DEVELOPMENT OF THE SENSORY ORGANS

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DEVELOPMENT OF THE SENSORY ORGANS
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DEVELOPMENT OF THE EYE
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DEVELOPMENT of the EYE OVERVIEW

• 22 Days Optic Groove Appears
• 24 Days Optic Vesicle
• 26 Days Optic Cup Lens Placode
• 28 Days Further folding Optic Cup Lens Placode
• 33 Days Sensory Pigmented Retina
• 33 36 Days Lens distinct

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EYE FIELDS

• Day 17
• The eyes begin to develop from a population of
  cells in the anterior neural plate.
• These cells make up the eye fields.
• Day 20
• The eye fields are in the prosencephalon
  (forebrain).

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• Day 21
• Rapid growth of the prosencephalon carries this
  region of the brain forward, along with the eye
  fields.
• Day 22
• Optic grooves (sulci) form as some of the cells
  in the eye fields invaginate.

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Formation of the Optic Cup and Lens Vesicle

• The developing eye appears in the 22-day embryo
  as a pair of shallow grooves on the sides of the
  forebrain.
• With closure of the neural tube, these grooves
  form outpocketings of the forebrain, the optic
  vesicles.
• Lens placode forms from epithelial ectoderm
• Lens placode infolds as future lens

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• Day 24
• The optic groove is easily visualized in this
  fronto-lateral view.
• The edges of the cranial neural folds (arrows)
  approach each other in the midline as .

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Development of the optic cup

• By day 24, the optic vesicles have evaginated
  from the diencephalic region of the neural tube,
  with their distal surfaces,
• The retinal discs, apposed to the inner surface
  of adjacent ectoderm.

Relationship of the optic groove to this
ectoderm.
The optic grooves form the optic stalks and the
optic vesicles
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Development of the optic cup

• Contact between the neural ectoderm of the optic
  vesicle and the surface ectoderm results in
  induction of the lens placode
• The lens placode and the adjacent portion of the
  optic vesicle as it begins to invaginate.

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Development of the optic cup

• On day 32, the retinal disc indents to form the
  goblet shaped optic cup that will eventually form
  the retina,
• While the optic vesicle has narrowed into a thin
  optic stalk and is the beginnings of the optic
  nerve.

The invaginating lens placode forms the lens
vesicle that pinches off the surface ectoderm.
Invagination of the optic vesicle forms the
bilayered optic cup that remains connected to the
forebrain via the optic stalk.
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• On the ventral surface of the optic cup, the
  choroidal fissure transmits
• The hyaloid vessels into the interior of the cup.

The optic vesicle and the optic stalk invaginate,
forming the choroid fissure inferiorly.
The hyaloid artery courses through the choroid
fissure.
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Development of the lens

• Meanwhile, a thickening called the lens placode
  develops in the surface ectoderm as a result of
  induction by the adjacent optic vesicle.
• While the optic cup forms, the lens placode
  invaginates, forming a lens pit, and then pinches
  off from the surface ectoderm to form the lens
  vesicle, sitting within the rim of the optic cup.
  
• Between the lens vesicle and the inner wall of
  the optic cup, the lentiretinal space, a
  gelatinous matrix is secreted, which will form
  the vitreous body.

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• On day 33, the cells of the posterior wall of the
  lens vesicle differentiate into primary lens
  fibres, filling up the lumen of the lens vesicle,
  and will make up the central lens body of the
  mature lens.
• Cells on the anterior wall of the lens vesicle
  differentiate into a simple epithelium, and in
  the 8th week, cells at the periphery of this
  epithelium differentiate into secondary lens
  fibres.

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• Both the lens and retina are supplied by the
  hyaloid branch of the ophthalmic artery, which
  occupies the lentiretinal space in fetal life,
• But the mature lens loses its blood supply, so
  part of the hyaloid vessels degenerate, leaving
  the hyaloid canal in the vitreous body.
• The lips of the choroidal fissure fuse to enclose
  the portions of the hyaloid vessels in the optic
  stalk, transforming them into the central artery
  and vein of the retina.

