Eye development

As eye development in vertebrates, the structural and functional development and education of their visual organs is called. The investigation of these processes is part of developmental biology. The vertebrate eye is on the structure and performance quite marked differences depending on the species, while the stages of its genesis have basic similarities. In terms of its development process, the vertebrate eye is an example of an organ that is formed by a concatenation of genetic trigger events. These so-called inductions are so interlinked that the various components of the eye - such as lens, cornea and retina - in a decreasing order of development steps strictly ordered and mutually interrelated and thus act as an overall system. Evolution critics took a long time, that this would only independently may arise and consequently ontogenetic (individual) would also develop independently. That this view is now obsolete, to the findings have contributed to the development of the eye.

The vertebrate eye is a part of the brain, from which it grows as the optic vesicle (optical vesicles). In the head region, this results in a series of tissue interactions, which initiated the formation of the lens from the surface and their design is established. The development of the retina to several layers of light and color-sensitive photoreceptor cells and associated nerve cells and their connections with other brain shares is the most complex operation for eye development. Here there is an inverted, facing away from the light arrangement of the photoreceptors. The axons are looking for in a self-organizing process based on chemical signals their way in the brain, where their main strands partially cross over to the other side. The formation of appendages such as eye muscles, eyelids and lacrimal apparatus are subordinate processes, complete the development of the eye. It was not until long after birth this is completed with the coordination of eye movements, especially in animals with binocular vision, as well as the optimization of visual acuity.

• 4.6.1 External eye muscles
• 4.6.2 eyelids
• 4.6.3 lacrimal apparatus
• 4.6.4 pupil and inner eye muscles

The Evolution of the eye

Since the anatomy of the eye is not fossil- handed in detail and also the fossil record of the earliest vertebrates and their immediate ancestors is virtually unknown, based provided the following statements about the evolution of the vertebrate eye to

The evolution of the vertebrate eye can be roughly divided into six phases ( Fig. 1). Then in a first phase had already 600 million years ago simple bilateral animals rhabdomerartige ( brush-like ) and ciliary ( equipped with eyelashes ) developed photoreceptors with appropriate early forms of visual pigment protein opsin. In this light-sensitive visual pigments dyes ( chromophores ) are integrated, which determines the light - perception in animals ( phototaxis ) are. The receptors can thereby have been concentrated in so-called eye spots ( ocelli ) or distributed over the body.

In a second phase, 580-550 million years ago (late Proterozoic ) had developed the direct ancestors of the first vertebrates advanced ciliary photoreceptors with corresponding opsin protein. These were the photoreceptors of the living today closest relatives of vertebrates, the lancelet ( Branchiostoma ) and those of lanzettfischchenähnlichen larvae of tunicates ( Tunicata ), probably very similar.

In phase three, in about 550-530 million years ago (early Cambrian ), there was already a photoreceptor type with outer membrane and a suitable for a graded signal transmission at the synapse output. The tissue of the nervous knot in the head region ( "brain" ) was formed on both sides equipped with photoreceptors protuberances ( vesicles, optic vesicles ). This eye sores began to turn below einzustülpen cup- shaped, with the inside of the cup, the earliest form of the retina ( retina) represents. The invagination of the vesicle has been accompanied the addition of an early form of the retinal pigment epithelium to the " proto- retina ". In addition, the lens placode was homologous to the same embryonic lens system of higher vertebrates. However, the lens placode at first prevented only pigmentation above the Augenvesikel outer skin of the head, so that the outer skin was translucent at these locations. This early eye, before about 530 million years ago, even without the imaging capabilities of the retina that can be used to the recent hagfish ( Myxinoidea ), the most primitive extant vertebrates compared.

