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  • Mark Hill's site for this lecture
  • This lecture is about the development of the face and ear
  • Everyone has virtually the same external anatomy, but the one unique feature is the appearance of your face (there is a lot of neurological input into recognition of faces)
    • But the way in which peoples' faces develop is virtually the same (and is similar to that seen in other species)
    • It has very different in its development to other parts of the body
    • Many deformities are associated with this area of the body (e.g. cleft lip or cleft palate)
  • The face has a complex origin arising from a number of head structures. Therefore the face is susceptible to a number of teratogens that act during various different critical periods of development
    • The upper lip and palate contribute to the majority of face deformities
  • Animation shows you the contribution to the elements of the face based on embryological origins
  • The pharyngeal arches are components that arise in week 4, and contribute the structures of the head and neck.
    • Head and neck are derived from pharyngeal arches 1-6, with the face forming from arches 1 and 2 and the frontonasal prominence. Each arch contains similar arch components, derived from endoderm, mesoderm, neural crest and ectoderm.
  • Portions of the ear appear very early in development as a specialised region (the otic placode) on the embryo surface that sinks into the mesenchyme to form a vesicle (the otic vesicle or otocyst) that forms the inner ear.

Objectives

To introduce the developmental embryology of both the face and ear, and their associated abnormalities.

  1. To understand the formation and contribution of the pharyngeal arches to face and neck development.
  2. To know the main structures derived from components of the pharyngeal arches (groove, pouch and arch connective tissue).
  3. To know the 3 major parts (external, middle and inner) of hearing development and their embryonic origins.
  4. To briefly understand some abnormalities associated with face and hearing development.
  • (memorise a table in Larsen's for objective number 3)

Week 3

Degeneration of the buccopharyngeal membrane

Picture (first one under "buccopharyngeal membrane"): the majority of the head is formed by the neural tube with the neural pore (?) at the end which will close over. The indentation on the anterior surface beneath the heart is the stomedeum. On the floor of the stomedeum is the buccopharyngeal membrane that has started to break down, allowing the amniotic fluid to enter to foregut.

  • On either side of the stomedeum is a ridge where the first pharyngeal arch will begin to form. It has a smaller superior component and a larger inferior component.

Pharynx and pharyngeal arches

  • Look at the red and yellow picture, showing the beginnings for the first pharyngeal arch to the side of the stomedeum relative to the neural tube and the pharynx
  • The neural crest (dorsal to the neural tube) contributes important nervous structures in the head (different source to in the rest of the body)
  • Pharyngeal arches develop in a rostrocaudal sequence (like somites - 1st, 2nd, 3rd, 4th then 6th)

Week 4

Pharyngeal arch components

  • Pharyngeal arches contribute to the formation of the head and neck, and in the human appear at the 4th week. The first arch contributes the majority of upper and lower jaw structures
  • If you cut through an arch, you'll see his cross section. In the middle will be the endoderm lining of the pharynx. This shows the 1st, 2nd, 3rd arch. Each arch has the same components. Each of these components forms a different anatomical part of the head and neck.
  • Arch consists of mesenchyme, consisting of mesoderm and neural crest cells. The neural crest contributions will be in a segmental pattern as well (a rostrocaudal sequence).
  • There is an indentation of the ectoderm between the arches - called a groove or a cleft. Numbering: 1st groove = beneath the 1st arch and so on.
  • Going down the pharynx there is similar endoderm indentation between arches (called pouches, numbering as above).
  • Between the pharyngeal arches, the endoderm and ectoderm come close together with little or no mesoderm in between (called a pharyngeal membrane, numbering as above).
  • Pharyngeal arches can also be called branchial arches.
  • All three trilaminar embryo layers (+ neural crest) contribute to each of the pharyngeal arches
  • Video - showing dorsal side of embryo. Labeled the neural ectoderm. Where the neural tube has closed, we can see the neural crest cells migrating out to enter pharyngeal arch cells. Each of the pharyngeal arches is populated by neural crest cells from specific sections of the ectoderm (called rhombomeres - each has a different rhombomeric contribution)
    • The mesenchyme is invaded by neural crest cells, which contribute to adult CT components (cartilage, bone and ligaments)
    • These neural crest cells arise from the midbrain and hindbrain region
  • Each pharyngeal arch contains
    • Artery (bilaterally, so they are paired)
    • Cartilage
    • Nerve
    • Muscle
  • Arches and pharynx form the face, tongue, lips, jaws, palate, pharynx and neck, cranial nerves, sense organ components, glands
  • Notable features:
    • Arch
    • Groove (cleft; ectoderm) - externally separates each arch (only the first pair persist as the external auditory meatus)
    • Pouch (endoderm) - internally separates each arch (pockets out from the pharynx)
    • Membrane - ectoderm and endoderm contact regions (only first pair persist as tympanic membrane)

