Lecture - Head Development

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Contents

Head Development

Stage14 sem2cl.jpg

Introduction

Human face development (Week 6 to 7)

The face is the anatomical feature which is truly unique to each human, though the basis of its general development is identical for all humans and similar to that seem for other species. The face has a complex origin arising from a number of head structures and sensitive to a number of teratogens during critical periods of its development. The related structures of upper lip and palate significantly contribute to the majority of face abnormalities.

The head and neck structures are more than just the face, and are derived from pharyngeal arches 1 - 6 with the face forming from arch 1 and 2 and the frontonasal prominence. Each arch contains similar Arch components derived from endoderm, mesoderm, neural crest and ectoderm. These components though will form different structures depending on their arch origin. Because the head contains many different structures also review notes on Special Senses), Respiratory, Integumentary (Teeth), Endocrine (thyroid, parathyroid, pituitary, thymus) and Ultrasound- Cleft lip/palate.

Lecture Plan

  • Origins of the head and neck tissues
  • Pharyngeal arches
  • Development of the face
  • Development of the palate
  • Development of the tongue and pharynx
  • Derivatives of the pharyngeal arches and pouches
  • Development of the skull

Lecture Date: 2013-09-10 Lecture Time: 12:00 Venue: Wallace Wurth LG03; Speaker: Professor Ken Ashwell

The Powerpoint file used to present this lecture is available as a pdf document HERE

A recording of the lecture will be available on Lectopia - Lectopia Login page

Pharyngeal arch structure cartoon.gifStage13 pharyngeal arch excerpts.gif

Textbooks

Pharyngeal arch cartilages.jpg

The Developing Human: Clinically Oriented Embryology

The Developing Human, 9th edn.jpg Citation: The Developing Human: clinically oriented embryology 9th ed. Keith L. Moore, T.V.N. Persaud, Mark G. Torchia. Philadelphia, PA: Saunders, 2011. (links available to UNSW students)

Larsen's Human Embryology

Larsen's human embryology 4th edn.jpg Citation: Larsen's human embryology 4th ed. Schoenwolf, Gary C; Larsen, William J, (William James). Philadelphia, PA : Elsevier/Churchill Livingstone, c2009. (links available to UNSW students)

Embryology

Logo.png Hill, M.A. (2012) UNSW Embryology (12th ed.). Sydney:UNSW.
Head Links: Introduction | Medicine Lecture | Medicine Lab | Science Lecture | Science Lab | Craniofacial Seminar | Palate | Tongue | Placodes | Skull Development | Head and Face Movies | Abnormalities | Category:Head
  • The Developing Human: Clinically Oriented Embryology (8th Edition) by Keith L. Moore and T.V.N Persaud - Moore & Persaud Chapter Chapter 10 The Pharyngeal Apparatus pp201 - 240.
  • Larsen’s Human Embryology by GC. Schoenwolf, SB. Bleyl, PR. Brauer and PH. Francis-West - Chapter 12 Development of the Head, the Neck, the Eyes, and the Ears pp349 - 418.

Movies

Stage16-18 face 01 icon.jpg Stage15-22 head 01 icon.jpg Face 001 icon.jpg Palate 001 icon.jpg Palate 002 icon.jpg Tongue 001 icon.jpg
Face

Stage 16 to 18

Head

Stage 15 to 22

Face Palate 1 Palate 2 Tongue

Animation of Face Development

<Flowplayer width="420" height="500" autoplay="true">Face_001.flv</Flowplayer>

Quicktime version

Development of the Face


This animation shows a ventral view of development of the human face from approximately week 5 through to neonate.

The separate embryonic components that contribute to the face have been colour coded.

  • Frontonasal Prominence (white)
  • Frontonasal Prominence - Lateral nasal (purple)
  • Frontonasal Prominence - Medial nasal (green)
  • Pharyngeal Arch 1 - Maxillary prominence (yellow)
  • Pharyngeal Arch 1 - Mandibular prominence (orange)
  • Stomodeum (black)

The stomodeum is the primordial mouth region and a surface central depression lying between the forebrain bulge and the heart bulge. At the floor of the stomodeum indentation is the buccopharyngeal membrane (oral membrane).

