Ectodermal Derivatives

Development of the Ear

Regions of the Ear (mammalian)

1. external2. middle 3. internal

Regions of the Ear (mammalian)

• The external Ear consist of the auricle or pinna and the external auditory canal which

functions as a sound collecting funnel

Regions of the Ear (mammalian)

• The middle Ear has a sound transmitting

mechanism which involves a chain of 3 auditory ossicles which pick up the vibrations received by the eardrum and transmits them across the middle ear or the tympanic cavity, to the receptive mechanism of the internal ear

Regions of the Ear (mammalian)• The internal Ear

composed of an elaborate system of fluid filled, epithelially lined chambers and canals constituting the so called membranous labyrinth

Regions of the Ear (mammalian)• The internal Ear

the membranous labyrinth lies within the temporal bone in a similarly shaped, but larger, series of cavities consisting the bony labyrinth

the perilymphatic space which is filled with the perilymphatic fluid is the narrow space between the walls of the bony labyrinth and the membranous labyrinth

Cochlea and Vestibular Complex• Cochlea sound receiving portion of the

membranous labyrinth which is shaped like a snail shell

• Vestibular complex concerned with equilibrium, its membranous labyrinth is composed of the sacculus, utriculus, and the 3 semicircular ducts or canals

the most primitive part of the ear and the only part the ear that has been differentiated in fishes

Cochlea and Vestibular complex

Formation of the auditory vesicle• stimulated by and inductive action of the

hindbrain upon the overlying ectoderm

• In the 3rd week (2 somite stage), in human embryos, the embryological changes are first noticeable and it is when the superficial ectoderm on the either side of the still-open neural tube thickened

Formation of the auditory vesicle• By the middle of the third week (7 somite

stage), the auditory placode start to thicken and becomes quite clearly marked

Formation of the auditory vesicle• 3rd week> the auditory placode has taken

shape as a sharply circumscribed thickening on the either side of the developing myelencephalon

• 4th week>, the placode has invaginated to form the auditory pit

• As soon as it has been closed off from the surface, the auditory pit constitute of the auditory vesicle or otic vesicle

Formation of the auditory vesicles

• The auditory vesicle induces the mesenchyme around it to form the cartilaginous ear capsules

• And the origin of this mesenchyme is the mesoderm

• As the auditory vesicle enlarges, it changes from its originally spheroidal shape and becomes elongated dorso-ventrally• The area where the epithelium of the

auditory vesicle was separated from the superficial ectoderm, a tubular extension of the vesicle develops which is known as the endolymphatic duct• The endolymphatic duct disappears in adults

• As the auditory vesicle expands laterally, the endolymphatic duct is left occupying a progressively more median position in relation to the rest of the vesicle

the auditory vesicles divide into • the ventral component which gives

rise to the saccule and the cochlear duct• The dorsal component forms the

utricle, semi-circular canals and the endolymphatic duct

Saccule, Cochlea and Organ of Corti

• 6th week> the saccule forms the tubular outpocketing at its lower pole, which is called the cochlear duct;

– which penetrates the surrounding mesenchyme in a spiral fashion until the end of the 8th week it has completed 2 and a half turns

Cochlear duct

Saccule, Cochlea and Organ of Corti

• Cochlear duct connection with the remaining portion of the saccule is then confined to a pathway – the ductus reuniens

• 10th week> the differentiated cartilagenous shell from the mesenchyme undergoes vacoulization forming the two perilymphatic spaces–scala vestibuli and the –scala tympani

• The cochlear duct is then separated from the scala vestibuli by the vestibular membrane and the scala tympani by the basicular membrane–The spiral ligament attaches the lateral

wall of the cochlear duct to the surrounding cartilage–The median angle of the cochlear duct is

conneected to and partly supported by a long carilagenous process, the modiolus, the future axis of the bony cochlea

• The epithelial cells of the cochlear duct then forms two ridges: • inner ridge, (future spiral limbus

) and the

• outer ridge

• The outer ridge forms one row of inner and 3 or 4 rows of outer hair cells, the sensory cells of the audotory system which will be covered by the tectorial membrane

• tectorial membrane fibrilar, gelatinous substance attached to the spiral limbus that rests with its tip on the hair cells