Contributions of the pharyngeal arches

  • Pharyngeal arch 1 (mandibular arch) has 2 prominences:
    • Smaller upper prominence is the maxillary prominence. This forms the maxilla, the zygomatic bone and squamous part of the temporal bone
    • Large lower prominence is the mandibular prominence. This forms the mandible
  • Pharyngeal arch 2 (hyoid arch)
    • Forms most of the hyoid bone
  • Pharyngeal arches 3 and 4
    • Form neck structures
  • Slide shows to the right the upper opening in the buccopharyngeal membrane and the left = towards the glottis. Pharyngeal arches are between these
  • Humans have 5 arches: 1, 2, 3, 4, 6 (arch 5 is a very transient structure)
  • They form in a rostrocaudal sequence (from 1-6) starting in week 4.
  • Arch 1 and 2 occur around the same time as the closure of the cranial neuropore (opening in the neural tube)
  • The face is contributed by arches 1 and 2 (the rest contribute neck structures)
    • Neck components - arches 3 and 4 (arches 4 and 6 fuse)
  • Labelled ventrolateral view of the embryo (stage 14, day 32).
    • Can't see the 6th unless you go back earlier
    • The 4th and 6th arch fuse together to form a single arch.
    • Can still see the stomedeum. By this stage, the buccopharyngeal membrane has broken down,
    • Frontonasal prominence: at the front of the head, and includes where the nasal cavity will form
    • The first arch has 2 components (never misnumber the arches!) - a smaller cranial component (maxillary) and a much larger caudal component (mandibular component)

Pharyngeal arch arteries

  • Back to cross section - in the mesenchyme we see structures: an artery, a bar of cartilage and a nerve - these are paired (as in, there is one artery in the left part of the arch, and a corresponding artery in the right part the arch. These fuse in the midline to form adult structures). Arteries are continually remodelled. The significance of remodelling is especially important in areas like the aortic arch that are scarcely symmetrical, where the left and right contributions to the artery/ies develop in different patterns.
  • Contributions of each arch artery to the final CVS:
    • Arch 1 - mainly lost, forms part of the maxillary artery
    • Arch 2 - stapedial arteries
    • Arch 3 - common carotid arteries, internal carotid arteries
    • Arch 4 - left forms part of the aortic arch, right forms part of the right subclavian artery
    • Arch 6 - left forms part of the left pulmonary artery, right forms part of the right pulmonary artery
    • Memorise the contributions of each arches to the cardiovascular system (the lower 3 arches form very important CVS components, but the top 2 only form small things)

placental vein -> liver -> heart -> truncus arteriosus -> aortic sac -> arch arteries -> dorsal aorta -> placental artery

Pharyngeal arch cartilages

The cartilage of each arch is given a different name.

  • Arch 1 - Meckel's cartilage, horseshoe shaped
    • Meckel's cartilage is the cartilage in arch 1, forming a ventral horseshoe, making the mandibular template.
    • Midpart forms ligaments (anterior malleus, sphenomandibular)
    • The dorsal ends of Meckel's cartilage also contribute to the auditory ossicles (malleus an incus; which form as a single bone, and are later separated by a joint between them)
    • The Meckel's cartilage running in the anterior region does not ossify - the mandible forms later by intramembranous ossification (not endochondrial ossification)
  • Arch 2 - Reichert's cartilage
    • The second arch cartilage is Reichert's cartilage, contributing the superior part of the hyoid, and contributes the stapes.
    • Dorsal ends form the stapes and temporal bone styloid process
    • Ventral part ossifies to form hyoid bone components (the lesser cornu and superior body)
    • The rest of these cartilages (not contributing things) are lost later
  • Arch 3 forms the greater cornu and inferior part of hyoid
  • Arch 4 and 6 - form laryngeal cartilages, except epiglottis (from hypobranchial eminence)