Note the complex origin of the maxillary region (upper jaw) requiring the fusion of several embryonic elements, abnormalities of this process lead to cleft lip and cleft palate.

See also the movie (Quicktime | Flash) showing a similar view of human embryo faces between Carnegie stage 16 to 18.



Links: Quicktime Development Animation - Face | Quicktime version | Head Development

Buccopharyngeal Membrane

These images of the Stage 11 embryo show the breakdown of the buccopharyngeal membrane.


The Pharynx

Head arches cartoon.jpg Stage13 B2 excerpt.gif Pharynx cartoon.jpg

The cavity within the pharyngeal arches forms the pharynx.

  • begins at the buccopharyngeal membrane (oral membrane), apposition of ectoderm with endoderm (no mesoderm between)
  • expands behind pharyngeal arches
  • narrows at glottis and bifurcation of gastrointestinal (oesophagus) and respiratory (trachea) systems
  • regions on roof, walls and floor have important contributions to endocrine in oral and neck regions
  • also contributes to tongue development

Pharyngeal Arch Components

Pharyngeal arch structure cartoon.gif

Major features to identify for each: arch, pouch, groove and membrane. Contribute to the formation of head and neck and in the human appear at the 4th week. The first arch contributes the majority of upper and lower jaw structures.

Pharyngeal Arch Development

  • branchial arch (Greek. branchia = gill)
  • arch consists of all 3 trilaminar embryo layers:
  1. ectoderm - outside embryo surface (and neural crest in core)
  2. mesoderm - core of mesenchyme
  3. endoderm - inside pharynx

Neural Crest

neural crest migration
  • Mesenchyme invaded by neural crest generating connective tissue components
  • cartilage, bone, ligaments
  • arises from midbrain and hindbrain region

Arch Features

Each arch contains: artery, cartilage, nerve and muscular component

Arches and Phanynx Form the face, tongue, lips, jaws, palate, pharynx and neck cranial nerves, sense organ components, glands

  • Humans have 5 arches - 1, 2, 3, 4, 6 (Arch 5 does not form or regresses rapidly)
  • from in rostro-caudal sequence, Arch 1 to 6 from week 4 onwards
  • arch 1 and 2 appear at time of closure of cranial neuropore
  • Face - mainly arch 1 and 2
  • Neck components - arch 3 and 4 (arch 4 and 6 fuse)
  • arch
  • groove
    • externally separates each arch
      • also called a cleft
    • only first pair persist as external auditory meatus
  • pouch
    • internally separates each arch
    • pockets from the pharynx
  • membrane
    • ectoderm and endoderm contact regions
    • only first pair persist as tympanic membrane
Stage13 B2 excerpt.gif

Pharyngeal Arch 1 (Mandibular Arch) has 2 prominences

  • smaller upper- maxillary forms maxilla, zygomatic bone and squamous part of temporal
  • larger lower- mandibular, forms mandible

Pharyngeal Arch 2 (Hyoid Arch)

  • forms most of hyoid bone

Pharyngeal Arch 3 and 4

  • neck structures

Arch Arteries

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

  • Arch 1 - mainly lost, form part of maxillary artery
  • Arch 2 - regresses to small stapedial arteries
  • Arch 3 - common carotid arteries, internal carotid arteries
  • Arch 4 - left forms part of aortic arch, right forms part right subclavian artery
  • Arch 6 - left forms part of left pulmonary artery, right forms part of right pulmonary artery

Arch Cartilage

Meckel's cartilage, first pharyngeal arch
Pharyngeal arch cartilages
  • Arch 1 - Meckel's cartilage, horseshoe shaped
    • dorsal ends form malleus and incus
    • midpart forms ligaments (ant. malleus, sphenomandibular)
    • ventral part forms mandible template
  • Arch 2 - Reichert's cartilage
    • dorsal ends form stapes and temporal bone styloid process
    • ventral part ossifies to form hyoid bone components (lesser cornu and superior body)
  • Arch 3 - forms greater cornu and inferior part of hyoid
  • Arch 4&6 - form laryngeal cartilages, except epiglottis (from hypobranchial eminence)