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Development of the neural and pigment retinas
Development of the retina Development of the retina
Embryological structure Mature retinal structure
Outer wall of optic cup Pigment retina,Melanin appears on day 33
Inner wall of optic cup Neural retina,Mostly developed by week nine,All layers present by 8 months

• Development of the neural retina
• The layer of cells adjacent to the lumen of the
  optic cup becomes the outer proliferative zone,
  producing waves of cells the migrate inward
  toward the lentiretinal space,
• Forming the layers of the neural retina in a
  similar fashion to the ventricular epithelium of
  the neural tube.

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Development of the neural and pigment retinas

• By the 9th week, there are 2 layers of blast
  cells
• The outer neuroblastic layer, producing the
  light-receptive rod and cone cells
• The inner neuroblastic layer, producing the
  ganglion and supporting cells.
• On the inner surface of the retina, axons grow
  from the ganglion cells to form the fibre layer 
• 2 thin membranes bound the neural retina
• The internal limiting membrane, separating the
  fibre layer from the vitreous body
• The external limiting membrane, just external to
  the rod and cone cell bodies.

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Development of the neural and pigment retinas

• The space between the neural and pigment retinas,
  the intraretinal space, is an extension of the
  3rd ventricle,
• The intraretinal space is obliterated by growth,
  disappearing in the 7th week as the retinal
  layers fuse.

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Development of the optic nerve

• In the 6th week, axons from the retinal ganglion
  cells reach and grow along the optic stalk to
  form the optic nerve (CN II),
• With axons on the nasal side of each retina
  crossing to the contralateral side at the optic
  chiasm,
• Finally, all axons synapsing in the lateral
  geniculate bodies of the diencephalon.

As the retina develops, the pigmented layer
becomes relatively thinner while the neural
portion thickens.
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Development of the mesenchyme around the eye

• In the 6th week, the mesenchyme encapsulating the
  optic cup forms
• The inner vascular choroid,
• The outer fibrous sclera.
• In the 6th week, the mesenchyme anterior to the
  lens splits into layers conforming to the choroid
  and sclera to form the anterior chamber of the
  eye.

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• The mesoderm of the anterior wall of the anterior
  chamber, with surface ectoderm, forms the cornea,
  consisting of 3 layers
• The superficial anterior epithelium, from surface
  ectoderm
• The substantia propria, from the mesoderm of the
  anterior wall
• The epithelium lining the anterior chamber, from
  the mesoderm of the anterior wall

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Development of the mesenchyme around the eye

• The mesoderm of the posterior wall of the
  anterior chamber forms
• The posterior chamber of the eye, via
  vacuolisation of the posterior layers of mesoderm
  in contact with the lens
• The pupil of the eye, after the breakdown of a
  thin layer of remaining mesoderm called the
  pupillary membrane.
• Initially separated the posterior and anterior
  chambers.

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Development of the mesenchyme around the eye

• The anterior rim of the optic cup, with overlying
  choroid, forms the iris, with its posterior
  surface coming from the 2 fused layers of the
  optic cup, and the pupillary muscle (sphincter
  and dilator pupillae) derived from neural crest
  origin.
• Just posterior to the iris, the optic cup forms
  the ciliary body,
• Including the suspensory ligament,
• And the ciliary muscle, which comes from neural
  crest origin

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DEVELOPMENT OF THE EAR
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DEVELOPMENT OF INNER EAR

• The first indication of the developing ear can be
  found in embryos of approximately 22 days as a
  thickening of the surface ectoderm on each side
  of the rhombencephalon.
• These thickenings, the otic placodes,
• Invaginate rapidly and form the otic or auditory
  vesicles (otocysts).

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DEVELOPMENT OF INNER EAR

• During later development, each vesicle divides
  into
• (a) a ventral component that gives rise to the
  saccule and cochlear duct and
• (b) a dorsal component that forms the utricle,
  semicircular canals, and endolymphatic duct.
• Together these epithelial structures form the
  membranous labyrinth.

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DEVELOPMENT OF 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.
• Its connection with the remaining portion of the
  saccule is then confined to a narrow pathway, the
  ductus reuniens.

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DEVELOPMENT OF SACCULE, COCHLEA, AND ORGAN OF
CORTI

• Mesenchyme surrounding the cochlear duct soon
  differentiates into cartilage.
• In the 10th week, this cartilaginous shell
  undergoes vacuolization, and two perilymphatic
  spaces,
• The scala vestibuli and
• The scala tympani, are formed.