Pharyngeal arch muscles

  • The mesoderm of these arches form the muscles of the face/neck
  • Arch 1 - muscles of mastication, mylohyoid, tensor tympanic, ant. belly digastric
  • Arch 2 - muscles of facial expression, stapedius, stylohyoid, post. belly digastric
    • Second arch grows up and spreads out and provides the muscles of facial expression. It also forms stapedius
  • Arch 3 - stylopharyngeus
  • Arch 4&6 - cricothyroid, pharynx constrictors, larynx muscles, oesophagus (st. muscle)

Pharyngeal arch nerves

  • Arch 1 - CN V (trigeminal nerve), responsible for the innervating caudal 2/3 of the maxillary and mandibular regions, and the cranial 1/3 sensory nerves of the head and neck, and for motor innervation for mastication
  • Arch 2 - CN VII (facial nerve)
  • Arch 3 - CN IX (glossopharyngeal nerve)
  • Arch 4 and 6 - CN X (vagus)
    • Arch 4 - superior laryngeal nerve
    • Arch 6 - recurrent laryngeal nerve
  • Note that the vagus spreads out and stretches to innervate body structure, but arch 6 forms only cranial structures

Pharyngeal arch pouches

  • Arch 1 - elongates to form the tubotympanic recess, tympanic cavity, mastoid antrum, eustachian tube
  • Arch 2 - forms the tonsillar sinus, mostly obliterated by the palatine tonsil
  • Arch 3 - forms the inferior parathyroid and thymus
  • Arch 4 - forms the superior parathyroid, parafollicular cells of the thyroid

Thyroid gland

  • Not a pouch structure
  • First endocrine organ to develop (day 24)
  • Develops from the floor of the pharynx
  • Descends through the thyroglossal duct (which closes)
  • Upper end of the thyroid is at the foramen caecum

Anterior pituitary gland

  • Not a pouch structure
  • Boundary epithelial ectoderm in the roof of the pharynx
  • Forms a pocket (Rathke's pouch) that comes into contact with the ectoderm of developing brain.
    • Rathke's pouch is named after German embryologist and anatomist Martin Heinrich Rathke (1793 — 1860).

Face development

Face development begins in week 4 centered around the stomadeum (the external depression at the oral membrane).

  • Forms from 5 prominences (5 initial primordia from neural crest mesenchyme (week 4)):
    1. Frontonasal (centre) -- forms forehead, the nose dorsum and apex. Nasal epithelium and external surface of nose.
    • Nasal placodes develop later bilaterally, and are pushed medially
    1. 2 Maxillary prominences (lateral) -- form upper cheek and upper lip; upper jaw and maxilla
    2. 2 Mandibular prominences (lateral) -- form lower cheek, chin and lower lip; lower jaw

These are formed around the stomedeum. Nasal epithelium comes from frontonasal prominence.

  • Doesn't look like a face by the end of week 4. Animation shows how each of the 5 component
  • Cranial vault is covered with the frontonasal prominence. All the features beneath it form from intramembranous ossification (?)
  • Stage 16-18 gif
  • The second arch contributes the musculature of the head, and grows over the top of the lower arches (which are lost from the surface). On the lateral part of the embryo we can see an indentation below the second arch (cervical sinus)

Head/skull

  • Chondrocranium forms base of the skull (endochondrial ossification)
    • In lower vertebrates encases the brain
  • Cranial vault, jaw (intramembranous ossification)
  • Calveria (upper part of the cranium and surrounds the cranial cavity containing the brain - made up of the frontal, occipital and right and left parietals)
  • Facial skeleton
  • Pharyngeal arches

Sensory placodes

  • During week 4 a series of thickened surface ectodermal patches form in pairs rostro-caudally in the head region
  • Recent research suggests that all sensory placodes may arise from common panplacodal primordium origin around the neural plate, and then differentiate to eventually have different developmental fates
  • The sensory placodes will later contribute key components of each of our special senses (vision, hearing and smell). Other species have a number of additional placodes which form other sensory structures (fish, lateral line receptor). Note that their initial position on the developing head is significantly different to their final position in the future sensory system.
  • Only talking about auditory, not visual
  • Placodes are surface ectodermal regions in the head
  • The first placode we see on the surface of the embryo is stage 12
    • It is visible as a dark spot behind the 2nd pharyngeal arch. This represents where the otic placode was - it folds in to form an indentation/vesicle (otic vesicle), that folds in to form the inner ear. This contributes all of the inner ear (membranous labyrinth)
    • Nasal placode appears next (can see on the surface - a thickened surface of ectoderm (lateral to the frontonasal prominence)
      • Later folds inwards and becomes in the interior surface of nose
    • Optic placode appears later, laterally to nasal placode
      • Also buds inwards to form a vesicle to form the sense of vision

