Download Development of the Face

Head and Neck
Embryology
The most distinctive feature in development of the head and
neck is the presence of pharyngeal arches (the old term for
these structures is branchialarches because they somewhat
resemble the gills [branchia] of a fish). These arches appear
in the fourth and fifth weeks of development. they consist
of bars of mesenchymal tissue separated by deep
clefts known as pharyngeal cleft.
With development of the arches and clefts, a number
of outpocketings, the pharyngeal pouches are
formed.
Pharyngeal arches not only contribute to formation of the neck, but
also play an important role in formation of the face.
Each pharyngeal arch consists of a core of mesenchymal tissue covered
on the outside by surface ectoderm and on the inside by epithelium
of endodermal origin. each pharyngeal arch is characterized
by its own muscular components.The muscular components of each
arch have their own cranial nerve, and wherever the muscle cells
migrate, they carry their nerve component with them.
PHARYNGEAL POUCHE
The human embryo has four pairs of pharyngeal pouches; the fifth is
rudimentary. Since the epithelial endodermal lining of the pouches
gives rise to a number of important organs.
PHARYNGEAL CLEFTS
The 5-week embryo is characterized by the presence of four pharyngeal
clefts. of which only one contributes to the definative structure of the
embryo. The dorsal part of the first cleft penetrates the underlying
mesenchyme and gives rise to the external auditory meatus. The
epithelial lining at the bottom of the meatus participates in formation
of the eardrum. the second, third, and fourth clefts lose contact with
the outside The clefts form a cavity lined with ectodermal epithelium,
the cervical sinus, but with further development, this sinus disappears
Thyroid•
􀂄􀂄 In
4th week begins as endodermal thickening in floor of •
primitive pharynx •
􀂄􀂄 The thickening becomes an outpouching: thyroid diverticulum •
􀂄􀂄 Thyroid descends anterior to hyoid and thyroid cartilage •
􀂄􀂄 Connected to tongue by thyroglossal duct •
􀂄􀂄 Week 7: Thyroid reaches final position Thyroglossal duct has •
degenerated
􀂄􀂄 Pyramidal lobe: Persistence of distal end of thyroglossal duct •
Present in 50% of people •
Tongue•
􀂄4th
week: elevation on floor of pharynx, just rostral to foramen
cecum:
Median Tongue Bud (Tuberculum impar)
􀂄 Distal Tongue Buds develop just lateral to median tongue
bud
􀂄Both of the above originate in mesenchyme of first branchial
arch
• Tongue muscles originate from the occipital somites which
• bring with them innervation (CN XII).
• Innervation to tongue:
• .... Ant 2/3: CN V
• .... Post 1/3: CN IX
Distal tongue buds overgrow the median tongue bud and merge with
each other These form the ant 2/3 of the tongue Median tongue bud
forms no adult structure.
At same time 2 elevations develop caudal
to foramen cecum:
1. Copula: from 2nd arch
2. Hypobranchial emminence: from 3rd & 4th arches
The hypobranchial eminence overgrows the copula which disappears
The post 1/3 of the tongue is formed bythe rostral part of the
hypobranchial emminence (Arch 3)
Caudal part of hypobranchial emminence(Arch 4) forms the epiglottis
Teeth develop from epithelial– mesenchymal interactions
between oral epithelium and neural crest–derived mesenchyme.
Enamel is made by ameloblasts It lies on a thick layer
of dentin produced by odontoblasts, a neural crest derivative. •
Cementum is formed by cementoblasts, another
mesenchymal derivative found in the root of the tooth. The first
teeth (deciduous teeth or milk teeth) appear 6 to 24 months
after birth, and the definitive
or permanent teeth, which supplant the milk
teeth, are formed during the third month of development
Development of the Face
Five facial primordia contribute to development of the •
face:
– The frontonasal prominence •
– Paired Maxillary prominences •
– Paired Mandibular prominences •
• 4th week: thickening of ectoderm in the ventrolateral parts of the
FNP: Nasal Placodes
• Mesenchyme on the edges of the placodes proliferates to form:
medial and lateral nasal prominences
• As a result the nasal placodes now lie in a depression called nasal pits
which enlarge dorsally to form the nasal cavities.
• These nasal cavities are separated from the oral cavity by the oronasal
membranes which rupture to form the primitive choana