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DEVELOPMENT OF SACCULE, COCHLEA, AND ORGAN OF
CORTI

• The cochlear duct is then separated from the
  scala vestibuli by the vestibular membrane and
• The cochlear duct is separated from the scala
  tympani by the basilar membrane.
• The lateral wall of the cochlear duct remains
  attached to the surrounding cartilage by the
  spiral ligament,
• Its median angle is connected to and partly
  supported by a long cartilaginous process, the
  modiolus, the future axis of the bony cochlea.

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DEVELOPMENT OF SACCULE, COCHLEA, AND ORGAN OF
CORTI

• The cochlear duct form two ridges
• The inner ridge, the future spiral limbus, and
• The outer ridge.
• The outer ridge forms one row of inner and three
  or four rows of outer hair cells.
• These cells are covered by the tectorial
  membrane, a fibrillar gelatinous substance
  attached to the spiral limbus that rests with its
  tip on the hair cells.

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DEVELOPMENT OF SACCULE, COCHLEA, AND ORGAN OF
CORTI

• The sensory cells and tectorial membrane together
  constitute the organ of Corti.
• Impulses received by this organ are transmitted
  to the spiral ganglion and then to the nervous
  system by the auditory fibers of cranial nerve
  VIII.

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DEVELOPMENT OF UTRICLE, AND SEMICIRCULAR CANALS

• During the fifth week of development,
  semicircular canals appear as flattened
  outpocketings of the utricular part of the otic
  vesicle.

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DEVELOPMENT OF UTRICLE, AND SEMICIRCULAR CANALS

• Central portions of the walls of these
  outpocketings eventually appose each other and
  disappear, giving rise to three semicircular
  canals.
• Whereas one end of each canal dilates to form the
  crus ampullare,
• The other, the crus nonampullare, does not widen.
• Since two of the latter type fuse, however, only
  five crura enter the utricle, three with an
  ampulla and two without.

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DEVELOPMENT OF UTRICLE, AND SEMICIRCULAR CANALS

• Cells in the ampullae form a crest, the crista
  ampullaris, containing sensory cells for
  maintenance of equilibrium.
• Similar sensory areas, the maculae acusticae,
  develop in the walls of the utricle and saccule.
• Impulses generated in sensory cells of the
  cristae and maculae as a result of a change in
  position of the body are carried to the brain by
  vestibular fibers of cranial nerve VIII.

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DEVELOPMENT OF UTRICLE, AND SEMICIRCULAR CANALS

• During formation of the otic vesicle, a small
  group of cells breaks away from its wall and
  forms the statoacoustic ganglion.
• Other cells of this ganglion are derived from the
  neural crest.
• The ganglion subsequently splits into cochlear
  and vestibular portions, which supply sensory
  cells of the organ of Corti.
• Those of the saccule, utricle, and semicircular
  canals, respectively.

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DEVELOPMENT OF INNER EAR
TIME EVENTS STRUCTURE
22 DAY Surface Ectoderm Thickening OTIC PLACODE
Surface Ectoderm Invagination OTIC PIT
OTIC VESICLE
Otic Vesicle Dorsal Part UTRICLE, SEMICIRCULAR CANAL
Otic Vesicle Ventral Part SACCULE, COCHLEAR PART
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DEVELOPMENT OF SACCULE, COCHLEA, AND ORGAN OF
CORTI
TIME EVENTS STRUCTURE
6 Week Saccule lower pole tubular outpocketing Begining cochlear duct formation
End 8 Week 2.5 Turn Cochlear Duct
10 Week Cochlear Duct Seperating Formation of Scala Tympani and Scala Vesitubuli
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DEVELOPMENT OF UTRICLE, AND SEMICIRCULAR CANALS
TIME EVENTS STRUCTURE
5 Week Utricle flattened outpocketing Begining semicircular canal formation
6 Week Outpocketing Central Portion Apposed 3 Semicircular canal Appear
8 Week 5 Crura Enter Utricle End of Formation Semicircular Canal
8 Week 3 Crura with Ampullae Enter Saccule End of Formation Semicircular Canal
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MIDDLE EARTYMPANIC CAVITY AND AUDITORY TUBE

• The tympanic cavity, which originates in the
  endoderm, 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, the tubotympanic
  recess, widens and gives rise to the primitive
  tympanic cavity,
• The proximal part remains narrow and forms the
  auditory tube (eustachian tube).
• The tympanic cavity communicates with the
  nasopharynx.

