The+Development+of+the+Eye+and+Ear

toc =**Objectives**=

**Eye**

 * 1. Define and/or describe:**

__Optic Vesicle__ A pair of shallow groove develop on the anteriolateral surface of the developing forebrain called the optic grooves. As the midline neural fold fuses, the optic grooves evaginate to form spherical optic vesicles projecting outwards towards the surface ectoderm of the head, eventually forming the optic cup. The hollow region inside is called the intra-retinal space and is continuous with the optic stalk

__Optic Cup__ As the optic vesicle continues to grow outward, it contacts the ectoderm of the head. This signals the optic vesicle in invaginate to form the optic cup. Eventually, it will form the 10 layers of the retina. The hollow region inside is called the intra-retinal space and is continuous with the optic stalk

__Optic Fissure (Choroid Fissure)__ The optic cup remains attached to the developing neural tube by the optic stalk. The optic stalk has a shallow groove on its ventral surface calls the optic fissure, hyaloids groove, retinal fissure, or choriod fissure. It extends through the ventral aspect of the optic cup and the hyaloids artery grows within this fissure. The fissure eventually closes in later development.

__Lens Placode__ The lens placode is a thickening of the surface ectoderm where the optic vesicle contacts it on the head. Induction of the lens placoid depends on the interaction between the optic vesicle tissues and overlying ectoderm.

__Lens Pit__ The lens pit is the lens placode as it grows inward towards the surface of the optic vesicle. It will continue to invaginate, forming the lens vesicle.

__Lens Vesicle__ The lens pit continues to invaginate, forming a spherical lens vesicle which later detaches from the surface ectoderm. The lens vesicle occupies a position within the concavity of the optic cup. The space between the lens and the inner wall of the optic cub is the lentiretinal space. It will become filled with the vitreous body.


 * 2. Define and/or describe:**

__Hyaloid Artery (Central Artery of Retina)__ The hyaloid artery is the blood supply for the developing optic cup and lens. It is a branch of the ophthalmic artery of the internal carotid artery. The optic cup remains attached to the neural tube by the optic stalk which has a ventral invagination called the optic or choroid fissure where the hyoid artery grows within. Eventually, this fissure will close.

The posterior portion of the hyaloid artery passes through the continuation of the optic or choroid fissure into the optic cup to supply the developing retina. The anterior portion of the hyaloid artery crosses the vitreous chamber to supply the developing lens.

The posterior segment of the hyaloid artery passes through the posterior wall of the optic cup and is maintained as the central artery of the retinal in the adult.

__Hyaloid Canal__ As lens development continues and the lens cells are transformed to lens fibers, the lens no longer needs the blood supply from the hyaloid artery. So the anterior segment of the hyloid artery degenerates, leaving the hyloid canal through the vitreous humor.


 * 3. Define and/or describe the three major layers (tunics) of the eye and list the component structures found in each.**

The eye has three major layers: the tunica fibrosa, the vascular or uveal layer, and the retinal layer. The tunica fibrosa layer contains the translucent cornea anteriorly and a tough elastofibrous sclera posteriorly. The vascular layer contains the iris anteriorly, the ciliary body which suspends the lens on the zonules of zinn, and the choroid. The retinal layer contains the retinal pigment epithelium, and the neural retina which together make up the 10 layers of the retina. See [The Histology of the Eye]].


 * 4. Describe the major steps that occur in the process of formation of the lens. What induces the formation of the lens.**

The optic vesicle grows outward until it contacts the surface ectoderm of the head. This interaction causes the thickening of the surface ectoderm to form the lens placode and the invagination of the optic vesicle to form the optic cup.

The lens placoid grows inward towards the surface of the optic vesicle to form the lens pit. The lens pit continues to invaginates, forming a spherical lens vesicle which later detaches from the surface ectoderm.

Induction of the lens depends on interaction between the optic vesicle tissues and overlying surface ectoderm. Removal of the optic vesicle prevents lens formation. Pax-6 is important for proper lens development.

As the lens invaginates, it cause an invagination of the optic vesicle and lies within the concavity of the collapsed optic vesicle, the optic cup.

At the earliest stage of development, the lens is consists of non-specialized lens epithelial cells. By week 6, the cells at the center of the lens are stimulated by fibroblast growth factor (FGF) and stops dividing to elongate along the anterior-posterior axis. They express crystalline proteins and become transparent lens fibers. The cells at the posterior pole of the pens elongate under the influence of the retina to form the lens nucleus which fills the lens vesicle as the primary lens fibers. These fibers persist throughout life. As development continues, the lens enlarges through division and elongation of cells at the equatorial margin of the lens which become the secondary lens fibers. These cells add new lens fibers to the lateral surface of the lens nucleus and continue to do so throughout life.


 * 5. Describe the formation of the anterior chamber and the cornea. What induces the differentiation of the cornea.**

During the 6th week, mesenchyme surrounding the optic cup fills the space between the developing lens and overlying corena. As development continues, vacuolization occurs via cell apoptosis to form the chambers of the eye.

This splits the mesenchyme into two layers to create the anterior and posterior chambers: an outer layer that will become part of the inside aspect of the corneal stroma, and an inner layer that forms a thin membrane over the papillary opening called the iridopupullary membrane. The iridopupillary membrane degenerates later in development to leave the cavity of the posterior chamber in its place.


 * 6. Describe the formation of the iris.**

The iris develops from the anterior rim of the optic cup which still contain both inside and outside folds. The pigmented cells of the posterior wall of the optic cup (RPE) and non-pigmented cells derived from retina form the posterior layers of the iris as it grows anteriorly and inwards to extend circumferentially around the anterior aspect of the eye, forming the pupil. The iris is a double cuboidal epithelium because it still contains the layers derived from the RPE and deneuralized neural retina which acquires pigmentation.

The iris is covered anteriorly by the ectomesenchyma cells which form the stromal Neuroectodermal cells (possibly neural crest derived) from the anterior epithelium of the iris migrate into the stroma and differentiate into smooth muscle to form the dilator pupillae and sphincter pupillae.


 * 7. Describe the formation of the ciliary body.**

The ciliary body develops from the anterior rim of the optic cup. As mesenchyme external to the optic cup begins to differentiate into the choriodal coat, cells on the anterior aspect proliferate to form a bulge on the choriod adjacent to the lateral margins of the lens. Simultaneously, the two layers of the optic cup RPE and deneuralized neural retina cells grow over to cover the bulge to form the ciliary body. In this portion of the eye, the RPE and deneuralized neural retinal cells become fused head to head with basement membranes around both sides to form the blood-aqueous barrier.

Mesenchymal cells within the bulk of the ciliary body and possibly neural crest cells differentiate into the smooth cilliary muscles. Differential expansion and folding of the deneuralized retinal cell layer results in the formation of ridges of the cilliary processes. These processes secrete the zonules of zinn fibers that form the suspensory ligament of the lens.


 * 8. Describe the formation of the retina.**

The two layers of the retina (RPE and neural retina) are derived from the two walls of the optic cup. Epithelial cells in the outer wall of the optic cup become the heavily pigmented retinal pigmented epithelium (RPE). The thick inner wall of the optic cup gives rise to the 9 layers of the neural retina comprised of the photoreceptor layer through the internal limiting membrane.

Differentiation of the neural retina occurs during week 6-7 when cells abutting the intraretinal space begin to proliferate forming two layers. The inner wall of the optic cup is composed of an outer neuroblastic membrane covered by the external limiting membrane separating it from the intraretinal space and an inner neurobastic membrane covered by the internal membrane membrane separating it from the vitreous space.

The photoreceptors develop from the outer neuroblastic layer, right up against the external limiting membrane and RPE. The retinal ganglion cells and modified neuron cells of the retina develop from the inner neuroblastic layer. The ganglion cells develop first and photoreceptors develop last.

The axons of the developing retinal ganglion cells collect on the inner surface of the retina and grow towards the optic stalk to exit the eyeball. These axons line the inner wall of the optic stalk thicken and fill up the lumen of the optic stalk, forming the optic nerve.


 * 9. Describe the development of the internal ear.**

The inner near forms as a thickening on the surface ectoderm dorsal to the 2nd pharyngeal arch on either side of the developing rhombencephalon called the otic placodes. The otic placodes invaginate to form otic pits that pinch off as otic vesicles. The otic vesicles elongate to form utricular and saccular compartments which form the membranous labyrinth. The saccular region elongates as a tubular outgrowth which coils as development proceeds to form the cochlear duct of the inner ear.

A small group of epithelial cells migrate medially toward the neural tube to give rise to the vestibular nerve (CN VIII) spiral ganglion. A separate group of epithelial cells migrate to the floor of the cochlear duct to form the sensory hair cells of the organ of corti. Neural crest derived cells migrate to the floor of the otic vesicle and give rise to the basilar membrane and support cells of the organ of corti.

The mesenchyme surrounding the developing cochlear duct begins to condense, forming a cartilaginous capsule around the cochlear duct. The dorsal and ventral regions of this capsule begin to vacualize forming spaces which will become the perilymph-filled scala vestibuli and scala tympani in the adult structure. Opposite poles of the cochlear duct become anchored to the developing cochlear canal at the region of the spiral lamina and spiral ligaments, stretching the cochlear duct when the cartilaginous encasement develops into the bony encasement of the bony labyrinth.

The vestibular apparatus develops from the dorsal utricular compartment of the otic vesicle which flattens. Part of the central walls collapse and fuse together and undergo programmed cell death resulting in three tubular semicircular canals. Each canal develops and expanded region at one end, forming the ampullae.

The middle ear forms from the first pharyngeal pouch. Mesenchyme adjacent to the ectoderm of the 1st and 2nd arches condense to form the cartilaginous precursors of the auditory ossicles. The first pharyngeal pouch elongates to form the middle ear, engulfing the forming ossicles and connecting the ossicles to the tympanic membrane and oval window. The invaginating ectoderm from the first pharyngeal groove continues inwards and contacts the lateral wall of the middle ear cavity, forming the tympanic membrane.

Thus, the tympanic membrane is formed by outer ectoderm, mesoderm, and inner endoderm.


 * 10. Describe the development of the external ear structures.**

The external auditory meatus develops from the invagination of the dorsal region of the first pharyngeal cleft. THe outer auricle and pinna of the ear form from the 6 nodular masses of the mesencyme called auricular hillocks, derived from the 1st and 2nd pharyngeal arches.

The 1-3 hillocks derived from the 1st pharyngeal arch form the anterior portions of the external ear. The skin overlying is innervated by the mandibular nerve.

The 4-6 hillocks derived from the 2nd pharyngeal arch form the posterior portions of the ear. The skin overlying is innervated by C2; the muscle underneath is innervated by the facial nerve CN VII (and some CN IX).

**Ear**

 * 1. Define and/or describe the three anatomical subdivisions of the ear.**

See the Histology of the Ear


 * 2. Define and/or describe the otic placode and list the structures that are derived from it.**

See the Histology of the Ear


 * 3. Define and/or describe the membranous labyrinth and the boy labyrinth**

See the Histology of the Ear


 * 4. Define and/or describe:**

See the Histology of the Ear

__Endolymph__

__Perilymph__


 * 5. Describe the relationships between the utricle and related semilunar ducts and endolymphatic duct. Describe the relationship of the saccule and the cochlear duct.**

The Endolymphatic sac and duct, Utricle portion, and Saccule portion of the Otic vessal form from the otic pit. The Utricle portion flattens and portions undergoe apoptosis to give rise to the the semilunar canals. The sacule portion elongates a tubular outgrowth that coils 2 3/4 turns.


 * 6. Briefly describe the formation or the neural transducers found in the semicircular ducts, the utricle, and cochlear duct.**

Epithelial cells migrate to form the neural-sensory components of the inner ear. Neural crest cells migrate to the floor of the otic vessicle and give rise to the Basilar membrane and suporting cells of the organ of corti.


 * 7. Briefly describe the formation of the tympanic cavity of the middle ear from the first pharyngeal (brachial) pouch. What is the developmental history of the middle ear ossicles (malleus, incus, stapes)?**

Mesenchyme adjacent to the ectoderm of the 1st and 2nd arches condense to form the cartilaginous precursors of the suditory ossicles. The 1st pharyngeal arch elongates to form the tubotympanic recess which engulfs the forming ossicles and lines the middle ear cavity.


 * 8. Briefly describe the formation of the external auditory canal from the first brachial cleft.**

The first brachial cleft invaginates and contacts the first pharyngeal pouch and the two membrane fuse to form the tympanic membrane.


 * 9. Define pinna (auricle). What is the developmental history of the auricle?**

The pinna or auricle developes from 6 nodular masses of mesenchyme called auricular hillocks. Hillocks 1-3 are derived from the 1st pharyngeal arch and form the anterior portions of the ear, innervated by CN V. Hillocks 4-6 are derived from the 2nd arch and form the posterior portion of the ear.


 * 10. Distinguish between “nerve deafness” and “conduction deafness.” List at least one congenital cause for each.**

Nerve deafness is the result of a failrure to detect or transmit auditory information to the brain, and can be caused by matrnal rubella infection during the 7th and 8th week of development affecting the development of the spiral organ. Conductive deafness is the result of a physical impediment to sound conduction and can be caused by several genitic disorders such as congenital fixation of the stapes.


 * 11. What is congenital cholsteatoma?**

Congenital cholesteatomas are benign tumors of the temporal bone that usually originate within the middle ear space. They classically appear as a white mass beneath an intact tympanic membrane, and early diagnosis on routine otoscopic examination is often possible.


 * 12. When is the ear most susceptible to teratogens?**

During weeks 4-9 the ear is highly sensitive.


 * 13. Explain the developmental history of any/all structures observed in microanatomy exercises.**

Table of structures and embryological origin should go here if I felt like making one.