Next, the fourth section before about 530-500 million years ago ( mid- Cambrian ) evolved five different novel photoreceptor cells, cones, each with its own ciliary opsin, as well as bipolar and novel retinal ganglion cells ( so-called biplexiforme retinal ganglion cells) as a prerequisite for the more sophisticated signal transduction to the optic nerve. Bipolar cells and ganglion cells here are organized in a three -layered neural structure within the retina. By invagination of the lens placode in the optic cup and subsequent constriction arises the lens. Accommodation and Iris ( and thus the possibility of a limited change in size of the pupil) were added later, as well, for the eye movement, extra - ocular muscles with nerve connections. During this period, about 500 million years, so there was already an eye that was the almost all modern vertebrates in broad terms comparable. It had the design of a simple camera, so could see pictures and was the eye of today's lamprey ( Petromyzon ) most similar.

In phase five, before 500-430 million years ago (late Cambrian to late Silurian ) evolved myelin, which provides a faster signal transmission throughout the nervous system. In addition, another new type of photoreceptor, the rods that allow vision in low light. With these, the characteristic of vertebrates Rhodopsin appeared. The iris was highly contractile and pupil size could now be optimally adapted to the lighting conditions ( adaptation). On the inside of the eyeball muscles caused for the lens, which allowed an improved accommodation. This eye already relatively sophisticated probably featured the now-extinct armored jawless fish ( " Ostracodermi " ) and probably it was that eye is very similar to that in many of today's fish, and thus in jawed vertebrates ( Gnathostomen ), is to be found.

During the sixth and final phase, which began 430 years ago ezwa Millionan, even the basic version of the Eye of terrestrial vertebrates arose and Others ( tetrapods ). In the course of numerous adaptations of the fish-like vertebrate organism to have a life outside of the water, which began before about 375 million years ago (late Devonian), lens participated in an elliptical shape in cross section. This was necessary, because the light is refracted at the transition from air strongly into the cornea, as the transition of water into the cornea. To protect the eyes from drying in air, the eyelid was born.

In summary it can be said that the vertebrate eye of the simplest, only light-dark distinctive predecessor forms an evolutionary period of about 200 million years needed to modern, to see high-resolution, color images capable lens eye of most Gnathostomen. All the basic features that also characterize the eye of man could after 50 million years ago, at the end of the Devonian, have been already available. More than 200 million years later reduced a series of endothermic and thus to the nocturnal lifestyle enabled terrestrial vertebrates (eg owls or cats) for some unnecessary photoreceptors again and adjusted their retina or otherwise on the night vision on. In addition, also occur in other lines of development in gnathostomes specializations of the eye with appropriate modification of the Gnathostomen basic type.

The eye as an example of networked trigger processes

Thus, the genotypic and phenotypic processes come in eye development in transition and run in the correct order, we require a cascade of organized tissue interactions in the form of successive and interconnected release ( inductions ) (Fig. 2 and 12). Three specific DNA sequences are at the beginning of the chain. They contain a significant for the whole course of further development of the eye Gentypus, referred to as Schaltergen, master gene, Masterkontrollgen or transcription factor. Here are the genes Rx1 ( retinal homeobox gene), Six3 ( sine oculis gene) and most importantly - as measured by the frequent occurrence in the literature - discovered by Gehring 1995 gene Pax6 ( paired box gene 6 ).

Furthermore, the induction of the lens placode ( lens induction ) and thus the formation of the lens is being driven by two primary factors: first, the presence of the expression of Pax6 in the epidermis of the head and secondly the presence of the specific ectodermal tissue. The costs associated with Pax6 and other genes stages of early development of the eye lens are evolutionarily deeply rooted and across types often coincidental. Pax6 itself is completely identical in mice and man. The genes called Pax6, Rx1 and Six3 are a necessary and sufficient control circuit for the induction vertebrate eyes. By using the mouse Pax6 eyes could be induced initially in the fruit fly in an experiment shifted to the legs (ectopic ). This spectacular experiment, with the function of the homologous Pax6 gene eyeless fly was completely satisfied, demonstrated the high degree of conservation of Pax6. Same later managed at least to some vertebrate, including the chicken (1995 ) or by means of Sox3 in the clawed frog (Xenopus laevis) ( 2000). In these experiments, it came to the formation of ectopic lenses or placodes. The fact that the experiments have not led to results so complete as in the fruit fly, suggesting that the greater complexity of vertebrates. In any case, the eye development is omitted entirely in the vertebrate when Pax6 is suppressed ( Fig. 3).

