HEAD AND NECK

• 4th and 5th week
• Pharyngeal arches: consists of bars of mesenchyme tissue separated by deep clefts,
pharyngeal clefts.
• Pharyngeal pouches: outpocketings along the lateral walls of the pharynx
• Resembles the formation of gills in fishes and amphibians.
• Contribute with the formation of the neck and head.
• 6th week: five mesenchymal prominences can be observed – mandibular, maxillary
and frontonasal

PHARYNGEAL ARCHES
• Composed of mesenchyme tissue derived from mesoderm, with surface
covered by ectoderm and inside with endoderm.
• Also has neural crest cells: skeletal components of face.
• Mesoderm: muscles of face and neck.
• Each arch have its own muscle, cranial nerve, artery

FIRST PHARYNGEAL ARCH
• Dorsal portion: maxillary process
• Ventral portion: mandibular process which contains the Meckel’s cartilage
• Meckel’s cartilage disappears except for two small portions at its dorsal end that form the
incus and malleus.
• Maxilllary process: its mesenchyme through membranous ossification form premaxilla,
maxilla, zygomatic and part of temporal bone.
• Muscles: muscles of mastication (temporalis, masseter, pterygoids), anterior belly of
digastric, mylohyoid, tensor tympani, tensor palatine
• Nerve: mandibular branch of the trigeminal nerve, sensory supply by ophthalmic, maxillary
and mandibular branches of trigeminal nerve.

SECOND PHARYNGEAL ARCH
• Cartilage: Reichert’s cartilage form the stapes, styloid process of the temporal
bone, stylohyoid ligament, lesser horn and upper part of the body of hyoid
bone.
• Muscles: stapedius, stylohyoid, posterior belly of the digastric, auricular,
muscles of facial expression.
• Nerve: facial nerve

THIRD PHARYNGEAL ARCH
• Cartilage: lower part of the body, greater horn of the hyoid bone.
• Muscles: stylopharyngeus muscles
• Nerve: glossopharyngeal

FOURTH AND SIXTH PHARYNGEAL
ARCHS
• Cartilages fuse: thyroid, cricoid, arytenoid, corniculate and cuneiform
cartilage of larynx.
• Muscles (4th): cricothyroid, levator platini, constrictors of pharynx
• Nerve: superior laryngeal branch of vagus
• 6th arch: intrinsic muscles of the larynx is innervated by the recurrent
laryngeal branch of vagus.

PHARYNGEAL POUCHES
• Has four pairs and the fifth is rudimentary
• FIRST POUCH
• Forms a stalk-like diverticulum, tubotympanic recess that comes in contact with the
epithelial lining of the 1st pharyngeal cleft to form the external auditory meatus.
• Distal portion of the diverticulum forms the primitive tympanic or middle ear cavity
• Proximal part remains narrow to form the auditory tube
• The lining of the tympanic cavity helps to form the tympanic membrane.

SECOND PHARYNGEAL POUCH
• Form buds that are invaded by mesodermal tissue forming the palatine
tonsils.
• During third and fifth months these are infiltrated by lymphatic tissue.
• Part of the pouch remains and forms the tonsillar fossa.

THIRD PHARYNGEAL POUCH
• Have a dorsal and ventral wing.
• Dorsal wing: inferior parathyroid gland
• Ventral region: thymus, which loses contact with the pharyngeal wall and migrates
in a caudal and medial direction pulling the inferior parathyroid with it.
• The main portion of the thymus moves to the anterior part of the thorax and fuses
with its counterpart from the opposite side. The tail sometimes remain embedded
in the thyroid gland or isolated thymic nests.
• The parathyroid tissue lies on the dorsal surface of the thyroid gland forming the
inferior parathyroid gland.

FOURTH PHARYNGEAL POUCH
• Dorsal wing: superior parathyroid gland which attaches itself to the dorsal
surface of the caudally migrating thyroid forming the superior parathyroid
gland.
• Ventral wing: ultimobranchial body which incorporates into the thyroid
gland.
• Ultimobranchial body: parafolicular or C cells of the thyroid gland.
• They secrete calcitonin.

PHARYNGEAL CLEFTS
• Four pharyngeal clefts
• Dorsal part of the first pharyngeal cleft penetrates the mesenchyme forming
the external auditory meatus.
• The second, third and fourth clefts fuse together and form a cavity lined with
ectodermal epithelium, the cervical sinus but with further development it
disappears.

