Embryology of the Eye and Ear PDF

DEVELOPMENT OF THE SENSE ORGANS  Developmen t of the eye Parts of the eye 1.Retina 2.Iris 3.ciliary body 4.Lens 5.Optic nerve etc 6.Sclera 7.Choroid 8.Cornea 9.Pupil 2 3  Development of the eye The eye is a highly specialized sensory organ derived from neural ect...

DEVELOPMENT OF THE SENSE ORGANS  Developmen t of the eye Parts of the eye 1.Retina 2.Iris 3.ciliary body 4.Lens 5.Optic nerve etc 6.Sclera 7.Choroid 8.Cornea 9.Pupil 2 3  Development of the eye The eye is a highly specialized sensory organ derived from neural ectoderm, mesoderm and surface ectoderm. The neural ectoderm gives rise to: a. Optic vesicle & optic cup b. Retina c. Optic nerve fibers d. Iris. The surface ectoderm give rise to: a. Lens b. Lacrimal & tarsal glands. c. Epithelium of the cornea d. Conjunctiva e. Epidermis of the eyelid The mesoderm give rise to: f. Cornea stroma g. Sclera h. Choroid i. Ciliary muscle j. Parts of vitreous k. Muscles lining anterior chamber. Formation of optic vesicle  On the 22nd day of embryonic development, the region of the neural tube destined to form the prosencephalon (diencephalon) shows a linear thickened area on either side  This area soon becomes depressed to form the optic sulcus / groove.  With closure of the neural tube, as these optic grooves deepens, the wall of the prosencehalon (future diencephalon) overlying the groove bulges outward to form hollow outpocketings (diverticula) called optic vesicles  Formation of the optic vesicles The proximal part of the optic vesicle becomes constricted and elongated to form the optic stalk As the optic vesicle grows laterally, their lateral walls make contact with the surface ectoderm The surface ectoderm overlying the optic vesicle, becomes thickened to form the lens placode 7 Formation of Optic Stalk & Lens Vesicle  The lens placodes invaginate as they sink deep to the surface ectoderm, forming lens pits.  The edges of the pits approach each other and fuse to form spherical lens vesicles which soon lose their connection with the surface ectoderm  Thus the lens vesicle becomes completely separated from the surface ectoderm  Formation of the optic cup  While the lens vesicle is forming, the optic vesicle begins to invaginate & becomes converted into a double-layered optic cup having an outer and inner layers  The conversion of the optic vesicle to the optic cup is as a result of differential growth of the wall of the vesicle. 9  The outer and inner layers of Formation of the optic this cup are initially cup separated by a lumen called the intraretinal space  but with development, the walls of the optic cup become apposed resulting in the gradual disappearance of the intervening space 10  Formation of the optic cup  The margins of the optic cup grows over the upper and sides of the lens to enclose it  Such overgrowth does not take place in the inferior aspect of the lens, as a result of which the wall of the cup shows a deficiency  This deficiency extends for some distance along the inferior surface of the optic stalk and is called the choroidal or fetal fissure. 11  The developing neural tube is surrounded by mesoderm, which consequently condenses to form the meninges An extension of this mesoderm covers the optic vesicle, thus the eye primordium is completely surrounded by mesoderm. 12  Later this mesoderm differentiates to form a superficial fibrous layer (corresponding to dura) and a deeper vascular layer (corresponding to pia-arachnoid) This mesoderm not only surrounds the eye primordium from the outside but also invades the inside of the optic cup by way of the choroid fissure 13 The mesoderm within the choroid fissure and the optic cup forms the hyaloid vessels which during intrauterine life supply the lens and forms the vascular layer on the inner surface of the retina. Lens A B Ora serrata located about here Ciliary Body Iris Lens Lens Iris Central artery C of retina D NEXT SLIDE The hyaloid vessels in this region are obliterated and disappear during fetal life, leaving behind the hyaloid canal The hyaloid vessels within the optic stalk becomes the central artery and vein of the retina Lens A B Ora serrata located about here Ciliary Body Iris Lens Lens Iris Central artery C of retina D NEXT SLIDE  It develops from the double layers Retina of the optic cup The outer wall of the optic cup remains thin and becomes pigmented forming the pigmented layer of the retina It also contributes to the formation of the ciliary body and iris The inner layer of the optic cup forms the neural layer of the retina 16  Inner neural layer of the retina can be divided into the anterior one-fifth and posterior four fifth. The anterior part remains thin and forms an epithelial covering for the ciliary body and iris 17  In the posterior part, the inner neural layer of the optic cup differentiates into matrix cells, mantle and marginal layers. The cells of the matrix layer forms the rods and cones The cells of the mantle layer differentiates into biploar cells, the ganglion cells and other neurons of the retina, and also the supporting elements. 