These three master control genes form a stabilizing, genetic network, initiated by the new inductions and hundreds of other genes are activated. In the eye of the fruit fly there are 2000 genes. Alone, about the pigmentation of the iris, which is the color of eyes, requires at least 16 different genes. More inductions join in following the course of eye development. They each conduct extensive development steps one involving many downstream genes, such as the formation of the lens and the cornea ( Fig. 2).

The role of the gene Pax6

The granted after its discovery extreme special position of Pax6 as the Masterkontrollgen for eye development may be after 20 years reassessed. For the feature of the master gene Pax6 speaking, first, that it is on the other hand expresses a early, namely an eye stem cells in many tissues throughout the eye development, specifically in the fruit, in humans and squid. In these species from different phyla eye development is assumed to be independent. Pax6 can be considered therefore conserved since a common ancestor. Secondly, the reduction of its expression in reduced eye size in Drosophila, mouse and human. Third, Pax6 misexpression in certain tissues, eg in Drosophilaflügel or leg ectopic eyes cause.

For an outstanding or even sole Mastergenstellung of Pax6 in eye development speak the following facts: First, the elimination of Pax6 or the homologous gene Eyeless leads in Drosophila, which also belongs to the Pax6 family and in the fly has similar function, not only the loss of the eye but also on other parts of the brain, in the extreme case in Drosophila to the total head loss. Second, take other genes in addition to Pax6 key positions in the early eye development one, such as addition to the aforementioned Rx1 and Sine oculis ( Six ) and Eyes absent ( Eya ) and dachshund ( roof). These genes can also induce ectopic eyes. Your loss of function also leads to the loss of the eye. They thus exhibit similar Masterkontrollgen properties such as Pax6.

The known tribal overarching characteristics of Pax6 are summary, from the present perspective less questioned. They are, however, offset compared to the capabilities of other master genes today. It must therefore be said in the present state of the science of evolutionary conservation of the regulatory network of an entire group of genes.

Phases of eye development

Early initiation of eye development field

The lens of the vertebrate eye can be seen as a regrowing from the brain sensory organ. Already at the end of gastrulation, the first course for the development of the eye are provided. This is still at an early stage of embryonic development of the case when the formation of the three germ layers endoderm, mesoderm and ectoderm (inner layer, middle layer, outer layer) comes to a conclusion. In the eye as in the other sense organs, the ectoderm is the essential germ from which to develop the structures. In humans, these first steps happen from the 17th day of pregnancy.

The development of the shoe sole shaped neural plate at the gastrula (Fig. 4, light gray area ), resulting from the first the neural tube (Fig. 4, vertical center stripe) and it later the brain and spinal cord, is caused by the underlying mesoderm (induced ) and results in the formation first of a uniform field eyes on this flap (Fig. 4, purple). Said switches genes Rx1, Pax6 Six3 and are essential for the initiating steps. During the formation of the neural tube, the eye box is divided into two outer eyes domains controlled by the gene Sonic hedgehog ( SHH), which is activated in a center line between these two domains, and to suppress Pax6. Sonig hedgehog thus provides the explanation for the fact that the vertebrate has two eyes. If his expression at this critical point from cyclopia be developed. A lack of activation ( expression ) of that switch genes leads to loss of eye formation.

Augenvesikel and lens placode

As a result, it occurs in humans about the beginning of the second month of pregnancy to the eye fields to a two-sided protrusion of the anterior ectoderm and their outgrowth as optical optic vesicle from the diencephalon (Fig. 6 ), called optic stalk. Accordingly, the information here about incoming light initially reach the diencephalon, the processing is done in the cerebrum.