Otic placode

  • Carnegie stage 12 - still visible on embryo surface
  • Carnegie stage 13/14 embryo (shown below) the otic placode has sunk from the surface ectoderm to form a hollow epithelial ball (the otocyst) which now lies beneath the surface surrounded by mesenchyme (mesoderm). The epithelium of this ball varies in thickness and has begun to distort. It will eventually form the inner ear membranous labyrinth

Lens placode

  • Lies on the surface, adjacent to the outpocketing of the nervous system (which will form the retina) and will form the lens

Nasal placode

  • Has 2 components (medial and lateral) and will form the nose olfactory epithelium

Hearing development

Many different embryonic components contribute to the sensory structures required for hearing. It is always considered/divided in terms of the 3 main divisions (external, middle and inner), each of which have their own separate components from different embryonic origins.

The complexity and time course of development leads to many associated abnormalities.

External Ear

  • Auricle - Pharyngeal Arches 1 and 2 (ectoderm, mesoderm)
    • form from 6 hillocks (week 5): 3 on each of arch 1 and 2
  • External Auditory Meatus - Pharyngeal Arch 1 groove or cleft (ectoderm)
  • Tympanic Membrane - Pharyngeal Arch 1 membrane (ectoderm, mesoderm, endoderm)

Middle Ear

  • Middle Ear Ossicles
    • Malleus and incus - Pharyngeal Arch 1 cartilage Neural crest (ectoderm)
    • Stapes - Pharyngeal Arch 2 cartilage Neural crest (ectoderm)
  • Middle Ear Muscles
    • Tensor tympani - Pharyngeal Arch 1 (mesoderm)
    • Stapedius - Pharyngeal Arch 2 (mesoderm)
  • Middle Ear Cavity
    • Pharyngeal Arch 1 pouch (endoderm)

Inner Ear

  • Inner Ear Labyrinth
    • Cochlea - Otic vesicle - Otic placode (ectoderm)
    • Semicircular canals - Otic vesicle - Otic placode (ectoderm)
    • Saccule and utricle - Otic vesicle - Otic placode (ectoderm)
  • Cranial Nerve VIII
    • Auditory component - Otic vesicle and neural crest (ectoderm)
    • Vestibular component - Otic vesicle and neural crest (ectoderm)
  • Review the anatomy of the ear, and then learn the embryonic contribution of everything
  • The outer ear is formed from 6 surface hillocks (pharyngeal arches)
  • External auditory meatus forms from the first groove (which is the only groove that forms anything)
  • First pharyngeal membrane forms the tympanic membrane
  • First and second pharyngeal cartilages contribute the ossicles. Initially these bones form within solid mesenchyme. It's only the expansion of the cavity that forms the open ear. Later, when we get a cavity, the cavity is fluid filled (normal function inhibited)
  • Eustachian tube comes from the first pharyngeal pouch. Unlike the grooves, the pouches have major contribution to the endocrine system
  • The membranous labyrinth of the inner ear is derived from the otic placode/vesicle
  • Cranial nerve is associated with hearing/balance

External ear (auricle)

  • Numbers show the Carnegie stages of development
  • First and second pharyngeal arches contribute this
  • There are 6 hillocks (bumps) on the surface of pharyngeal arches 1 and 2, which fuse together to form a donut-like structure that runs around the external auditory meatus (formin the pinna).
  • Note that the anatomical position of the ear is much lower down than it is in the adult (it's further down in the mandible than in the adult). As the head grows, it shifts down
    • Teratogens that affect head development will affect the position of the ear on the head (can use the levels of the ears as a clinical marker to see when a teratogen took affect)
    • Ear developmen occur at the same time as some renal/cardiac development - so clinical abnormalities can occur in all these systems at once

