Development of organs of special senses - Eye, ear Flashcards
I. Development of the ear
1. Development of Membranous labyrinth
At about 22 days
- The otic placode invaginates forming a pit that later becomes separated from the ectoderm, forming the otic vesicle (or otocyst) deep to the ectoderm.
- The otic vesicle is surrounded by mesoderm that will become the otic capsule, the cartilaginous precursor of the bony labyrinth.
- The upper parts of the otic vesicle will develop into the utricle, semicircular canals and endolymphatic duct (the part of the inner ear involved in balance).
- The lower portion will become the saccule, which develops a tubular outgrowth that becomes the cochlear duct (the part of the inner ear involved in hearing).
- Mesenchyme surrounding the cochlear duct becomes cartilaginous and two vacuoles appear within the cartilage. These vacuoles become the scala tympani and scala vestibuli. Positioned between them the cochlear duct (the scala media) remains
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I. Development of the ear
2. Development of Organ of Corti.
Epithelial cells of the cochlear duct differentiate into the sensory cells and tectorial membrane of the auditory system, collectively called the Organ of Corti. These cells transmit auditory signals to the vestibulocochlear nerve (CN VIII).
-> The foetus can hear external sounds by week 20.
I. Development of the ear
3. Development of semicircular canals
The saccule and utricle remain connected to the cochlear duct via the ductus reuniens and the utriculosaccular duct, respectively.
The saccule develops a group of sensory cells involved in translating vertical movements of the head.
Three flattened bud outgrowths protrude from the utricle to form three semicircular canals, each with an ampulla at one end, filled with endolymph. In each ampulla is a collection of sensory cells that aid balance. The utricle is involved in detecting horizontal movements of the head (Figures 46.3 and 46.4).
I. Development of the ear
4. Development of Bony labyrinth
The cartilage that surrounds the membranous labyrinth is ossified (weeks 16–24) and creates a perilymph‐filled protective space for the inner ear.
This area is connected to the subarachnoid space at the base of the brain through the cochlear aqueduct in the tempo- ral bone.
This is the petrous part of the temporal bone and is one of the hardest bones in the body.
The vestibule of the inner ear contains the oval window, which is in contact with the stapes bone of the middle ear (Figures 46.4 and 46.5).
I. Development of the ear
5. Development of middle ear
The middle ear consists of the tympanic cavity, the pharyngo- tympanic tube (or auditory or Eustachian tube) and the ossicles (Figures 46.4 and 46.5).
Endoderm of the first pharyngeal pouch (see Chapter 40) extends laterally and on contact with the ectoderm of the first pharyngeal cleft forms the tubotympanic recess. the tubotympanic recess becomes the tympanic cavity and the proximal part becomes the pharyngotympanic tube (Figure 46.4).
Mesenchyme of the first and second pharyngeal arches develops into the ossicles.
- the malleus and incus are derived from the first arch and the stapes from the second arch.
The tensor tympani muscle which inserts on to the malleus develops from the first pharyngeal arch and is innervated by the mandibular branch of the trigeminal nerve.
The stapedius muscle which inserts on to the stapes develops from the second pharyngeal arch and is innervated by the facial nerve
I. Development of external ear
The external ear begins internally with the tympanic membrane, also known as the eardrum
Developing from the first pharyngeal cleft, the external auditory meatus retains cells in its proximal part which form a plug until the seventh month. This disintegrates leaving a layer of epithelia to form part of the tympanic membrane.
The auricle (or pinna) develops from six swellings or hillocks (hillocks of Hiss) formed by proliferating cells which can be seen from week 6.
II. Development of the eye
1. Development of Optic cup and lens
As the neural tube closes these invaginations become the optic vesicles and remain continuous with the developing third ventri- cle (Figure 47.1).
Contact of these optic vesicles with the surface ectoderm induces the formation of the lens placodes (Figures 47.1 and 47.2).
As the optic vesicle invaginates it forms a double‐walled struc- ture, the optic cup
At the same time the lens placode invaginates and forms the lens vesicle which lies in the indent of the optic cup and is completely dissociated from the surface ectoderm.
Epithelial cells on the posterior wall of the lens vesicle lengthen anteriorly and become long fibres that grow forwards -> become primary lens fibres (after 2 weeks)
Secondary lens fibres form from epithelial cells located at the equator of the lens and are continuously added throughout life along the scaffold made by the primary fibres from the centre of the lens.
II. Development of the eye
2. Development of Retina
In the optic cup there is an outer layer that develops into the pig- mented layer of the retina and an inner layer that becomes the neural layer.
The posterior four‐fifths of the inner neural layer (Figure 47.4) consists of cells forming the rods and cones of the photoreceptive layer. Deep to this are the neurons and supporting cells. Deeper again lies a fibrous layer comprising the axons of these neurons, with axons leading towards the optic stalk that will develop into the optic nerve.
The anterior one‐fifth of the inner layer remains one cell thick. It becomes parts of the iris and ciliary body (
II. Development of the eye
3. Development of Optic nerve
The optic cup remains attached to the forebrain via the optic stalk (Figure 47.2). Axons from the photoreceptor cells and other neu- rons within the retina run along the inner wall of the optic stalk.
Within the optic stalk is a groove, the choroid fissure
The number of neurons running through the optic stalk increases and during week 7 the choroid fissure closes forming a tunnel for the hyaloid artery which becomes the central artery of the retina
Continually increasing numbers of neurons fill the stalk and the lumen of the optic stalk is obliterated. By week 9 it is the optic nerve.
II. Development of the eye
4. Development of the choroid and sclera
The choroid and sclera of the eye are comparable to the pia mater and dura mater of the brain, respectively.
They develop from the loose mesenchyme that surrounds the posterior part of the developing eye.
The choroid is highly vascular and pigmented whereas the sclera is tougher and more fibrous. The sclera is continuous with the dura mater that surrounds the optic nerve
II. Development of the eye
5. Development of Cornea
Loose mesenchyme surrounding the anterior part of the develop- ing eye is split by vacuolisation, forming the anterior chamber.
An inner iridopupillary membrane is created that degenerates, leaving open communication between the two fluid‐filled spaces of the anterior and posterior chambers (Figure 47.7).
The outer portion is continuous with the sclera and becomes the cornea.
(The cornea has three parts: an epithelia layer from ectoderm, the mesenchyme part mentioned above and another epithelial layer that lines the anterior chamber. Neural crest cells contribute to the sclera and the cornea.)
II. Development of the eye
6. Development of Extraocular muscles
Extraocular muscles include the inferior and superior oblique, medial, lateral, inferior and superior rectus and levator palpebrae superioris muscles. These muscles may develop from paraxial mesoderm of somitomeres 1–4 or from mesenchyme near the pre- chordal plate (a thickening of endoderm in the embryonic head) and are innervated by CN III, IV and VI.