The evagination of the optic vesicles ( vesicles), based on individual cell migration. As first discovered in fish, the protein Rx3 are the eyes of progenitor cells with molecular signpost. They give these cells the information how they can move from the center of the brain in the direction of eye field, where there is greater accumulation of these cells. The Sprouting optic vesicle interacts with the outer layer and triggers a new important step induction there the formation of the lens placode from a thickening of the ectoderm, and this indentation of the eyes pit (Fig. 5 and 6). Without the vesicles would result ( with the exception of amphibians) no thickener and no lens. Through various mesodermal signalings and signals of the optical vesicle, the surface ectoderm is increasingly being prepared for the prospective lens formation. The fabric is first referred for lensing as competent and lens specifically in further steps. The fabric may only be lens after contact with the vesicle and their signals. Only the epidermis of the head (the epidermis) is thus able to respond to signals from the optical vesicle. In empirical experiments it was shown that a vesicle is planting as the Kopfektoderm to another region and grow there leaves, does not lead to lensing. But also transplanted surface ectoderm of the head does not lead to lens when there is missing the contact with the optic vesicle.

The thickening of the ectoderm leads to deformation of the vesicle into a cup, the optic cup ( Fig. 7). This ensures through appropriate induction signals that the first is not transparent lens is formed. After their initial formation, the surface ectoderm closes over the vesicles. The lens vesicle separates from the ectoderm and sinks into the depth ( Fig. 8).

Lens and cornea

The early lens, the company resulting from the lens placode lens vesicle is initially a hollow ball of cells surrounding (Fig. 9.1). Each of these cells contains a nucleus with chromosomes and DNA. The anterior side to the outside to the inside of the posterior of the eye directed (fig. 9.2). The cells are surrounded by a capsule with proteinaceous material (only shown in Figure 9.1 and 9.6). In a first step, extended from the fifth week in humans, the posterior cells into the cavity (Fig. 9.2, blue-gray). They form primary lens fibers, the later lens nucleus. The Anschichten around the central core is always done from the lens equator (Fig. 9.4). In extending these fibers form a plurality of proteins, the crystalline. This will fill the cavity of the lens and subsequently form with 3 types and 90 % share of all proteins of the lens whose main components. First, they form the lens fibers. As a result, the lens fiber cells build their nucleus and other organelles, including the energy centers ( mitochondria) (Fig. 9.3, blue). Thus, the cell metabolism drastically reduces the light scattering is minimized. This procedure does not, as usual, to programmed cell death (apoptosis). Due to these processes can and the Linsenszellen must not renew until death.

The anterior cells remain as a single- cell layer on the outer surface of the lens consist ( lens epithelium ), and in the fully developed lens. You constantly continue to divide, with the upper end of the lower man from the seventh week of secondary lens fibers occur (Fig. 9.3, red). This lens fibers are very long and are superimposed on the lens in concentric rings onion skin -like in many layers. These always grow new secondary lens fibers of said positions up and down around the lens (Fig. 9.4), displace the secondary lens fibers previously formed from inward while always new secondary lens fibers are generated (Fig. 9.4, brown), the equally grow around the lens. Continuous forms the anterior outer layer by cell division supplies material for this process. Due to the continuous formation of new rings, the lens can grow (Fig. 9.6). During the entire time of prenatal lens development a vascular, blood vessel -containing network is spread over this posterior and lateral, the tunica lentis vasculosa that disappears shortly after birth.

The formation of new secondary lens fibers continues throughout the entire life of the organism. Here, the lens does not increase more substantially, but increases in density. The developed lens contains a nucleus of early cells ( Fig. 9.6, light blue). With age, the elasticity of the lens decreases, while it loses more and more accommodative. The finished lens is the only organic tissue from completely transparent living cells.

The lens can be regenerated at a salamander. This is done by transdifferentiation, a stepwise regression of mesodermal cells at the edge of the iris to a previous state ( Wolffian lens regeneration). The lens is up to 18 times regenerable. Also, certain tissues of the iris and the neural retina can be regenerated in salamanders.