Middle ear

  • Tensor tympani and stapedius minimise movements of ossicles during loud noise

Postnatal changes

  • In the adult, the eustachian tube (equaliser of pressure and a drain) is angled at 45 degrees from the horizontal (easy drainage). An infant's ear has the eustachian tube tilted at only about 10 degrees (it won't drain very well), and is much narrower than in the adult, and there are less muscles to pull the eustachian tube open (to open it during pressure equalisation on yawning). These features mean the eustachian tube can be easily blocked and infections can also spread back from the nasal cavity to the middle ear (middle ear infections are very common in children). Changes in anatomy later on allow the child to grow out of it. Treatment: grommits.
  • Adult - longer (twice as long), wider and runs at approximately 45 degrees to the horizontal, tube is opened by two separate muscles (tensor palati and levator palati)
  • At birth - shorter (17-18 mm), narrower and runs almost horizontal, tube is opened by a single muscle (tensor palati muscle)
  • Auditory tube = Eustachian, otopharyngeal or pharyngotympanic tube.
  • Connects middle ear cavity to nasopharynx portion of pharynx
  • Ventilation - pressure equalization in the middle ear
  • Clearance - allow fluid drainage from the middle ear. Tube is normally closed and opened by muscles

Skull

  • Head growth continues postnatally - fontanelles allow head distortion on birth and early growth
  • Bone plates remain unfused to allow growth. During puberty there is growth of the face.
  • Two methods of ossification
  • Everything in purple is cartilage, everything in blue is bone.
  • Can see the mandible ossifying, the maxilla ossifying, the baseplate of teh skull is ossifying in different centres
  • Spiderlike web: intramembranous ossification in the cranial vault.
  • Base of the skull: endochondral ossification (paraxial mesoderm; chondrocranium)
  • Skull vault and mandible: intramembranous
  • Neural crest contributions to the cranial vault and facial skeleton
  • Calveria - bone has no cartilage (direct ossification of mesenchyme).
    • In between ossification centres, there is no bone, leaving unossified fontanelles - makes childbirth easier
    • After birth, fontanelles allow postnatal growth of the brain (mainly white matter)
    • Posterior fontanelle closes at 3 months, anterior closes at 18 months
    • In old age - the sutures completely ossify
  • Chondrocranium - formed from paraxial mesoderm
    • Cranial end of vertebral column
    • Modified vertebral elements
    • Occipital and cervical sclerotome
    • Bone preformed in cartilage (endochondrial ossification)
  • Cranial vault and facial skeleton - formed from neural crest
    • Muscle is paraxial mesoderm
    • Somitomeres and occipital somites
  • Calveria - bone has no cartilage (direct ossification of mesenchyme)
    • Bones do not fuse, fibrous sutures
      1. Allow distortion to pass through birth canal
      2. Allow growth of the brain
    • 6 fontanelles - posterior closes at 3 months, anterior closes at 18 months

Palate

  • The palate has two key stages of development during embryonic and early foetal life involving fusion of structures (epithelium to mesenchymal)
  1. Embryonic
    • Primary palate, fusion in the human embryo between stage 17 and 18, from an epithelial seam to the mesenchymal bridge
  2. Foetal
    • Secondary palate, fusion in the human embryo in week 9. This requires the early palatal shelf's growth, elevation and fusion during the early embryonic period. The fusion event is to both each other and to the primary palate (therefore a cleft primary palate results in a cleft secondary palate).

Tongue development

  • Ectoderm of the first arch surrounding the stomodeum forms the epithelium lining the buccal cavity.
  • Also the salivary glands, enamel of the teeth, epithelium of the body of the tongue.
    • As the tongue develops "inside" the floor of the oral cavity, it is not readily visible in the external views of the embryonic (Carnegie) stages of development.
  • Contributions from all arches, which changes with time
  • begins as swelling rostral to foramen cecum, median tongue bud
  • Arch 1 - oral part of tongue (ant 3/2)
  • Arch 2 - initial contribution to surface is lost
  • Arch 3 - pharyngeal part of tongue (post 1/3)
  • Arch 4 - epiglottis and adjacent regions

Tongue muscle

  • Tongue muscles originate from the somites. Tongue muscles develop before masticatory muscles and are completed by birth
  • Masticatory muscles originate from the somitomeres. These muscles develop late and are not complete even at birth