TONGUE
• 4TH WEEK
• First pharyngeal arch: Two lateral lingual swellings + one medial swelling
(tuberculum impar)
• 2nd, 3rd and 4th arches: second median swelling, the copula (hypobranchial eminence)
• 4th arch: third median swelling forms the epiglottis
• Immediately behind the epiglottis is the laryngeal orifice flanked by the arytenoid
swellings.

• The lateral lingual swellings merge and form the anterior two thirds of the tongue,
innervated by mandibular branch of trigeminal nerve. (sensory)
• The body of the tongue is separated from the posterior third by a v shaped groove, the
terminal sulcus.
• The root of the tongue is formed from the copula and is innervated by the glossopharyngeal
nerve. (sensory)
• The epiglottis and extreme part of the tongue is innervated by superior laryngeal nerve.
• Tongue musculature is innervated by hypoglossal nerve.
• The special sensory innervation (taste) anterior two thirds by the chorda tympani branch of
the facial nerve and posterior third by the glossopharyngeal nerve.

THYROID GLAND
• Appears as an epithelial proliferation where the foramen cecum is.
• It descends infront of the pharyngeal gut as a bilobed diverticulum, keeping
attached to the tongue through the thyroglossal duct that later disappears.
• 7th week: reaches its definitive location infront of the trachea and has two
lobes and small median isthmus.
• 3rd month its functional, follicular cells produce colloid that forms the
thyroxine and triiodothyronine. Parafollicular or C cells are derived from
ultimobrachial body.

FACE
• 4th week
• Facial prominences made up mostly of neural crest derived mesenchyme of 1st pharyngeal arch appear.
• The maxillary, mandibular and frontonasal prominences and the nasal placodes that appear on both sides of
the frontonasal prominence.
• 5th week these invaginate to form the nasal pits, creating ridges that surround the pits, medial and lateral nasal
prominences.
• The maxillary prominences grow medially compressing the medial nasal prominences toward the midline
until they fuse together forming the upper lip.
• The upper lip is fomed by the two maxillay and two medial nasal pominences.
• The lower lip is formed by the merging of the two mandibular prominences on the midline.

• The maxillary and lateral nasal prominences are separated by the nasolacrimal groove.
• Ectoderm on the floor of this groove forms a solid epithelial cord that canalizes forming the
nasolacrimal duct.
• The upper end widens and forms the lacrimal sac.
• The nasolacrimal duct then runs from the medial corner of the eye to the inferior meatus of
the nasal cavity, and the maxillary prominences enlarge to form the cheeks and maxillae.
• Nose is formed by: frontal prominence that forms the bridge, the merged medial nasal
prominences form the crest and tip and the lateral nasal prominences form the sides (alae)

INTERMAXILLARY SEGMENT
• The medial growth of the maxillary prominences causes the two medial nasal
prominences to fuse deeper and form the intermaxillary segment:
• 1. labial component: philtrum of upper lip
• 2. upper jaw component: caries the four incisor teeth
• 3. palatal component: triangular primary palate
• This is continuous with the rostral portion of the nasal septum formed by
the frontal prominence.

SECONDARY PALATE
• The main definitive palate is formed by two shelf like outgrowths (palatine
shelves) of the maxillary prominences.
• 7th week the palatine shelves ascend horizontally above the tongue and fuse
forming the secondary palate.
• Anteriorly they fuse with the triangular primary palate and the incisive
foramen marks the midline between both.
• The nasal septum (frontonasal prominence) grows down and fuses with the
cephalic aspect of the palate.

NASAL CAVITIES
• 6th week
• Nasal pits deepen and is separated by the oronasal membrane from the oral cavity by way of
the newly formed foramina, primitive choanae.
• These choanae lie on each side of the midline and immediately behind the primary palate
but with the formation of the secondary palate and development of nasal chambers the
definitive choanae lie in the junction of the nasal cavity and the pharynx.
• Paranasal air sinuses develop as a diverticula of lateral nasal wall and extend into the maxilla,
ethmoid, frontal and sphenoid bones.
• They reach their definitve size during puberty and contribute to the definitive shape of the
face.

TEETH
• Arise from an epithelial-mesenchyme interaction between the overlying oral
epithelium and underlying mesenchyme derived from neural crest.
• 6th week basal layer of oral cavity forms a c shaped structure, the dental
lamina, along the length of the upper and lower jaws.
• This gives rise to dental buds, 10 on each jaw, the deep surface of the buds
invaginate forming the cap stage of tooth development.

• The cap consists of outer dental epithelium layer and the inner dental
epithelium layer and the central core of loosely woven tissue, the stellate
reticulum.
• The mesenchyme that originates in the neural crest in the indentation forms
the dental papilla.
• Bell stage: the dental cap grows and the indentation deepens appearing a bell.