The next process after lens induction again done an induction, this time the lens to the surface ectoderm. She runs to a new thickening of the cornea (Fig. 5 and 8). In contrast to the cells of the lens corneal cells have an extremely short life span and also postnatal week to renew. The cornea is heavily imbued with nerves. The front cup rim is to the pupil. The cornea ( cornea ) is caused by a transformation of the Oberflächenektoderms in anterior epithelium. The choroid ( choroid ), sclera ( sclera ) existeth from the mesodermal mesenchyme of the head region. With the formation of the dermis can use the formation of blood vessels that run through the retina.

Retina

Before it comes to the differentiation of the retina is the tissue of a retinal field of undifferentiated precursor cells. Similar to the previous phases of the vesicles or lens induction orderly steps of cell differentiation must be established. All these retinal precursor cells express this purpose a common suite of transcription factors, which are genes which re- express other genes. These are Pax6, Six3, Six6, Lbx2, Hes1. The cells at this stage are still multipotent stem cells, which means that they can differentiate themselves even to different target cells. These are in addition to the partially light-conducting Müller cells later especially the photoreceptor cells as well as different types of nerve cells, which they called horizontal cells with one another interconnect or downstream of the signal flow form, such as bipolar cells, and modulate, such as amacrine cells before it reaches the ganglion cells of the retina, whose projections then can forward signals from the eye to other areas of the brain. The mechanisms to ensure an accurate here cell differentiation to the development of the retina, are both gene activities from the optical vesicle (intrinsic ) and from mesenchymen regions outside of the eye ( extrinsic). Here, fibroblast growth factors ( FGF) play an important role. A self-reinforcing Sonic hedgehog expression wave, the " sloshing " through the ganglion cell layer, causing the first ganglion cells to differentiate. Another Shh - wave, which is expressed throughout the inner layer is the launching pad for further differentiation of neuronal cells of the retina. Both discoveries were made in zebrafish.

The wall of the optic cup now consists of an outer and an inner sheet in which more later retinal layers form (Fig. 7 easy Fig. 10 inner layer detail ). The thin outwardly facing sheet (Fig. 8) forms the retinal pigment epithelium, which darkens absorbs light and is used for regeneration of sensory cells. The structure of the inner sheet is thicker described in more detail below. This neural retinal layer consists of nerve cells and is further divided into inner and outer sub-layers ( Figure 10). In the course of development is located in the neural layer, a further intermediate sub- layer of the bipolar cells of the retina. Your task is to collect the information of the light-sensitive photoreceptors ( rods and cones ) to prioritize and forward them to the ganglion cells of the retina inward (Fig. 10 left). To sum up, develop in the retina of the eye similar to other sensory organs, such as the ear, mainly three, superimposed cell layers here: receptor cells, bipolar cells and ganglion cells, whose axons project to regions of the brain. This arrangement applies to humans as for other vertebrates.

The formation of rods and cones is made on the outer side of the inner layer (Fig. 10, right, nuclei of the photoreceptors in front of white background layer, light- sensitive, elongated appendages in front of brown background layer ). The three different types of cones in humans which distinguish shades of light. The rods convey the intensity alone as brightness. Since only one type of rod is present in humans, no color impression can arise with him in the dusk. Nocturnal vertebrates have developed more types of nanorods.

The majority of complex retinal trend is observed in human cell growth in a coordinated wave from the middle of the third month to the 4th month. Then the optic nerve for adequate signal routing is fully myelinated. The yellow spot ( macula lutea) with the highest density of specific cells ( cones) begins to form after 8 months. It grows to about the birth further out. After five months, as the compound nerve of the eye is completed with the brain. The embryo is already at 7 month of pregnancy certain types of eye movements, the so-called Rapid Eye Movement ( REM), which supports the synchronization of the retina to the visual cortex of the brain.