Salivary glands

  • Epithelial buds in oral cavity (week 6-7) extend into mesenchyme
  • Parotid, submandibular, sublingual salivary glands develop

Abnormalities

  • Clefting of the lip occurs during the embryonic period, as a problem in the frontonasal prominence (philtrum). The lateral components are from the maxillary prominences
    • Can be unilateral or bilateral
    • Occurs during stage 17-18. Need fusion of frontonasal prominence or maxillary prominence
  • Cleft palate forms in the foetal period (week 9) - maxillary part of oral cavity has 2 wings that grow out, initially downwards towards the midline, then during the foetal period they elevate away from the tongue then fuse in the midline. If you disrupt the process of fusion of shelves, you get cleft palate (in the midline)
    • Can have both cleft lip and palate together - if the lip is cleft, then the palate will not be able to fuse there
    • If you fuse the lip you can still have cleft palate as well

Cleft lip and palate

  • There are >300 abnormalities, different cleft forms and extent, upper lip and anterior maxilla, hard and soft palate
  • Cleft palate
    • The national rate for this abnormality in births is 4.8-6/10,000 births, representing 1,530 infants (5.5% were stillborn and 11.5% liveborn died during the neonatal period). Cleft palate is slightly more common in twin births than singleton
  • Cleft lip
    • 8.1-9.9/10,000 births. Of 2,465 infants, 6.5% were stillborn and 7.8% liveborn died during neonatal period and the rate was similar in singleton and twin birth

First arch syndrome

Two types of first arch syndrome (Treacher Collins syndrome and Pierre Robin syndrome), both of which result in extensive facial abnormalities

  • Pierre Robin syndrome results in hypoplasia of the mandible, cleft palate and eye and ear defects
    • The initial defect is small mandible (micrognathia) resulting in posterior displacement of the tongue and a bilateral cleft palate

DiGeorge syndrome

  • Absence of thymus and parathyroid glands.
  • 3rd and 4th pouch do not form
  • Disturbance of cervical neural crest migration

Foetal alcohol syndrome

Due to alcohol in early development (week 3+) leading to both facial and neurological abnormalities

  • Lowered ears, small face, mild+ retardation
  • Microcephaly - leads to small head circumference
  • Short Palpebral fissure - opening of eye
  • Epicanthal folds - fold of skin at inside of corner of eye
  • Flat midface
  • Low nasal bridge
  • Indistinct Philtrum - vertical grooves between nose and mouth
  • Thin upper lip
  • Micrognathia - small jaw

Exposure of embryos in vitro to ethanol simulates premature differentiation of prechondrogenic mesenchyme of the facial primordia (1999)

Others

  • Cysts - many different types
  • Facial clefts
    • Extremely rare
    • Holoprosencephaly -- shh abnormality
  • Maternal effects
    • Retinoic acid - present in skin ointments
    • 1988 associated with facial developmental abnormalities

Structures derived from pharyngeal arches

Structures derived from Arches

Arch Nerve Skeletal Structures Muscles Ligaments
1 (maxillary/mandibular) trigeminal (V) mandible, maxilla, malleus, incus   ant lig of malleus, sphenomandibular ligament
2 (hyoid) facial (VII) stapes, styloid process, lesser cornu of hyoid, upper part of body of hyoid bone   stylohyoid ligament
3 glossopharyngeal (IX) greater cornu of hyoid, lower part of body of hyoid bone  
4 & 6 superior laryngeal and recurrent laryngeal branch of vagus (X) thyroid, cricoid, arytenoid, corniculate and cuneform cartilages  


Structures derived from Pouches

Each pouch is lined with endoderm and generates specific structures.


Pouch
Overall Structure Specific Structures
1
tubotympanic recess tympanic membrane, tympanic cavity, mastoid antrum, auditory tube
2
intratonsillar cleft crypts of palatine tonsil, lymphatic nodules of palatine tonsil
3
inferior parathyroid gland, thymus gland
4
superior parathyroid gland, ultimobranchial body
5
becomes part of 4th pouch

Structures derived from Grooves

Only the first groove differentiates into an adult structure and forms part of the external acoustic meatus.

Structures derived from Membranes

At the bottom of each groove lies the membrane which is formed from the contact region of ectodermal groove and endodermal pouch. Only the first membrane differentiates into an adult structure and forms the tympanic membrane.

Images

External links

Larsen's: