontal and amacrine cells are located at the inner nuclear layer. The ganglion cell layer is made up of the ganglion cell bodies. These neurons are spherical and large, have a central euchromatic nucleus and numerous organoids in the cytoplasm. Their axons form the fibers of the optic nerve. Covering the axons there is a thick basement membrane that constitutes the inner limiting membrane. The retina has areas that differ from the rest by their sensitivity to light. The macula lutea (fovea) is the area of maximum acuity and contains almost only cones. The ciliary or blind retina is located in the rostral area of the retina, is considered as the thinner portion of the retina and is limited by the hora serrata. The blind spot of the retina lacks retinal cells of any type, on the contrary, it is constituted by the nerve fibers that make up the optic nerve. 6