Rear facing light (inverse ) position of the photoreceptors

The vertebrate eye is considered part of the brain, since its first plant seen from this. This is for example the octopus, not one of the vertebrates but the cephalopods, not the case where the eye is formed by invagination of the outer surface. The development process in the vertebrate with an inverted retina has several consequences: First, the inwardly bundled, leading to the brain optic nerve generates a blind spot because no light- sensitive photoreceptor cells are located at the point where it exits from the eye. Second, the nerve fibers, the nerve cells and blood vessels are located on the inner side towards the light -directed, so that the light it must pass through before reaching the photoreceptors. Third, the long extensions of the photoreceptor rods and cones are directed towards the pigment epithelium to the outside - that is away from the light. Thus, both the light must therefore pass through the overlying layers and unscattered the photoreceptors themselves, before it reaches the light-sensitive outer segments ( Fig. 10). In the octopus, the path simpler; with him the light hits directly on the receptors.

All other things being equal and well-trained components of the eye, the inverted retina structure of the vertebrate indicates a " suboptimal" evolutionary solution. The Octopus could possibly look better, since the incoming light signals are less obstacles in low light. However, evolutionary solutions need not be perfect according to the theory of evolution, they have to be so good that the species is sufficiently well adapted to their respective environments, in order to survive. The inverted lens eye is adapted for the night birds by improving the retinal characteristics at seeing in the dark.

The structural differences between vertebrate and octopus point at least in the structural component of the retina go to an independent, convergent history of this eye types. On the other side lie with the switch genes before matching, but at least similar and so homologous genetic basis. The developmental genetics of the eye are with respect to the simultaneous convergence and homology thus ambiguous clues to its evolutionary history. In other words, the eye photoreceptors and initiating gene networks to one or more times have occurred, certain design elements of the eye, such as lens or multilayer retina, multiple times, independently formed in each case.

Visual pathways and their components

In addition to rods and cones as photoreceptors of the eye, the retina is also several million nerve cells for a first information processing. In order for the eye to function as a sense organ, the incoming light information must be transmitted to the brain as a " higher-order evaluation stations ". First of ganglion cells form on the inner retinal layer (Fig. 11, left). These cells form Nerve fibers ( axons) that need to penetrate the layer of the retina and looking in the following specific target areas in the brain and find. The control of these topographical achievement is a self-organizing process ( axon guidance ). Complex chemical processes are responsible for: molecules in the retina and in the midbrain ( tectum ) form of stepped chemical gradients. Their help caused by diffusion concentration gradient to guide the growth direction of the axons. The axons are bundled into the blind spot and in mammals from there as the central nerve cord, the optic nerve ( optic nerve ), about the visual pathway with different neural structures to the visual center ( visual cortex) continued ( Fig. 11). Is just a stopover first the primary visual center for pre-processing and then the secondary visual center. In this way, there is a partial chiasm ( optic chiasm ). The nerve cells of the left eye reach the primary visual center of both the left and the right brain. The same applies to the nerve cells of the right eye. In the reception area of ​​the brain arriving in several single strands of nerve cells need to be further fanned out so that a precise processing is possible. Depending on the Origin lead the axons in various narrowly defined areas. The process is called retino - tectale projection. He is largely controlled by ephrins (gradients) and ephrin receptors. A map on the retina corresponds to a copy of this map in the brain. When not in mammals ( fish, amphibians, reptiles and birds) is a complete intersection of nerves is formed. In this case, all the axons of eye side are performed on each opposite side of the brain. The effect of the optic chiasm in the clawed frog Xenopus laevis can be shown experimentally by an eye cup is removed and reimplanted vice versa. There is an uncrossed mapping of retinal regions in the midbrain. The animal moved in search of food his tongue to the wrong places and learn only with time a correct orientation.

Appendages and pupil

Exterior eye muscles

In vertebrates, the inner distinguished depending on their function and location of the extraocular muscles. The outer, responsible for eye movements eye muscles arise together with the Tenon's capsule ( part of the ligaments ) and the fatty tissue of the eye socket (orbit ). They are common descendants of the embryonic connective tissue ( mesenchyme ) that surrounds the Augenvesikel, and are made from so-called somitomeres, certain Mesodermsegmenten of said body region in the embryo that grow on both sides ( Fig. 12). The later supplied by the oculomotor nerve eye muscles (upper straight muscle, lower straight muscle, inner, nasal situated, straight muscle and lower, oblique muscle) hail from, together with the levator of the foremost two somitomeres 1 and 2, the upper oblique muscle the third and lateral straight muscle, and the no longer existing in humans retractor of the eye, from the fifth Somitomer. The muscle cells of the myotomes of the somites migrate here in their target areas in the eyes, where then the muscle structures are formed.