Arch Muscle

  • Arch 1 - muscles of mastication, mylohyoid, tensor tympanic, anterior belly digastric
  • Arch 2 - muscles of facial expression, stapedius, stylohyoid, posterior belly digastric
  • Arch 3 - stylopharyngeus
  • Arch 4&6 - crycothyroid, pharynx constrictors, larynx muscles, oesophagus (st. muscle)

Arch Nerve

  • Arch 1 - CN V trigeminal, caudal 2/3 maxillary and mandibular, cranial 1/3 sensory nerve of head and neck, mastication motor
  • Arch 2 - CN VII facial
  • Arch 3 - CN IX glossopharyngeal
  • Arch 4&6 - CN X vagus, arch 4 - superior laryngeal, arch 6 - recurrent laryngeal

Arch Pouches

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

Thyroid Gland

Stage13 and 22 thyroid development
  • not a pouch structure
  • first endocrine organ to develop day 24
  • from floor of pharynx
  • descends thyroglossal duct (which closes)
  • upper end at foramen cecum

Anterior Pituitary

Pituitary
  • not a pouch structure
  • boundary epitheilal 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

Stage16-18 face animation.gif

Begins week 4 centered around stomodeum, external depression at oral membrane

5 initial primordia from neural crest mesenchyme

  • single frontonasal prominence (FNP) - forms forehead, nose dorsum and apex
  • nasal placodes develop later bilateral, pushed medially
  • paired maxillary prominences - form upper cheek and upper lip
  • paired mandibular prominences - lower cheek, chin and lower lip

File:Face animation.gif

Head/Skull

  • chondrocranium forms base of skull
    • in lower vertebrates encases brain
  • cranial vault
    • calveria
  • facial skeleton
    • pharyngeal arches

Sensory Placodes

MH - will cover sensory placodes in Senses Lecture.

  • 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.
  • These 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 postion on the developing head is significantly different to their final position in the future sensory system

Otic placode

  • in the 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 epithelia of this ball varies in thickness and has begun to distort, it will eventually form the inner ear membranous labyrinth.
Stage12 sem6.jpg

Stage 12 Embryo - otic placode

Stage13 otocyst.jpg

Stage 13 Embryo - otocyst

Lens placode

Stage 14 Embryo - - optic and nasal placodes

lies on the surface, adjacent to the outpocketing of the nervous system (which will for the retina) and will form the lens.

Nasal placode

has 2 components (medial and lateral) and will form the nose olefactory epithelium.

Head Growth

Fetal Head (12 weeks) showing bone and cartilage
  • continues postnatally - fontanelle allow head distortion on birth and early growth
  • bone plates remain unfused to allow growth, puberty growth of face

Skull Overview

Lateral view of the cranium of a human fetus 80 mm long.

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
Skull anterior.gif Skull superior.gif Skull lateral view.gif
anterior view superior view lateral view
showing anterior fontenelle, sutures, mandible showing anterior fontenelle, sutures showing suture, mandible

Palate

The palate has two key stages of development during embryonic and an early fetal involving the fusion of structures (epithelia to mesenchymal).

Embryonic

Primary palate, fusion in the human embryo between stage 17 and 18, from an epithelial seam to the mesenchymal bridge.

Stage17-18 Primary palate.gif

Fetal

Stage 22 image 061.jpg

Secondary palate, fusion in the human embryo in week 9. This requires the early palatal shelves growth, elevation and fusion during the early embryonic period. The fusion event is to both each other and the primary palate. palatal shelf elevation | secondary palate

Ear Auricles

  • form from 6 hillocks (week 5)
  • 3 on each of arch 1 and 2

Tongue Development

Tongue1.png Tongue2.png Tongue3.png

  • 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.
  • 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 development animation

Tongue muscle

tongue muscle
  • Tongue muscles originate from the somites. Tongue muscles develop before masticatory muscles and is 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 (wk 6-7) extend into mesenchyme
  • parotid, submandibular, sublingual

Abnormalities

Cleft Lip and Palate

  • 300+ different abnormalities, different cleft forms and extent, upper lip and ant. maxilla, hard and soft palate

Cleft Palate

  • Cleft palate has the International Classification of Diseases code 749.0.
  • In Australia the national rate (1982-1992) for this abnormalitity in births was 4.8 - 6/10,000 births, which represented 1,530 infants 5.5% were stillborn and 11.5% liveborn died during neonatal period and slightly more common in twin births than singleton.

