Download DEVELOPMENT OF THE SENSORY ORGANS

DEVELOPMENT OF THE
SENSORY ORGANS
DEVELOPMENT OF THE EYE
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
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).
•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.
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
•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 .
Development of the optic cup
Relationship of the optic
groove to this ectoderm.
•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.
The optic grooves form the optic
stalks and the optic vesicles
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.
Development of the optic cup
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.
•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.
•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.
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.
•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.
•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.
Development of the neural and pigment retinas
•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.
Development of the retina
Embryological
structure
Mature
structure
retinal
Outer wall of optic Pigment
retina,
cup
Melanin appears on
day 33
Inner wall of optic Neural
retina,
cup
Mostly developed by
week
nine,
All layers present by
8 months
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
retina, axons grow from
ganglion cells to form
fibre layer
•2 thin membranes bound
neural retina:
the
the
the
the
–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.
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.
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.
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.
•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
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.
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
DEVELOPMENT OF THE EAR
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
DEVELOPMENT OF INNER EAR
TIME
EVENTS
22 DAY Surface Ectoderm Thickening
Surface Ectoderm Invagination
STRUCTURE
OTIC PLACODE
OTIC PIT
OTIC VESICLE
Otic Vesicle Dorsal Part
UTRICLE, SEMICIRCULAR CANAL
Otic Vesicle Ventral Part
SACCULE, COCHLEAR PART
DEVELOPMENT OF SACCULE, COCHLEA, AND ORGAN OF CORTI
TIME
6 Week
EVENTS
Saccule lower pole tubular
outpocketing
End 8
Week
10 Week
STRUCTURE
Begining cochlear duct formation
2.5 Turn Cochlear Duct
Cochlear Duct Seperating
Formation of Scala Tympani and Scala
Vesitubuli
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
MIDDLE EAR
TYMPANIC 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.
MIDDLE EAR
OSSICLES
•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.
MIDDLE EAR
OSSICLES
• 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.
Development of the Tympanic cavity
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 structure
Middle ear structures
1st pharyngeal cleft
External acoustic meatus
1st pharyngeal membrane
Tympanic membrane
1st pharyngeal pouch
Tubotympanic recess
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.
EXTERNAL EAR
EARDRUM 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
EXTERNAL EAR
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.
Development of the External Ear
Differentiation of the auricle
Pharyngeal arch
Hillocks ---> Resulting part of
pinna
(from ventral to dorsal on
pharyngeal arch
1st
Tragus
Helix
Cymba concha
2nd
Antitragus
Antihelix
Concha
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