The further development is controlled by three growth centers, each associated with a nerve. This creates the subsequent motor nerve supply ( innervation ) of the eye muscles through the three cranial nerve oculomotor nerve (III) trochlear nerve (IV) and Abducens (VI). The development of the extraocular muscles is dependent on a normal development of the eye socket, while the formation of the ligaments is independent. The eye muscles develops in humans late, only in the fifth month. A complete coordination of all forms of eye movements occurs only after birth in infancy.

Eyelids

In the 7th week the eyelids occur in the form of two folds of skin that grow from above and below the eye and are closed due to the bonding of their Epithelränder between the 10th week and the 7th month. At its margins will appear on the lashes, and it comes through Einsprossungen of epithelial cords in the mesenchyme to form the meibomian glands and minor. In this phase also arises as the " third eyelid " called the nictitating membrane in the nasal canthus. At the same forms from the head mesenchyme, the conjunctiva.

Lacrimal apparatus

In the 9th week of pregnancy attracts a number of Epithelsprossen from the lateral conjunctival sac into the underlying mesenchyme, from which the system of the lacrimal glands are formed. They are shared by the tendon of the levator palpebrae superioris in two different sized plants. From the so-called nasolacrimal groove that forms on the outer nasal wall approximately in the 7th week of pregnancy, caused the lacrimal drainage system. Although their erosion begins in the third month of pregnancy, but their discharge locations open until the 7th month of pregnancy.

Pupil and inner eye muscles

At about the eighth week of pregnancy forms in humans by the rounding of the optic cup opening, the pupil, which reacts under other than pinhole dynamically to light. Between the optic cup and the surface epithelium of the internal eye muscles, sphincter pupillae and dilator pupillae muscle arise. Your cells are derived from the ectodermal epithelial cells of the optic cup. The ciliary muscle of the eye is adjusted continuously on the different object distances, arises from the mesoderm in the choroid, and is considered to be a derivative of the neural crest.

In the final stage of pregnancy occurs in the embryo to pupillary responses that are already in the uterus are possible and necessary, contrary to previous perception. A dilation of the pupil by the softphone dilator muscle of the pupil, which is controlled by the sympathetic nervous system, a portion of the autonomic nervous system, can also be an expression of the extent of emotional arousal. The light- controlled reaction, the number of neurons in the retina. At the same time it regulates the development of blood vessels in the eyes. The photons in the womb activate in the mouse embryo, a protein melanopsin, which sets the normal development of blood vessels and neurons in motion.

Further development after birth

The development of the eye is not completed at birth. It has reached its full size until the beginning of puberty and learns in the first year, a number of changes ( Fig. 13). Thus, the field increases; the crystalline lens and macular pigmentation of the iris experienced structural improvements. A complete coordination of all forms of eye movements and thus the formation of binocular vision lasts until some months after birth. Many cells of the lateral geniculate nucleus, a part of the visual pathway, can not respond to input from the ganglion cells of the retina light stimuli. The visual acuity (visual acuity ) is not fully developed at birth on the basis of a still unstable central fixation. In fact, the visual acuity developed until about the age of 10.

Pathology

The most spectacular malformation is the Cyclops already mentioned, the cyclopia. By any failure of the divergence of the two eyes soaking facilities, a conglomerate of eye parts is in the middle of the upper half of the face (Fig.). Because of the associated brain malformation, the fetus is not viable. Incomplete closure of the embryonic optic cup results in cleft palate, coloboma, varying degrees - iris, choroid, Netzhautkolobom -. Virus diseases of the mother during the first trimester of pregnancy, but also taking some medications that can lead to developmental disorders. Well known is the clouding of the lens in addition to other damage caused by rubella infection in the 4th to 8th week of gestation, ie at the stage of lens development. Not infrequently and harmless are persistent remnants of the pupillary membrane as inhibition deformity.