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MIDDLE EAROSSICLES

• The malleus and incus are derived from cartilage
  of the first pharyngeal arch,
• The stapes is derived from that of the second
  arch.
• Although the ossicles appear during the first
  half of fetal life,
• They remain embedded in mesenchyme until the
  eighth month, when the surrounding tissue
  dissolves.
• The endodermal epithelial lining of the primitive
  tympanic cavity then extends along the wall of
  the newly developing space.
• The tympanic cavity is now at least twice as
  large as before.
• When the ossicles are entirely free of
  surrounding mesenchyme,
• The endodermal epithelium connects them in a
  mesentery-like fashion to the wall of the cavity.
• The supporting ligaments of the ossicles develop
  later within these mesenteries.

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MIDDLE EAROSSICLES

• Since the malleus is derived from the first
  pharyngeal arch, its muscle, the tensor tympani,
  is innervated by the mandibular branch of the
  trigeminal nerve.
• The stapedius muscle, which is attached to the
  stapes, is innervated by the facial nerve, the
  nerve to the second pharyngeal arch.
• During late fetal life, the tympanic cavity
  expands dorsally by vacuolization of surrounding
  tissue to form the tympanic antrum.
• After birth, epithelium of the tympanic cavity
  invades bone of the developing mastoid process,
• Epithelium-lined air sacs are formed
  (pneumatization).
• Later, most of the mastoid air sacs come in
  contact with the antrum and tympanic cavity.
• Expansion of inflammations of the middle ear into
  the antrum and mastoid air cells is a common
  complication of middle ear infections.

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Development of the Tympanic cavity
Pharyngeal arch derivatives in the middle ear Pharyngeal arch derivatives in the middle ear
Pharyngeal arch Middle ear structures
1st Cartilage malleus, incus Mesoderm tensor tympani
2nd Cartilage stapes Mesoderm stapedius muscle
Pharyngeal arch derivatives in the middle ear Pharyngeal arch derivatives in the middle ear
Pharyngeal arch structure Middle ear structures
1st pharyngeal cleft External acoustic meatus
1st pharyngeal membrane Tympanic membrane
1st pharyngeal pouch Tubotympanic recess
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EXTERNAL EAREXTERNAL 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.

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EXTERNAL EAREARDRUM OR TYMPANIC MEMBRANE

• The eardrum is made up of
• (a) ectodermal epithelial lining at the bottom of
  the auditory meatus,
• (b) endodermal epithelial lining of the tympanic
  cavity, and
• (c) an 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.
• The remaining portion forms the separation
  between the external auditory meatus and the
  tympanic cavity

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EXTERNAL EARAURICLE

• 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.

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Development of the External Ear
Differentiation of the auricle Differentiation of the auricle
Pharyngeal arch Hillocks ---gt Resulting part of pinna(from ventral to dorsal on pharyngeal arch
1st Tragus Helix Cymba concha
2nd Antitragus Antihelix Concha
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Formation of the Optic Cup and Lens Vesicle

• These vesicles subsequently come in contact with
  the surface ectoderm and induce changes in the
  ectoderm necessary for lens formation.
• Shortly thereafter the optic vesicle begins to
  invaginate
• Forms the double-walled optic cup.
• The inner and outer layers of this cup are
  initially separated by a lumen, the intraretinal
  space,
• But soon this lumen disappears, and the two
  layers appose each other.
• Invagination is not restricted to the central
  portion of the cup but also involves a part of
  the inferior surface that forms the choroid
  fissure.
• Formation of this fissure allows the hyaloid
  artery to reach the inner chamber of the eye
• During the seventh week, the lips of the choroid
  fissure fuse, and
• The mouth of the optic cup becomes a round
  opening, the future pupil.
• During these events, cells of the surface
  ectoderm, initially in contact with the optic
  vesicle,
• Begin to elongate and form the lens placode.
• This placode subsequently invaginates and
  develops into the lens vesicle.
• During the fifth week, the lens vesicle loses
  contact with the surface ectoderm and lies in the
  mouth of the optic cup