18  The axons of the ganglion cells grow into the marginal layer to form the layer of nerve fibers These nerve fibers in the marginal layer converge toward into the optic stalk , which gradually forms the optic nerve 19  Ciliary Body & Iris Develop from the inner & outer layers of the optic cup. Mesenchyme Lens Lens A B Ciliary Body Iris Lens Lens Iris Central artery C of retina D NEXT SLIDE  Ciliary Body [= processes + muscle] From e c hym mesenchyme se n me Anterior chamber Ciliary epithelium Primary [pigmented & (posterior) Posterior non-pigmented lens fibers chamber Anterior lens fibers Ciliary body develops from the anterior part of the two layers of the optic cup. Stroma of the ciliary boy, Ciliary muscle (smooth muscle), and blood vessels develops from vascular mesenchyme surrounding the optic cup  Iris hy me s e nc me From neuroectoderm Anterior chamber Primary (posterior) Posterior lens fibers chamber Anterior lens fibers STROMAL LAYER OF IRIS Is formed by the marginal region of the optic cup. (Epithelial layer forms from the most rostral aspect of optic cup; 2 cell layers thick; posterior side of iris; blue pigment) Stromal or connective tissue layer and blood vessels of the iris develops from mesenchyme present anterior anterior side of iris (optic cup). Sphincter & Dilator muscles of the pupil develop within the stromal layer of the iris from neuroectoderm of the optic cup.  Lens The lens develops from the lens vesicle The vesicle is first lined by a single layer of cuboidal cells The cells in the anterior wall of the vesicle remains cuboidal 23  Lens Those in the posterior wall gradually elongates, thus encroaching upon the cavity of the vesicle and the cavity is eventually obliterated The elongated cells of the posterior wall lose their nuclei and are converted into the fibers of the lens. The anterior layer remains as the epithelium covering this aspect of the lens 24  The mesoderm surrounding the optic Choroid & Sclera cup differentiates into an inner pigmented vascular layer and an outer fibrous layer The inner pigmented layer forms the choroid The choroid is in contact with the outer pigmented layer of the optic cup The outer fibrous layer forms the sclera The sclera is continuous with the dura mater which ensheath the optic nerve at the point where the nerve enters the optic foramen of the skull 25 Vitreous Body The loose mesenchyme tissue which surrounds the optic cup migrates into the cavity of the optic cup (position between the retina and lens) forming a delicate network of fibers between the lens and retina. The interstitial spaces of this network is later filled with a transparent gelatinous substance, forming the vitreous body. 26 Anterior chamber Vacuolation occurs in the mesenchymal cells in the region between the lens and the surface ectoderm forming a cavity The resultant cavity forms the anterior chamber which split the mesenchyme into two layers 27 Anterior chamber The outer wall of the chamber is continuous with the sclera and the inner wall is continuous with the choriod. The outer layer of the mesoderm gives rise to the substantia propria and the mesothelium of the cornea 28  The outer epithelial layer of the cornea is derived from the surface ectoderm The inner layer lies in front of the lens and iris forms the irido- pupillary membrane which, in its central part is in contact with the anterior surface of the lens. 29  Mesenchymal tissue on the anterior portion of the retina contributes to the formation of the connective tissue element of the iris and the dilator and sphincter muscles of the pupil The region of the non visual retina between the iris and the visual retina becomes folded forming the ciliary processes Mesenchymal cells between the ciliary processes and the lens form radially-arranged fibers, the suspensory ligament of the lens 30  During the late foetal period, the irido-pupillary membrane breaks down and the space bounded by the lens, suspensory ligament and iris forms the posterior chamber of the eye Ciliary muscles control the tension of the