Sensory cells + tectorial membrane = Organ of Corti

• Impulses received by this organ are transmitted to the spiral ganglion and then to the nervous system by the auditory fibers by the cranial nerve VIII

Utricle and semicircular canals

• 6 weeks of development, the semicircular canals appear as flattened outpocketings of the utricular part of the otic vesicle

• The central portions of the walls of these outpocketings eventually appose each other and disappear giving rise to the semicircular canals

• One end of each canal dilates to form the crus ampullare, and the other the crus nonampullare does not widen

Utricle and semicircular canals• Crista ampulllaris, which are the cells in the

ampule which form a crest containing sensory cells

• Similar sensory areas, the maculae acuosticae, develop in the walls of the utricle – The impulses generated in the sensory cells of

these two as a result of the change in the position of the body are carried to the brain by the vestibular fibers of the cranial nerve 8

Utricle and semicircular canals

• During the formation of the otic vesicle, a small group of cells break away from its wall and forms the statoacuostic ganglion

• Other cells of this ganglion are derived from the neural crest

• This ganglion subsequently splits into cochlear and vestibular portions, which supply sensory cells of the organ of Corti and those of the saccule, utricle, and semi-circular canals respectively

Middle ear

Tymphanic Cavity and Auditory tube• The tympanic cavity originates from the

endoderm and is derived from the 1st pharyngeal pouch

• Auditory tube or Eustachian tube is the proximal part of the pouch which remains narrow

• Tubotympanic recess is the distal part of the pouch which widens and give rise to the primitive tympanic cavity

Ossicle

• Malleus, Incus are derived from the cartilage of the 1st pharyngeal arch• Stappes are derived from the 2nd

pharyngeal arch

External EarExternal Auditory Meatus–Develops from the dorsal portion of the

pharyngeal cleft–3rd month, the epithelial cells at the bottom

of the meatus proliferate, forming a solid epithelial plate, the meatal plug–7th plug, this plug dissolves and the

epithelial lining of the floor of the meatus participates in the formation of the definitive ear drum

Eardrum or tympanic membrane–Made up of (a) ectodermal epithelial lining

at the bottom of the auditory meatus, (b) endodermal lining of the tympanic cavity and (c) intermediate layer of connective tissue that forms the fibrous stratum– The major part of the eardrum is attached to

the handle of the malleus and the remaining portion forms the separation between the external auditory meatus and the tympanic cavity

• Auricle–Develops from 6 mesenchymal proliferation

at the dorsal ends of the first and 2nd pharyngeal arches, surrounding the 1st pharyngeal cleft– This swellings (auricular hillocks), three on

each side of the external meatus, later fuse and form the definitive auricle– Externally the ears are in the lower neck

region, but with the development of the mandible, they ascend to side of the head at the level of the eyes

Variation in the development of the auditory placode

• Amniotes – the whole epidermal layer is involved with the formation of the auditory placode and it later invaginates to form a sac which is at leant temporarily open to the exterior

• Frogs – the auditory placode is formed by the thickening of the interior “sensory” layer of the epidermis, while the external covering layer is not involved at all, as a result, when the placode invaginates, there is no opening or pit at the surface of the skin

• Bony fishes – the auditory organ is formed not by invagination but as a solid mass of cells on the inner surface of the epidermis and is hallowed out secondarily

Q: So what will happen if the ear vesicle is removed?

A: the ear capsule and the superfluous cartilage in the area does not develop but middle ear still develops

In the absence of the vesicle, the proliferation of the procartilage cells falls short of the normal

Chain of inductions provided in the development of the ear

1. the primary inductor – the roof of the archenteron, consisting of the presumptive chordomesoderm – causes the development of the hindbrain

2. The hindbrain, as a secondary inductor, stimulates the development of the ear vesicle (in conjunction with the direct action of the mesoderm on the presumptive ear ectoderm)

3. The ear vesicle, as a tertiary inductor, causes the formation of the cartilagenuous capsule

Deafness and external ear abnormalities

Congenital Deafness – associated with deaf-mutism, maybe caused by abnormal development of the membranous and bony labyrinth or by malformations of the auditory ossicles and ear drum• In extreme cases, the tympanic cavity and external

meatus are absentPreauricular 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

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Amu lang naDuru gid nga salamat :P :j