Embryology - Eye and Ear Flashcards
Define placodes and pharyngeal apparatus
- Placodes are thickened ectodermal patches on the developing head
- The pharyngeal apparatus is the series of ridges and furrows, with corresponding internal pouches, which go on to form structures in the head and neck.
When does development of the eye begin?
4th week of gestation
Describe the development of the eye.
The forebrain forms outpocketings, the optic vesicles (which are hollow since the developing forebrain is also a tube – the neural tube). The optic vesicles grow and make contact with the overlying ectoderm – the optic placode. The optic placode will become the lens.
The lens placode then invaginates and the optic vesicles surround them. The fissure between the two edges of the optic vesicle/optic stalk as it encloses the lens is the choroid fissure – the hyaloid artery runs in it, until after the fissure has fused when it degenerates distally, and the proximal part becomes the central artery of the retina.
What are the derivatives of the optic cup?
• Ciliary body forms at the edge of the lens, connects the choroid to the lens
o Muscular and vascular structure connecting choroid to the lens
• Retina – consists of neural and pigmented layers, have to fuse the layers
• Iris – contractile diaphragm with central aperture
Describe how binocular vision is enabled?
The eyes are initially positioned on the side of the head but move to the front of the face as the facial prominences go, which allows for binocular vision
What is coloboma?
• A coloboma is a failure of the choroid fissure to close (which is the fissure as the optic vesicle surrounds the lens)
o Hyaloid artery, which ran in the choroid fissure, degenerates distally.
o Proximal portion becomes the central artery of the retina.
What is congenital cataracts?
• Congenital cataracts (opacity of the lens) can be genetic, or as the result of a teratogen like rubella (also causes microcephaly and PDA)
What is detached retina?
• Detached retina – retina develops from two layers separated by a space. The space is obliterated as the two layers fuse. Pigmented layer and sensory layer separate in detachment of the retina.
What does the adult ear consist of? Functions?
• The external ear, which “funnels” sound towards the ear drum
• The middle ear, which conducts sound (via the ossicles)
• The inner ear, which transduces sound energy into action potentials
o In the cochlea
• The inner ear is responsible for balance as well, via the vestibular system
o Semicircular canals
Describe development of the inner ear.
In development the inner ear begins as the otic placodes (thickened ectodermal patches) on the back of the head, which invaginate and form the auditory, or otic vesicles (star). The surface ectoderm closes over. These give rise to the membranous labyrinth. The otic vesicle’s fate is as follows:
• It has a saccule which elongates and curls round to form the cochlea
• And an utricle which undergoes a smidgen of apoptosis to form the semicircular canals
Describe development of the inner ear. What are the ossicles derived from?
The middle ear is derived from the pharyngeal pouch and pharyngeal arch (cartilage bar).
The ossicles are cartilage bar derivatives:
• Malleus and incus are derived from Meckel’s cartilage (first pharyngeal arch)
• Stapes is derived from Reichert’s cartilage (second pharyngeal arch)
And the tympanic cavity and Eustachian tube are derived from the first pharyngeal pouch. The pouch expands distally and creates the tympanic cavity, whilst proximally it remains narrow (the Eustachian tube).
Describe the development of the external ear.
The external auditory meatus develops from the first pharyngeal cleft, whilst the auricles develop from proliferations in the first and second pharyngeal arches (auricular hillocks), which surround the first pharyngeal cleft.
The ears develop in the neck, then ascend to lie in line with the eyes as the mandible grows. Interestingly almost all common chromosomal abnormalities have external ear abnormalities.
describe the innervation of the ear.
- Special sensory innervation from the vestibulocochlear nerve (CNVIII)
- Tensor tympani is innervated by CNV3
- Stapedius is innervated by CNVII
- Sensory innervation of the external ear is mainly CNV and cervical spinal nerves
What causes congenital middle ear deafness? Inner ear deafness?
o Middle ear deafness
• Due to problems with the first and second pharyngeal arch
o Inner ear deafness
• Maldevelopment of the organ of Corti can be due to many teratogens, especially rubella.