suspensory ligament. This ligament in turn regulates the shape of the lens and thereby visual accommodation 31  Eyelid Is formed by reduplication of the Accessory surface ectoderm above and below the cornea structures of eyeball The ectodermal folds formed contain some mesoderm that gives rise to muscle and to tarsal plates As the folds enlarge, their margins approach each other , meet and fuse together The fusion of the eyelid is a temporary union as they separate again before or shortly after birth 32  Separation of the eyelids occurs around the 20th week of gestation The fusion of the lids cut off a space called the conjunctival sac. The conjuctiva is thus of ectodermal origin The conjunctiva lining the eyelids is referred to as the palpebral conjunctiva and that covering the sclera is known as the bulbar conjunctiva33  The lacrimal gland is formed from a number of buds that arises from the upper angle of the conjunctival sac. Soon after birth, the lacrimal gland begins to secrete a watery fluid into the conjunctival sac which lubricates the cornea Both deep and superficial lacrimal glands may develop in association with the third eyelid 34  Superficial rod-like cords of ectoderm extends from the medial canthi of the eyelids to the developing nasal pit. As these cord lose their contact with the surface and move deeper into the underlying mesenchyme, they become canalized to form the naso-lacrimal duct. The rostral end of each duct opens into the corresponding developing nasal cavity The proximal part bifucates at the medial canthus giving a branch to each eyelid. These duct become the lacrimal canaliculi 35  The opening of these canaliculi on the eyelids are named the lacrimal puncta A dilatation of the nasolacrimal duct close to its bifurcation forms the lacrimal sac. The lacrimal glands and their associated duct systems constitute the lacrimal apparatus 36  Anomalies of the eye Coloboma Non-closure of the lips of the choroidal fissure results in a defect on the ventral surface of the eyeball referred to as coloboma. Failure of the fissure to close may occur anywhere along its length. Coloboma iridis: is a common eye abnormality frequently asscociated with other eye defects Colobomas (clefts) of the eyelids may also occur 37  Anomalies of the eye Corneal opacity Results from persistent iridopupillary membrane instead of being resorbed during formation of the anterior chamber. Congenital cataract In congenital cataracts the lens becomes opaque during intrauterine. The condition though genetically determined, many children of mothers who have had German measles between the 4th & 7th weeks of pregnancy have cataracts. 38  Anomalies of the eye Anophthalmia Failure of the eyeball to develop due to developmental failure of the optic vesicle Microphthalmos: In microphthalmia, the eye is too small. (Abnormal reduction in ocular size). It may be unilateral or bilateral, sometimes in association with other ocular abnormalities. 39  The hyaloid artery may persist to form a cord or cyst Congenital aphakia (absence of the lens) & aniridia (absence of the iris) are rare anomalies 40  Cyclop (single eye) & synophthalmia (fusion of the eyes). It comprise a spectrum of defects in which the eyes are partially or completely fused. These defects are invariably associated with cranial defects, such as holoprosencephaly, in which the cerebral hemispheres are partially or completely merged into a single telencephalic vesicle Blue sclera Thin sclera through which the pigment of choriod can be seen Anomalies of pigmentation/albinism 41 42 Embryology of the ear DEVELOPMENT OF EARS The ears are composed of three parts: 1. External ear 2. Middle ear 3. Internal ear Each part embryologically different 44 DEVELOPMENT OF EARS The external ear directs sound towards the middle ear and is formed from the first pharyngeal cleft and its surrounding mesoderm The external ear consist of the auricle (pinna), external auditory meatus and the outer lining of the tympanic membrane 45 DEVELOPMENT OF EARS The middle ear conducts sound from external to the inner ear and is derived from the first pharyngeal pouch and its surrounding mesoderm The components of the middle ear are: auditory tube (Eustachain tube), tympanic cavity and its associated auditory ossicles 46 DEVELOPMENT OF EARS The inner ear consist of vestibulocochlear organ, which functions in hearing and balance 47 Embryology of the inner ear  The internal ears are the first to develop. Internal ears Early in the 4th week , the surface ectoderm overlying the developing hindbrain (mylencephalon) on its lateral aspect shows a bilateral thickenings, the otic placodes The otic placode soon invaginates and sinks below the surface ectoderm into the underlying mesoderm to form an otic pit 49  The edges of the otic pit come together and fuse to form an otic vesicle or auditory vesicle or otocysts The otic vesicle is the primordium of the membranous labyrinth The otic vesicle soon loses its connection with the surface ectoderm and a diverticulum grows from the vesicle and elongates to form endolymphatic duct and sac. 50 The cavity of the otic vesicle is filled with fluid referred to as the endolymph 51  The otic vesicle differentiates into two distinct regions; a dorsal utricular portion and a ventral saccular portion. a. A ventral component – gives rise to the saccule, cochlear duct and spiral ganglion of vestibulocochlear nerve b. A dorsal component – form the utricle, semicircular canals, endolymphatic duct and sac. 52  In the 6th week of development, three disc-like diverticula / outpocketings grow out from the utricular parts of the otic vesicle Central portions of the these diverticula fuse to each other and then disappear The end of each semicircular duct The peripheral unfused portions of shows localized dilatations, the the diverticula becoes the ampullae. semicircular ducts which are attached to the utricle and are later enclosed in the semicircular canals of the bony labyrinth 53  Specialized receptor areas differentiate in the ampullae, the utricle and the saccule Ampullae – Cristae ampullares Utricle - Maculae utriculi Saccule - Maculae sacculi Impulses generated in sensory cells of the cristae and maculae as a result of change in position of the body are carried to the brain by vestibular fibers of cranial nerve Vlll 54  In the 6th week of development, the saccule forms a tubular-shaped outpocketing at its lower pole, the cochlear duct The duct elongates progressively and penetrates the surrounding mesenchyme 2.5 turns in a spiral manner by 8 weeks. Its connection with the remaining portion of the saccule is the confined to a narrow pathway, the ductus reuniens 55  The spiral organ differentiates from cells in the walls of the cochlear duct Ganglion cells of the vestibulocochlear nerve migrates along the coils of the membranous cochlear and forms the spiral ganglion Nerve processes extends from this ganglion to the spiral organ where they terminate on the hair cells 56  The membranous labyrinth is surrounded by mesoderm. Inductive influences from the otic vesicle stimulate the mesoderm around the otic vesicle (membranous labyrinth) condense and differentiates into a cartilaginous otic capsule / shell At 10 week , the cartilage shell undergoes vacuolation resulting in the formation of a space between it and membranous labyrinth, the perilymphatic space. 57  This space becomes filled with fluid referred to as perilymph The membranous labyrinth is now suspended in the perilymph 58  In the cochlear region, the perilymphatic space becomes sub- divided into two distinct spaces, the scala tympani and scala vestibuli. The cochlear duct is seperated from the scala vestibuli by a vestibular membrane and from the scala tympani by a basilar membrane 59  The lateral wall of the cochlear duct is attached to the cartilaginous shell by the spiral ligament, while its medial angle is connected to and supported by a cartilaginous process, the modiolus. Later, the cartiliginous capsule surrounding the membranous labyrinth of the inner ear is replaced by bone forming the osseous labyrinth within the petrous temporal bone of the skull 60 EMBRYOLOGY OF THE MIDDLE EAR Development of the Middle Ear Tympanic cavity Auditory tube/ Eustachian tube/ pharyngotympanic tube Ossicles (malleus, incus & stapes) Is derived from the tubotympanic recess. Development of the Middle Ear This recess develops from the first pharyngeal pouch The first pharyngeal pouch develops as an out-pouching between the first and second pharyngeal arches. The pouch grows rapidly in a lateral direction and comes in contact with the floor of the 1st pharyngeal cleft (deep end of the external auditory meatus) Development of the Middle Ear The proximal portion remains narrow and forms the auditory or Eustachain tube / Pharyngotympanic tube The Eustachain tube is a channel through which the tympanic cavity communicates with nasopharynx The tubes orifice is surrounded by a considerable amount of lymphoid tissue, a tubal or pharyngeal tonsil (adenoids)  Nasal inflammation associated with tubal tonsillar swelling often result in occlusion of the tube and inflammation of the tympanic cavity resulting in otitis media Development of the Middle Ear The distal part of the tubotympanic recess widens and gives rise to the tympanic cavity The tympanic cavity gradually envelops the small bones of the middle ear (auditory ossicle {malleus, incus and stapes}, their tendons and ligaments , and the chorda tympani nerve  The Malleus and Incus are formed from the cartilage of the Ossicles first pharyngeal arch (Meckel’s cartilage) Stapes are formed from the dorsal end of the cartilage of the second pharyngeal arch (Reichert’s cartilage) 67  During late fetal life, expansion of the tympanic cavity dorsally and posteriorly by vacuolization of surrounding tissue gives rise to tympanic/mastoid antrum Mastiod antrum is located in the permastoid part of the temporal bone.  After birth, the epithelium of the tympanic cavity invades bone of the developing mastoid process Later most of the mastoid air sacs come in contact with the antrum and tympanic cavity, as a result, infection from the throat and ear may spead to the mastiod process and causes pus to form there  They ossicles remain embedded in mesenchyme until the 8th month, then the surrounding tissue dissolve The endodermal epithelial lining of the primitive tympanic cavity then extends along the wall of the newly developing space connecting the ossicles in a mesentry-like fashion to the wall of the cavity The supporting ligament of the ossicles develop later within the mesenteries 71  The dorsal blind end of the recess Tympanic grows toward the first pharyngeal membrane cleft. The inner ectodermal wall of the first cleft and the endodermal wall of the tympanic cavity are separated by a layer of mesenchyme. The three tissues fused together to form the tympanic membrane This sturcture forms a partition between the external and middle ear 72  Since malleus is derived from the 1st pharyngeal arch, its muscles, the tensor tympani is innervated by the mandibular branch of trigeminal nerve The stapes is of 2nd pharyngeal arch, its muscle the stapedius is innervated by the facial nerve.  At birth, the cavities of the middle ear is fill with air via the eustachian tube, with the ossicles, they form the system used for transmitting vibrations to the inner ear The part of the osseous labyrinth opposite the stapes remains thin and becomes the oval window (fenestra vestibuli) which opens into the vestibule Below this, another thinning of the bony labyrinth forms the round window (fenestra cochlea) which opens into the scala tympani. Both are closed by membrane Tympanic Membrane/Eardrum Thus, the handle of the malleus and the chorda tympanic nerve are trapped between the ectoderm of the meatal plug and the endoderm of the tympanic cavity The major part of the eardrum External ear Development of the External Ear The external ear consists of the auricle or pinna and the external auditory meatus The external ear develops from mesenchymal proliferation in the first and second branchial arches (auricular hillocks) Auricle Develops around 6th week of intrauterine life Auricle develops from a series of mesodermal thickenings (about 6) called tubercles or auricular hillocks which appears around the 1st pharyngeal cleft or groove.  These tubercles or hillocks, three on each side of the external acoustic meatus later fuse to form the definitve auricle The first hillock forms the 1. tragus 2 – Crus of helix 3 – Helix 4 – Anti helix 5 – Anti tragus 6 – Ear lobule External Acoustic Meatus Develops from the dorsal part of the first ectodermal cleft/groove At the beginning of the 3rd month, epithelial cells at the bottom of the meatus proliferate, forming a solid epithelial plate, the meatal plug or cellular cords. The cord reaches the the tympanic cavity. External Acoustic Meatus In the 7th month, the meatal plug canalized And the epithelial lining of the floor of the meatus then participates in the formation of definitive ear drum. External Acoustic Meatus Occasionally, the meatal play persist until birth, resulting in congenital deafness. Hairs and ceruminous glands are develop as ingrowths of the epithelial lining  Anomalies of the Ear 1. The auricle may be totally or partially absent if its development is arrested at any stage 2. There may be stenosis or atresia of the external auditory meatus over its whole length or over part of it 3. The ossicles may be malformed 4. Various parts of the membranous labyrinth may remain undeveloped. In some cases the cochlea alone is affected. These anomalies lead to congenital deafness 83 84 85 86