Cleft Lip

  • The International Classification of Diseases code 749.1 for isolated cleft lip and 749.2 for cleft lip with cleft palate.
  • In Australia the national rate (1982-1992) for this abnormalitity was 8.1 - 9.9 /10,000 births. Of 2,465 infants 6.2% were stillborn and 7.8% liveborn died during neonatal period and the rate was similar in singleton and twin births.

First Arch Syndrome

  • There are 2 major types of associated first arch syndromes, Treacher Collins (Mandibulofacial dysostosis) and Pierre Robin (Pierre Robin complex or sequence), both result in extensive facial abnormalites.

Treacher Collins Syndrome

Pierre Robin Syndrome

  • Hypoplasia of the mandible, cleft palate, eye and ear defects.
  • Initial defect is small mandible (micrognathia) resulting in posterior displacement of 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

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

Fetal Alcohol Syndrome

FASface.jpg

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)

Fetal Alcohol Syndrome

Table - 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 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  

Skeletal Structures


File:Mandible sm.jpg mandible
File:Hyoid bone sm.jpg hyoid
File:Larynx cartilage sm.jpg  

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.

References

Textbooks

  • The Developing Human: Clinically Oriented Embryology (8th Edition) by Keith L. Moore and T.V.N Persaud - Moore & Persaud Chapter Chapter 10 The Pharyngeal Apparatus pp201 - 240.
  • Larsen’s Human Embryology by GC. Schoenwolf, SB. Bleyl, PR. Brauer and PH. Francis-West - Chapter 12 Development of the Head, the Neck, the Eyes, and the Ears pp349 - 418.

Online Textbooks

Search

Movies

Face 001 icon.jpg Palate 001 icon.jpg Palate 002 icon.jpg Tongue 001 icon.jpg
Face Palate 1 Palate 2 Tongue

External Links

Embryo Images Unit: Embryo Images Online | Craniofacial Development | Cell Populations | Pharyngeal Arches | Tongue | Nose and Upper Lip | Palate Development

Research Labs: tzahor lab

Terms

palate

The roof of the mouth (oral cavity) a structure which separates the oral from the nasal cavity. Develops as two lateral palatal shelves which grow and fuse in the midline. Initally a primary palate forms with fusion of the maxillary processes with the nasal processes in early face formation. Later the secondary palate forms the anterior hard palate which will ossify and separate the oral and nasal cavities. The posterior part of the palate is called the soft palate (velum, muscular palate) and contains no bone. Abnormalities of palatal shelf fusion can lead to cleft palate. (More? Head | Head Abnormalities | Medline Plus - Cleft Lip and Palate)

palatogenesis

The process of palate formation, divided into primary and secondary palate development. (More? Head | Head Abnormalities | Medline Plus - Cleft Lip and Palate)

pharyngeal arch

(branchial arch, Greek, branchial = gill) These are a series of externally visible anterior tissue bands lying under the early brain that give rise to the structures of the head and neck. In humans, five arches form (1,2,3,4 and 6) but only four are externally visible on the embryo. Each arch has initially identical structures: an internal endodermal pouch, a mesenchymal (mesoderm and neural crest) core, a membrane (endoderm and ectoderm) and external cleft (ectoderm). Each arch mesenchymal core also contains similar components: blood vessel, nerve, muscular, cartilage. Each arch though initially formed from similar components will differentiate to form different head and neck structures. (More? | Head Development | Endocrine | Neural Crest)

