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Embryology
Lec7
Dr.Ban
Development of face and nasal cavity
The external human face develops between the 4th and 6th weeks of embryonic
development. Facialswellings arise on the frontonasal process (2 medial nasal and
2 lateral nasal processes) and the firstpharyngeal arch (2 mandibular and 2
maxillary processes). By a process of merging and some localizedfusion these
processes come together to form the continuous surfaces of the external face. The
primary palate is formed in this period by fusion/merging of the medial nasal and
maxillary processes.
Sequence of developmental events :
During the 3rdweek of development an oropharyngeal membranebuccopharyngeal
,or oralmembrane) is first seen at the site of the future face, between the
primordium of the heart and the rapidlyenlarging primordium of the brain. It is
composed of ectoderm externally and endoderminternally. It lies at the beginning
of the digestive tract and breaks down during the 4th week in orderto form the
opening between the future oral cavity (primitive mouth or stomodeum) and the
foregut. Theoropharyngeal membrane breaks down when it stops growing. While
tissues around it expand veryrapidly, the oropharyngeal membrane’s nonproliferating cells are gradually pulled apart because theycannot fill the expanding
area.
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Origin of the human face and mouth. The facedevelops from five primordia that appear in the fourth week:
thefrontonasal prominence, the two maxillary swellings, and thetwo mandibular swellings. The
buccopharyngeal membranebreaks down to form the opening to the oral cavity.
Subsequently, between 6th and 12th embryonic/fetal weeks, the secondary palate is
formed as the result of fusion between palatal processes growing from the oral
surfaces of the maxillary processes. Each merging and fusion site is also the site of
a potential facial or palatal cleft.
The human face begins to form during the 4thweek of embryonic development. By
the 6th weekthe external face is completed. Between the 6th and 8thweeks the
development of the palate subdividesnasal and oral cavities. This development
continues into the 12th week with completion of the soft palate.
Occurrence of abnormalities throughout a complicated developmental process is
inevitable. Theincidence of congenital malformations of the face is approximately
1 in 700 births. This numberincludes major defects incompatible with life and
minor defects that are surgically correctable. The mostsevere congenital problems
are those that develop early in facial development (4th–8th weeks); relatively
minor problems develop later (8th–12th weeks). Clinically, it is important to realize
that the tissuessurrounding the forebrain: frontonasal process develop separately
from the tissues of thefirst pharyngeal arch: mandibular and maxillary processes .
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Therefore you may find adevelopmental problem in one or the other but usually
not in both places.
Development of the face
At the end of the 4th week, the center of the face is formed by the stomodeum ,
surrounded by the first pair of pharyngeal arches.
Face initially formed by 5 mesenchymal swellings ( prominences):
• Two mandibular prominences (right and left, from 1st arch neural crest
mesenchyme) .
• Two maxillary prominences (right and left, from 1st arch neural crest
mesenchyme). The maxillary prominence is initially located superior/lateral
to the stomodeum while the mandibular prominence is located inferior to it.
• Frontonasal prominence (midline structure, from cranial neural crest
mesenchyme)is a single structure that is ventral to the forebrain. It is derived
from neural crest cells, which have an ectodermal origin.
• Nasal placodes originate on the frontonasal prominence from the ectoderm.
They thicken and sink in to form nasal pits. At the same time, mesodermal
cells proliferate around the placodes, and the sides of these swellings form
the medial and lateral nasal prominences.
• As the maxillary prominences continue growing they merge laterally with
the mandibular prominences to form the cheeks.
The cheeks
After formation of the upper and lower lips,thestomodium is very broad .In
it’s lateral part,it is bounded above by the maxillary process & below by the
mandibular process.These processes undergo progressive fusion with each
other to form the cheeks. Macrostoma (incomplete lateral merging of
maxillary and mandibular processes).
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• The growthofmaxillary prominences compresses the medial nasal
prominences and causes them to fuse around the 10th week of development.
This establishes a- the bridge of the nose and b-the intermaxillary segment,
which is the region of the medial nasal prominence located inferior to the
bridge of the nose and superior to the mandibular prominence.
• The intermaxillary segment yieldsa- the portion of the upper lip containing
the philtrum, b-the upper jaw with 4 incisors, and c-the primary palate.
•
The medial prominence fuses with the maxillary prominence, giving rise to
a smooth upper lip while fusing the primary and secondary palate.
• Meanwhile, the lateral nasal prominence gives rise to the alae of the nose
and fuses with the maxillary prominence, forming the nasolacrimal duct.
This duct is formed when the ectoderm thickens into a cord and sinks into
the underlying mesenchyme.
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Development of the nasal cavity
The formation of the lateral and medial nasal prominences makes the nasal
placodes lie in the floor of the depression, called nasal pits. The nasal pits deepen
and develop the nasal sacs in the 6th week. These new structures grow
dorsocaudally in front of the forming brain. In the beginning, the nasal sacs are
separated from the oral cavity by the oronasal membrane. This membrane
disappears in the 7th week leaving a connection between the nasal cavities and the
oral cavity, called the primitive choanae. Later, when the development of the
secondary palate occurs, the choanae changes its position and locates at the
junction of the nasal cavity and the pharynx. The nasal septum grows as a
downgrowth from the merged nasal prominences and fuses with the palatine
process between the 9thand 11thweek. Finally, the superior, middle and inferior
conchae develop the lateral wall of each nasal cavity.
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The epithelial covering of the medial nasal and maxillary processes normally
contact and create a zone of fusion named nasal fin. This epithelial fin is soon
presented by connective tissue growth, which binds together the two maxillary and
medial nasal parts of the lip. If this penetration were not to occur, the lip could pull
apart.
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Schematic depiction of breakdown of nasal fin and formation
of nostrils. Arrows indicate disintegration of the nasal fin
between the medial nasal and maxillary prominences
External ear
External ear is formed around the dorsal part of the 1st ectodermal cleft.A series of
mesoderal thickenings appear on the mandibular and hyoid arches where they
adjoin this cleft.The pinna is formed by the fusion of these thickenings,when first
formed the pinna lies caudal to the developing jaw. It is pushed upward and
backward to it’sdefinitve position due to the great enlargement of the mandibular
process.If the mandibular process fails to enlarge ,the ears remain low down.
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