Special features of selected vertebrates

Vertebrate eye must meet specific requirements, such as for the perception in the dark (cats, birds of night ) or a sharp vision at a great distance ( birds of prey ). Especially cats, but also dogs, horses and cattle have developed, for example, as a residual light amplifier for enhanced night vision capability, a retroreflective layer behind or in the middle of the retina, the tapetum lucidum ( mirror eye) ( Fig. 14). In raptors emerge other developmental differences. Their eyes are relatively large, which allows a high incidence of light and thus a large image of the Sehobjekts on the retina and brain. The large-area division of the floating object to a higher number of retinal cells leads to a more detailed and colorful image.

The eyes of birds of prey are on the front head side, so trained frontally, which allows the simultaneous perception of an object with both eyes. This arrangement permits binocular single vision, this is how the people who prerequisite for spatial vision.

For optimized sharp vision raptors develop a highly specialized neuromuscular accommodation. These fit fine ciliary muscles to the curvature of the lens at varying object distances. In addition, raptors develop next to the fovea a second, lateral fovea in the retina. Here is how in the central fovea, a compression of cones before. Finally, all birds have a comb-like eyes compartments inside the vitreous, the pecten oculi. This criss-crossed with narrow capillaries structure provides increased blood flow and nutrients to the retina.

The man looks sharp at different distances by changing the radius of curvature of the lens and in this way shifts the focus. The same effect is achieved snakes and fish, by changing the distance from the lens to the retina. A special muscle fish can pull the lens from hibernation towards the retina, snakes forward. Snakes do not have eyelids. Rather, the ocular surface is covered by a transparent scale. Differences also prevail in color perception. While man three types of cones forms ( trichromatic vision), most mammals, however, develop only two types of receptor ( dichromatic vision), reptiles and birds which they give four ( tetrachromatic vision), pigeons even five. Birds can unlike humans see ultraviolet light. Sharks, whales, dolphins and seals are color blind and have only one green - sensitive cone type.

Unique in the eye development of vertebrates is the migration of the two eyes in flatfish. Here, an eye during the early growth of the dorsal fin over or migrate through the base on the upper side of the body later. The hike can run both on the left side ( turbot ) and on the right side ( plaice, sole ).

Some turtles, including the False Map Turtle humps ( Graptemys pseudogeographica ) can turn their eyes around an imaginary axis connecting the pupil (Fig. 16 and 17). The center line of the eye thus remains mostly focused on the horizon, even if the animal is swimming up or down while looking into the swimming direction. At the level of the black center line of the retina has the highest density of receptors, thus the low to the ground or in the water living animal for seeing along the Horizontalinie is best adapted. This unique development is coordinated, probably by the sense of balance in the brain ( vestibular ) that controls specific muscles of the eye for it. A major challenge is the adaptation of the eyes of vertebrates that have to look good both under and above water, such as the four- eye. His cornea develops in two parts: the upper half is strongly curved for seeing through water, the lower half only very slightly curved for seeing under water ( Fig. 16). Thus, the different refractive power of air and water is taken into account and simultaneously good vision in air and water possible. The retina of the eye develops four divided in two. The responsible for seeing into the air side has twice as many pins as for seeing in the water.

Chameleons develop several outstanding characteristics of their eyes. These are independent of each mobile. It is suspected that there is an independent and separate processing of the information from both eyes in the brain. Chameleons also achieve an additional pinhole camera effect that allows them to focus on a kilometer through the little eye opening. Your focus speed is about four times faster than that of humans. Other features in vertebrate eyes are the spherical, focused on short distance at rest lens in fish, multifocal lenses in some cat species, the inclination of the retina to the lens in horses in a Gleitsichteffekt causes or the protective nictitating membrane in frogs, birds and dogs, rudimentary also in the nasal canthus in humans. Development processes and genetics of eye components described herein and differences in vertebrates are only poorly understood.

Chronology of scientific discoveries for eye development