pharyngeal arch artery

Each early developing pharyngeal arch contains a lateral pair of arteries arising from the aortic sac, above the heart, and running into the dorsal aorta. later in development these arch arteries are extensively remodelled to form specific components of the vascular system. Pharyngeal Arch 1 arteries are mainly lost and forms part of maxillary artery. Pharyngeal Arch 2 arteries remains to form the stapedial arteries. Pharyngeal Arch 3 arteries forms the common carotid arteries, internal carotid arteries in the neck. Pharyngeal Arch 4 arteries will form part of aortic arch (left arch artery) and part right subclavian artery (right arch artery) Pharyngeal Arch 6 arteries form part of left pulmonary artery (left arch artery) and part of right pulmonary artery (right arch artery). (More? | Head Development | Cardiovascular)

pharyngeal arch cartilage

Each early developing pharyngeal arch contains a horseshoe shaped band of cartilage that acts as a template and contributes to the development of head and neck bony and cartilagenous features, including the middle ear bones. Pharyngeal Arch 1 cartilage (Meckel’s cartilage) dorsal ends form malleus and incus midpart forms ligaments (ant. malleus, sphenomandibular) ventral part forms mandible template. Pharyngeal Arch 2 cartilage (Reichert’s cartilage) dorsal ends form stapes and Temporal bone styloid process, ventral part ossifies to form hyoid bone components, lesser cornu and superior body. Pharyngeal Arch 3 cartilage forms hyoid components, greater cornu and inferior part of hyoid. Pharyngeal Arch 4 and 6 cartilage forms laryngeal cartilages except epiglottis (from hypobranchial eminence). (More? Head Development | Middle Ear)

pharyngeal arch nerve

Each early developing pharyngeal arch contains the developing cranial nerves, as a pair, within the arch mesenchyme. Each cranial nerve is numbered (roman numeral) in rostrocaudal sequence and also has a specific name. The cranial nerve within each arch often relates to the other structures formed from taht arch. Pharyngeal Arch 1 contains the trigeminal nerve (CN V, cranial nerve 5). Pharyngeal Arch 2 contains the facial nerve (CN VII, cranial nerve 7). Pharyngeal Arch 3 contains the glossopharyngeal nerve (CN IX, cranial nerve 9) Pharyngeal Arch 4 and 6 contains the Vagus (CN X cranial nerve 10), forming the adult superior laryngeal and recurrent laryngeal branches. (More? | Head Development | Neural | Neural Crest)

pharyngeal arch pouch

An out-pocketing of the endoderm lined pharynx occurring between each developing pharyngeal arch. Each of the pharyngeal arch pouches contributes different components of the head and neck, either cavities or endocrine tissues. Pharyngeal Arch 1 pouch elongates to form tubotympanic recess tympanic cavity, mastoid antrum and auditory tube (Eustachian tube). Pharyngeal Arch 2 pouch forms the tonsillar sinus and is later mostly oblierated by palatine tonsil. Pharyngeal Arch 3 pouch forms the inferior parathyroid and thymus. Pharyngeal Arch 4 pouch forms the superior parathyroid, parafollicular cells of Thyroid. (More? Middle Ear | Thyroid | Parathyroid | Thymus‎ | Endocrine | Head Development

pharyngotympanic tube

(auditory tube, eustachian tube, otopharyngeal tube) A narrow canal connecting the middle ear space to the back of the oral cavity. The tube allows ventilation, protection and clearance for the middle ear cavity. Ventilation is the pressure equalization in the middle ear. Clearance is to allow fluid drainage from the middle ear. Embryonic origin is from the first pharyngeal pouch. In development, the canal is initially both horizontal, short and very narrow leading to poor drainage and easy blockage. (More? Middle Ear | Hearing | Hearing Abnormalities)

pharynx

(throat) Forms the initial segment of the upper respiratory tract divided anatomically into three regions: nasopharynx, oropharynx, and laryngopharynx (hypopharynx). Anatomically extends from the base of the skull to the level of the sixth cervical vertebra. (More? Respiratory System Development)

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Cite this page: Hill, M.A. (2014) Embryology Lecture - Head Development. Retrieved July 26, 2014, from //php.med.unsw.edu.au/embryology/index.php?title=Lecture_-_Head_Development

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Dr Mark Hill 2014, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G