Development of the eye and ear

Development of the eye I. First noticeable at the beginning of the 4th wk Develop from the optic grooves—developing neural tube II. As neural folds fuse (= forebrain formation) optic vesicles—evaginations of forebrain

IIIa. Induction of lens placode (surface ectoderm) IIIb. Formation of optic stalk and optic cup from optic vesicle

Continued development of optic cup and lens Optic cup —invagination of distal optic vesicle to form a double walled“cup” Optic (choroid) fissure —sulcus on ventral aspect optic cup/stalk (allows passage of vasculature to lens & layers of cup) Lens placode —ectodermal thickening Lens pit—invaginates to form lens vesicle

Development of the Retina Develops from the walls of the optic cup Inner wall develops into nervous retina. Outer wall develops into pigment epithelium of retina

Congenital anomalies Retinal detachment: between inner and outer portions of the optic cup derivatives •congenital: failure of fusion •acquired: trauma

Cornea Sclera Epithelium from surface ectoderm Bowmans,descements, endothelium are derived from the fibrous layer of mesenchyme Is developed from fibrous layer of mesenchyme Sclera

Lens The lens placode in the surface ectoderm invaginates as the lens vesicle supplied by hyaloid artery

Congenital anomalies Aphakia —absence of the lens (extremely rare) Congenital cataracts—(e.g., rubella virus)

Iris Anterior epithelial layer & post. pigmented epith. layer are derived from the optic cup Sphincter and dilator pupillae are derived from the anterior epithelial layer Stroma and blood vessels from vascular mesenchyme Anomalies Defects in closure of optic fissure • iridial coloboma • retinal coloboma This person has near normal vision, but is light sensitive because of the extra light the notch in the iris lets into the back of the eye With Retina Coloboma a section of the lower retina never developed. This person has near normal vision, but is missing the upper field of vision.

Aniridia: Extremely rare: 1 in 75,000 Heterochromia Different color of both iris

Development of the ear Internal Ear In embryos 22 days a thickening of the surface ectoderm on each side of the rhombencephalon, is the first indication of the developing ear. These thickenings, the otic placodes, pits ,invaginate rapidly and form the otic or auditory vesicles (otocysts). Each vesicle divides into; Ventral component that gives rise to the saccule and cochlear duct . Dorsal component that forms the utricle, semicircular canals, and endolymphatic duct). Together these epithelial structures form the membranous labyrinth.

24 days 27 days 4.5 weeks Transverse sections through the region of the rhombencephalon showing formation of the otic vesicles. Note the statoacoustic ganglia.

SACCULE, COCHLEA, AND ORGAN OF CORTI In the sixth week of development, the saccule forms a tubular outpocketing at its lower pole. This outgrowth, the cochlear duct, penetrates the surrounding mesenchyme in a spiral fashion until at the end of the eighth week it has completed 2.5 turns. The ductus reuniens connects the remaining portion of the saccule with utricle.

C to E. Cochlear duct at 6, 7, and 8 weeks, respectively C to E. Cochlear duct at 6, 7, and 8 weeks, respectively. Note formation of the ductus reuniens and the utriculosaccular duct. A and B. Development of the otocyst showing a dorsal utricular portion with the endolymphatic duct and a ventral saccular portion.

Middle Ear TYMPANIC CAVITY AND AUDITORY TUBE The tympanic cavity is derived from the first pharyngeal pouch. This pouch expands in a lateral direction and comes in contact with the floor of the first pharyngeal cleft. The distal part of the pouch gives rise to the tubotympanic recess. The proximal part gives rise to the auditory tube (Eustachian tube).

External Ear EXTERNAL AUDITORY MEATUS The external auditory meatus develops from the dorsal portion of the first pharyngeal cleft. At the beginning of the third month, epithelial cells at the bottom of the meatus proliferate, forming a solid epithelial plate, the meatal plug. In the seventh month, this plug dissolves and the epithelial lining of the floor of the meatus participates in formation of the definitive eardrum. Occasionally the meatal plug persists until birth, resulting in congenital deafness.

EARDRUM OR TYMPANIC MEMBRANE The eardrum is made up of; Ectodermal epithelial lining at the bottom of the auditory meatus, Endodermal epithelial lining of the tympanic cavity. Intermediate layer of connective tissue that forms the fibrous stratum. The major part of the eardrum is firmly attached to the handle of the malleus, and the remaining portion forms the separation between the external auditory meatus and the tympanic cavity

Middle ear showing the handle of the malleus in contact with the eardrum. The stapes will establish contact with the membrane in the oval window. The wall of the tympanic cavity is lined with endodermal epithelium.

AURICLE The auricle develops from six mesenchymal proliferations at the dorsal ends of the first and second pharyngeal arches, surrounding the first pharyngeal cleft. These swellings (auricular hillocks), three on each side of the external meatus, later fuse and form the definitive auricle. As fusion of the auricular hillocks is complicated, developmental abnormalities of the auricle are common. Initially,the external ears are in the lower neck region, but with development of the mandible, they ascend to the side of the head at the level of the eyes.

A. Lateral view of the head of an embryo showing the six auricular hillocks surrounding the dorsal end of the first pharyngeal cleft B to D. Fusion and progressive development of the hillocks into the adult auricle

E. The six auricular hillocks from the first and second pharyngeal arches. H, heart; NP, nasal placode F. The hillocks becoming more defined. Note the position of the ears with respect to the mouth and eyes (e). G. External ear nearly complete. Growth of the mandible and neck region places the ears in their permanent position.

Anomalies Microtia: Underdeveloped external ear Bat ears: One of the most common anomalies Anotia: Complete absence of external ears

Preauricular appendages and pits Preauricular appendages and pits are skin tags and shallow depressions, respectively, anterior to the ear. Pits may indicate abnormal development of the auricular hillocks, whereas appendages may be due to accessory hillocks.